Predation of Rhopalosiphum padi (Homoptera: Aphididae) by polyphagous predatory arthropods during the aphids' pre-peak period in spring barley

Polyphagous predators (e.g. Araneae, Carabidae and Staphylinidae), collected from spring barley fields during 1981-85, were examined by either gut dissection or a R. padi-specific antiserum in enzyme linked immunosorbent assay (ELISA) in order to detect predation of Rhopalosiphum padi during the aphids' establishment and exponential growth phases. Overall 18% of c. 3000 carabids dissected were shown to feed on R. padi during the aphids' pre-peak period. No overall relationship was found between percentage carabids with R. padi in the diet and peak R. padi densities. Relatively high proportions of Bembidion spp. (particularly B. lampros) and Pterostichus cupreus fed on R. padi during the aphids' establishment phase, and proportions of those predator taxa containing R. padi increased with increasing R. padi densities in both high and low aphid density years. P. melanarius and Harpalus rufipes mainly fed on R. padi during the aphids' exponential growth phase. Overall 11% of c. 1350 predators examined in ELISA gave positive reactions to the R. padi antiserum. Relatively high proportions of linyphiid and lycosid spiders were positive throughout the aphid pre-peak period. Several Acari, Opiliones, Trechus spp. (Carabidae), Philonthus spp. (Staphylinidae), Cantharidae and Chilopoda were positive mainly during the aphids' exponential growth phase. Sample sizes were small, however. Very few of the Bembidion spp. tested in ELISA were positive compared with those examined by gut dissection. The maximum period of R. padi protein (antigen) detection in B. lampros was related to temperature, i.e. 8.5 h at 30°C, 20.5 h at 20°C and 34.5 h at 10°C, respectively. It is suggested that the rate of R. padi protein digestion in B. lampros is faster at higher temperatures than the rate of elimination of prey solids from the guts. Several key predators (in this case B. lampros, P. cupreus and linyphiid spiders) which are abundant in spring cereal fields at a time when R. padi emigrants arrive in the crop and which feed on R. padi during the aphids' establishment phase, are identified. The results are compared with those from similar investigations elsewhere with predators of Sitobion avenae and Metopolophium dirhodum.