Plant domestication is assumed to result in reduced levels of defensive compounds in crops. A reduction in plant defensive traits is often expected to result in increased insect performance. However, recent studies have shown that this pattern is not ubiquitous.
The reason for this may lie in the purpose or the organ targeted by domestication. Such that, a positive relationship between reduction in plant defense and increased insect performance will be only, or more evident for cultivars and plant organs that have been selected for consumption. We tested this hypothesis by investigating the relationship between the physical and chemical defense and insect performance for squash varieties (genus Cucurbita) selected for different purposes, for fruit and seed consumption and ornamentals. The roots of these varieties were analyzed for their content in cucurbitacins, known to be toxic to generalist herbivores. In parallel, we conducted laboratory assays to determine the preference and performance of larvae of two generalist herbivores, Spodoptera latifascia that feeds on leaves and Diabrotica balteata that feeds on the roots of these plants.
Results/Conclusions
We found partial support for our hypothesis. Trichome density and cucurbitacin content in the roots were lower for varieties selected for fruit consumption. However, larval performance for both insects was not better on fruit varieties. For the leaf-herbivore this is not surprising, as the leaves do not contain cucurbitacins. For the roots, our results suggest that D. balteata is adapted to feed on plants containing high levels of this toxic compound. Furthermore, choice experiments revealed that larvae selected roots with high cucurbitacin content.
We conclude that although domestication has reduced the cucurbitacin content in the fruit varieties, this reduction does not result in increased insect performance. Purpose-specific effects of plant domestication on plant defenses can be expected for other squash varieties and other crops as well.