Food Conditioning Affects Expression of Insect Resistance in Diploid Willows (Salix spp.)

The high energy quota and versatility of use make willows (Salix spp.) attractive as bioenergy crops. Insect defoliation constitutes a threat to the profitability of willow growers. Hitherto, the breeding for resistance against the main insect pests has been hampered by the fact that all known resistant willow clones are polyploids, and existing molecular breeding tools work most effectively for diploids. Here, we firstly report diploid willows highly resistant to the main insect defoliator, the leaf beetle (Phratora vulgatissima), offering new opportunities for breeding resistance. Leaf beetles exposed to three resistant clones (two S. purpurea one S. eriocephala) laid three to 27 times fewer eggs than females on a susceptible S. viminalis clone. Secondly, we show that beetles laid significantly more eggs on resistant clones if they were fed the susceptible clone prior to the oviposition monitoring test compared to when they pre-fed on resistant clones. Nevertheless, the differences observed between resistant and susceptible clones were pronounced in all cases. The food conditioning effect means that small differences in resistance among clones may be undetected.