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Genetics of Bacillus thuringiensis resistance in insect pests.

Bacillus thuringiensis is an insect pathogen which is used as a bioinsecticide for the control of certain insect species, particularly those belonging to the orders Lepidoptera, Diptera and Coleoptera. Laboratory-selected resistance to B. thuringiensis has been documented in a number of pest species and field-evolved resistance has been detected in Plutella xylostella. As a consequence, the development of resistance is a major concern for the continued success of B. thuringiensis bioinsecticides.

This month's feature highlights a paper from the Bulletin of Entomological Research 89, issue 5, by F. Huang, R. A. Higgins and L. L. Buschman on Heritability and stability of resistance to Bacillus thuringiensis in Ostrinia nubilalis (Lepidoptera: Pyralidae). The authors' abstract from the paper is as follows:

Realized heritability, h², of resistance in European corn borer, Ostrinia nubilalis (Hübner), to Bacillus thuringiensis Berliner ssp. kurstaki endotoxins was examined in five resistant laboratory colonies. These colonies were reared on a meridic diet that incorporated a commercial formulation of B. thuringiensis, Dipel ES. Resistance in these colonies reached 42-67x by the seventh to twentieth selected generations and then plateaued. The realized heritability of resistance averaged 0.17-0.31 over all selected generations for the five colonies. In the three Iowa colonies, the highest realized heritability, 0.18-0.33, occurred during the second period of selection (seventh to thirteenth selected generations). In the two Kansas colonies, the highest realized heritability, 0.36 and 0.46, occurred during the first period of selection (first to sixth selected generations). In the absence of selection pressure, resistance in the southwest Kansas colony decreased from 62x to 42x after two generations, and remained at about that level for the next five generations.

The full version of the paper is available on PEST CABWeb^®.

A selection of abstracts from Review of Agricultural Entomology is given below, relating to the genetics of resistance to B. thuringiensis in insect pests.

Related items included in PEST CABWeb^®:

• Bulletin of Entomological Research

• Review of Agricultural Entomology

• Biocontrol News and Information

Also of interest:

• Insecticide resistance: from mechanisms to management. I. Denholm, J. A. Pickett and A. L. Devonshire (Editors), Department of Biological and Ecological Chemistry, IACR Rothamstead, Harpenden, UK.

TI: Inheritance of resistance to Bacillus thuringiensis toxin (Dipel ES) in the European corn borer.
AU: Huang, F.\ Buschman, L. L.\ Higgins, R. A.\ McGaughey, W. H.
JN: Science (Washington)
YR: 1999
VL: 284
NO: 5416
PP: 965-967
LA: En
MS: 22 ref.
AA: Department of Entomology, Kansas State University, Manhattan, KS 66506, USA.
AB: Resistance in the European corn borer, Ostrinia nubilalis, to a commercial formulation of Bacillus thuringiensis (Bt) toxin, Dipel ES, appears to be inherited as an incompletely dominant autosomal gene. This contrasts with the inheritance of resistance to Bt in other insects, where it has usually been characterized as a recessive trait. The proposed high concentration/refuge strategy for resistance management in Bt maize depends on resistance being recessive or partially recessive. If field resistance turns out to be similar to this laboratory resistance, the usefulness of the high concentration/refuge strategy for resistance management in Bt maize may be diminished.
DE: Bacillus\Bacillus thuringiensis\toxins\Zea mays\maize\Ostrinia nubilalis\inheritance\insecticide resistance\genes\insect control\chemical control
AN: 0E08708145\6P01502140

TI: Levels, inheritance and stability of resistance to Bacillus thuringiensis formulation in a field population of the diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae) from Thailand.
AU: Imai, K.\ Mori, Y.
JN: Applied Entomology and Zoology
YR: 1999
VL: 34
NO: 1
PP: 23-29
LA: En
MS: 27 ref.
AA: Pesticide Section, Agricultural R & D Center, ZEN-NOH, Hiratsuka, Kanagawa 254-0016, Japan.
AB: Levels, genetic traits and stability of resistance to Bacillus thuringiensis in the Bang Bua Thong(BS) colony of the diamondback moth, Plutella xylostella, collected in Thailand, was investigated. The BS colony exhibited 668 and 66.5-fold resistance to formulations derived from B. thuringiensis subsp. kurstaki (ToarowCT) and B.thuringiensis subsp. aizawai (XenTari), respectively. In order to determine the genetic traits of resistance, two reciprocal crosses and their offspring were checked for sensitivity to B  thuringiensis using the laboratory susceptible (H) and the resistant BS colonies. Analysis of the  dose-mortality relationship of their progeny suggested that resistance to B. thuringiensis in the BS colony might be polyfactorial. Susceptibility to B. thuringiensis (ToarowCT and XenTari) in the BS colony was not quickly restored in the absence of B. thuringiensis selection.
DE:Bacillus\Bacillus thuringiensis\Plutella xylostella\inheritance\Lepidoptera\Plutella\Plutellidae\Thailand\colonies\formulations\susceptibility\
entomopathogenic bacteria\Bacillus thuringiensis subsp. kurstaki\Bacillus thuringiensis subsp.
aizawai\resistance
AN: 0E08707475

TI: Genetic mapping of resistance to Bacillus thuringiensis toxins in diamondback moth using biphasic linkage analysis.
AU: Heckel, D. G.\ Gahan, L. J.\ Liu YongBiao\ Tabashnik, B. E.
JN: Proceedings of the National Academy of Sciences of the United States of America
YR: 1999
VL: 96
NO: 15
PP: 8373-8377
LA: En
MS: 35 ref.
AA: Department of Biological Sciences, Clemson University, Clemson, SC 29634, USA.
AB: The major gene conferring resistance to B. thuringiensis (Bt) toxins in diamondback moth (Plutella xylostella) was mapped. The biphasic nature of Lepidopteran genetic linkage was used to map the gene with 207 amplified fragment length polymorphism markers. An amplified fragment length polymorphism marker for the chromosome containing the Bt resistance gene was cloned and sequenced. It is concluded that the results provide a powerful tool for facilitating progress in understanding, monitoring and managing resistance to Bt. 
DE: Bacillus thuringiensis\Plutella xylostella\linkage\mapping\DNA\plant pests\insect
pests\genetics\amplified fragment length polymorphism\nucleotide sequences\toxins\pesticide
resistance\genes\Plutellidae

