Biological Weapons Show Promise In Onion Growers' Long Battle Against A Plague Of Maggots, Cornell Researchers Say

New York's 12,000 acres of commercial onion fields annually produce a cropwith a value of between $50 million and $75 million. If a field getsinfested with the maggot, between 20 percent and 90 percent of unprotectedonion seedlings can be wiped out.

Until now the maggot has resisted attempts at control. But CornellUniversity agricultural researchers are reporting that two biological toolsare showing promise in field tests against the onion maggot: a funguscalled "Beauveria bassiania" and a bacterium known as "Bacillusthuringiensis "(Bt).

"The research is encouraging," says Charles J. Eckenrode Jr., professor ofentomology and researcher at Cornell's New York State AgriculturalExperiment Station in Geneva, N.Y. "We've had very good success in thelaboratory and Jan van der Heide had good results in the field."Beauveria" looks exciting, but we have to work out more details." Van derHeide is a Cornell Cooperative Extension (CCE) agent in Oswego County, N.Y.

Cornell laboratory experiments have shown that when "Beauveria",commercially available as Mycotrol ES, made by Mycotech, is sprayed onseedlings, onion maggot damage is reduced to 8.1 percent from 30.3 percentin untreated plots. The researchers believe that this is the result ofincreased fly mortality and thus leaves fewer maggots on the seedlings."We do have evidence that "Beauveria" kills flies, but this is indirectevidence," says van der Heide. The researchers first reported theirfindings in February at the New York State Vegetable Conference in Syracuse.

In a New York growing season, the onion maggot has three generations. Thefirst is the most destructive because the young plants are very susceptibleto maggot damage caused by larval feeding after emergence in late May toearly June. Farmers' major line of defense has been soil insecticides,specifically an organophosphate that is applied in the seed furrow atplanting. Pyrethroid insecticides are labeled for use on onions, and aresometimes used in an attempt to kill adult onion maggot flies later in theseason. Unfortunately, these pyrethroid applications only kill a verysmall percentage of the flies, because most seek refuge from warm, dryconditions in weedy borders and hedge rows, and consequently spend littletime on onion field seedlings.

Since 1996, as an alternative to organophospate, onion growers havereceived Environmental Protection Agency permission during each growingseason to use seed pellets augmented with cyromazine, an insect-growthregulator.

In controlled field plots without soil insecticides, "Beauveria "wiped outthe first generation of onion maggots, limiting seasonal damage to between2 and 10 percent, the point at which the maggots become only a minornuisance, the Cornell researchers say.

"We still have to learn how to use "Beauveria", and when to use it, thusenabling the growers to reduce their dependence on more conventionalpesticides," says Eckenrode. "New control approaches such as Bt and"Beauveria" are urgently needed, so we must continue to invest significantamounts of research and time and attention on the ones that show promise."

Eckenrode says that "Beauveria" fungi occur naturally and are believed notto affect humans. Flies killed by these fungi can readily be found inhomes and gardens each year. "By taking one of nature's epidemics againstflies, and using it commercially, we hope to speed up the process,"Eckenrode says.

In New York, onions are grown very intensively in a highly organic,peat-type soil, known in the industry as muck. This usually requires ayearly grower investment of $3,000 to $3,500 an acre before harvest. Themuck soil holds water well and allows the onion bulbs to expand. Even inthis favorable environment, plant nutrients must be added at strategictimes, and a wide array of pests -- including the onion maggot -- plantdiseases and weeds must be controlled.

Cornell researchers recommend that growers rotate out of onions, if theycan. One possibility for rotation is sorghum sudan grass. Planting it notonly breaks the maggot's life cycle, but reduces the nematode population inthe muck soil. The grass roots also aerate the ground, allowing increasedonion harvests in subsequent growing seasons.

Laboratory and field work discussed here were conducted by Eckenrode andvan der Heide; Mary-Lou Hessney, entomologist at the Geneva ExperimentStation; Kathleen Hahn, CCE researcher in Oswego County; Mark Ramos, USDAAgricultural Research Service; and John Dunsmoor, an onion grower inOswego.

Related World Wide Web sites: The following sites provideadditional information on this news release.

Cornell College of Agriculture and Life Sciences: http://www.cals.cornell.edu