Variability in susceptibility among cucumber and tomato strains of Tetranychus urticae Koch to 2-tridecanone from tomato trichomes: effects of host plant shift

Abstract

The effects of secondary plant compounds on different host races/strains of a herbivorous arthropod are not easy to interpret based on dose–response tests alone. This difficulty arises because the responses in a population to a given dose are dictated not only by genetic factors, but also by factors such as feeding history, age and plant. To discriminate between these possibilities different strains of a herbivorous mite (Tetranychus urticae Koch) were exposed to one relevant toxin from glandular hairs on tomato (the methyl ketone, 2-tridecanone) and these strains had a known feeding history on either or both of two host plants (tomato and cucumber). It was hypothesized that tomato is a relatively hostile host plant to spider mites partly due to methyl ketones from glandular hairs and that consequently there will be stronger selection on the tomato strains than on the cucumber strains. However, the expected differences between the tomato and cucumber strains did not show up; three spider mite strains collected from tomato and two strains collected from cucumber appeared to be equally susceptible to 2-tridecanone. This unexpected result cannot result from selection for resistance to 2-tridecanone, but it may be the consequence of cross-resistance to other ketones in cucumber or the bio-accumulation (i.e. metabolic load) of 2-tridecanone prior to the toxicity test. To test this hypothesis, one of the tomato strains was released on cucumber for different time intervals and one of the cucumber strains on tomato. It was found that the resistance of the tomato strain to 2-tridecanone increased 6 months after transfer to cucumber. This increased resistance cannot be the result of selection because 2-tridecanone is absent from cucumber. Hence, it may be due either to selection for resistance to another ketone in cucumber, possibly leading to cross-resistance, or to the absence of 2-tridecanone bioaccumulation on cucumber. Transfer of the cucumber strain from cucumber to tomato also increased the resistance to 2-tridecanone. As this was accompanied by high mortality directly after the transfer, selection for resistance may have played a role. Alternatively, the increased resistance may be due to induction of resistance to secondary plant compounds of tomato, including 2-tridecanone. In conclusion, experiments on host plant transfer show that the tomato strain and the cucumber strain are not equal in their resistance to 2-tridecanone.