Species

Maggots are the common name given to flies of the order Diptera that are in the larval stage. However, the term maggot is often given to any insect in the larval stage. Species include the cabbage maggot (Delia radicum), the seedcorn maggot (Delia platura) and the onion maggot (Delia antiqua).

Host/Site

The seed corn maggot attacks the germinating seedling and early roots of soybean and corn. Onion maggots live in the soil and attack the seedlings, roots, and bulbs of onions. The cabbage maggot can harm parsnips, turnips, radishes, cabbage, broccoli, and other vegetables.

Identification

Appearance
Maggots, the larvae of Delia fly insects, are creamy-white in color, and are about 0.4 inch in length (10mm). Microscopic observation and tests are generally required to identify between species.

Indicators
Maggots will tunnel in the roots of plants below the ground so their presence may not be overly apparent. Unusual wilting or death of plants demands investigation of the soil. Maggots live at relatively shallow soil depths so if their presence is suspected one can dig down and discover if they are in fact in the soil.

Life Cycle

Female flies lay eggs at the base of a plant's stem, or in the soil of a recently planted field. Soil with high organic content is preferred by the flies. Eggs hatch in the spring and the larvae reach full maggot stage in about 3 weeks. The following pupal stage lasts around 16 days, after which adult flies emerge. Each year can produce a few generations of maggots.

Natural Enemies

Predaceous Ground Beetles and [toxipedia:Rove beetles] are effective at consuming flies at the egg and maggot stages. Parasitic Nematodes can control the larva stage of flies (the maggots), and are generally harmless to plants. Birds also feed on maggots.

Monitoring

Plants must be monitored to prevent severe attacks from maggots. Plants can display visual signs of stress from maggots such as wilting, yellowing, and unexpected death. Roots may be severed or blackened. The soil around the roots may contain maggots themselves and their feeding tunnels.

Action Threshold

Plants that are especially susceptible to maggot damage should be protected early on prevent future attacks. Maggots will emerge in the spring and can continue to inflict damage to plants throughout the summer. If the presence of maggots is spotted in the spring, action should be taken to avoid further damage further in the summer. Due to the maggots' ability to destroy the root system of plants, it is imperative to control them once they become apparent.

Physical/Mechanical Controls

Preventing the fly eggs from being initially buried around the plants is the best method for controlling maggots. One method is to wrap a plastic sheet around the plant that covers the stem just under the soil layer. This barrier will prohibit the eggs to be sufficiently buried in the soil around the plant.

Maggots can also be controlled through the use of colorful sticky traps or buckets of soapy water that attract maggots and trap them. Pheromone traps have also been successful in controlling maggots. These are also effective in catching adult flies, limiting future generations of maggots.

Biological Controls

Parasitic Nematodes attack maggot larvae. By attacking the larvae nematodes can successfully destroy a generation of new flies thus decreasing the amount of new maggots in the future.

Chemical Controls

There are few chemical control options that are recommended for treating maggots. The Monterey Garden Insect Spray, contains Spinosad, and is considered to have low human toxicity. The substance is derived from a bacterium called Saccharopolyspora spinosa.

Chemicals should only be used if other controls prove ineffective. Use of insecticides on maggots, while initially being effective has been found to be ineffective in the long term as maggots adapt to the spray and become resistant. Insecticides will also kill natural enemies of maggots.

References

Cabbage Maggot. Aug. 2008. Cornell Cooperative Extension- Suffolk County. 11 Aug. 2008.
<http://counties.cce.cornell.edu/suffolk/HortFactSheets/factsheets/Cabbage%20Maggot.pdf>.

Hoffman, Carl. "July is the time for apple maggot control." Hutchinson Leader. 30 June 2008.
<http://www.hutchinsonleader.com/july-time-apple-maggot-control-8720>.

"Onion and Garlic Maggots." 17 June 2008. University of California. 11 Aug. 2008.
<http://www.ipm.ucdavis.edu/PMG/r584300211.html>.