INSECTICIDE RESISTANCE AND RESISTANCE MANAGEMENT Heritability of Tolerance to the Cry1Ab Toxin of Bacillus thuringiensis in Chilo suppressalis (Lepidoptera: Crambidae) FARAMARZ ALINIA, 1, 2 MICHAEL B. COHEN, 1 AND FRED GOULD 3 J. Econ. Entomol. 93(1): 14Ð17 (2000) ABSTRACT Heritability of Chilo suppressalis (Walker) tolerance to the Cry1Ab toxin of Bacillus thuringiensis Berliner was estimated using a half-sibling design. ArtiÞcial diet with and without Cry1Ab was infested with progenies of 20 males, each mated with 2 females, and mortality was scored 5 d after infestation. The progeny of each female was reared and scored separately. Mean mortality of the 20 families on the Cry1Ab diet was 46.5%. The effects of both male parent and of female parent within male parent were signiÞcant. Heritability was estimated to be 0.52, suggesting that a high proportion of phenotypic variation was because of genetic differences. Mortality on the Cry1Ab diet was not correlated with mortality on control diet, indicating that differences among families in tolerance to Cry1Ab were not attributable to differences in general Þtness. Our results indicate that “high dose” Bt rice plants may be particularly important for Cry1Ab resistance management in C. suppressalis populations. KEY WORDS Bacillus thuringiensis, Chilo suppressalis, insecticide resistance, heritability, rice Chilo suppressalis (WALKER), generally known as the Asiatic rice borer, is a major pest of rice in Europe and temperate Asia and is also widespread in tropical Asia. Many rice varieties with moderate resistance to stem borers have been released (Khush 1995), but high levels of resistance to lepidopterous stem borers and foliage feeders have not been identiÞed in rice germ- plasm (Heinrichs et al. 1985). Consequently, there has been substantial interest in genetic engineering of rice with toxin genes from Bacillus thuringiensis Berliner to enhance resistance to lepidopterous pests, especially stem borers (Fujimoto et al. 1993, Wunn et al. 1996, Ghareyazie et al. 1997, Nayak et al. 1997, Cheng et al. 1998, Datta et al. 1998). As yet Bt rice varieties are not available to farmers, but Þeld tests have begun in China (G. Ye, personal communication). The long-term effectiveness of Bt rice will depend in part on the genetic potential for the evolution of resistance to Bt toxins in stem borer populations and the implementation of suitable resistance manage- ment strategies. Methodologies have been developed to help predict the rate of evolution of insect resis- tance to insecticides (Roush and McKenzie 1987, Firko and Hayes 1990). Most models assume that re- sistance is attributable to a mutation at a single locus. In contrast, quantitative genetic models make no as- sumption regarding the number of genes involved, and the expression of a trait is assumed to depend on environmental as well as genetic factors (Firko and Hayes 1990, Tabashnik 1992). As a tool for resistance risk assessment, quantitative genetics enables predic- tions to be made regarding the speed and magnitude of genetic change associated with resistance. Quanti- tative genetic techniques allow the heritability of a trait (i.e., the proportion of the total phenotypic vari- ation as a result of the average effects of genes [Fal- coner and Mackay 1996]) to be measured. The heri- tability of a quantitative variable can be estimated using offspring-parent regression, threshold trait anal- ysis (Firko and Hayes 1990, Omer et al. 1993), or sibling analysis (Bull et al. 1982, Tabashnik and Cush- ing 1989, Firko and Hayes 1991). In full-sibling anal- ysis, 1 male is mated with 1 female and variation within and among families of offspring is compared (Firko and Hayes 1990). In half-sibling analysis, .1 female is mated with each male to produce both full and half- sibling families. Variation among males and among females within males are compared by nested analysis of variance (ANOVA). In this study, we estimate the heritability of toler- ance to the Cry1Ab toxin of B. thuringiensis in a pop- ulation of C. suppressalis. We use the term “tolerance” in the sense of the response to a toxin by an individual, family, or population. Cry1Ab is the toxin that has been most often used in rice transformation for stem borer resistance (Fujimoto et al. 1993, Wunn et al. 1996, Ghareyazie et al. 1997, Nayak et al. 1997, Cheng et al. 1998, Datta et al. 1998). Materials and Methods Insects. Rice stubble (straw remaining rooted in the soil after harvest) was collected from 5 rice Þelds ’10 km apart from each other, in Laguna Province, Phil- ippines. The straw was dissected and C. suppressalis pupae were collected and sexed under a dissecting 1 Entomology and Plant Pathology Division, International Rice Re- search Institute, P.O. Box 3127, Makati Central Post OfÞce, Makati City 1271, Philippines. 2 Rice Research Institute of Iran, P.O. Box 1958, Rasht, Iran. 3 Department of Entomology, North Carolina State University, Ra- leigh, NC 27695. 0022-0493/00/0014Ð0017$02.00/0 q 2000 Entomological Society of America