WEB COSTS AND WEB DEFENSE IN THE PARSNIP WEBWORM (LEPIDOPTERA, OECOPHORIDAE)

The parsnip webworm, Depressaria pastinacella (Duponchel), webs togeth er developing umbels of its host plant, Pastinaca sativa L., throughou t the course of its larval life. We estimated energetic, time, and mat erial costs of web-spinning behavior for this species. Webworms spent almost-equal-to one-third of their time spinning silk; of digested foo d biomass not expended on respiration, one-fourth was incorporated int o 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 conspeci fics. Of 63 staged interactions between web occupants and intruders, 5 2 were initiated by the residents, and all but one resident successful ly defended the web. Finally, when larvae were displaced from their ow n webs and ''squatters'' installed, aggressive attacks were equally li kely to be initiated by web-builders and squatters. These behaviors ar e consistent with a high cost of web building because costs and risks associated with aggressive defense of a web presumably do not exceed t hose expended in constructing a new web or in being without a web.