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Overcoming Onion Maggot Resistance

The biopesticide Bt: an option for onion maggot flies?

photo of field worker

The onion maggot is one of the most important onion pests for New York growers. It can cause 100 percent losses in untreated onion fields. But growers need more options for their control than they currently have. Through years of exposure, onion maggots have built up significant resistance to insecticides. Furthermore, the most successful treatments for onion maggot only target the first-generation immature life stages. Insecticides are used for adult flies of the second and third generations, but the flies are elusive targets. Fewer than 20 percent of them are in the field at any one time.

With these problems in mind, Jan van der Heide, Cooperative Extension educator in Oswego County--in close cooperation with Charles Eckenrode, of the entomology department in Geneva--initiated an IPM project on the use of Bt, a biopesticide, as an alternative treatment for adult onion maggot flies. Bt (Bacillus thuringiensis pv. Israelensis) has the advantage of being highly specific in its effects. It has been shown to have little effect on insects that are natural enemies of insect pests in onion and other vegetable crops.

Onion grower Dan Dunsmoor, of Southwest Oswego, provided space in his fields for small wire cages, placed over portions of onion rows. Plants in four of the cages were sprayed with sugar water laced with Bt; four other cages received plain sugar water. Adult flies from a laboratory colony were placed in all eight cages.

When onion maggot damage to the caged plants was assessed after 17 days, the differences were dramatic: 41-82 percent of the plants were damaged by the onion maggot flies in the four "no-Bt" cages versus 11-28 percent in the "Bt cages."

"The damage level is not low enough to make the Bt bait solution a stand-alone method of control," explains van der Heide. "But I see it as a good fit within an integrated pest management strategy." The next step is to determine whether the Bt will still cause substantial fly mortality at a lower, more affordable dose.