The biology of the white clover seed weevil Apion dichroum Bedel (Col. Curculionidae).
Abstract
The following is based largely on the author's summary. The bionomics of Apion dichroum Bedel in fields of white clover [Trifolium repens] in south-eastern England were investigated with special reference to the distribution of spring migrants in the breeding fields and to oviposition. Much of the information was derived from random sampling carried out during 1964-66, primarily to investigate the population dynamics of the weevil. A quadrat method of sampling adult populations is described, and its efficiency is compared with that of a sweep-net method. The density of migrants increased in the breeding fields from mid-May to early July. The latter part of this period was characterized by an increasing proportion of females, while the estimated density of the males generally diminished. Densities were considerably higher in plots fenced off from grazing stock than in unfenced plots. Regression analyses showed that in sample unit areas of 0.1 m2 weevil density was unrelated to the height of the sward and the densities of the male and female populations were strongly associated. The density of the females, but not of the males, was related to that of the clover flower-heads [cf. RAE A 57 1951]. After migration, the wing muscles degenerated and the gonads developed. On average, males retained their wing muscles for about 10-45 days longer than females. Adults of both sexes were parasitized and made sterile by a Braconid tentatively identified as Microctonus aethiops (Nees), while the larvae were parasitized by an unidentified Eulophid of the genus Entedon. Ovipositing females selected flower-heads in which up to 25% of the flowers were open and preferred those high up in the sward. The density of the eggs, based on the numbers of flowers and eggs in the head, remained the same in heads of any size. The females showed a slight preference for flowers towards the centre of the head and tended to reject those already containing an egg. The numbers of oviposition punctures made by the female per flower-head decreased in the latter half of the oviposition period (mid-July to late August), and the number of eggs deposited per puncture also diminished during this time. The rate of oviposition, calculated from the census data, was of the order of 8 eggs per female per day, attained in early July, in all three years, with a mean rate of about 4 eggs per female per day for an oviposition period of approximately 50 days.