Biology and Ecology of the Garden Chafer,Phyllopertha horticola(L.). vi.—The Flight Season: Reproductive State of Females

Abstract

This part of the study of the garden chafer, Phyllopertha horticola (L) in the English Lake District deals with the reproductive state of the females during the flight season. Total egg-production depends entirely on stores (fat-body) accumulated in the final larval instar. On the average, the fat-body enables about 16 or 17 mature eggs to be manufactured per female, the usual range being 9–32. Rate of reproductive development is the same in the largest and smallest female adults. The outstanding mass features of the flight season are the two roughly equal and half-overlapping Phases: Phase 1, swarming over the grass sward, followed by Phase 2, swarming on the surrounding bracken, hedges and trees. Mass aspects of the female reproductive state throughout the two Phases are as follows.At primary emergence on the grass sward, females always contain some fat-body together with some fully developed and/or immature eggs. On the average they have about one-third of their full complement of eggs matured, the range being from none (with about 50% of the original fat-body still remaining) up to four-fifths (with about 10% of the fat-body). Since primary emergences in a homogeneous population extend over about a fortnight, females taken from the grass sward throughout Phase 1 are in various reproductive states. Thus after the first few days of the Phase and until the last individual makes its primary emergence, sample females may have half their fat-body and no eggs yet fully matured or no fat-body and all their eggs actually laid, or any intergrade between these extreme states. Towards the end of Phase 1 (which finishes about a week after the last primary emergence), females have no fat-body remaining and have already laid most or all of their eggs; and a small proportion of them now has food in the gut.In Phase 2, about 99 per cent, of females on bracken, hedges and trees contain no fat-body and therefore cannot manufacture any more eggs. On the average they have two mature or very nearly mature eggs left (range 0–26). This egg content is much the same as at death. The remaining 1 per cent, of females are aberrants with some fat-body still unconsumed. ‘ Bee-liners ’, the females which shoot out bullet-like from the bracken, hedges or trees to alight comparatively far afield, either have some fat-body remaining, or, if not, considerable numbers of eggs (mean about 9 as compared with the general mean of 2). Thus bee-liners are the phase-2 individuals with most eggs still unlaid. Obviously they are a minority.Deductions as to individual behaviour are made from the mass aspects of the reproductive state and are to be examined in the next paper of this series.Since females of the garden chafer have already laid all or most of their eggs before appearing on hedges and trees, gardeners will not control this pest by killing all they find on their hedges and trees.