TI: Insect resistance to Bacillus thuringiensis: uniform or diverse?
CT: Insecticide resistance: from mechanisms to management [edited by Denholm, I.; Pickett, J. A.; Devonshire, A. L.].
AU: Tabashnik, B. E.\ Liu YongBiao\ Malvar, T.\ Heckel, D. G.\ Masson, L.\ Ferre, J.
JN: Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences
YR: 1998
VL: 353
NO: 1376
PP: 1751-1756
LA: En
MS: 45 ref.
AA: Department of Entomology, University of Arizona, Tucson, AZ 85721, USA.
AB: This paper reviews the biology of the insecticidal proteins produced by B. thuringiensis (Bt), resistance to Bt, variation in the genetic basis of resistance to Bt among 3 populations of Plutella xylostella and other moths, and implications for Bt resistance management. Resistance to Bt has been documented in more than a dozen species of insect. Nearly all of these cases have been produced primarily by selection in the laboratory, but one pest, P. xylostella, has evolved resistance in open-field populations. Insect resistance to Bt has immediate and widespread significance because of increasing reliance on Bt toxins in genetically engineered crops and conventional sprays. Furthermore, intense interest in Bt provides an opportunity to examine the extent to which evolutionary pathways to resistance vary among and within species of insect. One mode of resistance to Bt is characterized by more than 500-fold resistance to at least one CrylA toxin, recessive inheritance, little or no cross-resistance to  CrylC, and reduced binding of at least one CrylA toxin. Analysis of resistance to Bt in P.xylostella and two other species of moths suggests that although this particular mode of resistance may be the mostcommon, it is not the only means by which insects can attain resistance to Bt.
DE: Bacillus thuringiensis\microbial pesticides\bacterial insecticides\bacterial toxins\Plutella
xylostella\genetic engineering\inheritance\Lepidoptera\insecticides\insecticide resistance\resistance
mechanisms\resistance management\proteins\insect pests\plant pests\pest control\genes\insect
control\biological control
AN: 0E08704391

TI: Biochemistry and genetics of insect resistance to Bacillus thuringiensis insecticidal crystal proteins.
AU: Ferre, J.\ Escriche, B.\ Bel, Y.\ Rie, J. van
JN: FEMS Microbiology Letters
YR: 1995
VL: 132
NO: 1/2
PP: 1-7
LA: En
MS: 30 ref.
AA: Departamento de Genetica, Fac. CC Biologicas, Universitat de Valencia, Av. Dr. Moliner 50, 46100-Burjassot (Valencia), Spain.
AB: Current knowledge of biochemical mechanisms of insect resistance to Bacillus thuringiensis is reviewed. Available information on resistance inheritance and on patterns of cross-resistance is included. Modification of the binding sites for B. thuringiensis insecticidal crystal proteins has been  found in different populations of three insect species (Plodia interpunctella, Plutella xylostella, Heliothis virescens). This resistance mechanism seems to be inherited as a single recessive or partially recessive major gene, and the resistance levels reached are high. Altered proteolytic processing of B. thuringiensis crystal proteins has been suggested to be involved in one case of resistance. From the available data it seems that binding site modification is the most significant resistancemechanism under field conditions.
DE: cross resistance\crystal proteins\binding sites\bacterial toxins\bacterial insecticides\genetic
resistance\defence mechanisms\population genetics\biological controlagents\Bacillus
thuringiensis\resistance\Plodiainterpunctella\Plutella xylostella\Heliothis
virescens\reviews\Lepidoptera\genetics\disease resistance\resistance\genetics\insecticide resistance
AN: 0E08501309\7E01800671

TI: Evolution of resistance to Bacillus thuringiensis.
AU: Tabashnik, B. E.
JN: Annual Review of Entomology
YR: 1994
VL: 39
PP: 47-79
LA: En
MS: 167 ref.
AA: Department of Entomology, University of Hawaii, Honolulu, HI 96822, USA.
AB: This review examines the following aspects of resistance to B. thuringiensis (Bt): laboratory selection, resistance risk assessment, variation among conspecific populations (including Lepidoptera, Coleoptera and Diptera), mechanisms of resistance, cross-resistance, genetics, stability, fitness costs, and resistance management. The author concludes that although genetic variation for resistance may be somewhat less for Bt than for conventional insecticides, the limited use and low persistence of Bt are probably the primary reasons for the scarcity of resistance in the field so far. 
DE: Diptera\Coleoptera\Lepidoptera\resistance mechanisms\cross resistance\resistance
management\bacterial insecticides\entomopathogenic bacteria\reviews\Bacillus thuringiensis\insect
pests\resistance\insecticide resistance\insects\resistance\microbial pesticides\resistance
AN: 0J08302308\7E01601084\0E08307382

