Parasitoids Reduce Growth Rate of Oak-Feeding Caterpillars

Koinobiont parasitoids have been shown to both reduce and increase the growth rate of caterpillars. However, no comprehensive study has been conducted on several caterpillar hosts of a given plant species. Moreover, most of the existing case studies are on large caterpillars attacking crop plants. This may not be representative of the effect of parasitoids on the growth of caterpillars that feed on trees. We sampled caterpillar communities from the canopy of oak trees (Quercus robur and Q. petraea) in a forest in France and from one in Poland, measured their growth rate, and noted parasitoid exit. The details of the methods varied between study years and locations, and this was partially accounted for statistically. Across six species of Lepidoptera, parasitoids reduced caterpillar growth rate on average. The effect of parasitism appeared to vary widely within and across lepidopteran species, indicating diversity in the outcome of plant–herbivore–parasitoid interactions. Further studies should verify this result and determine if the reduced growth rate is accompanied by lower plant consumption rates. If so, plants would immediately benefit from attracting parasitoids when they are attacked by caterpillars, and forestry practices that increase parasitoid abundance could reduce folivory partly without delay.