The effects of predator exclusion and caging on cereal aphids in winter wheat.
Changes in numbers of cereal aphids (mainly Sitobion avenae (F.) and Metopolophium dirhodum (Wlk.) but including also Rhopalosiphum padi (L.)) were measured inside and outside cages designed to exclude aphid-specific predators, including coccinellids (mainly Coccinella septempunctata L.), syrphids and chrysopids, in a field of winter wheat in southern England in 1976-79. In each year, 3 phases of aphid population development were distinguished: an initial phase of rapid growth, a divergence phase and a decline phase. Populations in the cages increased at the same rate as outside during the growth phase, and during the divergence phase continued to increase to a peak, while populations outside increased at a slower rate or decreased. As a result of the divergence, a difference in peak numbers of up to 6 times was recorded. Populations outside fell rapidly during the decline phase, while a slower reduction occurred in the cages. In the 3 years, the divergence phase coincided with an increase in numbers of predators. The consumption rate by predators that would be required to bring about the observed differences between cages and outside was calculated and found to lie within published values. It was concluded that predation was likely to be the major cause of the differences between cages and outside to the time of the population peak. Predation, parasitism (mainly by Aphidius spp.), diseases caused by (Entomophthora planchoniana, Zoophthora neoaphidis (Erynia neoaphidis) and Conidiobolus obscurus) and emigration all contributed to the decline phase. Although there was no difference in plant growth stage inside and outside the cages, and the divergence phase occurred at different growth stages in the 3 years, it was not possible to rule out a subtle effect of the cageing technique itself on the physiology of the plant or aphids, acting only during the divergence phase.