TI: Inheritance and stability of resistance to Bacillus thuringiensis formulations of the diamondback moth, Plutella xylostella (Linnaeus) (Lepidoptera: Yponomeutidae).
AU: Hama, H.\ Suzuki, K.\ Tanaka, H.
JN: Applied Entomology and Zoology
YR: 1992
VL: 27
NO: 3
PP: 355-362
LA: En
MS: 18 ref.
AA: National Institute of Agro-Environmental Sciences, Tsukuba, Ibaraki 305, Japan.
AB: Genetic traits, stability and degree of resistance to Bacillus thuringiensis (BT) in the Kohno populations RO and ROO of Plutella xylostella were investigated using the leaf-dip method. The RO and ROO populations exhibited 78.5 and 704-fold resistance to B. thuringiensis (formulated as Toarrow CT), resp., high resistance to other formulations derived from B. t subsp. kurstaki and low resistance to B. thuringiensis subsp. kurstaki and aizawai. Analysis of dose-mortality relationships of F(1), F(2) and backcross progenies derived from crossing susceptible and ROO populations revealed that B. thuringiensis resistance was primarily controlled by an incompletely recessive, autosomal single allele. High B. thuringiensis resistance in the plutellid have decreased within generations in the absence of insecticidal selection. The RO population exhibited 46- and 52-fold resistance to the tertiary amines cartap and thiocyclam, resp. Resistance to tertiary amines has remained high without insecticidal selection.
DE: insect pests\Plutellidae\Lepidoptera\pesticides\bacterial insecticides\pesticide resistance\cross
resistance\formulations\Plutella xylostella\Bacillus thuringiensis subsp. kurstaki\Bacillus thuringiensis
subsp. aizawai\Bacillus thuringiensis\cartap\thiocyclam\resistance\insecticides\insecticide
resistance\entomopathogenic bacteria\Bacillus thuringiensis\genetics\Bacillus
thuringiensis\resistance\microbial pesticides
AN: 0E08200113\7E01500137

TI: Inheritance of resistance to the Bacillus thuringiensis CryIC toxin in Spodoptera littoralis
(Lepidoptera: Noctuidae).
AU: Chaufaux, J.\ Muller-Cohn, J.\ Buisson, C.\ Sanchis, V.\ Lereclus, D.\ Pasteur, N.
JN: Journal of Economic Entomology
YR: 1997
VL: 90
NO: 4
PP: 873-878
LA: En
MS: 28 ref.
AA: Institut National de la Recherche Agronomique, Station de Recherches de Lutte Biologique, La
Miniere, 78285 Guyancourt Cedex, France.
AB: A strain of Spodoptera littoralis, selected for resistance to the Bacillus thuringiensis CryIC toxin, was used to analyse the inheritance of resistance. This strain resulted from a laboratory selection with spore-crystal preparations of B. thuringiensis CryIC toxin. At generation 12, resistance was 500 times the resistance ratio of the control. Dose-mortality curves of the resistant population, the susceptible strain, the F1 and the backcrosses from reciprocal crosses were compared. The resistance was partially recessive. F(1) offspring from resistant males and susceptible females were more resistant than those from susceptible males and resistant females, indicating a clear paternal effect. Segregation of resistance was investigated in the 8 possible backcrosses between F(1) and one of the parental strains. Dose-mortality responses significantly differed from those expected under the hypotheses of a major resistance gene that was either autosomal or sex linked. It was concluded that resistance was most probably polyfactorial. These results are discussed in relation to the nature of bacterial toxin resistance genes in various insects. 
DE: spodoptera littoralis\bacillus thuringiensis\lepidoptera\noctuidae\inheritance\resistance\insecticide
resistance\microbial pesticides\bacterial insecticides
AN: 0E08600498

TI: Genetic basis of diamondback moth resistance to Bacillus thuringiensis toxin Cry1C.
AU: Liu YongBiao\ Tabashnik, B. E.
JN: Resistant Pest Management
YR: 1997
VL: 9
NO: 2
PP: 21-22
LA: En
MS: 16 ref.
AA: Department of Entomology, University of Arizona, Tucson, Arizona 85721, USA.
AB: A resistant colony of diamondback moths [Plutella xylostella] to the toxin Cry1C of Bacillus thuringiensis subsp. aizawai (Bta) was established in the laboratory from a resistant field population from Hawaii, USA. It was then attempted to increase resistance through selection, and assess the genetic basis of resistance. After 6 selections with Cry1C, the resistance level increased from 22-fold to 62-fold. Resistance was autosomally inherited as no significant difference in LC50 occurred in either reciprocal crosses carried out between resistant and susceptible strains. Resistance to Cry1Ab was also tested and found not to be linked with resistance to Cry1C. This indicates the gene(s) which confer resistance to Cry1C segregate independently of those for resistance to Cry1Ab. It is concluded that the most successful pest management programme may be to use Cry1C and Cry1A in rotation or mixtures to delay resistance development. 
DE: molecular genetics\Plutella xylostella\insect pests\plant pests\insecticide resistance\bacterial
insecticides\Bacillus thuringiensis\genetics\toxins
GL: Hawaii\USA
AN: 0E08606083

TI: Global variation in the genetic and biochemical basis of diamondback moth resistance to Bacillus thuringiensis.
AU: Tabashnik, B. E.\ Liu YongBiao\ Malvar, T.\ Heckel, D. G.\ Masson, L.\ Ballester, V.\ Granero, F.\ Mensua, J. L.\ Ferre, J.
JN: Proceedings of the National Academy of Sciences of the United States of America
YR: 1997
VL: 94
NO: 24
PP: 12780-12785
LA: En
MS: 41 ref.
AA: Department of Entomology, University of Arizona, Arizona, Tucson, AZ 85721, USA.
AB: Populations of Plutella xylostella from Hawaii and Pennsylvania share a genetic locus at which a recessive mutation associated with reduced toxin binding confers extremely high resistance to four Bacillus thuringiensis (Bt) toxins. In contrast, resistance in a population from the Philippines shows multilocus control, a narrower spectrum, and for some Bt toxins, inheritance that is not recessive and not associated with reduced binding. The observed variation in the genetic and biochemical basis of resistance to Bt, which is unlike patterns documented for some synthetic insecticides, affects the choice of strategies for combating resistance.
DE: Bacillus thuringiensis\resistance mechanisms\Plutella xylostella\toxins\genetics\insecticide
resistance\biological control agents
GL: USA\Hawaii\Pennsylvania
AN: 0E08604293\7E01901088

