Reduced aggressive behavior: A benefit of silk-spinning in the parsnip webworm, Depressaria pastinacella (Lepidoptera : Oecophoridae)

The parsnip webworm, Depressaria pastinacella, spins a silken web within th e umbels of its host plant, the wild parsnip Pastinaca sativa, and aggressi vely defends this web against conspecifics. We first established experiment ally that the number of aggressive interactions between caterpillars with t heir webs removed was significantly higher than for webworms with intact we bs. In order to determine whether web-spinning acts to divide food resource s and reduce aggressive interactions, we measured relative weight gain and total silk production of parsnip webworms isolated from one another grouped together with webbing undisturbed, and grouped together with webbing remov ed daily. Parsnip webworms isolated from one another and therefore unable t o engage in aggressive interactions attained the highest pupal weights and spun the smallest amount of silk; caterpillars with webs removed daily and therefore with frequent aggressive interactions until territories were rees tablished had the lowest pupal weights and spun the greatest quantity of si lk. Our findings indicate that, for the parsnip webworm, constructing a sil ken web reduces aggressive encounters among conspecifics.