Differential behavior in specialist willow sawfly and its effect on secondary metabolites on wild and domesticated salix species

Abstract

Plant domestication has resulted in the modification of specific plant traits to increase their yield and quality. However, selective breeding may lead to a reduction of chemical defenses, usually affecting parameters of herbivore behavior. Furthermore, domesticated plants can become more susceptible to damage by herbivore insects compared to their wild relatives. In Argentina, willow plantations (Salix spp) are important in forest production and can be attacked by the sawfly Nematus oligospilus. Here, we compared N. oligospilus preference and performance when it feeds either on the native and non-domesticated S. humboldtiana, or on the high domesticated S. babylonica (var. Sacramenta). We studied oviposition preference of N. oligospilus between both willow genotypes, either undamaged or after egg laying and larval feeding. We also compared the performance of larvae that initiated their development on the leaf, where eggs were laid, and measured secondary metabolites as willow response. Our results showed a marked preference for laying eggs on native S. humboldtiana regardless of prior insect damage. Emission profiles of volatile compounds varied between willow species and changed after larval feeding, especially in S. babylonica, which released nitrogenous compounds (an aldoxime and benzyl cyanide). Larval development time was shorter, and mass of larvae and pupae were lower when they fed on S. humboldtiana than on S. babylonica. However, constitutive and inducible salicinoids levels were higher on S. humboldtiana than on S. babylonica. Preliminary analysis of condensed tannins showed no differences between species. Our results showed that the higher preference of oviposition of this specialist sawfly on S. humboldtiana may be positively related with the larval tolerance to salicinoids, increasing mass of larvae and pupae. We suggest that the differential performance might be due to a lower nutritional value of S. babylonica rather than higher chemical defense levels, such as phenolic compounds.