TI: Inheritance of resistance to the Bacillus thuringiensis toxin Cry1C in the diamondback moth.
AU: Liu YongBiao\ Tabashnik, B. E.
JN: Applied and Environmental Microbiology
YR: 1997
VL: 63
NO: 6
PP: 2218-2223
LA: En
MS: 47 ref.
AA: Department of Entomology, University of Hawaii, Honolulu, HI 96822, USA.
AB: Laboratory selection increased resistance to the Bacillus thuringiensis toxin Cry1C in a strain of Plutella xylostella. The selected strain was derived from a field population that had evolved high levels of resistance to B. thuringiensis subsp. kurstaki and moderate resistance to Cry1C. Relative to the responses of a susceptible strain of P. xylostella, the resistance to Cry1C of the selected strain increased by 62-fold after 6 generations of selection. The realized heritability of resistance was 0.10. Analysis of F(1) hybrid progeny from reciprocal crosses between the selected and a susceptible strain showed that resistance to Cry1C was autosomally inherited. The dominance of resistance to Cry1C depended on the concentration; inheritance was increasingly dominant as the concentration decreased. Responses of progeny from single-pair families showed that resistance to Cry1C and Cry1Ab were inherited independently, enhancing opportunities for managing resistance. However, the potentially dominant inheritance of resistance to Cry1C could accelerate evolution of resistance.
DE: cross resistance\microbial pesticides\Bacillus thuringiensis subsp. kurstaki\toxins\Bacillus
thuringiensis\pesticide resistance\Plutella xylostella\Bacillus thuringiensis\resistance
AN: 7E01802712\0E08600479

TI: One gene in diamondback moth confers resistance to four Bacillus thuringiensis toxins.
AU: Tabashnik, B. E.\ Liu YongBiao\ Finson, N.\ Masson, L.\ Heckel, D. G.
JN: Proceedings of the National Academy of Sciences of the United States of America
YR: 1997
VL: 94
NO: 5
PP: 1640-1644
LA: En
MS: 42 ref.
AA: Department of Entomology, University of Hawaii, Honolulu, HI 96822, USA.
AB: Environmentally benign insecticides derived from Bacillus thuringiensis (Bt) are the most widely-used biopesticides, but their success will be short-lived if pests adapt quickly to them. The risk of evolution of resistance by pests has increased, because transgenic crops producing insecticidal proteins from Bt are being grown commercially. Efforts to delay resistance with two or more Bt toxins assume that independent mutations are required to counter each toxin. Also, it is generally assumed that resistance alleles are rare in susceptible populations. These assumptions were tested by conducting single-pair crosses with Plutella xylostella, the first insect known to have evolved resistance to Bt in open field populations. An autosomal recessive gene conferred extremely high resistance to four Bt toxins (Cry1Aa, Cry1Ab, Cry1Ac, and Cry1F). The finding that 21% of the individuals from a susceptible strain were heterozygous for the multiple-toxin resistance gene implied that the resistance allele frequency was 10 times higher than the most widely cited estimate of the upper limit for the initial frequency of resistance alleles in susceptible populations. These findings suggest that pests may evolve resistance tosome groups of toxins much faster than previously expected.
DE: microbial pesticides\natural enemies\biological control agents\entomopathogens\transgenic
plants\bacterial toxins\alleles\gene frequency\entomopathogenic bacteria\Bacillus thuringiensis\Plutella
xylostella\toxins\resistance\insecticides\insecticide resistance\genetics\molecular genetics\genes\toxins
AN: 0E08508104\7E01802818

TI: Inheritance, stability, and lack-of-fitness costs of field-selected resistance to Bacillus thuringiensis in diamondback moth (Lepidoptera: Plutellidae) from Florida.
AU: Tang, J. D.\ Gilboa, S.\ Roush, R. T.\ Shelton, A. M.
JN: Journal of Economic Entomology
YR: 1997
VL: 90
NO: 3
PP: 732-741
LA: En
MS: 40 ref.
AA: Department of Entomology, Cornell University, New York State Agricultural Experiment Station, Geneva, NY 14456, USA.
AB: A colony of Plutella xylostella, established from crucifer fields in Florida, was used to investigate resistance to Bacillus thuringiensis subsp. kurstaki. From an initial level of more than1500-fold, resistance fell within 3 generations in the absence of selection to approx.300-fold compared with susceptible larvae. Unlike previous cases of resistance to B. thuringiensis in P. xylostella, resistance in the Florida colony was stable at approx.300-fold without additional selection in the laboratory. High levels of resistance (more than1000-fold) recurred after a single exposure to B. t. subsp. kurstaki in the 4th generation. High levels of resistance did not recur after a 2nd selection in the 8th generation. Cage studies and genetic analysis of F1 larvae and backcross progeny, where the resistant parents were characterized by stable levels of resistance, showed that resistance was an incompletely recessive, autosomal trait probably controlled by a single allele that did not confer detectable levels of reduced fitness in the absence of exposure to B. thuringiensis. As one of the few studies to demonstrate stable resistance to B. t. subsp. kurstaki from insects that were collected from the field and not subject to further selection in the laboratory, these results clearly emphasize the need to develop specific resistance management strategies for B. thuringiensis before there is widespread evolution of resistance.
DE: Brassicaceae\insect pests\plant pests\microbial pesticides\bacterial insecticides\entomopathogenic
bacteria\Bacillus thuringiensis subsp. kurstaki\Plutella xylostella\resistance\insecticides\insecticide
resistance
GL: USA\Florida
AN: 0E08510457\7E01900128

