Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu
The unusually warm and dry winter and spring we have had up to now has allowed large populations of seed and root maggots to invade our vegetable fields. Some farms have been hit particularly hard in their onion crop this early season by maggots. These maggots include seedcorn maggot Delia platura (SCM), onion maggot Delia antiqua (OM) and cabbage maggot Delia radicum (CM), the latter being a specialist of the cabbage family. All three species overwinter in the soil as a maggot inside a brown pupal case. In March and April small, grayish-brown flies (Fig. 1) emerge, which are usually SCM or CM. OM flies usually peak 2-3 weeks later. Adult flies are most active from 10 a.m. – 2 p.m. and are inactive at night, in strong winds and when temperatures are below 50°F or above 80°F. Adults live 2-4 weeks and females lay hundreds of eggs.
Figure. 1. Root maggot adult on yellow sticky card
Seedcorn maggot eggs are oviposited in soils with decaying plant material or manure. Onion maggot females lay eggs in soil near onion plants. Female cabbage maggot flies seek out and lay eggs on the lower portions of stems of young host seedlings or in nearby cracks in the soil. Some wild crucifers, such as yellow rocket, are important hosts for cabbage maggot and are especially important for their overwintering success; when these weeds are abundant they can lead to heavy infestations in spring crucifers. Combine this with the very mild winter we had and infestations are almost assured in some fields. The adults are also attracted to the organic media around the roots of transplants and germinating seeds. Within a few days the eggs hatch and the tiny maggots burrow down to the roots and into stems and begin feeding. Larvae of seedcorn maggots attack seedlings, feeding on the developing roots and stem. Their damage is usually restricted to the early seedling stage. SCM larvae will move into small stems and move up the plant causing a swelling of the stem just above ground level, while also causing root collapse and decay (Fig. 2). If these stems are split you usually can find the white cylindrical larvae (Fig. 3). Onion maggots inflict similar damage (Fig. 4) but usually continue to feed on the expanding bulb during later stages of growth. A single maggot can destroy up to 20 small seedlings. Either SCM or OM can attack onion bulbs, while SCM also can attack vegetable seeds and transplants. Complete larval development requires 2-4 weeks. Maggots then enter a pupal stage that lasts another 2-4 weeks. There are 3-4 generations per season in our area, with the most destructive being the spring and fall generations. When wilted transplants or newly emerging seedlings are inspected in the field, maggots are sometimes not found (they have already pupated), but their tell-tale damage appears as hollowed out seeds or stems and roots held together by a few strands of plant material.
Figure 2. Swollen stem of cucurbit plant with collapsed rotting roots.
Figure 3. When stem from Figure 2 is cut open the white maggots often can be found.
Figure 4. Maggots found in base of onion plant
Cultural controls: Avoid planting in soils that have a great deal of non-decomposed organic matter, such as fields with a heavy cover crop or are very weedy. Rotate early season crops away from any areas that had onions or crucifers last fall. Early spring-planted crops are more likely to be damaged when the soil is too cool for rapid germination and emergence. If serious infestations are expected, wait until the soil warms up in the spring. You can get an idea of how serious a possible infestation could be by using yellow sticky cards that attract adult flies and can be put out a few weeks ahead of time. It may take a few seasons of using the cards to readily recognize when a certain fly population on the cards represents a significant possibility of a heavy crop infestation. Recently seeded or transplanted crops should be covered with floating row covers, which act as barriers against any of the root maggot flies. Do not use row covers where onions or brassicas were grown the previous year. When soil temperatures increase and maggot first-flights end, the row covers can be removed.
Chemical controls: The use of treated seed (Trigard ST- commercially treated onion seed only) or banding of an insecticide (diazanon as a preplant application, Cyantranilprole as a soil or foliar application for CM or chlorpyriphos as a post plant drench for dry bulb onions only) gives some control of SCM, CM and OM, however, replacing dead transplants is the only solution after these maggots are inside a plant. Once seedcorn maggot or onion maggot damage is noticed, it is too late to apply control procedures. Thus, economic thresholds are not useful and all management options are preventative.
The adult flies can often be found dead; stuck to vegetation during periods of warmer weather. These flies have been infected by a fungus, Entomophthora sp. These infected flies usually are found at the top of a tall object in the field such as a grass seed head or a wire field-flag (Fig. 5). Just before the fungus kills them the flies cement their body via their mouthparts to the tall object and die. If you look closely you’ll see the fly’s body is filled with a white fungus that has ruptured between the segments (Fig. 6). Being on a tall object allows the spores of the fungus to move longer distances and infect more flies than if the fly had died on the ground. Unfortunately, the infection rate is not enough to reduce seed or root maggot populations and stop infestations.
Figure 5. Two SCM flies killed by a fungus stuck to a wire field-flag via their mouthparts
Figure 6. Adult SCM killed by a fungus – white strands coming out of abdomen
