The parsnip webworm,Depressaria pastinacella(Duponchel), webs together developing umbels of its host plant,Pastinaca sativaL., throughout the course of its larval life. We estimated energetic, time, and material costs of web-spinning behavior for this species. Webworms spent ≈one-third of their time spinning silk; of digested food biomass not expended on respiration, one-fourth was incorporated into silk, and almost 18% of ingested nitrogen was incorporated into silk. Three experiments examined webworm behavior with respect to occupied and unoccupied webs. Caterpillars displaced from their webs occupied empty webs provided to them 87.5% of the time instead of spinning new webs. Moreover, webworms defended their webs against usurping conspecifics. Of 63 staged interactions between web occupants and intruders, 52 were initiated by the residents, and all but one resident successfully defended the web. Finally, when larvae were displaced from their own webs and ‘squatters’ installed, aggressive attacks were equally likely to be initiated by web-builders and squatters. These behaviors are consistent with a high cost of web building because costs and risks associated with aggressive defense of a web presumably do not exceed those expended in constructing a new web or in being without a web.