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SUMMARY A model based on the concept of the circle of influence of a plant is used to estimate the relationship between yield and area/plant for tobacco crops with gaps in stand. The model predicts the average yield of plants in any configuration that includes missing plants and is used to predict the economic consequences of loss in stand from pest or other damage.

SUMMARY Plants surrounding a gap in stand compensated to some extent for leaf yield lost to missing plants. The contribution of plants in an adjacent row was much less than that of plants next to the gap within the row. Losses were reduced and quality was improved by replacing destroyed plants within two weeks of the original planting.

SUMMARY A wide range of morphological differences, from apterae to alatae, and of differences in the behaviour of alatae were produced by isolating alatiform nymphs of Aphis fabae on successive days after their birth. Alatiform. nymphs isolated soon after birth developed into the extreme apterous form, whereas when kept in a crowd until shortly before their final ecdysis a large proportion became alatae that flew before depositing young (migrants). Alatiform nymphs isolated on successive days after birth developed either into alatae that flew after depositing nymphs (flyers) or did not fly (non-flyers). Aphids thus show a continuous polymorphism, comparable to that of locusts, and possibly related to concentrations of juvenile hormone present in the aphid during larval development.

SUMMARY Size, relative wing length and numbers of antennal sensoria of alate Aphis fabae caught in a suction trap 12.2 m above ground and of migrants, flyers and non-flyers among alate A. fabae from field infestations on beans and beet are compared. The size of aphids in successive samples decreased throughout the period of colonization but, on average, beet aphids were bigger and broad bean aphids were smaller than field bean aphids. There were very few migrant alatae among the very large or the very small aphids from any of the crops, neither were aphids of these extreme sizes caught in the suction trap. Non-flyers and beet aphids generally had relatively shorter wings than migrants and bean aphids respectively. The numbers of antennal sensoria differed more between aphids from the three crops than between classes of alatae from the same crop, although trap aphids, presumed to be migrating from field beans had more sensoria than alatae taken from that crop. The mean dry mass of flyers was usually less than that of migrants of a similar size, although if the mass of nymphs deposited before flight is added, it is apparent that a flyer would be heavier than a migrant of the same size at maturity. This additional mass is thought to be due to increased development of the embryos in the ovarioles of flyers at this time and indicative of their greater adaptation for reproduction.

SUMMARY Alate Aphis fabae from field infestations on beans and sugar beet were tested for flight before parturition and divided into migrants, flyers and non-flyers. More alatae were produced, fewer nymphs were deposited before flight and the proportions of migrants among alatae increased with population density on each of the three crops. After the population peak, proportions of migrants decreased, more interforms were produced and more nymphs were deposited before flight.