Delia radicum Linnaeus Diptera Anthomyiidae

Natural History

Distribution. Cabbage maggot is known as a pest throughout the northern hemisphere. Not native to North America, it apparently was introduced accidentally from Europe in the early 1800s. Cabbage maggot thrives under cool conditions, and it is a pest only in the northern portions of the United States. It rarely is reported to be a pest south of about latitude 45° North, and when it is, it usually occurs at a high elevation. In Canada, it is found almost everywhere, including some of the northernmost agricultural regions. Beirne (1971) attributed greatly reduced production of cruciferous root crops in eastern Canada to this insect.

Host Plants. Cabbage maggot commonly attacks cruciferous vegetable crops, including broccoli, Brussels sprouts, cabbage, cauliflower, collards, kale, kohlrabi, mustard, radish, rutabaga, turnip, and watercress. It has been reported from noncrucifer crops on occasion, but these are misidentifications that stem from the difficulty in accurately identifying this fly. Cruciferous weeds apparently do not play a significant role in the biology of this insect; although some appear to be suitable hosts, they rarely are mentioned in the economic entomology literature.

Natural Enemies. Important natural enemies include staphylinids (Coleoptera: Staphylinidae) of several genera, particularly Aleochara sp.; a wasp, Trybliographa rapae (Westwood) (Hymenoptera: Eucoi-lidae); and a mite, Trombidium sp. (Acari: Trombi-diidae). Aleochara bilineata is unusual in that though larvae parasitize the pupal stage of cabbage maggot, the adults feed on maggot eggs and young larvae (Royer et al., 1998; Royer and Boivin, 1999). Trybliographa attacks the larvae, and the mite destroys the eggs. Clausen (1978) provides a useful synopsis of the aforementioned beneficial insects.

Natural control has been studied extensively in Europe, and this is relevant because many of the same biotic natural mortality factors are also found in North America. Egg predation by staphylinids and carabids may reach 90-95% annually (Hughes, 1959; Coaker and Williams, 1963). Aleochara are very effective predators, but become active too late in the spring to have much effect on first generation cabbage maggot. Trybliographa is fairly effective at high host densities, often parasitizing in excess of 50% of available hosts (Miles, 1956; Hughes and Mitchell, 1960; Jones and Hassell, 1988). Other natural enemies of the immature cabbage maggot include numerous hymenopter-ous parasitoids of questionable economic importance, carabids (Coleoptera: Carabidae) and ants (Hymenop-tera: Formicidae). General predators undoubtedly attack the adults, but are not considered important.

Fungi are commonly observed infecting flies. Entomophthora muscae and Strongwellsea castrans cause epizootics among adults during wet weather, and though impressive, act too late to prevent early season crop damage. Entomophthora muscae has been discussed further in the section on onion maggot, Delia antiqua (Meigen).

Weather. The spring generation tends to appear consistently, but latter generations are greatly influenced by weather. Rain and cool weather may decrease egg production and egg predation, and cause starvation of flies (Miles, 1951; 1956), but optimal egg production is associated with temperatures of 18-21°C, which corresponds well with the weather occurring during most spring generations (Miles, 1954a,b). Pupal development is particularly susceptible to delay caused by hot temperature, which normally is associated with summer generations (Finch et al., 1986). Dry soil is lethal to eggs.

Life Cycle and Description. The number of generations occurring annually varies from one in the far north to three in optimal climates, though there are occasional reports of four generations. Two generations occur in southern Alberta (Swailes, 1958), and two and three in eastern and southwestern Ontario, respectively (Mukerji and Harcourt, 1970). Three generations are documented from New York (Schoene, 1916), though part of the second generation enters diapause rather than going on to form a third generation. Sandy soils in Canada's Atlantic Provinces have two generations, whereas heavy soil in the same area usually support only one generation (Beirne, 1971). In New York, the adults of the first brood were observed in May to early June, the second during mid-June to early July, and the third during late August to early October. A very similar pattern occurs in British Columbia. The generations may overlap considerably. A developmental threshold of about 6°C has been determined for most life stages (Eckenrode and Chapman, 1971a). The time required for a complete generation is estimated at 40-60 days.

