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.

SUMMARY Sitona lineatus displays two distinct flight periods; a post‐teneral flight in the late summer and a post‐diapausal flight in the spring. The late summer flight begins as soon as adults start to emerge from pupal cells during late July and continues until mid‐October. Maximum aerial density is observed in late August. Less than 10% of newly emerged adults leave the crop by flight, the rest remain in the soil or move to overwintering sites by walking. Spring flight begins in late March if temperatures are above the 12.5°C flight threshold and reaches a peak in mid‐April. Most migratory flight is downwind. Little trivial flight occurs and once individuals have located a crop, redistribution is by walking. Laboratory studies indicate that adults fly most often during the day, particularly at noon. The size of infestations appeared to be independent of the area of legumes grown in a locality with large differences occurring even between neighbouring fields. Examination of the yields of field experiments at Rothamsted demonstrate an unexpected trend for higher yields in later sown spring bean crops. This may be because late sowing avoids infestation by spring migrants.

The insecticides, pirimicarb and dimethoate are commonly applied to cereal crops in the UK and when this study was begun the fungicide benomyl was also regularly used. The effects of these chemicals on natural enemies of cereal aphids were examined in replicated plots of winter wheat.Benomyl did not affect any of the groups examined but it was applied early in the season (April/May) when most natural enemies were few or absent from the crop. Numbers of carabid beetles, staphylinid beetles and spiders, sampled using pitfall traps and a vacuum net sampler, were all reduced by dimethoate but not by pirimicarb, although these effects were only detected by pitfall traps when the small plots were surrounded by polyethylene barriers to prevent inter-plot movements. Parasitic Hymenoptera were decreased by both insecticides, partly due to direct toxicity and partly to host removal. Data on the aphid predators, Coccinellidae, Syrphidae and Chrysopidae were inconclusive because there were very few in the crop due to the scarcity of aphids.Ways in which pesticides can affect natural enemy populations other than by direct mortality and the dangers associated with routine, prophylactic pesticide applications are discussed.

SUMMARYIn two experiments done in successive years to compare the growth and yield of Cappelle wheat either protected from or exposed to attacks by larvae of wheat bulb fly, the plots were previously fallowed, but egg laying was prevented on half of each plot by using Polythene soil covers. In the first season wheat was sown at the end of October, November and December and in the second season in late October and in early January, when there were two sowings, one with and one without a spring application of herbicide. The infestation rate was 1·1‐1·7 million eggs per acre, typical of a moderate attack. Larvae had little effect on the yield of October‐sown wheat as the plants had two shoots each when first attacked and few were killed. On plots sown late, yield was decreased by up to 22%, as plants had only a single shoot when attacked and many were killed.The main effect of wheat bulb fly was to reduce the number of ear‐bearing shoots by killing plants and restricting the production of new shoots. Surviving plants partially compensated by producing more ear‐bearing shoots with heavier ears and slightly heavier seeds than normal.Killed plants were not distributed uniformly but were often in patches several feet across. Wheat on the attacked plots ripened more slowly and unevenly than on the unattacked plots.Weather affects the growth of the plants and activity of the larvae and thus partly determines percentage shoot survival.

Larvae of Plutella maculipennis (Lepidoptera; Plutellidae) and Phaedon cochleariae (Coleoptera; Chrysomelidae) were fed on the leaves of both turnip Brassica rapa and radish Raphanus sativus. Both the weight of the food eaten and the area of leaf injured were measured. The weight eaten depended on the nutritive value of the food, whereas the area of leaf injured depended on the leaf thickness and the method of feeding of the insect. Both species developed fastest on young plants, which contained the most protein. They ate a greater weight of old than of young leaves, and injured a greater area of radish than of turnip leaf, which had more dry matter/unit area. The area of leaf injured by P. maculipennis was equivalent to the weight eaten, but P. cochleariae which had a different method of feeding destroyed much more leaf area than was expected from the weight of food eaten. ZUSAMMENFASSUNG BLATTSCHÄDEN UND NAHRUNGSVERBRAUCH DER LARVEN VON PHAEDON COCHLEARIAE (COLEOPTERA: CHRYSOMELIDAE) UND PLUTELLA MACULIPENNIS (LEPIDOPTERA: PLUTELLIDAE) BEIM FRASS AN RÜBSEN UND RETTICH Larven des Meerrettichkäfers, Phaedon cochleariae Fab., und der Kohlschabe, Plutella maculipennis Curtis, wurden an intakten Pflanzen von Rettich (Rhaphanus sativus L., var. French Breakfast) und Rübsen (Brassica rapa L., var. Early Milan White) gehalten. Größe und Trockengewicht der verzehrten Nahrung wurden während der gesamten Larvalentwicklung ermittelt. Die Versuche verliefen bei einer konstanten Raumtemperatur von 20°. Beide Insekten fraßen eine größere Gewiohtsmenge von Rübsen‐ als von Rettichblättern und von alten mehr als von jungen Blättern. Die mittlere Umsatzrate von P. maculipennis betrug für Rübsen und Rettich 0,41 und der Ausnutzungskoeffizient war an jüngeren Pflanzen sowohl bei Rettich wie Rübsen größer als an älteren. Die Umsatzrate von P. cochleariae variierte zwischen 0,11 und 0,27 in Abhängigkeit von Art und Alter der Nährpflanze, aber es war nicht möglich, den Ausnutzungskoeffizienten zu messen, weil der Kot nicht gesammelt werden konnte. Das Trockengewicht der von P. maculipennis gefressenen Nahrung auf den jüngsten und ältesten Pflanzen variierte bei Rübsen zwischen 10,52 und 23,77 mg und bei Rettich zwischen 9,45 und 15,28 mg; und bei P. cochleariae zwischen 12,24 und 15,70 für Rübsen und 9,79 und 12,29 für Rettich. Beide Insekten fraßen von Rettich eine größere Blattfläche als von Rübsen, wahrscheinlich weil Rübsen einen größeren Gehalt an Trockensubstanz pro Flächeneinheit enthält. Die von P. maculipennis beschädigte Blattfläche entsprach dem Gewicht der gefressenen Nahrung, da sie sauber durch das Blatt hindurch frißt und die Blattadern meidet. P. cochleariae beschädigte eine größere Blattfläche als erwartet wurde, da er die untere Blattoberfläche beschabt, oft nicht eindringt, aber die Blattadern durchnagt, so daß das Gewebe infolge Austrocknung und Ernährungsstörungen abstirbt.