Seedcorn Maggot as a Pest of Field Corn
Delia platura (Meigen)
The seedcorn maggot, a pest imported from Europe, is now established and distributed throughout the United States and southern Canada. Corn and soybeans are the major crops attacked. The incidence of injury from this pest has been on the increase in recent years. The increase may be associated with an increase in reduced tillage practices and a reduction in soybean seedling rates. Seedcorn maggots, however, often cause extensive damage to a large number of vegetable crops.
Seedcorn maggot lavae on sprouting soybean seed. © Dennis Calvin Penn State University
Image of mature seedcorn maggot.
The Problem in Pennsylvania
Each spring a few corn and soybean fields are severely attacked by seedcorn maggots. Most fields in the Commonwealth lose a few plants to the pest each year. Usually no more than 2 percent of the seedlings are infested, but occasionally fields may suffer a plant loss of 30 to 60 percent from this pest.
The occurrence of this pest is not 100% predictable, but its occurrence is more common under some soil and cultural practices. From observations made over the past couple of decades, seedcorn maggots tend to cause greater losses in fields with an abundance of decaying organic matter such as manure and plant residue, and during years when the early growing season is cool and damp. Therefore, heavily manured fields, old pastures and hay fields that have recently been plowed under, and fields with naturally high organic matter levels are at the highest risk of severe infestations. In years where injury is widespread within a local area, these rules may not hold up.
Description & Life Cycle
Tiny, white, elongated eggs are deposited in loose groups among debris and around plant stems near the soil surface. Maggots are dirty white with a yellowish tinge. They are legless, cylindrical, and tapered. Full-grown maggots are 1/5- to 1/4-inch in length. They have no visible head, but two tiny black hooks on their front end are a part of their mouth to feed in the seed or on the underground stems of seedlings. Adults are brownish-gray flies and closely resemble common houseflies except that they are about half the size
Maggots pupate inside a dark brown capsule-like puparium that resembles a grain of wheat in size and shape. Puparia are found in the soil where the maggots developed. Seed corn maggot puparia can be found throughout the year.
The adult flies emerge from dark brown, capsule-like puparia during late April and early May. Eggs are deposited on or near the soil surface. Eggs of later generations frequently are deposited around plant stems at the soil surface. Eggs hatch in a few days and the maggots work their way into the soil in search of food. They complete their development in a week to 10 days and then pupate in the nearby soil. The pupal stage requires about 10 days, then a new generation of adult flies emerges. The seedcorn maggot survives the winter in the pupal stage.
The time required to grow from egg to adult is between 3 to 4 weeks. There are 3 to 5 generations each year in Pennsylvania.
Maggots burrow into the seeds and destroy the seed germ. Damaged seed may germinate, but there are not enough food reserves left in the seed for the plant to survive. Maggots also attack the underground stems of sprouted corn and soybeans resulting in weakened seedlings that seldom survive. Reduced plant stands are evident about a week after the corn plants start emerging.
Any cultural practice that will speed up germination and plant emergence will help reduce crop losses from maggots. Another tactic that can be used is to plant the field after 450 degree days (using a base temperature threshold of 40° F) have elapsed since the organic matter was plowed down. This is the time required for the larvae to complete development and move to another host.
One of the most reliable control methods now in general use is to plant seeds that are treated with an appropriate insecticide in the seed box at planting. Practically all commercially available seed corn is treated with an insecticide to protect the seed from insect pests during storage. However, this treatment is not sufficient to provide field protection against seedcorn maggots. A planter box seed treatment insecticide at planting is recommended. If a soil insecticide with activity against the pest is used, then a seed box treatment is not needed. If treating soybean seed with a planter box seed treatment, make sure it will not affect the inoculant on the seed. Order of mixing can be important to prevent killing the rhyzobium inoculant.
Pesticides are poisonous. Read and follow directions and safety precautions on labels. Handle carefully and store in original labeled containers out of the reach of children, pets, and livestock. Dispose of empty containers right away, in a safe manner and place. Do not contaminate forage, streams, or ponds.
Authored by: Stanley Gesell, Extension Entomologist Revised by Dennis Calvin, Professor of Entomology
Last updated 8/15/2000
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