Fecundity in the garden chafer, phyllopertha horticola (L.), is extremely variable. Fecundity depends entirely on reserves laid down by the larva.

Previous work in Britain showed that (a) early larval feeding periods in the field produce heavy pupae; (b) in an experiment comparing early with late-hatched larvae, the heavier diapause larvae developed from the early batch; (c) in the field, the earliest larvae to enter diapause were the heaviest.

Further data (hitherto unpublished) showed that larvae reared at 25°C. entered diapause heavier than larvae reared at field temperature, although mortality was high. The above data also indicated that time of entry into diapause was fixed and unaffected by temperature.

A repeat of the field experiment (b) showed the same result and also showed how a population of feeding third-instar larvae can be destroyed by frost while larvae in diapause deeper in the soil are unaffected. Thus a late season produces light larvae and a high risk of death before diapause.

Observations on larvae kept at constant temperatures (12, 16 and 18°C.) showed that the temperature threshold of development is about 12°C. in the first instar but is lower in succeeding stages; the length of the first and the second instars decreases with rise in temperature; high temperature probably produces larger larvae by the end of the second instar, but the effect was small and none at all was detected at the first moult.

The length of the third-instar feeding period is not affected by temperature in the range 12–20°C. Kate of growth (increase in weight) increases with temperature. The higher temperatures (perhaps as a result) produce heavy diapause larvae and pupae in both sexes. Entry into diapause is hindered at 20°C. The weight of a larva at the second moult is positively correlated with its diapause weight.

It is suggested that the temperature effects described can be most simply explained by postulating the existence of differential temperature coefficients for the various growth processes.