  1. The eggs normally are laid in the soil around the stem of cruciferous plants, but sometimes they are deposited directly on the stem of plants. The elongate eggs are white in color, and taper markedly at both ends, but one end is more blunt than the other. One side of the egg is flattened or slightly concave, with the opposite side convex. The eggs measure about 1.1 mm long and 0.34 mm wide. They are often laid in clumps of a few eggs, but sometimes hundreds of eggs are found at the same location—evidence that more than one female may oviposit at the same spot. Females commonly produce 300-400 eggs during their life span of 30-60 days. Eggs hatch in 3-5 days, averaging about 3.5 days at 20°C.
  2. There are three instars. The length of the mouthparts (cephalopharyngeal skeleton) can be used to differentiate instars, with mean lengths of 0.44,0.80, and 1.24 mm, respectively (Miles, 1952). The larvae are white and attain lengths of about 1.5, 3.7, and up to 8 mm, respectively. The mouth hooks are black. Located immediately behind the head are a pair of brownish fan-like spiracles, each of which is divided into about 12 lobes. Larvae feed externally and internally on roots, and internally on stem tissue. The larval period requires about 18-22 days under field conditions, but development time may be altered by weather. Exposure of first and second instar larvae to cool temperatures or short photoperiods seems to induce diapause in the pupal stage (Koni, 1976), but the relative importance of each factor in stimulating diapause is disputed.
  3. The puparium is oval, bluntly rounded at both ends, and brown in color. The average length is 5.5 mm, (range 3.5-6.5 mm). The duration of the pre-pupal stage is about 3-5 days, and the pupa requires 12-25 days during the summer. However, this is the overwintering stage, so it is prolonged for 5-8 months in the overwintering generation. Overwintering pupae require at least 22 weeks of temperature less than 6°C to complete diapause development (Collier and Finch,

Cabbage maggot larva.

Cabbage maggot larva.

Delia Radicum Larvae
Cabbage maggot larva, posterior view showing caudal spiracles.
  1. The puparia from the summer generations are usually found in the soil immediately adjacent to the root on which the larvae last fed. Sometimes they occur within the plant tissue, including the above-ground stem tissue. Aestivation occurs in response to warm temperature above 20°C, and especially in response to hot temperature, 27-30°C (Collier and Finch, 1983). In preparation for overwintering, the larvae seem to disperse further from the plant, and deeper into the soil. Overwintering puparia may be found 12-15 cm from the plant, and commonly are found in the soil at a depth of 10 cm.
  2. The adults are dark with gray markings. The male bears three blackish longitudinal bands on the thorax, but these markings are less distinct on the female. The flies are quite bristly, and measure 5-7 mm long. The male cabbage maggot is distinguished from its common relatives, seedcorn maggot (Delia platura (Meigen) and onion maggot, by the presence in cabbage maggot of a dense tuft of bristles at the base of the underside of the hind femora. Adults feed on nectar from flowering plants. If they obtain adequate food they may persist for 2-4 weeks, whereas they perish in 2-3 days if denied food. Adults are highly attracted to crucifers for oviposition (Eckenrode and Chapman, 1971b). The pre-oviposi-tion period of adults is about six days.

The biology of cabbage maggot was given by Schoene (1916) and Fulton (1942), and information on rearing by Harris and Svec (1966) and Finch and Coaker (1969). Mass rearing technology was presented by van Keymeulen et al. (1981). Keys to distinguish eggs, larvae, and adults of cabbage maggot from other anthomyiids associated with crucifers were provided by Brooks (1951). An interesting account of Delia ecology, and implications for management from a British perspective, was found in Finch (1989).


