Induced resistance and herbivore population dynamics 

Many plants are able to increase their level of resistance to herbivores in response to herbivore damage (induced resistance). I am interested in the consequences of induced versus unchanging (or constitutive) resistance strategies for herbivore population dynamics, since induced resistance is a potential source of density dependent regulation for herbivores. 

Although it has been suggested that induced and constitutive plant resistance should have different effects on insect herbivore population dynamics, there is little experimental evidence that plant resistance can influence herbivore populations over longer than one season. We (Underwood and Rausher 2000) used a density manipulation experiment and model fitting to examine the effects of constitutive and induced resistance on herbivore dynamics over both the short and long-term. We used likelihood methods to fit population dynamic models to recruitment data for populations of Mexican bean beetles on soybean varieties with no resistance, constitutive resistance or induced resistance. We compared model configurations that fit parameters for resistance types separately with models that did not account for resistance type. Models representing the hypothesis that the three resistance types differed in their effects on beetle dynamics received the most support. Induced resistance resulted in lower population growth rates and stronger density dependence than no resistance. Constitutive resistance resulted in lower population growth rates and stronger density dependence than induced resistance. Constitutive resistance had a stronger effect on both short-term beetle recruitment and predicted beetle population dynamics than induced resistance. The results of this study suggest that induced and constitutive resistance can differ in their effects on herbivore populations even in a relatively complex system.

                                                                                 
    Mexican bean beetle eggs...               larva (note ferocious spines)...                                 and adult