Biology and Ecology of the Garden Chafer, Phyllopertha horticola (L.). I.—The Adult and Egg Production
Authors: Milne A. , Laughlin R. 
Published: July 2009

The Garden Chafer, Phyllopertha horticola (L.) is the chief British representative of a type of world-wide pest. The following summary concerns the adult and its reproduction under laboratory conditions.

Adult feeding affects neither the number of eggs produced by the ovaries (productivity) nor the number laid (fecundity). Productivity depends entirely on the amount of fat-body stored away by the third-instar larva before hibernation.

The adult stage lasts about three weeks (3 to 35 days) in both males and females. Starvation shortens the life by a day or two. Females which lay no eggs (barren) are usually short-lived, i.e., they die before oviposition is due to start, and comprise 14 per cent, of the population.

The fat-body is exhausted and the eggs are all matured by about the end of the first half of the adult stage. Oviposition and feeding apparently do not begin until the manufacture of eggs is completed. Individually, oviposition lasts five or six days (1 to 23 days), the more eggs laid, the longer does the oviposition period usually last. The female dies two or three days (0 to 10 days) later. Starved females have a rather shorter oviposition period. Unmated females may lay a few non-viable eggs just before they die.

The average female lays 13 eggs (0 to 46 eggs). More than 70 per cent, of females lay all or nearly all of the eggs manufactured from the fat-body. Apart from barren females, there is no significant difference in length of life between females that lay all of their eggs and those that lay only some.

Females are larger than males, being on the average 1·2 to 1·3 times heavier. Productivity and fecundity are positively correlated with female body weight (which is, of course, proportional to weight of fat-body). Thus regressions may be calculated such as:

F = 0·113W − 6·60

where F = number of eggs laid and W = weight of pupa in milligrammes. Female pupal weights vary from 80 to 310 mg. Body weight is not associated with the occurrence of barrenness. If all individuals always fulfilled themselves, a population would increase eight-fold annually.

There is not the slightest evidence of a fixed oviposition pattern. Four eggs may take nine days to lay while 27 may take only 24 hours. The same number of eggs may take a longer or shorter time to lay in larger or smaller driblets, the numbers of eggs and the intervals between them being apparently quite random.

A female burrows down at a different point each time she has eggs to lay. At each burrowing she deposits her eggs within a radius of one inch from the vertical line passing through her entrance hole on the soil surface. Eggs are laid singly in small cavities about quarter of an inch apart (3/16 to 1 in.).

Eggs hatch out in 4 to 6 weeks at temperatures of 15 to 17°C. Eggs failing to hatch vary between 4 and 28 per cent, of the total laid. They comprise both unfertilised eggs and, to a lesser extent, eggs which have been fertilised but die from other causes. The proportion of unfertilised eggs increases with the age of the female at oviposition.