Larvae damage crucifers by feeding on the roots and, to a much lesser extent, the stems or petioles of plants. Damage to leaf crops such as cabbage is most evident in the late spring; signs of feeding damage are initially seen as drooping or wilting of a few leaves, and then perhaps the entire plant. Delayed maturity and stunting are common responses to root maggot injury. Plant death often coincides with drought or water stress, when the injury to roots is fully expressed. When plants are small, 5-10 maggots are necessary to kill the seedling. However, later in the season densities of 100 maggots or more may be supported satisfactorily if the plant has adequate water. In

Structure of spiracles on cabbage maggot.
Cabbage Maggot
Adult cabbage maggot.

California, Zalom and Pickel (1985) found that once Brussels sprouts plants attained an age of about four weeks they were not very susceptible to injury.

Cabbage maggot larvae feed on the rootlets or feeder roots, but invariably move to the main or tap root as they mature. They scar the surface and burrow into the root. For crops that are harvested for their root, such as radish and turnip, damage results in severe crop loss, and in this case the autumn generation may be quite important. The summer generation causes little damage. Damage tends to be greater on loamy-sand soil than on sand or clay soil (Friend and Harcourt 1957), but as a general rule light soils are more problem prone (Beirne, 1971).

Although most eggs are laid on the soil, a small number are sometimes deposited on plant tissue, resulting in injury by larvae to leaflets, especially to Brussels sprout buttons. Occasionally the growing points of plants are attacked, resulting in multiple heads (Gratwick, 1992).


Sampling. The adult flight periods can be monitored using cone-screen traps baited with crucifers. Baits are more effective than yellow-sticky traps (Dapsis and Ferro, 1983), though sticky trap captures are correlated with egg deposition rates (Sears and Dufault, 1986). Horizontal surfaces are more suitable than vertical for landing of flies (Finch and Collier, 1989). Dispensers that release isothiocyanates, natu rally occurring odors released by crucifers, can be used as lures (Finch and Skinner, 1982). Color, but not leaf pattern, also can influence host selection (Prokopy et al., 1983). Bracken (1988) found that water traps baited with isothiocyanate could be used to monitor population trends in Manitoba, but that trap catches did not predict egg numbers accurately. Finch (1991) increased selectivity of yellow water-pan traps by modifying the paint pattern. Alternatively, flight activity can be predicted from thermal unit accumulations. Eckenrode and Chapman (1972), working in Wisconsin, reported that the overwintering (partly developed) generation required about 300 degree-days (above a base temperature of 43° F). Subsequent generations required about 1200 degree-days. Similar results were reported by Bracken (1988). However, accurate prediction is complicated by bimodality of emergence in both diapausing and nondiapausing populations, with emergence peaks separated by at least 20 days when flies are reared at 20°C (Biron et al., 1998).

Sampling of immature stages is labor intensive. Mukerji and Harcourt (1970) estimated that at densities of about six eggs, larvae, or pupae per plant, sample sizes of about 50 plants might be required to obtain adequate precision. The optimal sampling unit was the plant root together with a 10-15 cm core of soil.

Insecticides. With the advent of chlorinated hydrocarbon insecticides, damage by cabbage maggot was greatly decreased. Long-lasting insecticides, applied to the soil at planting, protect the roots from larvae (Doane and Chapman, 1962b). This remains the principal method of plant protection in commercial crucifer production, but the insecticides have been changed over time as resistance to insecticides is developed. Loss of insecticide efficacy is due not only to selection for insecticide-resistant insects, but enhanced degradation of insecticide by soil microbes. Suett et al. (1993), for example, suggested that there should be at least a three-year interval in applications of the same insecticide for cabbage root maggot control.

