Estimating predation of the grain aphid Sitobion avenae by polyphagous predators

1. The numbers of dead and live aphids falling to the ground in a cereal crop were estimated. The estimates showed that more live aphids fall to the ground than dead ones. 2. The availability of live aphids to predators which forage on the ground was low because aphids quickly returned to the crop canopy. This increased the relative availability of dead aphids. 3. Estimates of predation indicated that the staphylinids Tachyporus spp., which climb plants, were the most effective polyphagous predators. 4. Models comparing the potential availability of live aphids to climbing predators and ground predators indicated that predators such as Tachyporus spp. were able to contact more live aphid prey than were ground predators. 5. A comparison between polyphagous predators and aphid-specific hoverfly (Diptera: Syrphidae) larvae suggested that densities of syrphid larvae as low as 0.1 m-2 resulted in similar levels of predation to that accounted for by the polyphagous species measured in these field studies.