Resistance to 2-tridecanone in Tetranychus urticae:: effects of induced resistance, cross-resistance and heritability

Earlier studies of resistance to the tomato toxin 2-tridecanone revealed similar levels of resistance among tomato and cucumber strains of Tetranychus urticae, whereas we anticipated the tomato strains to have a higher resistance. Between-strain differences, however, may be concealed by toxin accumulation and/or cross-resistance. Mites do accumulate 2-tridecanone to a considerable extent, but we have shown before that this cannot explain the lack of difference in resistance. In this paper, we provide evidence against cross-resistance: selection for resistance to a homologous compound from cucumber, 2-nonanone, did not confer resistance to 2-tridecanone. Observed increases in resistance levels over generations may be due to induction or selection. To study induction, adult females from a cucumber strain and a tomato strain were exposed to a sublethal dose of 2-tridecanone. One day after exposure, the cucumber strain gained in resistance to 2-tridecanone (c. 8-fold), whereas the tomato strain showed a decrease in resistance. These changes in resistance disappeared within three days after exposure, the time the mites need to dispose themselves of the toxin. The results support earlier findings that resistance is induced in the cucumber strain, but not in the tomato strain. To assess the heritability of resistance to 2-tridecanone, offspring was obtained from individual females, and both the mothers and their offspring were tested for resistance. For the cucumber strain, offspring from susceptible or resistant mothers did not differ in resistance. For the tomato strain, however, offspring from resistant mothers were significantly more resistant to the toxin than offspring from susceptible mothers. This result provides unambiguous evidence that resistance to 2-tridecanone can be selected for.