Naomi Finson

Fitness costs of resistance to Bacillus thuringiensis in the diamondback moth (Plutella xylostella)

Does adaptation to stress entail fitness costs in optimal environments? Antagonistic pleiotropy sometimes causes trade-offs in fitness across different environments in which the agent of stress is a pathogen (Lenski 1988) or a novel diet (Pashley 1988; Via 1991). It is commonly believed that evolution of resistance to insecticides also involves such trade-offs. Alleles conferring resistance are rare before exposure to insecticide and have been presumed to exert negative effects on fitness in the absence of insecticide (Crow 1957; Uyenoyama 1986; Hoffman and Parsons 1991). However, fitness costs associated with resistance to conventional synthetic insecticides appear to vary considerably among species and insecticides (Roush and McKenzie 1987; Roush and Daly 1990; Denholm and Rowland 1992). Moreover, little is known about fitness costs of resistance to insecticidal proteins from Bacillus thuringiensis, a common soil bacterium (McGaughey and Whalon 1992; Tabashnik 1994). Toxins from B. thuringiensis kill susceptible insects by binding to and disrupting the integrity of the midgut epithelium (Gill et al. 1992). Reduced binding of toxins is a primary mechanism of resistance in the diamondback moth, Plutella xylostella (Ferre et al. 1991), a major pest of cruciferous vegetables (Talekar 1992). The normal function of toxin-binding sites is not known but seems to be important because such sites are found in numerous insects (Feitelson et al. 1992). Several authors have suggested that alteration of toxin-binding sites may interfere with normal physiological functions (Van Rie et al. 1990; MacIntosh et al. 1991). Instability of resistance to B. thuringiensis in the absence of selection

Instability of resistance to Bacillus thuringiensis

The continued efficacy of environmentally safe biopesticides derived from Bacillus thuringiensis (Bt) is threatened by the potential for development of resistance in pest populations. Instability of resistance is defined here as the tendency for the frequency of resistant genotypes to decrease in a population beyond effects directly attributable to immigration or emigration. Instability can be quantified as R, the average rate of change in the logarithm of the LC50 (concentration killing 50% of individuals) per generation, which is analogous to the average rate of response to selection per generation. In seven strains of Plutella xylostella, the first insect with field populations reported to be resistant to Bt, resistance declined when exposure to insecticide ceased (mean R = − 0.19). In four other pests, resistance to Bt declined slowly or not at all (mean R = − 0.02) in the absence of exposure to Bt Reduced biotic fitness associated with resistance is the most likely cause of instability of resistance ...

Oviposition preference of the diamondback moth (Plutella xylostelld) unaffected by the presence of conspecific eggs orBacillus thuringiensis

Neither toxins fromBacillus thuringiensis Berliner nor conspecific eggs deterred oviposition by the diamondback moth,Plutella xylostella (L.) (Lepidoptera: Plutellidae), in laboratory choice tests. The finding that toxins did not deter oviposition by moths from a susceptible line shows that oviposition preference and larval survival were not associated in this line. Selection for larval resistance to toxins did not significantly alter oviposition preference, which rules out a strong genetic correlation between larval performance and oviposition preference. Failure of conspecific eggs to deter oviposition may not represent lack of association of preference and performance because other evidence suggests that larval performance may not be greatly affected by larval density. These results suggest that the ability of refuges to slow evolution of physiological resistance toB. thuringiensis toxins will not be magnified by an oviposition preference for untreated foliage.