TI: A genomic approach to understanding Heliothis and Helicoverpa resistance to chemical and biological insecticides.
CT: Insecticide resistance: from mechanisms to management [edited by Denholm, I.; Pickett, J. A.; Devonshire, A. L.].
AU: Heckel, D. G.\ Gahan, L. J.\ Daly, J. C.\ Trowell, S.
JN: Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences
YR: 1998
VL: 353
NO: 1376
PP: 1713-1722
LA: En
MS: 37 ref.
AA: Department of Biological Sciences, Clemson University, Clemson, SC 29634, USA.
AB: Genomics is the comparative study of the structure and function of entire genomes. Although the complete sequencing of the genome of any insect pest is far in the future, a genomic approach can be useful in the study of mechanisms of insecticide resistance. The authors describe this strategy for Heliothis and Helicoverpa. Genome-wide linkage mapping provides the location of major and minor resistance genes. Positional cloning identifies novel resistance genes, even when the mechanisms are
poorly understood, as with resistance to Bacillus thuringiensis toxins. Anchor loci provide the reference points for comparing the genomes and the genetic architecture of resistance mechanisms among related species. Collectively, these tools enable the description of the evolutionary response of related, but independent, genomes to the common selective pressure of insecticides in the environment. They also provide information that is useful for targeted management of specific resistance genes, and may even speed the search for families of novel insecticidal targets in Lepidoptera.
DE: Helicoverpa\insecticides\Bacillus thuringiensis\cloning\genes\genomes\insecticide
resistance\Lepidoptera\linkage\mapping\Heliothis\pesticide resistance\molecular genetics\random
amplified polymorphic DNA\restriction fragment length polymorphism\resistance mechanisms\DNA
sequencing\nucleotide sequences
AN: 0E08703963

TI: Resistance risk assessment for single and multiple insecticides: responses of Indianmeal moth (Lepidoptera: Pyralidae) to Bacillus thuringiensis.
AU: Tabashnik, B. E.\ McGaughey, W. H.
JN: Journal of Economic Entomology
YR: 1994
VL: 87
NO: 4
PP: 834-841
LA: En
MS: 35 ref.
AA: Department of Entomology, University of Hawaii, Honolulu, HI 96822, USA.
AB: Criteria for comparing the risk of resistance development among single insecticides and between mixtures and sequences of 2 insecticides are described. The rate of development of resistance to an insecticide is proportional to the population's heritability (h(2)) of resistance to that insecticide. When cross-resistance is absent, a sequence of 2 insecticides is expected to be more durable than a mixture unless the population's h(2) of resistance to the mixture is less than half of the mean of the population's h(2) of resistance to the 2 individual components of the mixture. These criteria were applied to 11 previously reported selection experiments with the biopesticide Bacillus thuringiensis and Plodia interpunctella, a major pest of stored grain. The risk of resistance development did not differ significantly between the HD-1 strain of B. thuringiensis subsp. kurstaki and 3 other strains (HD-112, HD-133 and HD-198) of B. thuringiensis. Significant declines in realized h(2) of resistance during individual selection experiments suggest that the initial frequency of resistance of resistance alleles was much higher than previously assumed. This analysis also suggests that a mixture with a sequence of HD-1 followed by HD-133. Rapid evolution of resistance to the mixture of HD-1 + HD-133, which contained at least 6 different toxins, contradicts the claim that multiple toxins prevent or greatly retard resistance development.
DE: Lepidoptera\Pyralidae\insect pests\stored products pests\microbial pesticides\entomopathogenic
bacteria\Bacillus thuringiensis subsp. kurstaki\insecticide resistance\Plodia
interpunctella\resistance\insecticides\Bacillus thuringiensis\resistance
AN: 0E08304206\7E01601097

TI: Problems of insect resistance to Bacillus thuringiensis.
CT: Proceedings of a workshop on Bacillus thuringiensis, 24-26 September 1991, Canberra, ACT, Australia [edited by Milner, R. J.].
AU: McGaughey, W. H.
JN: Agriculture, Ecosystems & Environment
YR: 1994
VL: 49
NO: 1
PP: 95-102
LA: En
MS: 43 ref.
AA: US Grain Marketing Research Laboratory, USDA, ARS, 1515 College Avenue, Manhattan, KS 66502, USA.
AB: The implications of the resistance of insect pests to Bacillus thuringiensis, resistance discovered
to date, the genetics, characteristics, mechanism and management of insect resistance (involving techniques that minimise selection pressure, such as providing untreated refuges, and the use of multiple toxins in various mixtures or spatial and sequential patterns) are discussed. Since 1985, the potential for resistance has been demonstrated in at least 5 species, with high resistance among field populations reported in Plutella xylostella in Hawaii. In P. xylostella and Plodia interpunctella, the potential for resistance has been found to be widespread among diverse populations, with laboratory studies suggesting that it could reach high levels within only a few generations. The mechanism has been found to involve a change in the binding affinity of the insects' midgut membrane that is specific for the particular toxin type used in selecting the resistant population. 
DE: insect pests\bacterial insecticides\Plutellidae\Lepidoptera\Pyralidae\biological control
agents\conferences\Bacillus thuringiensis\entomopathogenic bacteria\resistance\Plutella
xylostella\Plodia interpunctella\insecticides\insecticide resistance\resistance
management\insects\microbial pesticides
AN: 0E08208967\0J08205867\7E01502796

