OBSERVATIONS ON THE EMERGENCE PERIODS, LARVAL POPULATIONS AND PARASITISM OF THE CABBAGE ROOT FLY (ERIOISCHIA BRASSICAE BCHE)

Abstract

At Wye the overwintering generation of Erioischia brassicae emerged in captivity in a sharply defined period lasting from 4 April to 3 May 1954 and 75% (53 flies) emerged from n to 19 April.The emergence period for first‐generation flies in 1954 was from 28 June to 24 September. There was no well‐defined peak period; flies emerged almost every day from 28 June to 16 August, with the highest numbers (11 flies per day) on 16–17 July and the greatest concentration of emergences (76 flies, 41%) from 11 to 21 July. In 1955, first‐generation flies emerged from 24 June to 23 August, with a peak emergence period from 6 to 16 July when 50% (348 flies) emerged. The differences between the times of peak periods of emergence in the 2 years were associated with differences in temperature in June and July; cooler weather appeared to delay emergence.Counts of first‐generation larvae and puparia of E. brassicae in the latter half of June 1955 showed average populations of 59 per plant with a maximum of 132 per plant. Counts after exposure to attack during July and from mid‐August to November showed an average infestation of less than two per plant. Plants exposed to attack from mid‐May onwards until December showed average populations of 15 puparia per plant only two of which were viable. This gave further evidence that high infestation by E. brassicae occurs only in spring.Larvae and puparia collected in June were allowed to complete their development in captivity. In 19–54 the emergences from 350 puparia were as follows: 188 flies (54%); 145 (41%) Trybliographa rapae Westw., a Cynipid (Hymenoptera) parasite; and 17 (5%) Aleochara bilineata Gyll., a Staphylinid beetle parasite. In 1955 the emergences from 990 puparia were 693 (70%) flies, 296 (30%) Trybliographa rapae and one Aleochara bilineata. The samples consisted of larvae and puparia in a ratio of about 3 : 1 and were therefore biased in favour of Trybliographa rapae. Observations on dates of emergence showed that in samples consisting wholly of viable puparia drawn after mid‐July, there is likely to be a bias in favour of the parasites because the peak of emergence of first‐generation flies has already passed.