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Research peels back onion maggot resistance puzzle

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Written Tuesday, March 09, 2004  

Onion maggots are a crawling nightmare for Ohio green onion growers. The larvae can cause stand losses in excess of 50 percent and have developed resistance to soil insecticides farmers commonly apply to reduce crop damage.

That's where research by Casey Hoy, an entomologist with the Ohio Agricultural Research and Development Center (OARDC) in Wooster, Ohio, can make a difference. Hoy and Erol Yildirim a visiting entomologist from Ataturk University in Erzurum, Turkey treated green onion seeds with cyromazine, an insect growth regulator, in an effort to both kill more maggots and use less pesticide.

The results of their experiments have shown promising results.

"Onion maggots are a very serious pest of Ohio onion crops and can only be controlled effectively at some times of the year with insecticides," Hoy said. "Very small amounts of the insecticide are needed for onion maggot control when it is placed directly on the seed."

Cyromazine seed treatments have been effective in controlling onion maggots on dry onions and leeks, but the application rates hadn't been calibrated for green onions before.

"Bulb onion growers in other states have used these treatments with good results," Hoy said. "But it still hasn't been used a lot in Ohio and hasn't been available for green onions. The growers have been keenly interested in onion maggot control and any new ideas."

The only onion maggot control tools available to growers are soil insecticides applied at planting, such as organochlorine, organophosphate and carbamate compounds. Unfortunately, maggots have shown resistance to these chemicals, which also are environmentally disruptive as they kill many other soil organisms.

Unlike those insecticides, cyromazine works by stiffening the larval cuticle, therefore interrupting key developmental processes that take place during the juvenile stages of onion maggots and other pests. The product also has low toxicity to mammals and beneficial insects.

Hoy and Yildirim conducted both laboratory and field studies to compare the effects of cyromazine at different rates on maggot mortality and behavior. Maggots were collected from Celeryville (Huron County) and Hartville (Stark County), two important green onion production areas in Ohio.

With technical assistance from David Tay of the U.S. Department of Agriculture's Ornamental Plant Germplasm Center, raw seed was treated with cyromazine in a polymer film coating. This coating helps encapsulate the insecticide, making its placement around the germinating seed more exact and efficient. The researchers also experimented with a cyromazine soil drench treatment to compare the results of both application methods.

Lab studies showed that the film-coated seed treatment required less insecticide than the soil drench treatment to attain the same level of mortality. These results, Hoy explained, suggest that the cyromazine on each seed provides better control than cyromazine diffusing through the soil, most likely because of the close association between the insecticide and the roots of the treated seeds.

Meanwhile, field studies at OARDC's Muck Crops Agricultural Research Station in Celeryville, Ohio, showed that cyromazine-treated seeds provide effective onion maggot control, even at application rates lower than those recommended for use on dry onions. For example, a cyromazine rate of 12.5 grams per kilogram of seed a fourth of the cyromazine rate labeled for dry onions significantly lowered plant damage.

Onion maggots are cream-colored fly larvae that emerge in the spring and feed on the onion roots and bulb, killing seedlings or providing a site of entry for pathogens. This pest is particularly damaging where green onions are grown intensively on high organic matter soils, as is the case in Huron and Stark counties in Ohio.

According to a 1998 grower survey conducted by Hoy, Ohio green onions have a production value of more than $2 million. Ohio ranks seventh in the country in green onion acreage, according to the 1997 U.S. Census of Agriculture.

OARDC is the research arm of Ohio State's College of Food, Agricultural, and Environmental Sciences.

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