Effects of parasites and predators on the cereal aphids Metopolophium dirhodum (Wlk.) and Macrosiphum avenae (F.) (Hem., Aphididae)

In a study of aphids and their parasites, hyperparasites and predators in a field of cereals at Rothamsted Experimental Station in 1970–71, samples were taken by counting specimens in 0·3-m lengths of row and by means of a suction trap set 12·2 m above ground and 0·5 km away from the field. Migrant aphids (Metopolophium dirhodum (Wlk.) and Macrosiphum avenae (F.)) and Braconid parasites (Aphidius spp., Ephedrus plagiator (Nees) and Praon volucre (Hal.)) were caught from April–May to August–September, with most from mid-June to August, whereas hyperparasites (Phaenoglyphis sp., Lygocerus aphidivorus Kieff., Asaphes vulgaris Wlk., Coruna clavata Wlk. and Alloxysta sp.) occurred for more of the year. Of the six parasite species attacking aphids on cereals in 1970–71, three Aphidius spp. were more common than E. plagiator and Praon volucre. Parasitism was higher and aphids were fewer in 1971 than in 1970. In the field, more than 80% of the mummies were of old nymphs; parasitism of live old nymphs and apterae was similar, larger than for alatae, and larger for Metopolophium dirhodum than for Macrosiphum avenae. Hyperparasitism by five species affected parasitism, especially in 1970. Migrant Coccinellids were trapped at the start and end of aphid infestations on cereals, Syrphids mainly in July and August, and other predators for longer. The commonest predators were Coccinella septempunctata L. and Propylea quatuordecimpunctata L. in 1970 and Syrphus balteatus (Deg.) in 1971; the others were scarce. July populations of aphids were primarily reduced by the Syrphids.