VALUE OF THE APHID RHOPALOSIPHUM-PADI AS FOOD FOR CEREAL SPIDERS

1. Laboratory experiments were carried out to analyse various aspects of spider performance under different diets, focusing mainly on the value of the aphid Rhopalosiphum padi. Aphis fabae and a sciarid midge were considered in single experiments. Laboratory fruit flies were used as a standard prey for comparison. The spider species used were Erigone atra, Pardosa amentata and P. prativaga. 2. In preference experiments P. amentata ranked prey types in the order: fruit flies > sciarid > R. padi > A. fabae. 3. In P. prativaga consumption of fruit flies increased with the length of prior starvation period; consumption of aphids was much lower and independent of hunger. 4. First instar spiderlings of P. amentata and Erigone atra were unable to develop on a pure diet of R. padi; they died without moulting. 5. Egg production in wild caught E. atra females stopped after 2-4 egg-sacs on a pure diet of R. padi, whereas females given fruit flies or fruit flies + aphids produced an average of eight sacs. Egg numbers were high on a pure diet of fruit flies, but hatching success was best with fruit flies supplemented with aphids. The size of hatched young depended on both parent size and diet. 6. It was concluded that R. padi represented a low quality prey for cereal spiders which obtain very little energy but essential nutrients from the aphids. It remains to be established whether this has consequences for the prospects of managing spider communities with the view of improving their effectiveness against aphids. It is argued that with a low preference for aphids, improving availability of alternative prey in more diverse agrosystems may possibly reduce spider predation on aphids.