Distribution. Carrot rust fly apparently originated in Europe, and was first discovered in North America in 1885 at Ottawa, Ontario. It reached the east coast (Maine) in 1893, and western North America in the 1920s. It is now widely distributed in the northeastern United States and eastern Canada, and in the northwestern United States and western Canada. It is absent from the arid central areas of Canada and the United States, as well as northern Canada and the southern United States. Carrot rust fly has also successfully invaded New Zealand. Generally, cool and moist habitats are favorable for this insect.
Host Plants. Carrot rust fly larvae feed on umbelliferous vegetable crops, including carrot, celery, celer-iac, chervil, parsnip, and parsley, but this insect is considered principally a carrot pest. It also is recorded from umbelliferous herbs such as caraway, dill, and fennel, and from weeds such as wild carrot, Daucus carota, and water hemlock, Conium maculatum. The adults feed on the flowers of numerous plants.
The host range has been studied intensively in England, where about 100 umbelliferous weeds have been identified as suitable hosts. Only a few plants of this family have been determined to be unsuitable (Hard-man and Ellis, 1982; Hardman et al., 1990). Annual, biennial, and perennial species can support carrot rust fly, but fly larvae perform poorly on plant species that are not actively growing. Annual species tend to diminish in suitability as the season progresses. A small number of nonumbelliferous plants favor the growth of carrot rust fly larvae, particularly chicory, endive, and lettuce, but flies do not deposit eggs on such plants. Records indicating these Compositae as host plants apparently are based on unusual situations where ovipositing flies perceived chemical stimuli derived from the earlier presence of umbelliferous plants.
Natural Enemies. Much of our knowledge relating to natural population regulation has been derived from British studies. Life-table studies conducted by Burn (1984) demonstrate the importance of egg and early larval mortality in determining population trends. The egg loss was due to both desiccation and predation. The larval parasitoid Chorebus gracilis (Nees) (Hymenoptera: Braconidae) and the pupal parasitoids, Eutrias tritoma (Thomson) (Hymenoptera: Eucoilidae), and Aleochara sparsa Heer (Coleoptera: Staphylinidae), are the most abundant parasitoids, but none is considered a key element in population regulation. The wasps were introduced into Canada in the early 1950s, but failed to establish. The biology of these insects was given by Wright et al. (1947). Burn (1982) also reported egg predation by beetles (Coleoptera: Carabidae and Staphylinidae), and estimated that they often account for 20-40% egg loss.
Fungi, particularly Entomophthora muscae, cause epizootics in carrot rust fly. Also reported infecting adults are Conidiobolus apiculatus and Erynia sp. None of the fungi are known to regulate fly populations (Eilenberg and Philipsen, 1988). However, Eilenberg (1987) observed modified oviposition behavior by flies infected with E. muscae early in the adult stage, suggesting they contribute few eggs to the next generation.
Life Cycle and Distribution. The number of generations varies from one to three depending on climate. All generations may not be damaging, because one may occur before the crop is present in the field or after it is harvested. In Massachusetts, adults developing from overwintering stages usually become active during the spring in May-June, with first generation flies appearing in August (Whitcomb, 1938). Total generation time in the field is usually about 9-11 weeks. Stevenson (1981) measured egg-to-adult development periods of 59, 70, and 81 days at 20°, 17.5°, and 15°C, respectively. Summer flies initiate another generation by depositing eggs between August and September. Two periods of adult activity are also known from Quebec (Boivin, 1987). Getzin (1982) reported a third flight in October during studies in western Washington; this was also the case in nearby British Columbia (Judd and Vernon, 1985). Although the diapausing pupae are normally considered to be the overwintering stage, larvae also survive cold weather, and proceed in development without a period of diapause. This can result in a protracted or bimodal emergence of flies in the spring. In some cases, a higher proportion of larvae than pupae overwinter (Collier et al., 1994). Developmental thresholds are quite low for this cold-adapted species; egg, larval, and pupal developmental thresholds are estimated at 4.5°, 2.0°, and 1.5°C, respectively (McLeod et al., 1985).
food plant, though some disperse distances of 10 cm before pupation, and most pupate at depths of 1015 cm. Duration of the pupal stage is about 25 days in the summer, but extends for several months during the winter. The early portion of the pupal stage is sensitive to environmental conditions, and temperature exposure at this stage determines diapause induction. If young pupae are exposed to temperature below 10°C, they can enter diapause (Stevenson and Barszcz, 1991).
Adult. Adults measure 4.5-5.0 mm long. The head and legs are yellowish-brown, and the thorax and abdomen are shiny black. A fine layer of yellowish hairs covers the thorax and abdomen. The wings are slightly iridescent. The tip of the abdomen tapers to a point in females, but is bluntly rounded in males. Cool weather and moist soil favors adult emergence.
