The garden chafer, P. horticola (L.) is widely distributed in England and Wales but is more abundant in the western half of the country. Its natural habitat is poor quality permanent grassland characterised by a diversity of species of flowering plants and a high proportion of weeds. The soil of the infested areas is invariably light, the land is usually hilly and the rainfall relatively high.

Adults emerge suddenly in May or June according to the weather. Males emerge some days before females. Daily activity increases during the flight period which lasts 3–4 weeks. Beetles only fly in warm sunny weather.

The females lay an average of 14 eggs each. Fecundity is correlated with weight and is related to the food supply of the larvae. In nature, the weight of the adults may be limited through the exhaustion of the food supply of the larvae. Females will mate and lay fertile eggs without feeding. In general, feeding by the adult does not affect fecundity but in one experiment beetles fed on salad burnet produced and laid more eggs than beetles fed on bracken, blackberry, grass or kept without food. Over 90 per cent. of the eggs are fertile ; fertility decreases with the age of the adult. The incubation period of the eggs is about 22 days in the laboratory but about 28 days in the field.

The spatial distribution and seasonal changes in population of P. horticola have been studied within an infested field on the Tickenham Ridge, Somerset. The eggs are laid in groups from which the larvae disperse. At the end of the feeding period the larvae may be distributed at random but there is some evidence that regrouping at feeding sites occurs. Most of the eggs are laid in the top 3 ins. of soil. The larvae feed in the root zone but go deeper in the soil in winter to hibernate. The population fell in successive years within the area studied. Initially there was a higher infestation in part of the field where the turf was damaged by the larvae but the relative intensity of infestation changed and subsequently more eggs and larvae were found in part of the field where the turf was undamaged. There was a higher mortality among larvae in the damaged part of the field. These population changes have been related to a number of environmental factors which may account for them, in particular to dispersal of the adults, density of the vegetation, consolidation, soil moisture and predators. In a field at Alton Pancras, Dorset, predation by birds reduced the larval population by 50 per cent. between 1st September and 1st November.

A study of the effect of infestation upon the vegetation showed that damage was largely due to the destruction of the grasses. Weeds were unaffected and there was no evidence of an increase of weed area by colonisation of bare patches resulting from damage. Brachypodium pinnatum resisted attack owing to its tough rootstock. Dactylis glomerata and Lolium perenne were relatively resistant, Poa spp. and Festuca spp. were damaged but regenerated when feeding stopped and the population declined. Salad burnet, a favoured food plant, was more abundant in damaged areas and its distribution may influence the distribution of P. horticola.

When infestation is severe and the grass roots are destroyed, the soil aggregates are broken down. The soil structure was restored when the grasses regenerated, and by consolidation.

Field populations were reduced by ploughing and by reseeding. Dusting with 3·5 per cent. benzene hexachloride at 70 lb. per acre during the flight period gave promising results when applied in favourable weather.