In field populations of cereal aphids parasitism levels declined through the season as fungal infection increased. In laboratory trials the fungus Erynia neoaphidis Remaudiere & Hennebert took 3 to 4 days to kill the rose-grain aphid, Metopolophium dirhodum (Walker), whereas the parasitoid Aphidius rhopalosiphi De StefaniPerez took 8 to 9 days at 20 ~ When aphids were infected by the fungus less than 4 days after being parasitized the parasitoids were prevented from completing their development. Conversely, when infection-occurred more than 4 days after parasitization development of the fungus was significantly impaired. There was no histological evidence that the fungus invaded the tissues of the parasitoid when both attacked the same aphid.Interference between parasitoids and fungal pathogens must be taken into account when estimating the impact of these mortality agents on pest populations.

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S U M M A R YThe population of Aphis fabae on field beans at a site in Highfield, Rothamsted in 1973 reached its peak 1 wk earlier than that at an equivalent site in Mill Dam Close, Woburn, 29 km NW of Rothamsted. Epizootics of Entomophthora caused weekly maximum mortalities of adult apterae of 71% at Highfield and 67% at Mill Dam Close. These epizootics and the periodicity of Entomophthora conidia in the air closely paralleled the development of the aphid population. There was little evidence of a consistent relationship between Entomophthora infection and any of the weather factors considered. At both sites most mortality was caused by E. planchoniana though many aphids were killed by E. aphidis and E. obscura. E. fresenii and E. virulenta killed very few aphids. Most conidia in the air were of the E. aphidis-type. Up to 44% of alatae emigrating from bean crops were infected with Entomophthora, confirming that aphid migration is an important means of distributing the fungi. Aphid numbers rose to more than 1600/plant at both sites, in spite of the action of Entomophthora, and would probably have been less had the fungi been more abundant earlier in the season.

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