Insecticides are typically applied as a granular formulation over the seed bed or incorporated into the soil, or as a liquid drench. Foliar applications are sometimes made to suppress adults. Foliar application of insecticides, timed according to temperature accumulations, can be superior to soil applications or calendar-based sprays (Wyman et al., 1977). In studies conducted in Ontario, incorporation of sucrose bait into foliar insecticide formulations did not increase efficacy (Dufault and Sears, 1982). Seed treatment can be an effective method to provide protection to seedlings, but it does not work well for all insecticides

(Ester et al., 1994). Naphthalene has been investigated in Europe for repellency to ovipositing flies; though good protection occurs for about six weeks, the cost of application is high (den Ouden et al., 1984).

Cultural Practices. Modification of planting and tillage practices is often recommended for reduction in cabbage maggot damage. Delayed planting is reported to allow the young plants to escape oviposi-tion by the spring adults. Sanitation also is quite important, as the roots and stems of crucifers left in the field can be very suitable for autumn and early spring generations of cabbage maggot. Crop residues should be deeply buried, or be pulled and allowed to dry completely. High plant densities are more attractive than low to flies, but because there are more plants on which to distribute the eggs, yield may be equivalent at both plant densities (Finch and Skinner, 1976). Introducing diversity into the landscape, as by under-sowing portions of a crucifer crop with clover, will disturb the normal host orientation pattern and will reduce oviposition (Kostal and Finch, 1994). Similarly, single-row intercropping of crucifers with unrelated plants will greatly decrease the oviposition rate on cru-cifers (Tukahirwa and Coaker, 1982).

Physical manipulations of the crop environment also assist pest suppression. Crops covered tightly with row covers escape injury by cabbage maggot (Schoene, 1916; Millar and Isman, 1988), though it is difficult to separate the benefit of insect damage reduction from improved microenvironment under the row cover (Matthews-Gehringer and Hough-Goldstein, 1988). Even surrounding a crop with a window screen barrier, without covering the top of the crop, provides some benefit. For example, Pats and Vernon (1999) reduced cabbage maggot fly numbers by 90% in radish plantings surrounded by a 1.2 m high barrier. Presumably, the barrier interferes with fly host location. Tar paper or cardboard discs, or collars made of other weather-resistant material, placed around the stem of seedlings have long been recommended as a physical barrier to reduce the ability of females to deposit eggs at the soil-stem interface (Schoene, 1916). A precise fit is required, however, or the flies will circumvent the barrier (Matthews-Gehringer and Hough-Goldstein, 1988). Mulch that is painted blue also reduces the incidence of infestation by cabbage maggot (Liburd et al., 1998).

Biological Control. Reliable biological control techniques have not yet been developed. Entomo-pathogenic nematodes (Nematoda: Steinernematidae and Heterorhabditidae) have been evaluated for suppression of larvae. Although heterorhabditid nema-todes are attracted to cabbage maggot larvae and pupae (Lei et al., 1992), under field conditions they have not been shown to be effective (Simser, 1992). Steinernematid nematodes provide some suppression of cabbage maggot larvae in pot and field trials, but very high densities of nematodes are needed, at least 100,000 nematodes per plant (Schroeder et al., 1996). This is not entirely surprising because fly larvae are less susceptible to nematodes than many other insects. In Europe, the potential of using the predatory beetle Aleochara bilineata (Gyllenhal) (Coleoptera: Staphylini-dae) to achieve biological suppression of cabbage maggot is being studied; while technically feasible, the costs thus far are high (Finch, 1993).

Host-Plant Resistance. Despite the common name, cabbage is less attractive to cabbage maggots than some other crucifer crops. Chinese cabbage, mustard, rutabaga, and turnip tend to be more severely injured than cabbage (Radcliffe and Chapman, 1966). Doane and Chapman (1962a) reported that a larger numbers of eggs were associated with radish, rutabaga, and turnip than with mustard and cauliflower. There also is some variation within vegetable crops in resistance to attack (Ellis et al., 1979); fast-growing varieties seem most injured (Swailes, 1959). Finch (1993) reported that despite considerable effort, not much progress has been made on finding cultivars resistant to cabbage maggot.

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