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 va lue of the aphid Rhopalosiphum padi. Aphis fabae and a sciarid midge w ere considered in single experiments. Laboratory fruit flies were used as a standard prey for comparison. The spider species used were Erigo ne atra, Pardosa amentata and P. prativaga. 2. In preference experimen ts P. amentata ranked prey types in the order: fruit flies > sciarid > R. padi > A. fabae. 3. In P. prativaga consumption of fruit flies inc reased with the length of prior starvation period; consumption of aphi ds was much lower and independent of hunger. 4. First instar spiderlin gs of P. amentata and Erigone atra were unable to develop on a pure di et of R. padi; they died without moulting. 5. Egg production in wild c aught 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 produc ed an average of eight sacs. Egg numbers were high on a pure diet of f ruit flies, but hatching success was best with fruit flies supplemente d 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 p rey for cereal spiders which obtain very little energy but essential n utrients from the aphids. It remains to be established whether this ha s consequences for the prospects of managing spider communities with t he view of improving their effectiveness against aphids. It is argued that with a low preference for aphids, improving availability of alter native prey in more diverse agrosystems may possibly reduce spider pre dation on aphids.