by Deborah Jeanne Sergeant

Seed corn maggot has begun to resurge in New York State, according to Elson Shields, field crop entomologist at Cornell University. Shields presented at the 2022 Corn Congress.

“People think seed corn maggots are not a big issue, but those of us in the field who are over 50 remember before neonic and how devastating it can be,” Shields said.

He walked dozens of fields last summer and encountered issues because of seed corn maggot. Because it was previously treated successfully with neonicotinoid seed treatments, the pest had all but disappeared. Because seed corn maggot populations had decreased so dramatically, many producers question whether it’s worth going with insecticide-treated seeds.

Shields said that continuous corn and following corn after cover crops are reasons to consider using treated seeds.

In seed corn maggot trials, Cornell researchers looked at single row plots using a four-row corn planter. The treatments were a single row of non-Bt-RW without neonic seed treatment; non-Bt-RW with neonic seed treatment; Bt-RW without neonic seed treatment; and Bt-RW with neonic seed treatment.

With six planting pairs, researchers observed a 25% loss (25,600 plants). Assuming a 20-ton yield and $40/ton value, a 1% yield loss is $8/acre. A 5% loss is $40 loss per acre. The cost of seed corn maggot seed treatment is $5/acre. Even at a mere 1% loss, it appears that the seed treatment is worthwhile.

Shields assumes the target corn stand is 32,000 plants/acre. With 9/24 planting pairs in seven years of continuous corn, his research team found a 38% reduction in the stand (only 28,800 plants/acre). With 8/24 planting pairs, the stand went down by 33% (27,500 plants).

Seed corn maggot affected test plots of corn following a cover crop plus dairy manure. Fifty-four percent of the test area suffered a 10% stand loss or greater with 13/24 planting pairs. Among 9/24 planting pairs, 38% experienced a 14% loss or greater. In 7/24 planting pairs, 29% had 20% or greater loss.

“What do we do if we lose neonic seed treatment?” Shields asked. He answered his question with a few scenarios: do nothing and suffer losses; use diamides as a seed treatment; use liquid insecticide with the popup fertilizer at planting; or use in-furrow soil insecticide.

“The diamides are touted as the replacement seed treatment and currently are three times the cost of the neonic,” Shields said.

He said corn has a very broad window of vulnerability, depending upon emergence. “Some of my colleagues suggest we can do risk assessment,” Shields said. “What if you planted your corn without flies around and suddenly, there’s an invasion of flies? We cannot risk that.”

Once seed corn maggot invades, the field would have no other rescue treatment. Cornell research showed that seed corn maggot persists across corn rotations.

Trials of the effects of entomopathogenic nematodes on Western corn root worm on non-root worm trait corn from 2016 – 2020 show 50% – 89% reduction of damage compared with areas without entomopathogenic nematodes. The exceptions were 2018, which had no corn root worm pressure, and 2019 in areas with very dry soil, which had only 24% reduction of damage.

Shields said that nematodes proved efficacious on corn root worm, wireworms, root weevils (such as alfalfa snout beetle, black vine weevil and strawberry root weevil) and multi-year white grubs. Just one application offered multi-year suppression.

During the same timeframe, Cornell researchers looked at entomopathogenic nematodes with 34/35 trait corn Herculex. The results ranged from 0% reduction of damage in 2016 to 86% in 2018.

Drought appears to reduce the effectiveness of entomopathogenic nematodes. “In 2020, it was very dry in the research farm in Pennsylvania,” Shields said. “The Bts didn’t even work. In this case, the corn was looking pretty tough anyway and there wasn’t much to harvest.”

Overall, the research showed that biocontrol nematodes offer 50% – 90% reduction in corn root worm feeding damage for two to six years after one application. These biocontrol nematodes work seamlessly with Bt-RW technology.

“I’ve had 35 years’ experience now,” Shields said. “I feel comfortable saying biocontrol nematodes offer an alternative to Bt-RW technology in rotated corn.” He added that in continuous corn in New York, nematodes must be used with Bt-RW corn varieties because of the increased corn root worm pressure on the crop.

“Biocontrol nematodes will help us where Bt corns are failing but you don’t dare use them by themselves,” Shields said.

He said nematodes are “easy to apply with existing equipment” but producers should remove screens and filters and use fertilizer stream nozzles. The application rate is 50 gallons/acre, ideally applied late in the day. He estimates that more than 30,000 acres of alfalfa and corn in New York have been inoculated.

Mike Hunter, a Cornell Extension agent, has combined nematode application with liquid manure application to save passes around the fields. Shields said that this method also provides nematodes with a UV block and lots of water. Famers can apply it at any time of the day.

In non-irrigated systems, farmers can expect 30% – 50% activity in year one and 100% in year two. With irrigated application, it increases to 50% – 75% activity in year one and 100% activity in year two.

The cost is $60/acre, plus shipping and application costs; however, Shields said this is a one-time expense, as the nematodes’ positive effects last for many years. Even in dry areas like New Mexico, nematodes “show excellent persistence” in test plots, Shields said.

He advised using nematodes with other corn root worm management tools to prevent resistance. Shields added that the ag science community is moving forward with additional testing on efficacy in problem areas across the country where Bt is failing.