S U M M A R YSeveral factors influencing the efficiency of water-traps in capturing cabbage root flies were studied at Wellesbourne in 1971 and 1972. In both the laboratory and field, approximately twice as many flies were caught in fluorescent as in non-fluorescent yellow traps. Depending upon trap density, addition of a source of the attractant allylisothiocyanate (ANCS) increased the numbers of females captured by approximately twofold in fluorescent traps and from two-to sevenfold in non-fluorescent traps. Traps were equally efficient irrespective of whether the ANCS was renewed every 2, 3, 4 or 5 days. On the first day of trapping, the number of flies caught per unit area was linearly related to the square root of the number of traps in that area. On the following days the rate was probably in equilibrium with the combined effect of immigration and the rate of development of responsive flies in the trapping zone. Most males were caught 30cm above the soil surface and most females at soil level. Traps 120 cm above the soil surface caught few flies.Populations of marked flies were released into large field cages containing both a section of hedgerow and a plot of cauliflowers. Even after a week, only 81 yo of the males and 55 yo of the females had been recaptured from the most responsive of these captive populations. Furthermore, only 30 % of females were recaptured when they were more than 8 days old, the age at which most probably enter the new host-crop.

In fourteen releases, most female D.radicum (L.) (Diptera, Anthomyiidae) flew upwind or at an angle to it of less than 77q regardless of the presence of host-plant odour. Females ready to lay eggs flew upwind without prior stimulation by odours from either a host crop or a trap releasing up to 3ml/day of the attractant allylisothiocyanate. Upwind flight was more pronounced in flies from a diapause than from a continuous, non-diapause culture. Males from the nondiapause culture dispersed upwind and downwind in more or less equal numbers; old males flew mainly downwind. But, like the females, most males from the diapause culture flew upwind. Long-distance, odour-modulated anemotaxis did not appear to be used to locate distant host crops by either sex. It is concluded that the distances of insect orientation to plant odours recorded to date are only of intermediate range, and that long-range orientation to the odours of a host-plant still has to be proven. It is suggested that host-plant volatiles are involved not only in the final stage of host location but also in the first, and probably most important stage of host selection whilst the insect is still in flight.

Yellow water traps were more effective than yellow sticky traps in capturing female cabbage root flies, Delia brassicae (Wied.). The water vapour from the traps preferentially attracted 9 9.Experiments showed that the cabbage root fly can locate sources of the cruciferous plant chemical, allylisothiocyanate (ANCS) solely by olfactory cues.Attempts to make yellow traps more effective by adding different amounts of synthetic and naturally occuring isothiocyanates were unsuccessful. There was no synergistic/coalitive effect when effective isothiocyanates were released from the traps as mixtures. Complex mixtures of mg amounts of volatiles extracted from six cuitivars of swede, a favoured host plant of the cabbage root fly, were as effective in traps as 7 ml of ANCS. It is possible that ANCS has to be released at this high rate because the fly normally responds only to mixtures of volatile cruciferous chemicals rather than to individual components of brassica odours. An extract from the swede cv. Wilheimsburger was ineffective, correlating with its partia ! resistance to this pest in the field.None of the traps tested caught 5 to 10 x more <2 9 than the standard ANCS trap and so they would not be able to control this pest.

The dispersal rates of wild and culture cabbage root flies Erioischia brassicae were determined in release-recapture experiments at Wellesbourne in 1971-3. The experiments were concerned mainly with the first 7 days of adult life. The flies, released from nine locations in the area, were recaptured in yellow water-traps.Dispersal was affected by wind, rain and the terrain the flies were crossing. The flies least often recaptured were those released into the host crop when 6-12 days old. The results indicated the following pattern of behaviour.Flies moved little during the first 2 days of adult life but by the third day both sexes had dispersed to c. IOO m from the release point. Flies are known to mate about the fourth day and after this the males continued to disperse at c. IOO m per day for the three subsequent days. 'Wild' females from fieldcollected pupae carried out a 'migratory' flight, however, and dispersed at c. 1000 m per day during the fifth and sixth days, the days preceding the start of oviposition. Similar rates of dispersal were recorded from flies released across host crop and non-host crop areas.Some females did not stop at the first crop they encountered. The culture females from the laboratory-reared pupae dispersed only c. one-third of the distance of the wild females. There was considerable intermingling of local populations. The percentage recapture of young culture and wild flies released I A P B 8 1

ABSTRACT. Emergence of cabbage root fly, Delia radicum (L.), from overwintering populations of puparia collected from twenty‐one sites in south‐west Lancashire, was extremely variable. The patterns of emergence indicated that there were two extreme biotypes, one with early‐ and the other with late‐emerging flies. There was also evidence of an intermediate biotype, tending more to early than to late emergence. This gradient of biotypes, or clinal divergence, was maintained by populations breeding at different times and by females mating close to their sites of emergence. Non‐dispersive females then perpetuated their genotype within their own locality. The time of emergence was not obviously associated with the type of host‐crop on which larvae had developed. The late‐emerging biotype was most prevalent around Halsall. The minimum distance between populations of the late‐ and the early‐emerging biotypes was 16 km. 20 km south‐east from Halsall only half of the fly population was early‐emerging, possibly a result of a displacement of the Halsall biotype by the prevailing NW wind. Regional‐based forecasts will need to take into account the emergence characteristics of the populations to predict the peak periods of cabbage root fly activity adequately in south‐west Lancashire and other areas where emergence patterns differ.