Local population differences in emergence of cabbage root flies from south‐west Lancashire: implications for pest forecasting and population divergence

Abstract

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.

Behavioural and chemosensory responses of the turnip root fly (Delia floralis) to glucosinolates

“…Finch and co-workers have shown that the volatile isothiocyanates influence the behaviour of Delia radicum, which prefers to oviposit near plants that contain a diverse array of these volatiles (Finch et al, 1986). Finch and co-workers have shown that the volatile isothiocyanates influence the behaviour of Delia radicum, which prefers to oviposit near plants that contain a diverse array of these volatiles (Finch et al, 1986).…”

Emergence Patterns of Delia radicum (Diptera: Anthomyiidae) Populations from North Carolina and New York

“…This observation, coupled with the uncharacteristic emergence pattern of the 1988 Geneva population, suggests that the spring emergence pattern of a population is capable of changing over a relatively short period of time. Finch et al (1986) concluded that weather conditions exert little selection pressure for early or late emergence in England; rather, agricultural practices that affect the availability of host plants were believed to be the primary selection factor.…”

“…In the laboratory, biotypes could be differentiated following the cold phase of diapause development, because >90% of the flies of the early-emerging biotype emerged from pupae within 14 d when held at 20°C, whereas emergence in the lateemerging biotype was protracted and completed over a period of 60-100 d. Subsequent work by Collier et al (1989) demonstrated that postdiapause development of early biotypes occurred after the cold phase of diapause development was completed and temperatures rose above 4.3°C (Collier & Finch 1985), but late bio-0046-225X/93/0559-0566$02.00/0 types required an additional period at temperatures above 7°C before post-diapause development occurred (Collier et al 1989). In addition, an intermediate biotype was also described that consisted of different proportions of individuals with slightly later emergence, but tending more toward the early than late spectrum of emergence (Finch et al 1986). Finch & Collier (1983) also demonstrated that populations exhibiting early-, intermediate-, or late-emergence patterns could be selected within one generation from a parental population that was heterogeneous with respect to emergence.…”

“…Finch & Collier (1983) also demonstrated that populations exhibiting early-, intermediate-, or late-emergence patterns could be selected within one generation from a parental population that was heterogeneous with respect to emergence. The dominance of either early-or lateemerging biotypes in various localities was presumed to be maintained by temporal, and thus reproductive, isolation (Finch et al 1986). …”

“…Pupae were then placed at 20°C and a photoperiod of 16:8 (L:D)h, and fIy emergence was recorded daily. Emergence patterns were classified according to Finch et al (1986): percentage of early emergers (percentage of flies emerging within 14 d), median time for emergence (Tso), and time elapsing between emergence of 10 and 90% of the flies (T 10-90)' Inheritance of Early (E) and Late (L) Emergence. To determine whether early-or lateemerging traits of flies from the Scaly Mountain population were genetically controlled, crosses were made among individual male and female flies emerging from pupae collected in December 1988. To do this, both early-and lateemerging flies had to be available at the same time; thus, pupae for the late-emerging flies were removed from 4°C =25 d before the pupae for the early-emerging flies.…”