Intraguild predation between erigonid spiders, lacewing larvae and carabids

Petri dish experiments were conducted to study intraguild predation (IGP) between erigonid spiders (Erigone aim (Blackwall), Oedothorax apicatus (Blackwall), a carabid beetle (Pterostichus melanarius (Ill.) and lacewing, Chrysoperla carnea (Stephens) larvae. Erigonid spiders increased the mortality rates of lacewing larvae to different degrees, depending on the species and sex of spider concerned. The effect of spiders on lacewing larvae was highest (up to nearly 100% mortality) in the absence of alternative prey. In the presence of three or 20 aphids (Sitobion avenae Fab.) female erigonids increased the mortality rates of lacewing larvae compared to controls, but predation on lacewing larvae decreased in the presence of increasing numbers of aphids. Males of O. apicatus caused no significant increase in mortality rate of lacewing larvae in the presence of aphids. Lacewing larvae (L2-larvae and L3-larvae) did not kill erigonid spiders. IGP between the female erigonids E. ari a and O. apicatus was not found. P. melanarius fed on lacewing larvae and on erigonid spiders, and E. atra was preferred over O. apicatus. The sex-specific and species-specific IGP rates of spiders on lacewing larvae and the great influence of alternative prey on IGP rate reveal a general problem when data resulting from one sexual stage or one species or one feeding regime are generalized to a whole arthropod group. Moreover, the results suggest that an increase of top predators, such as carabid beetles, which prey only to a limited extent on the pest species itself, may affect natural insect pest control by IGP.