Effects of genotypic variation in stem solidity on parasitism of a grass-mining insect

Host plant traits can play a significant role in influencing the importance, direction and intensity of tri-trophic interactions by both direct and indirect pathways. A major goal in applied tri-trophic research has been to determine whether breeding for host plant resistance traits can be combined with biological control to develop a more comprehensive control strategy. An important component of developing such a strategy is understanding how host resistance traits affect natural enemy–prey interactions for important pest insects. Here we examine the influence of genotypic variation in stem solidity, the primary trait conferring resistance against the wheat stem sawfly, Cephus cinctus, on parasitism of this major pest of wheat by its native braconid parasitoids. To do so, we conducted a field experiment in which we established replicate plots of 23 wheat genotypes that varied in levels of stem solidity, and quantified herbivore abundance and levels of parasitism across three sites in two years. Increasing stem solidity was associated with an approximately four-fold reduction in average parasitism rates, both across experimental plots and across wheat genotypes. Our analyses suggest that these effects were primarily direct, rather than indirectly mediated via effects of stem solidity on herbivore infestation levels or density. Interestingly, wheat genotype also had a significant influence on levels of parasitism, independent of its effects on stem solidity. Overall, our results suggest that although increasing stem solidity generally reduces parasitism, significant genotypic variability in average parasitism levels exist within solidity categories. Thus it may be possible to select resistant solid stemmed genotypes that also maintain relatively high parasitism levels. To our knowledge, our study is among the first to demonstrate a strong direct effect of genotypic variation in stem solidity on parasitism of grass mining insects, with important applied implications.