Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu
The unusually warm and dry spring we have had up to now has led many growers to transplant some of their melon and other vegetable crops early. The cool wet weather we have had in the last few days will make some of these fields vulnerable to seed corn maggots Delia platura (SCM) or less commonly found in cucurbit fields, cabbage maggots, Delia radicum (CM). Both species overwinter in the soil as a maggot inside a brown case. In March and April small, grayish-brown flies emerge. Adult flies are most active from 10 a.m. to 2 p.m. and are inactive at night, in strong winds and when temperatures are below 50o F or above 80o F. 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. Within a few days the eggs hatch and the tiny maggots burrow down to the roots and begin feeding. SCM eggs are oviposited in soils with decaying plant material or manure. The adults are also attracted to the organic media around the roots of transplants and germinating seeds. That is why fields that have been fumigated can still have problems with SCM. Maggots 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. If these stems are split you will usually find the white cylindrical larvae (Photos 1, 2 and 3).
Photos 1, 2 and 3. Swollen stem of cucurbit plant with collapsed rotting roots. When stem is cut open the white maggots often can be found.
The adult flies are often found dead, stuck to vegetation during periods of warm wet weather (like we had in early April). These flies have been infected by a fungus, Entomophthora sp. These infected flies usually will be found at the top of a tall object in the field such as a grass seed head or a wire field-flag (Photo 4). Just before the fungus kills them they cement their body via their mouthparts to the tall object and die. If you look closely you’ll see the body is filled with the white fungus that has ruptured between the segments (Photo 5). 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. Even though we have had a dry spring, I still have seen many fungus infected dead flies this year. Unfortunately, the infection rate is not enough to reduce the SCM population and stop infestations.
Soil temperatures two inches deep in the planting hole that are at or above 70o F reduce SCM egg laying and larval survival. If soil temperatures are above 70o F at planting but fall below this level for several days in a row (which they have just done), SCM adults will begin to oviposit eggs at the base of transplants. When wilted transplants are inspected in the field, maggots are often not found (they have already pupated), but their tell-tale damage can be seen as a hollowed out stem or root held together by a few strands of plant material. The use of treated seed or in-row banding of an insecticide gives some control of SCM, however, replacing dead transplants is the only solution after SCMs kill a plant. Once seed corn maggot damage is noticed, it is too late to apply control procedures. Thus, economic thresholds are not useful and all management options are preventative.
Photo 4. Two SCM flies killed by a fungus stuck to a wire field-flag via their mouthparts
Photo 5. Adult SCM killed by a fungus – white strands coming out of abdomen