Bottom-Up Mechanisms Generate the Same Temporal Pattern of Attack by a Specialist and a Generalist Caterpillar on Short-Lived Plants.

The local population dynamics of insect herbivores in ephemeral patches of short-lived plants are poorly known. We investigated whether a specialist and a generalist caterpillar exhibit contrasting temporal patterns of attack during plant development and also assessed bottom-up forces related to plant ontogeny that govern such population trends. Immature stages of the polyphagous Trichoplusia ni (Hübner) and the oligophagous Plutella xylostella (L.) were sampled throughout the development of cabbage (Brassica oleracea L. var. capitata L.) crops. We measured protein and glucosinolate contents and insect performance with regard to plant age and leaf strata. The populations of both caterpillar species changed in close parallel throughout plant development, and a nonlinear temporal pattern of egg laying was reproduced in sequential population patterns of the larval stages until pupation. Reduced protein availability and insect performance coincided with a decline in egg laying and subsequent larval abundance in mature plants. By standardizing the plant size, we found that young and nutritious plants support proportionately more insects than large and mature plants. In our models of the population oscillations, the interaction between plant size and quality provided a strong causal explanation for the densities of both oligophagous and polyphagous caterpillars. Patches of fast-growing herbaceous plants are very common worldwide in the form of crop fields, and a generalized temporal pattern of attack may be widespread among caterpillars, regardless of their feeding specialization. Our results highlight the role of bottom-up forces in shaping the population dynamics of caterpillars in such systems.