The value of three cereal aphid species as food for a generalist predator

The value of the cereal aphid species Metopolophium dirhodum (Wlk.), Sitobi on avenae (F.) and Rhopalosiphum padi (L.) as prey for the linyphiid spider Erigone atra (Bl.) was assessed. Fecundity of females was determined for s piders fed on eight experimental diets: three single-species aphid diets, a mixed diet of all three aphid species, three mixed diets with each aphid s pecies in combination with fruit flies Drosophila melanogaster (Meig.), and pure D. melanogaster as a high quality comparison diet. The development an d survival of first-instar juveniles fed on three diets of single aphid spe cies, and on a diet of Collembola were compared with those subjected to sta rvation. Prey value for adult females was assessed by egg production, hatch ing success and offspring size. In pure diets all three aphid species were of low value to the spiders, cau sing a rapid decline in egg production and supporting no growth of signific ance of first-instar juveniles. No difference in value of aphid species of single-species aphid diets was found in the fecundity experiment, while a r anking of aphid species of M. dirhodum > R. padi > S. avenae was revealed i n the survivorship experiment. A mixed-aphid diet was not found to be advan tageous compared with single-species aphid diets, and no advantage of inclu ding aphids in mixed diets with fruit flies was found. Metopolophium dirhod um and R. padi were neutral in mixed diets, while a diet of S. avenae and f ruit flies caused reduced egg production compared with the pure diet of fru it flies, revealing a toxic effect of S. avenae on the spider. The value-ra nking of aphid species in mixed diets was similar to that of single-species diets. A similar ranking of aphid species was found for different fitness parameters (fecundity of adult females and development of juveniles). A ranking of aphids by offspring size of mothers on aphid-only diets was S. avenae > M. dirhodum > R. padi. All aphid-fruit fly diets resulted in larg er offspring than a diet of only D. melanogaster, with the overall largest offspring being produced on the diet of M. dirhodum and fruit flies.