PARENT SESSION
Poster Session #34: Agroecology.
Thursday, August 9, 2001. Presentation from 10:30 AM to 12:00 PM. Exhibition Hall

Trophic positions of generalist predators in detritus-enriched cucurbit gardens: patterns from stable isotope analysis.

McNabb, Denise¹, Halaj, Juraj ¹, Wise, David¹, ¹

ABSTRACT- Generalist predators are potential biocontrol agents because they prey upon herbivores in the grazing food web. These generalist predators also feed on detritivores and other arthropods in the detrital food web; therefore, understanding the link between these two webs may be instrumental in understanding how manipulating detrital webs may enhance the effects of generalist predators on crop pests. By determining concentrations of stable isotopes in predators and potential prey, we inferred the trophic position of generalist predators in cucurbit gardens containing a rich detrital food web. The resource base and trophic position of species in a food web can be inferred from differences between species in the ratios of stable isotopes of carbon and nitrogen (¹³C and ¹⁵N). Spiders and Collembola, an abundant detrivitove, had similar ¹³C values, suggesting these detritivores are a significant source of energy to spiders in this system, specifically for small sheet-web spiders (Linyphiidae) and juvenile wolf spiders(Lycosidae). The ¹⁵N values indicate that spiders are 1.6 trophic levels above Collembola. Spiders may be more than one trophic level above detritivores due to cannibalism and intraguild predation among spider species, or due to feeding on other arthropod natural enemies. The carabid beetles Scarites spp. and Harpalus pennsylvanicus DeGeer had ¹³C and ¹⁵N isotopic values consistent with known feeding behaviors.

KEY WORDS: detrital food web, stable isotopes, generalist predators, detritivores