Effects of secondary plant metabolites on predators via their prey


Abstract: One of the most important alternatives to pesticides is biological control of harmfularthropods on crops. Recent pesticide bans have also given new momemtum to the efforts ofthe plant breeding industry to search for novel traits that increase crop resistance. There isgeneral belief that combining these two practices will be largely additive. However, we arguethat this is not necessarily the case because biological control agents, like their prey, can behampered by plant resistance traits. Such effects can be direct, for example via sequestration ofplant toxins by the prey, or be indirect via a decrease in prey abundance, size orenergetic/nutritional value. To study the interaction of plant resistance with natural enemyperformance, we infiltrated bean leaves with different amounts of nicotine, a plant secondarymetabolite that can be easily detected in biological samples. We then assessed the effect ofnicotine on the performance of the spider mite Tetranychus urticae Koch (Acari:Tetranychidae) feeding from these leaves. Subsequently, we offered the eggs of these spidermites to the predatory mite Phytoseiulus persimilis Athias-Henriot (Acari: Phytoseiidae) toevaluate their predation and oviposition rates. We also assessed the fate of the nicotine. Wedetected it in the treated beans, spider mite adults and in trace amounts in spider mite eggs, butnot in the predatory mites. Spider mites produced lower numbers of eggs when exposed tohigher amounts of nicotine and these eggs were on average smaller. The predatory mitesconsumed more prey eggs when these came from treatments with nicotine, but theirreproductive performance was unaffected by the treatment. Taken together, our results suggestthat differences in egg predation can be partly attributed to the indirect effect of nicotine onprey egg size. We discuss the importance of evaluating both direct and indirect effects of plantresistant traits on predators and the consequences this could have for successfully aligningbiological pest control and plant resistance breeding programs.

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