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A knowledge of the often complex relationship between insect populations and their effects on the yield-forming processes of crops is useful for assessing pest status and for devising methods of minimising the effects of infestation on yield. After a preliminary section on definition of terms, consideration is given to ways in which insects injure plants and crops, particularly annual crops. Investigations into the yield-forming processes of unattacked crops can often provide useful insights into the likely effect of insect injuries on yield. The differences between the characteristics of individuals and populations are discussed, both of insects attacking plants, and plants responding to the attacks of insects. The distribution of insects on plants and on crops and the effect that this has on damage is considered, as is the time and duration of the attack in relation to the growth cycle of the crop. The components of plant resistance to insect attack are then discussed, especially tolerance and the various ways in which plants and crops compensate for injury. The review concludes with an examination of the quantitative relationships between numbers of insects or injuries and their effects on yield. The generalised response is a sigmoid curve relating an upper and lower plateau of yield where changes in the numbers of insects or injuries have little effect. Most relationships conform to some part of this curve, though few show all its features.

Experiments started in 1976Experiments started in , 1977Experiments started in and 1978 on Clay-with-Flints soil at Rothamsted tested the effects of combinations of eight two-level factors on spring-sown field beans. Factors tested, presence v. absence, were irrigation, nitrogen fertilizer, aldicarb, fonofos (dieldrin in 1976), benomyl to the seed bed, permethrin (fenitrothion in 1976), pirimicarb, benomyl foliar spray (not tested in 1976).The main pests and diseases present were nematodes of the genus Pratylenchus, the pea aphid Acyrthosiphon pisum, the pea and bean weevil Sitona lineatus, root blackening associated with the fungal genera Pythium and Fusarium, the foliar diseases chocolate spot, Botrytis spp., rust, Uromyces fabae and bean leaf roll virus.Incidence of these pests and diseases varied between years. Controlling those present increased yield by about 0-7 t grain/ha each year. The difficulty of apportioning this increase to particular pests and diseases is discussed. ' Irrigation increased total dry-matter production and grain yield in 1976 and 1978 but only total dry-matter production in 1977, when grain yield was lost because of lodging. Nitrogen fertilizer had little or no effect.The most favourable combinations of treatments gave yields of 3-4, 5-0 and 6-4 t grain/ha in the 3 years respectively. Small yields in 1976, despite irrigation, were attributed to premature senescence caused by exceptionally high temperatures. It is suggested that with good control of pests and diseases yields of at least 5 t/ha should be attainable on Clay-with-Flints soil without irrigation in years of average temperature and rainfall and yields in excess of 6 t/ha when the soil-moisture deficit is lessened by either above-average rainfall or irrigation.Treatments applied to the beans had little or no effect on two following crops of winter wheat. INTRODUCTIONseasonal variations in yield and to determine the size of yields when known pathological constraints The introduction of residual weedkillers and were removed. The pests under study were nemainsecticides able to control black bean aphids todes and insects, and the pathogens were root and (Aphis fabae Scop.) increased the prospects of foliar fungi and viruses. growing satisfactory crops of spring-sown field Of the migratory nematode genera present in beans (Viciafaba L.). Despite these introductions Rothamsted soils, Pratylenchus Filipjev and small average yield and large seasonal variations Tylenchorhynchus Cobb are known to damage roots are still major problems. Recently additional pests of field beans (Oostenbrink, 1954; Whitehead & and pathogens, whose incidence and ability to Fraser, 1972). Pratylenchus Micoletzsky and Helicause damage differ each season, have been shown cotylenchus Steiner are known to attack barley to attack beans. The experiments described here (Coursen, Rohde & Jenkins, 1958;Taylor, 1960). attempted to control a wide range of these to At Rothamsted the predominant genus attacking determine the extent to which this could lessen field be...

SUMMARY Counts of Aphis fabae Scop, eggs in winter and of the active stages in May on the spindle bush, Euonymus europaeus L., are used to predict the need for chemical control of the black bean aphid on spring‐sown field beans, Vicia faba L. For the purposes of forecasting, most of England south of the Humber is divided into 18 areas for which separate forecasts are made. A level of 5 per cent of plants colonized on the SW. headlands of fields in early to mid‐June is used as an economic threshold above which control measures are justified. From 1970 to 1975, area forecasts of the probability of attack were successful. According to the sampled fields there was considerable annual variation in the size of infestations, ranging from widespread, potentially damaging populations in 1973 and 1974 to very small populations in 1975. There was also considerable variation between areas: for example, in East Anglia and the East Midlands treatment was justified only in 1973 and 1974, whereas in Hampshire and Worcestershire/Herefordshire treatment was justified in all years except 1975. It was estimated that, from 1970 to 1975, control based on forecasting would have led to a gain of £11·4/ha over no treatment, and £3·8/ha over correctly timed routine preventive treatment applied annually irrespective of the aphid situation.

SUMMARYThe flight activity and local distribution of adult D. coarctata were studied on Rothamsted Farm during 1970‐5 using several trapping methods. Changes in the activity and distribution of the flies were followed as the population aged, and in females these changes were associated with maturation of their eggs. Most females appeared to stay at their emergence sites in winter wheat until egg‐laying started about 1 month later. They then dispersed and both sexes were found in previously uninfested cereals and grass as well as on fallow land where the eggs are laid. Flight occurred chiefly in the late afternoon and evening. The concentration of flies at their emergence sites for several weeks before egg laying could facilitate chemical control, provided the practical difficulties of application can be overcome.