TI: Inheritance of resistance to a Bacillus thuringiensis toxin in a field population of diamondback moth (Plutella xylostella).
AU: Martinez-Ramirez, A. C.\ Escriche, B.\ Real, M. D.\ Silva, F. J.\ Ferre, J.
JN: Pesticide Science
YR: 1995
VL: 43
NO: 2
PP: 115-120
LA: En
MS: 32 ref.
AA: Departamento de Genetica, Facultad de Ciencias Biologicas, Universitat de
Valencia,46100 Burjassot, Valencia, Spain.
AB: The inheritance of resistance to the Bacillus thuringiensis CryIA(b) crystal protein was studied in Plutella xylostella. A field population 50-fold more resistant to CryIA(b) than a control susceptible strain was used. Dose-mortality curves of the resistant population, the susceptible strain and the F(1) from the 2 reciprocal crosses were compared. Resistance transmission to the F(1) was dependent on the sex of the resistant progenitor. The sex ratio of the survivors to high doses of  CryIA(b) in the F(1) of the 2 reciprocal crosses did not corroborate the preliminary hypothesis, being due to a recessive sex-linked allele. Since, in a previous work, the loss of CryIA(b) binding capacity of resistant insects had been demonstrated, binding to midgut tissue sections from F(1) individuals was also analysed. The presence of binding in all of the F(1) preparations showed that, at least, a recessive autosomal allele was responsible for the loss of binding capacity in the resistant population.
DE: insect pests\Plutellidae\Lepidoptera\natural enemies\microbial
pesticides\insecticides\pesticides\biological control agents\pesticide resistance\bacterial
toxins\alleles\genes\insecticide resistance\Plutella xylostella\Bacillus
thuringiensis\toxins\resistance\genetics\molecular genetics\entomopathogenic bacteria\bacterial
insecticides\resistance
AN: 0E08306248\7E01601934

TI: Genetics of Heliothis and Helicoverpa resistance to chemical insecticides and to Bacillus
thuringiensis.
CT: Resistance '97. Integrated approach to combating resistance.
AU: Heckel, D. G.\ Gahan, L. J.\ Gould, F.\ Daly, J. C.\ Trowell, S.
JN: Pesticide Science
YR: 1997
VL: 51
NO: 3
PP: 251-258
LA: En
MS: 82 ref.
AA: Department of Biological Sciences, Clemson University, Clemson, SC 29634, USA.
AB: Genetic linkage maps of Heliothis virescens and Helicoverpa armigera are being used to identify and characterize resistance-conferring genes. The insensitive acetylcholinesterase conferring resistance to organophosphorus insecticides and the insensitive sodium channel conferring resistance to pyrethroids have both been mapped in H. virescens. The linkage mapping approach permits a genetic dissection of resistance, even when the mode of action and lethal target are not precisely known, such as for the insecticidal toxins from the bacterium Bacillus thuringiensis (Bt). A major Bt-resistance locus in a strain of H. virescens exhibiting up to 10<thin>000-fold resistance to Cry1Ac toxin has been identified and mapped. The authors are currently developing a linkage map for H. armigera with a set of 'anchor' loci to facilitate comparison with H. virescens. Both species are currently experiencing their first significant selective pressure in the field by transgenic cotton expressing Cry1Ac, and timely identification of resistance mechanisms and their underlying genetics basis will be essential in successfully managing the Bt resistance that will eventually appear.
DE: linkage\resistance\acetylcholinesterase\genetics\insecticides\Heliothis virescens\Helicoverpa
armigera\Bacillus thuringiensis\bacterial insecticides\conferences\Resistance '97. Integrated approach
to combating resistance
AN: 0E08604381

TI: Field-evolved resistance to Bacillus thuringiensis toxin CryIC in diamondback moth (Lepidoptera: Plutellidae).
AU: Liu YongBiao\ Tabashnik, B. E.\ Pusztai-Carey, M.
JN: Journal of Economic Entomology
YR: 1996
VL: 89
NO: 4
PP: 798-804
LA: En
MS: 39 ref.
AA: Department of Entomology, University of Hawaii, Honolulu, HI 96822, USA.
AB: Previous results have shown that populations of the plutellid Plutella xylostella resistant to toxins from Bacillus thuringiensis subsp. kurstaki were susceptible to the toxin CryIC. Use of commercial formulations of B. thuringiensis subsp. aizawai that contain CryIC has increased recently. Analysis of 2 commercial formulations by HPLC showed that CryIC accounted for 26% of the CryI protein in the B. thuringiensis subsp. aizawai formulation, but did not occur in the B. thuringiensis subsp. kurstaki formulation. CryIAb was the most abundant CryI protein in the commercial formulations of B. thuringiensis subspp. aizawai and
kurstaki Resistance was found to CryIC in a field population of P. xylostella from Hawaii that had been treated with B. thuringiensis subsp. aizawai. Leaf residue bioassays showed that at 5 days after treatment with CryIC, LC50 for colonies derived from this population in 1993 and 1995 were approx. 20-times greater than the LC50 for a susceptible laboratory colony. For a nearby population that had not been treated with B. thuringiensis subsp. aizawai, responses to CryIC did not differ significantly from those of the susceptible laboratory colony. Resistance to CryIAb was lower in a CryIC-resistant colony than in a CryIC-susceptible colony that had been selected with B. thuringiensis subsp. kurstaki. The results suggested that the gene(s) conferring resistance to CryIC segregate independently from the gene(s) conferring resistance to CryIAb. In contrast with previous results with colonies derived in 1989, resistance to B. thuringiensis subsp. kurstaki in a colony derived in 1993 from the same field population did not decline when exposure to B. thuringiensis stopped. Thus, stability of resistance is not necessarily a fixed character, even for a specific population and pesticide. Despite substantial resistance to CryIC and B. thuringiensis subsp. kurstaki, resistance to a spore-crystal formulation of B. thuringiensis subsp. aizawai was only 2- to 4-fold. 
DE: insect pests\natural enemies\biological control agents\entomopathogens\bacterial
insecticides\microbial pesticides\bacterial toxins\endotoxins\HPLC\analytical
methods\bioassays\toxicity\crystal proteins\entomopathogenic bacteria\Bacillus thuringiensis subsp.
kurstaki\Bacillus thuringiensis subsp. aizawai\insecticide resistance\Plutella
xylostella\Bacillus thuringiensis\formulations\USA\Hawaii\toxins\resistance\insecticides
\genetics\molecular genetics\genes\toxins\resistance\Bacillus thuringiensis\pathogens\Bacillus
thuringiensis
AN: 0E08501346\7E01800530

