SEED MAGGOTS
(Diptera: Anthomyiidae)

Seed maggots include two very closely related maggot fly species, the seed corn maggot Delia platura (Meig), and the bean seed maggot Delia liturata (Meig). These two species attack the same crops at the same time and have identical seasonal histories.

CROPS ATTACKED

In Ontario, beans and soybeans are the most important hosts attacked, with all commercial and garden varieties showing susceptibility. Corn is only occasionally infested.

Peas are usually well established before egg lying by the overwintering generation of seed maggots; however, a slight shift in the weather pattern in the spring could result in severe infestations and crop loss.

Radishes are considered to be the most susceptible host of the crucifers, whereas the seed maggot only invades the other crucifers secondarily. All cucurbits are very susceptible to attack.

Occasionally tobacco fields may need replanting when transplanted seedlings are severely infested. Other crops attacked by seed maggots include: barley, oats, rye, alfalfa, onion, pepper, and asparagus.

INJURY

On beans, leaf damage ranges from a few holes in the first true leaves to complete destruction of the growing point. When primary leaves are destroyed, these "bald-headed" seedlings seldom develop into bearing plants, or if they do, they mature late, the pods are dwarfed, the yield is low, and the seed is of poor quality.

Feeding injury on soybeans usually appears as dark brown surface streaks on the outer parts of the cotyledons, and the seedlings may become bald-headed. On corn, visible leaf damage seldom occurs, therefore, seed examination for tunnels and maggots, is the only reliable method of determining infestation.

On cucurbits, feeding damage appears on the margins of the first true leaves. Occasionally leafless plants emerge. Heavy feeding produces ragged cotyledons or completely destroyed growing points; however, tunnelling is difficult to detect in the damaged seeds.

On tobacco, feeding damage appears as the maggot tunnels in the stems of transplants. Visual symptoms other than slowness of growth are usually absent.

With the exception of cucurbits, maggot infestations are seldom severe enough to actually reduce plant stands. However reduced yield and delayed maturity often necessitate some control measure.

BIOLOGY AND LIFE CYCLE

Seed maggot, Delia sp. ©1999 Zunke & Eisenback*

The seed maggot generally overwinters as a mature larva in a puparium (pupal case) 8-15cm (3-6 in.) deep in the soil. Adults occasionally hibernate and become active very early in the spring.

The adult is a greyish-brown fly, similar to a housefly, but only one half the size. They become active in late April, and their activity peaks through May and June. There are four generations in Southwestern Ontario. The generations overlap so that adults are present from late April to early December.

Seed maggots infesting early seeded crops such as peas, corn, early cole crops, radishes, onions and spring cereals are usually those from the overwintering generation. When the season is wet and cold, the first generation is delayed and can cause serious damage to beans.

In normal years the second generation has the greatest impact on bean culture and is the most destructive in Southwestern Ontario. Neither the third nor the fourth generations are of economic importance, although fall seeded cereals and legumes are sometimes slightly infested.

Seed maggots are attracted to freshly cultivated soil. The warm, moist earth that has been exposed attracts the flies and stimulates them to lay eggs. Often before seeding, eggs or maggots are present in the soil as a result of egg lying during seedbed preparation.

Decaying organic matter has also been associated with seed maggot egg lying. Often, high infestations follow spring plough-down of green manure or early crops such as spinach. Other egg-laying attractants are certain germinating seeds especially legumes, corn and cucurbits. The stage of germination also has an effect on the degree of attraction.

CULTURAL CONTROL

• Avoid planting at least 2 weeks after green manure plough-down to allow organic matter decomposition. Fall ploughing is preferred.
• Avoid spring applications of manure on fields that will be planted to susceptible crops.
• In tobacco, well-hardened, sturdy transplants are less apt to be retarded by maggot feeding.
• In small gardens, window screen tents may be erected over the crops at seeding to exclude egglaying flies until seedlings have become established.
• Late crops sown after June 1 in southern Ontario (June 10 in northern areas), are damaged less since egg laying by the first generation is already completed, and seedlings can emerge before the second generation already is active.
• Avoid planting seeds in cool, wet weather, which delays emergence and renders the germinating seeds more susceptible. Shallow planting to promote rapid seedling emergence is important.

CHEMICAL CONTROL

Even with heavy populations of seed maggots, infestations are often sporadic and difficult to predict, since weather and soil conditions influence the degree of injury. This pest has the potential of causing severe economic losses, necessitating protective seed treatment at planting time. These treatments are adequately described in Ontario Ministry of Agriculture and Food (OMAF) Publication 363, Vegetable Production Recommendations, and OMAF Publication 296, Field Crop Recommendations. These publications can be ordered through your local OMAF office or through the OMAF website.

No chemical control measures are available for home gardeners. The use of entomogenous nematodes to treat the soil to help control seed maggots may prove useful in some circumstances.

When using pesticides follow label directions to ensure safety and efficacy of each product.

Image source: *© 1999 J.D. Eisenbach & Zunke, EntoPix, A Journal of Entomological Images, Vol. 1.

A. Schaafsma/W.A. Attwater/R.T. Wukasch

PDCF-108 (Revised 2003/11/07)