This part of the study of the garden chafer, Phyllopertha horticola (L.), in the English Lake District covers the flight seasons 1948–1952. Annual sex ratios in the population of pupae (just before ecdysis to adult) ranged from 34·6 to 46·9 per cent. females.

The flight season begins with the emergence of some adult males upon the sward. If the weather continues favourable, the earliest females appear on the second or third day of the season. Unsuitable weather delayed the advent of females until the sixth day in two seasons. In a homogeneous population, primary emergence is complete in just under a fortnight, at the end of which time numbers of beetles seen on the grass sward are at their peak. Phase 1, swarming over the sward, continues, with dwindling numbers, for just over a week more (overlapping Phase 2, swarming on the bracken).

The female/male ratio rises throughout Phase 1 except for a tendency to fall in the last day or two. But generally for most and sometimes the whole of Phase 1, the proportion of females above ground is far below the true population ratio. This is partly because population emergence in the female is 1–2 days behind emergence in the male; but much more because after primary emergence the male is fully involved in above-ground activity every day until the beginning of Phase 2 or end of Phase 1 (depending on whether he emerges earlier or later), while the female is not.

In the same way, there are no females present on the bracken until after the first day of Phase 2, but the proportion of females rises more quickly and to a higher level than in Phase 1. By the middle of Phase 2, the proportion is close to the true population ratio. Since males are now confined to the bracken, this means that mass oviposition is more or less complete. During the last week of the season the female proportion rises steeply. This is because females live a few days longer than males besides being 1–2 days behind males in their original emergence.

After primary emergence, the average female lives about a fortnight, the male rather less. Approximately the first half of adult life is spent in the grass sward area, the second half on the surrounding bracken (or hedges or trees).

As Phase 1 proceeds, the pairing rate of females decreases. This is partly because the proportion of males is falling and therefore fewer males are available to search for each female over the grass sward, and partly because of a change in mating behaviour of the female as she ages. On the other hand, pairing rate is at its highest (average 82%, maximum 95%, of available females) in Phase 2 when the proportion of males is at its minimum (actually the true proportion in the population). This is because on the bracken the beetles are confined to a much smaller area than on the grass and therefore males find females more readily.

In flying weather, the proportion of beetles in the air at any moment is at best no more than about 70 per cent.; usually it is less than 50 per cent. On the whole males are very much more given to flying than females.