The parsnip webworm, Depressaria pastinacella, spins a silken web within the umbels of its host plant, the wild parsnip Pastinaca sativa, and aggressively defends this web against conspecifics. We first established experimentally that the number of aggressive interactions between caterpillars with their webs removed was significantly higher than for webworms with intact webs. In order to determine whether web-spinning acts to divide food resources 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 removed daily. Parsnip webworms isolated from one another and therefore unable to 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 reestablished had the lowest pupal weights and spun the greatest quantity of silk. Our findings indicate that, for the parsnip webworm, constructing a silken web reduces aggressive encounters among conspecifics.