DISENTANGLING EFFECTS OF INDUCED PLANT DEFENSES AND FOOD QUANTITY ON HERBIVORES BY FITTING NONLINEAR MODELS

Plants can respond to herbivore damage through both broad-scale (syste mic) and localized induced responses. While many studies have quantifi ed the impact of systemic responses on herbivores, measuring the impac t of localized changes is difficult because plant tissues that have su ffered direct damage may represent both a lower quality and a lower qu antity of food. This article uses nonlinear models to disentangle the confounding effects of prior herbivory on food quantity and quality. T he first (null) model assumes that herbivore performance is determined only by the quantity of food available to an average herbivore. Modif ied models allow two distinct effects of damage-induced defenses: an i ncrease in the amount of food each herbivore is required to consume in order to achieve maximum performance and a reduction in the maximum p erformance even when herbivores are fed ad lib. Maximum likelihood met hods were used to fit the models to data from held experiments ill whi ch Colorado potato beetle (Leptinotarsa decemlineata) larvae were rear ed on three varieties of potatoes that had been damaged to varying deg rees by adult beetles. Prior damage reduced the mean mass of beetles a t pupation, and this effect was due to both a decrease in food quantit y and induced changes in food quality. In contrast, beetle survival wa s affected in some cases by reduced food quantity but showed no respon ses that could be attributed to induced defenses. I discuss this resul t in the context of previous studies of induced (mostly systemic) resp onses in the potato-potato beetle system, and I suggest that detailed studies of particular chemical responses and the proposed method of co mbining bioassays with quantitative models should be used as complemen tary approaches in future studies of herbivore-induced defenses in pla nts.