Tetanops myopaeformis Roder Diptera Otitidae
Distribution. This native insect is found over a wide area of western North America. In the United States, it is known from Washington to Minnesota in the north, and from California to Colorado in the south. In Canada it occurs in the Prairie provinces.
Host Plants. Sugarbeet root maggot affects crops in the family Chenopodiaceae—beet, spinach, Swiss chard, and sugarbeet. Spinach appears the most suitable host, though it usually is not heavily infested because it is not normally grown in the summer months when flies are active. Not surprisingly, the herb garden orach, Atriplex hortensis, also support larval development, as it is also a member of this plant family. However, though flies oviposit on other cheno-pods such as lambsquarters, Chenopodium album, and Russian thistle, Salsola kali, larvae apparently fail to complete development. Flies also oviposit on other plants such as redroot pigweed, Amaranthus retro-flexus; curly dock, Rumex crispus; and black nightshade, Solanum nigrum; but also with unsatisfactory larval development. As all the known host plants of sugarbeet root maggot have been introduced to North America, the natural host remains curiously unknown (Mahrt and Blickenstaff, 1979).
Natural Enemies. The natural enemies of sugar-beet root maggot are poorly documented.
Life Cycle and Description. There is one generation per year, but adult emergence in the spring is protracted, often with a small secondary emergence peak that may give the impression of a second generation. Mature larvae overwinter. In Alberta, Colorado, and North Dakota, maggots pupate in the early spring, adults emerge in May-June, eggs are deposited beginning about 10 days after adult emergence, and large larvae burrow deep into the soil in preparation for overwintering beginning in July-August.
- The eggs are white, rounded at one end and tapering to a blunt point at the other end. They measure about 1.0 mm long and 0.25 mm wide, with one side convex and the opposite side concave. Eggs are deposited in clusters of about 10-25 in the soil at the base of plants at depths of up to 1 cm. Soil cracks are preferred oviposition sites. Mean fecundity is about 110 eggs, but some females produce up to 200 eggs. Duration of the egg stage is about 5-7 days.
- Larvae feed below-ground on plant roots, with the specific depth influenced by soil moisture conditions, but the majority at depths of 12-18 cm. The larva is elongate and cylindrical, with the anterior end tapering to a point and the posterior end blunt. There are three instars. The first instar is whitish and its length ranges from 0.75 mm at hatching to about 2.1 mm just before molting. The second instar is white to ivory and measures 2.0-3.0 mm long. The third instar is creamy-white to light-yellow, and ranges 33-11.0 mm long. The mouthparts (cephalopharyngeal skeleton) darken as the maggot matures, and are useful for distinguishing instars. The mouthparts measure about 0.24, 0.41, and 0.72 mm long, respectively. The posterior end of the maggot terminates in a pair of elongate, spine-like spiracles which enlarge and darken as the maggot matures. Larvae overwinter as third instars, usually at depths of 10-25 cm, moving toward the surface in the spring. They require a cold period to terminate diapause and tolerate temperature of — 5°C for brief period, but they survive better at warmer temperature (Whitfield and Grace, 1985). Post-diapause larvae develop at a temperature of about 8°C or greater (Whitfield, 1984).
- Pupation normally occurs at depths of 510 cm, with pupation occurring early in April even at northern latitudes. The brown puparium is elongate-elliptical and measures about 8 mm long. The pointed spiracles of the maggot are visible on the puparium. Duration of the pupal stage is 10-14 days.
- Sugarbeet root maggot adults measure about 6.1 mm long for males and 8.2 mm for females. They are shiny black with yellowish-brown legs and face. The wings are light-gray with brown or smoky spots basally and near the mid-point of the wing. Wing spots originate at the leading edge and cross about one-half the width of the wing. Perhaps the most distinctive feature is the elongate, pointed tip of the abdomen in the female. The abdominal tip of the male, in strong contrast to the female, terminates abruptly, and gives a squared-off appearance. Peak adult emer-
- Adult female of sugarbeet root maggot.
gence occurs at about 200 day-degrees when it is based on a developmental threshold of 9°C (Whitfield, 1984). Adults fly readily and disperse from field to field, but the air temperature of about 27°C is a threshold for their flight (Bechinski et al., 1990). Mean duration of the adult stage at temperatures of 20-25°C is 5-10 days (range 4-14 days).
A description of sugarbeet root maggot was given by Gojmerac (1956). Hawley (1922b) and Harper (1962) described the life history. The larva was described by Bjerke et al. (1992). A key to otidid genera based on adults was provided by Steyskal (1987).
The injury is caused by the larval stage. Maggots feed on the roots, burrowing completely through young roots, but feeding the surface of large roots. Black and necrotic lesions are developed on the surface of roots where larvae are feeding. A single larva may destroy a young plant, and about 3-4 maggots can cause severe damage even to mature plants. Above-ground symptoms of feeding include wilting of plants and eventually skips or bare spots in the fields.
- There is approximately a 10-day period between adult emergence and the initiation of ovi-position, so it is possible to forecast damage by maggots from adult population density (Bechinski et al., 1989). Adults densities are often determined with orange-sticky traps, though emergence, baited, water-pan, and blacklight traps operated during daylight hours also capture flies (Harper and Story, 1962; Swen-son and Peay, 1969; Blickenstaff and Peckenpaugh, 1976). The timing of adult flights can also be predicted. In Idaho, forecasting of adult emergence can be accomplished by accumulating day-degrees above a threshold of 8.6°C starting on April 1. Because flies are inactive at temperatures below about 25-27°C, however, this statistic must also be considered (Bechinski et al., 1990).
- Insecticides are directed at either the adult or larval stages for crop protection (Peay et al., 1969). For adult suppression, foliar applications of insecticide are timed for peak-fly emergence. This application is preferred to the other common practice, which is to apply granular insecticides to the soil at planting. Soil-applied insecticides prevent damage effectively, but may be unnecessary in many fields.
Cultural Practices. The most important cultural practice is crop rotation, because crops grown in fields previously infested with root maggots are most likely to be damaged. Flies will disperse a kilometer or more, however, so distance and isolation are important factors. Crops grown in light, sandy soil are often reported to be most heavily infested, so soil characteristics are also a consideration in choosing fields for susceptible crops. For small plantings, screen and row covers can be used to deny flies access to young plants.
Biological Control. Little work has been directed toward finding biological control agents for this pest. However, Wozniak et al. (1993) reported that several species and strains of entomopathogenic nematodes (Nematoda: Steinernematidae) could infect sugarbeet root maggot larvae. Rates of infection were low, but many survivors were deformed and became incapable of reproduction.
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