Distribution of aphids in spring cereals.
Observations were made in southern England in 1970-71 on aphid populations infesting the aerial shoots of spring cereals. Aphids mainly Acyrthosiphon (Metopolophium) dirhodum (Wlk.) with smaller numbers of Macrosiphum (Sitobion) avenae (F.) invaded spring cereals in May, were scarce until after mid-June, reached a maximum in mid-July and disappeared after early August. Aphid densities were greater in 1970 than in 1971, but they probably affected grain yield for only about a week in July 1970. In June, M. avenae occurred mainly round the edge of the crop, while A. dirhodum was widespread. In July, M. avenae infested the whole crop, but was most numerous in sheltered areas, whereas A. dirhodum was common everywhere. Both species occupied the leaves until after the heading stage, when most examples of M. avenae invaded the ears and most of those of A. dirhodum remained on the leaves. In 1970, groups of A. dirhodum were larger (maximum 79) than those of M. avenae (maximum 42), but in 1971 they were similar and smaller (maximum 38 for M. avenae). M. avenae occurred singly more often than A. dirhodum. In 1970, the largest groups of A. dirhodum appeared a week before and those of M. avenae up to 5 weeks before the maximum numbers of groups /0.3 m of row, whereas they coincided in 1971. Mean numbers of groups and of aphids/0.3 m were related, but group size was independent of population size. Statistical differences between the indices of aggregation for several aphids species could not be demonstrated.