Plant defence: Feeding your bodyguards can be counter-productive

Insect predators often utilize plant cues to locate herbivores and plant food for sustenance when prey is scarce. The capacity of many plants to attract and support predators has sometimes been interpreted as an indirect plant defence system, which has evolved because it reduces the plants' exposure to detrimental herbivores. In accordance with this hypothesis, plant breeders have begun to select for crop varieties that attract and support biocontrol agents. We have tested whether predator proneness to consume herbivores is affected by variation in the plant food quality provided by different Salix (willow) varieties. Plant suitability as food for the important biocontrol agent Anthocoris nemorum (Common Flower Bug) varied widely between the tested Salix varieties. The proneness of Anthocoris to consume eggs of the detrimental herbivore Phratora vulgatissima also varied between the varieties, consuming eggs at much lower rates on those that provided high quality plant food. The results suggest that plants that provide too good plant food may satiate their bodyguards, thereby reducing their motivation to hunt for herbivorous prey. Thus, the capacity of plants to support predators may sometimes result in a partially enemy-free space for herbivores an outcome conflicting with the notion of indirect defence.