Biology and Ecology of the Garden Chafer, Phyllopertha horticola (L.). IV.—The Flight Season: Introduction, and general Aspects
Authors: Milne A. 
Published: July 2009

Previous descriptions of the flight season of the Garden Chafer, Phyllopertha horticola (L.), are inadequate. From a study of five seasons, 1948–1952, the general or mass aspects may be outlined for the English Lake District as follows:—

The flight season starts in May or June, depending on soil temperature. When population is low the season lasts 3–4 weeks, when high it may last 5–6 weeks. Reasons for this are given.

The flight season has two overlapping phases. In Phase 1, beetles swarm close over the grass sward from which they emerged. As the season progresses, Phase 1 gradually gives way to Phase 2, in which the beetles now swarm closely on the bracken, hedges and trees surrounding the pasture. In the absence of bracken, the whole of Phase 2 takes place on hedges and/or trees. In the presence of bracken, phase-2 activity at first occurs on all three (bracken, hedges and trees) but, for some obscure reason, hedges and trees are forsaken in favour of bracken about halfway through the phase. Roughly speaking, phase-1 activity can be seen for the first two-thirds and Phase 2 for the last two-thirds of the flight season, i.e., there is about 50 per cent. overlap of the phases. The overlap is clearly due to earlier-emerging beetles entering phase-2 activity before later beetles have completed Phase 1.

Normal activity (i.e., flying closely over, or running or walking upon, the vegetation) is governed by weather. Both temperature and light are involved. Beetles are most active in warm bright conditions and, indeed, will fly and run only when the sun is shining. In warm bright-overcast conditions, they merely walk around. In cold, dull-overcast conditions (with or without wind or rain), they do not stir at all.

Activity starts any time between 8 a.m. and 2 p.m. (G.M.T.) according to weather (earliest recorded start being 7 a.m.). The first day of the flight season is usually marked by a short period of activity, 1–2 hours, in the forenoon, even when weather conditions continue favourable into late afternoon. As the days pass the period lengthens until beetles are active up to about 4 p.m. (latest recorded finish being 4.30 p.m.). The longest periods of activity, 8 a.m. to 4 p.m., occur when the sun shines all day after the first week of the season has passed. As the season draws to an end the activity period shortens until on the last day it is one hour or less. In general, rather more of the activity time occurs before noon than after.

The curve of numbers of beetles to be seen during the daily activity period is described for both phases. During the time of overlap of the phases, the curves are complicated by a certain amount of two-way traffic between grass sward and bracken (or hedges or trees). In addition, throughout Phase 2 a small proportion of individuals leaves the bracken flying high, straight and very fast, i.e., behaves abnormally; these individuals are termed “bee-liners” and will figure in a later paper.

For about the first week of the flight season all beetles disappear into the sward at the end of the day's activity. After that, a growing fraction of each day's active population spends the whole night in full view on the sward surface, bracken fringe, hedges and trees. All these “dormitories” begin to be used at the same time, i.e., about three days before Phase 2 starts. Passing the night on the sward surface ends with Phase 1. Roosting on hedges and trees ends simultaneously with activity there (first half of Phase 2). The bracken fringe alone is used right to the end of the season. Between sunset and sunrise, from 15 to 90 per cent, or more of resting beetles fall from bracken, hedges or trees to the ground. Reasons for this are given.

Beetles begin to feed when they stop spending the night below the sward surface. They feed after the day's activity is finished, and up to 8 p.m. at the latest. Their food is the herbage, bracken frondlets, leaves, blossoms, fruits, etc., on which they come to rest for the night.