TI: Prolonged selection affects stability of resistance to Bacillus thuringiensis in diamondback moth
(Lepidoptera: Plutellidae).
AU: Tabashnik, B. E.\ Finson, N.\ Johnson, M. W.\ Heckel, D. G.
JN: Journal of Economic Entomology
YR: 1995
VL: 88
NO: 2
PP: 219-224
LA: En
MS: 26 ref.
AA: Department of Entomology, University of Hawaii, Honolulu, HI 96822, USA.
AB: The effects were studied of prolonged selection with the microbial insecticide Bacillus thuringiensis on Plutella xylostella. A laboratory colony of P. xylostella derived from a moderately resistant field population was selected with B. thuringiensis during 21 of 40 generations. Continued increases in resistance after repeated exposure to concn of B. thuringiensis that were high enough to kill 100% of putative heterozygotes suggest that resistance was not controlled solely by one locus with two alleles. After an apparent plateau in response to selection occurred, the stability of resistance in the selected colony and in isofemale lines derived from the selected colony was evaluated. Resistance declined in most cases but in one of the isofemale lines,resistance remained extremely high (resistance ratio = 5800) after more than 20 generations without exposure to B. thuringiensis. These data show that at least one genotype conferring resistance to B. thuringiensis was not inherently unstable.
DE: genetics\resistance management\selection\stability\insect pests\plant pests\microbial
pesticides\insecticide resistance\Plutella xylostella\Plutella xylostella\Bacillus
thuringiensis\resistance\insecticides\entomopathogenic bacteria\resistance\resistance
AN: 0E083095

TI: Selection and genetic analysis of a Heliothis virescens (Lepidoptera: Noctuidae) strain with high levels of resistance to Bacillus thuringiensis toxins.
AU: Gould, F.\ Anderson, A.\ Reynolds, A.\ Bumgarner, L.\ Moar, W.
JN: Journal of Economic Entomology
YR: 1995
VL: 88
NO: 6
PP: 1545-1559
LA: En
MS: 34 ref.
AA: Department of Entomology, North Carolina State University, Raleigh, NC 27695, USA.
AB: Eggs of the noctuid Heliothis virescens were collected from 3 adjacent counties in North Carolina. A laboratory strain (SDK) was established from these eggs using precautions to avoid loss of genetic diversity. A subset of this laboratory strain (YHD2) was selected on artificial diet containing the Bacillus thuringiensis toxin, CryIA(c). In the 1st 2 episodes of selection, only moderate resistance (approx.7- to 8-fold) was found. However, after 19 episodes of selection, the strain had developed more than500-fold resistance to the CryIA(c) toxin. Further selection lead to higher levels of resistance with the greatest resistance ratio recorded being approx.10,000-fold. The YHD2 strain was cross-resistant to CryIA(a), CryIA(b) and CryIF. There was also some resistance to CryIB, CryIC and CryIIA, but the level of resistance to these toxins was more moderate. Reciprocal crosses between the resistant and control strains indicated that resistance to CryIA(c) and to CryIA(b) was partially recessive, but that low-level resistance to CryIIA was more dominant. Progeny from backcrosses of F(1) larvae to the resistant parent were placed on artificial diet containing a concn of CryIA(b) that had previously been found to slow the growth of F(1) larvae. When these larvae were weighed after 10 days, 2 clearly demarcated size classes
were found in approx.1:1 ratios as is expected in backcrosses when a single locus (or a set of tightly linked loci) is coding for a major component of a recessive trait. Adults that developed from the larger size class of larvae were mated and their offspring inherited the ability to grow well on CryIA(b). As expected from the single locus model, 25% of the offspring from matings of the small backcross larvae grew well on CryIA(b). The results of this selection experiment indicated that the initial frequency of this resistance trait could be approximately 10^-3, but field tests were required to confirm this rough estimate.
DE: insect pests\natural enemies\entomopathogens\pesticide resistance\biological control
agents\genetic analysis\selection\genetic diversity\genetic variation\synthetic diets\pesticide
resistance\backcrossing\cross resistance\entomopathogenic bacteria\Bacillus
thuringiensis\Heliothis virescens\diets\USA\North Carolina\genetics\insecticide
resistance\toxins\resistance\insecticides\resistance\microbial pesticides
GL: USA\North Carolina
AN: 0E08405064\7E01701277

TI: Initial frequency of alleles for resistance to Bacillus thuringiensis toxins in field populations of Heliothis virescens.
AU: Gould, F.\ Anderson, A.\ Jones, A.\ Sumerford, D.\ Heckel, D. G.\ Lopez, J.\ Micinski, S.\ Leonard, R.\ Laster, M.
JN: Proceedings of the National Academy of Sciences of the United States of America
YR: 1997
VL: 94
NO: 8
PP: 3519-3523
LA: En
MS: 35 ref.
AA: Department of Entomology, North Carolina State University, Raleigh, NC 27695, USA.
AB: The risk of rapid pest adaptation to an insecticide is highly dependent on the initial frequency of resistance alleles in field populations. Because empirical estimates of these frequencies are lacking, population genetic models of resistance evolution have relied on a wide range of theoretical estimates. The recent commercialization of genetically engineered cotton that constitutively produces an insecticidal protein derived from the biocontrol agent, Bacillus thuringiensis (Bt) has raised concern that data needed to quantify the risk of insect pests, such as Heliothis virescens, rapidly adapting to this ecologically valuable class of toxins is lacking. By individually mating over 2000 males of H. virescens collected in four states in the USA to females of a Bt toxin-resistant laboratory strain, and screening F(1) and F(2) offspring for tolerance of the toxic protein, the field frequency of alleles for resistance was estimated as 1.5 x10^-3. This high initial frequency underscores the need for caution in deploying transgenic cotton to control insect pests. The single-pair mating technique greatly increased the efficiency of detecting recessive resistance alleles. Because alleles that decrease target site sensitivity to Bt toxins and other
insecticides are often recessive, this technique could be useful in estimating resistance allele frequencies in other insects exposed to transgenic insecticidal crops, or conventional insecticides. 
DE: insect pests\plant pests\cotton\Gossypium hirsutum\alleles\gene frequency\transgenic
plants\bacterial insecticides\natural enemies\entomopathogens\biological control
agents\USA\entomopathogenic bacteria\Bacillus thuringiensis\Heliothis
virescens\toxins\resistance\insecticides\insecticide resistance\genetics
AN: 0E08509455

