Have you ever considered that insects feeding on plants early in the season may make the plants less susceptible to insects that feed later on? Wounding of leaves by plant-feeding insects such as caterpillars and beetles may induce localized or systemic changes in the levels of plant secondary metabolites or nutrients. This hypothesis was tested by researchers at the University of Kentucky by conducting laboratory no-choice and choice tests, and field evaluations to determine if early-season defoliation of crabapple trees, Malus spp., by eastern tent caterpillar, Malacosoma americanum, made the trees less palatable or more resistant to Japanese beetle, Popillia japonica, feeding.

This is not an unusual phenomenon, as spring defoliators can affect the quality of both undamaged and damaged leaves, as well as new emerging leaves, for subsequent plant-feeding insects later in the season. In fact, previous research has demonstrated that early-season plant-feeders can have positive effects on late-season plant-feeders. For example, fall webworm (Hyphantria cunea) grows faster and is larger as a result of feeding on the leaves of red alder (Alnus rubra) that had been previously fed upon by western tent caterpillar, Malacosoma californicum, compared to leaves that were not damaged. In contrast, late-season caterpillars feeding on spring-defoliated oaks suffered higher mortality on new leaves (regrowth) or insect-damaged leaves than on undamaged (intact) leaves. As both eastern tent caterpillar and Japanese beetle feed primarily on plants in the rose family (Rosaceae), it was logical to conclude that this phenomenon might occur between these two plant-feeding insects.

The bottom line, based on the results from the no-choice and choice tests and field evaluations, was that defoliation of crabapples by eastern tent caterpillars in early spring did not reduce feeding damage by Japanese beetles later in the growing season.