How Plants Benefit from Providing Food to Predators Even When It Is Also Edible to Herbivores

It is well established that plants provide alternative foods to predators of herbivorous arthropods. This provision may facilitate protection against herbivory. However, plants often cannot prevent other organisms from utilizing these foods as well. There are many examples of herbivorous arthropods that can feed on plant-provided foods such as extrafloral nectar and pollen. The question therefore arises whether individual plants still gain protection when not only the predators, but also the herbivores, can feed on these foods. We investigated this question using a mathematical model and experiments that assessed the impact of supplementary pollen on the dynamics of predatory mites (Iphiseius degenerans (Berlese)) and herbivorous thrips (Frankliniella occidentalis (Pergande)), two arthropods capable of using pollen for reproduction. Replicated greenhouse experiments showed that addition of pollen every two weeks to one young mature leaf of a male-sterile cucumber plant increased predator population growth and greatly reduced herbivore numbers. A stage-structured predator-prey-pollen model with experimentally established parameters gave reasonably accurate predictions of population trends observed in the greenhouse experiments with and without pollen. Model analysis yielded three important results. First, herbivore (prey) equilibria always settled to lower values in the presence of pollen. Second, mean herbivore numbers during the transient phase following predator release were not always lower under pollen supply, depending on the initial numbers of predators and prey. Third, limiting the plant area covered with pollen led to a decrease in mean herbivore numbers, provided that the predators aggregated in (and thereby "monopolized") pollen patches. The latter result may explain why plants provide alternative foods at specific sites.