TI: The genetic, molecular and phenotypic consequences of selection for insecticide resistance.
AU: McKenzie, J. A.\ Batterham, P.
JN: Trends in Ecology & Evolution
YR: 1994
VL: 9
NO: 5
PP: 166-169
LA: En
MS: 47 ref.
AA: Department of Genetics, University of Melbourne, Parkville, Victoria 3052, Australia.
AB: Studies of insecticide resistance allow theories of the adaptive process to be tested where the selective agent, the insecticide, is unambiguously defined. Thus, the consequences of selection of phenotypic variation can be investigated in genetic, biochemical, molecular, population biological and, most recently, developmental contexts. Are the options limited biochemically and molecularly? Is the genetic mechanism monogenic or polygenic, general or population/species specific? Are fitness and developmental patterns associated? These questions of general evolutionary significance can be considered with experimental approaches to determine how insecticide resistance evolves.
DE: reviews\population genetics\molecular genetics\phenotypes\selection\fitness\asymmetry\insecticide
resistance\insect pests\genetics\genetics\insecticides\resistance\genetics
AN: 0J08302331\0E08306282

TI: Resistance: a threat to the insecticidal crystal proteins of Bacillus thuringiensis.
CT: Myths of Managing Resistance held in Florida in 1994.
AU: Bauer, L. S.
JN: Florida Entomologist
YR: 1995
VL: 78
NO: 3
PP: 414-443
LA: En
LS: es
MS: 9 pp. of ref.
AA: USDA Forest Service, North Central Forest Experiment Station, Pesticide Research Center &
Department of Entomology, Michigan State University, East Lansing, MI 48823-5290, USA.
AB: Insecticidal crystal proteins (<delta>-endotoxins) synthesized by Bacillus thuringiensis are the active ingredient of various environmentally friendly insecticides. These insecticides are: highly compatible with natural enemies and other nontarget organisms; harmless to vertebrates; biodegradable in the environment; and highly amenable to genetic engineering. The use of transgenic plants expressing Bacillus thuringiensis <delta>-endotoxins has the potential to greatly reduce the environmental and health costs associated with the use of conventional insecticides. The complex mode of action of <delta>-endotoxin crystals is described. The recent escalation of commercial interest in B. thuringiensis has resulted in more persistent and efficacious formulations. For example, improved B. thuringiensis-based insecticides have allowed management of Plutella xylostella. Unfortunately this has resulted in the evolution of resistance to <delta>-endotoxins in P. xylostella populations worldwide. The recent appearance of B. thuringiensis resistance in the field demonstrates genetically-based resistance in several species of Lepidoptera, Diptera, and Coleoptera. In two strains of B. thuringiensis-resistant lepidopteran species, mechanisms of resistance involve reductions in the binding of toxin to midgut receptors. Research on other resistant strains suggests that other mechanisms are involved. Unfortunately, the high stability of the resistance trait, as well as broad spectrum cross-resistance to other <delta>-endotoxins, undermines many potential options for resistance management. Genetically engineered plants, expressing <delta>-endotoxin continuously and at ultra high doses, ensure intense and rapid selection of the target insect population. The efficacy of transgenic plants can be preserved only by developing an integrated pest management program that is designed specifically to reduce selection pressure by minimizing exposure to B. thuringiensis and increasing other mortality factors, thereby slowing the rate of pest adaptation to B. thuringiensis.
DE: microbial pesticides\insecticide resistance\genetic engineering\endotoxins\Bacillus
thuringiensis\resistance\insect pests\conferences\Myths of managing resistance\entomopathogenic
bacteria\resistance\reviews
AN: 0E08403419\7E01700817

TI: Development of resistance to Bacillus thuringiensis.
FT: Desarrollo de resistencia a Bacillus thuringiensis.
AU: Ibarra, J. E.\ Lopez-Meza, J. E.
JN: Agrociencia
YR: 1997
VL: 31
NO: 1
PP: 121-131
LA: Es
LS: en
MS: 54 ref.
AA: CINVESTAV, Unidad Irapuato, Depto. de Biotecnologia y Bioquimica, Apdo. Postal 629, 36500, Irapuato, Guanajuato, Mexico.
AB: Information is summarized on insect pests resistant to the toxins of B. thuringiensis. 6 lepidopteran species, 2 culicids, and 1 coleopteran species have been reported with varying resistance levels, to the 3 known pathotypes of B.thuringiensis. Only 4 Lepidoptera have developed resistance under commercial conditions (2 of them showing very low resistance levels). The remaining species have developed resistance only under laboratory conditions, through high selection pressures. It is clear that only two cases of resistance are relevant: Plodia interpunctella and Plutella xylostella. According to analysis of the mode of action of B. thuringiensis and preliminary evidence, a change of receptors of the midgut epithelial cells is the basis of the resistance mechanism. The theoretical basis of resistance development is reviewed, in order to suggest appropriate management strategies. These strategies are described for the use of B. thuringiensis as a bioinsecticide or in transgenic plants.
DE: insecticide resistance\Bacillus thuringiensis\toxins\Lepidoptera\Coleoptera\Plodia interpunctella\Plutella xylostella\Culicidae
AN: 0E08602363\0J08602479\7E01901068\7H00901101