Thorough treatment of carrot rust fly biology was given by Whitcomb (1938). A world review of biology and management was published by Dufault and Coa-ker (1987). Additional useful information and rearing procedures were provided by McLeod et al. (1985). Beirne (1971) gave a valuable Canadian perspective on this pest.
This is often considered as the most destructive pest of carrot. The larva is the only damaging stage. The larva mines the surface of the root, leaving trails or blotchy areas. The smaller roots or rootlets may be completely mined, but the principal economic effect is associated with the damage to the main root in crops such as carrot and parsnip. The feeding site often acquires a rust color owing to accumulation of rusty plant exudates; this is purported to be the basis of the insect's name. However, the foliage of affected plants also may develop a red or yellow color. Other signs of attack include wilting and forked or distorted
roots. The larval mines may be widely distributed, but tend to be concentrated in the lower or distal portions in carrot, and in the upper portion of the parsnip root. In celery, larvae will also burrow up into the crown and stalks, and in parsley they will mine the surface of the tap root and feed on the lateral roots.
Sampling. Various techniques have been developed to assess damage potential. Getzin (1982) used yellow-orange sticky traps for adult population sampling. Finch and Collier (1989) noted that a greater numbers of carrot rust flies were captured on the lower surface of yellow-sticky traps inclined at a 45° angle. Traps placed closer to the soil capture more flies than those that are elevated (Collier and Finch, 1990). Boivin (1987) used yellow-sticky traps for survey and quantitative estimates of fly abundance. Judd et al. (1985) reported successful use of sticky trap catches to monitor flies, resulting in appreciable reductions in insecticide use with nominal thresholds of about 0.4 flies per trap per 4-5 days.
Several authors have calculated day-degree accumulations necessary for carrot rust fly development, including Stevenson (1983), Judd and Vernon (1985), McLeod et al. (1985), and Boivin (1987). In England, developmental models were used to forecast the need for insecticide applications, and eliminated the need for several mid-summer insecticide applications on carrot (Finch, 1993).
Insecticides. Insecticides, usually in a granular formulation, are often incorporated into the furrow at planting time to protect roots from attack by larvae. This may be followed by several foliar applications to suppress adults. Insecticide resistance has been a problem, with chlorinated hydrocarbons losing their effectiveness in the 1950s (Howitt and Cole, 1959). Organophosphate and carbamate products vary in efficacy, and there is evidence that resistance has developed to some of these materials (Stevenson, 1976b; Harris et al., 1985).
Cultural Practices. Timing of planting and harvesting is commonly suggested as an effective means to avoid damage. This is more feasible for home garden production than commercial production. Carrot, for example, is a relatively short-season crop, so timing of planting can be adjusted to miss most of the flight of the flies from the overwintering insects, and harvesting planned to avoid the late summer flight. Cultivars vary somewhat in their maturity date, of course, and varieties that require long periods for growth are more susceptible to attack. Early season planting is particularly risky, as flies apparently select the largest carrots for oviposition. However, with long-season varieties, if planting is delayed to avoid the first flight of flies, then the carrots are susceptible to late season attack. To maximize production efficiency, commercial producers of carrots and other umbelliferous crops may stagger their planting dates, or at least have both an early season and a late season crop. Thus, it may be impossible for commercial carrot producers to avoid having at least some of the crop in a susceptible stage.
Because carrot rust fly is limited to umbelliferous crops, crop rotation is recommended. Although flies may move short distances, considerable benefit accrues from rotating fields. Intercropping of non-host crops such as onion, with a susceptible crop such as carrot, reduces the damage by carrot rust fly, but not all non-host plants are effective at disrupting fly injury (Uvah and Coaker, 1984).
Sanitation is also important. Crops left in the field may support overwintering insects, or be attractive oviposition sites in the spring.
Row covers can be used to prevent attack of crops by carrot rust fly in the spring. Finch (1993) indicated that though this is not economical under normal conditions, it is done for 10% of the British carrot crop to assure availability of insecticide-free carrots for young children.
Host-Plant Resistance. Some carrot varieties confer partial resistance to carrot rust fly. Finch (1993) indicated that partial resistance allows reduction in the intensity of insecticide application, and for this reason such partially resistant varieties now constitute two-thirds of the British carrot acreage. Partial resistance, combined with careful timing of harvest, is especially effective at reducing damage (Ellis et al., 1987). Phenolic acid levels in roots are correlated with plant resistance to some insects, and Cole et al. (1988) devised a rapid technique to screen carrot cultivars for resistance to carrot rust fly.
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