Biology and Ecology of the Garden Chafer, Phyllopertha horticola (L.). II.—The Cycle from Egg to Adult in the Field
Authors: Milne A. 
Published: July 2009

The Garden Chafer, Phyllopertha horticola (L.), has three larval instars, the third ending in hibernation, which gives way to a prepupal stage. The development cycle occupies 12 months and only one generation is present in the soil at any time. From a study extending over five years, 1948 to 1952, the cycle from egg to adult in the English Lake District may be outlined as follows:—At the earliest, oviposition starts in the latter half of May. In soil 3½ in. deep, eggs are laid at an average depth of 1½ in. (range ¾ to 2½ in.). Other authors report that where the soil is sufficiently deep, the maximum egg depth may extend to four inches in the Lake District and even eight inches elsewhere. The eggs are spaced about a quarter of an inch apart (max. 1 in.), each in a tiny earthen cavity, all more or less directly below the point where the female enters the soil. Incubation of the individual egg averages five weeks.The first instar occupies individually about three weeks on the average; the second instar about four weeks; and the third instar, up to the beginning of hibernation, eight to ten weeks. On the average also, first-instar larvae begin to appear in a population about the first week of July, second instars about the fourth week of July, third instars about the fourth week of August, and the earliest hibernators about the third week of October. Except for a few stragglers occasionally in early December, the entire population is generally hibernating by the end of November. The hibernation is a true diapause.A detailed description of the method of feeding is given. The larva consumes plant roots which it obtains by tunnelling through the soil. Since its natural habitat is pasture land, grass roots are the main food. It probably also eats invertebrate carrion occurring by chance in its path. The first and second larval instars are given over to growth, the third and final is occupied mainly in storing up fat-body. This store has to suffice for maintenance during the remainder of development and also for the entire egg or sperm production.On hatching, the larvae feed at about 1½ inches (¾ to 2½ in.) depth in the soil, i.e., at egg-level. As they grow, however, they ascend until latterly, as third instars, they are feeding about ½ inch (¼ to 1 in.) from the surface. This progressive rise is probably dictated by the increasing need for a more copious food supply. With the possible exception of a very prolonged drought, weather has no effect on the level at which larvae feed.Larvae hibernate at 2 inches (0·8 to 3·8 in.) below the surface of soil 4 inches (2·3 to 6·0 in.) deep, i.e., well above the “ pan ” (gravel bed, or rock). In other localities other authors have recorded hibernation at the same as well as greater depth in deeper soils. The existing data are insufficient to show what governs the choice of depth in the soil.On the average, prepupation begins in a population about the end of March and, individually, lasts between three and four weeks; pupation begins in the third week of April and lasts about four weeks. The pupa lies inside the last larval exuvium in the hibernation cell. Sex can easily be discerned in the pupa. There are always rather more males than females in a field population, considerably more in some years. Pupal sex ratios ranged from 1·13 to 1·89. On the average, male pupation precedes female by one day or a little more.Behaviour after the splitting of the pupal skin is the same in male and female. At first, for about four days (1 to 7), the adult remains motionless in the hibernation cell. Then, alternately burrowing and resting, it ascends to the base of the sward in about two days (¼ to 4). At the sward base it now halts for about two days (½ to 6) before emerging into activity upon the sward surface for the first time; this halt of two days may be prolonged by one or more days if weather is unsuitable when a beetle is ready to become active. In toto, given no weather hindrance, the individual transit from cell to sward surface usually occupies rather more than one week (7·7 days, range 6·0 to 8·5). On the average, the first active beetles are seen in the last week of May but may be as early as the third week of May or as late as the second week of June, according to the weather. The male precedence over females in development is maintained from pupation onwards, hence the first males are always active upon the sward at least one day before the first females.It takes some considerable time for all the individuals in a population to accomplish the change from any one particular stage of development to the next. There are, however, never more than two successive stages in the soil at the same time. The period of overlap of two stages in the population fluctuates widely (8 to. 41 days) from step to step in the development cycle. This is the result of the seasonal rise and fall in the soil temperature, and of diapause. The overlap, which is really a measure of range of developmental age among individuals, contracts to the minimum (8 days) as the population approaches maturity. This facilitates mating.