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Wisconsin corn crops see increase in flea beetles, potato leafhoppers

Grace Connatser
Wisconsin State Farmer
Flea beetles can be controlled the same way as cucumber beetles with row covers.

An integrated pest management update webinar from the University of Wisconsin-Madison brought new information to farmers on future EPA regulations and certain pest and disease populations across the state during this year's growing season.

Bryan Jensen, of UW-Madison's integrated pest management program, said some new insecticides are being introduced, including Sefina and Intrepid Edge, while Lorsban will no longer be produced.

Jensen said Sefina is the most specifically targeted with an approved label only for soybeans and designated to deal with bugs that affect the plant through mouth-sucking. Intrepid Edge, produced by Corteva, is available for use on corn and soybeans and is also designated for moth and butterfly problems.

Corteva has decided to discontinue Lorsban (rather than it being a decision from the US Environmental Protection Agency), Jensen added, because it's outdated and there are better products out there already. However, it's still available for purchase and use at the moment for farmers who already have it in their arsenal.

Jensen also said that new recommendations are being released on spraying and disposing of neonicotinoid pesticides, or neonics, due to continued pushback from the EPA on the ecological risks, especially around bodies of water. Farmers must make sure to pick up any spilled treated seeds during loading or planting, bury waste away from water and avoid contaminating bodies of water while washing planters.

"These neonics are very water soluble, and either through surface water contamination or groundwater contamination, they can move pretty quick," Jensen said. "Do not wash your planters off close to bodies of water so that that wash water cannot flow into our rivers, lakes (or) streams."

There is also an EPA-proposed label change on neonic products like Belay, Justice and Endigo to mitigate damage to pollinators, namely bees, because of how easily the insecticide travels in the air. Farmers may be subject to restrictions on spraying during certain wind conditions and using certain droplet sizes if the proposal is approved next year. Farmers would also not be able to spray neonics within 25 feet of a body of water.

Jensen said pest presence among crops this year was mixed, with some species being more prevalent than in past years, while others were not. He was particularly concerned about flea beetles because he saw more this year than in the past 15 years. Flea beetles can pose significant crop injury risks to seedling corn, especially during defoliation.

Flea beetles can also be a vector for Stewart's disease, caused by the bacteria Pantoea stewartii. A tell-tale sign of the disease is small yellow feeding scars along the leaves of the plant that gradually turn brown as the tissue of the plant dies around it. The crop will wilt if the stalk becomes infected.

"This past summer I had more calls on flea beetles than I've had in the last 10 or 15 years," jensen said. "Our greatest concern is not so much with production of field corn, but it is with the production of seed corn. There can be some vital sanitary issues if there's Stewart's disease in that seed."

Mat Wells uses a sweep net to scout for potato leafhoppers in an alfalfa stand.

Jensen was also worried about potato leafhoppers, explaining that the hot and dry weather in early and mid-summer drove population counts higher than in recent history. According to collected data, Jensen said late cuttings had more issues with potato leafhoppers than earlier cuttings and recommended that farmers make timely scouting rounds to check for the bugs, especially with a longer timeframe between cuttings.

"Rarely do we have, if ever, problems with first cutting, usually not second cutting. But third, fourth, fifth later cuttings can have some significant problems with leafhoppers," Jensen said. "Some sources told me they had situations where leafhoppers were so bad they required two applications."

Overall, farmers should schedule their pesticide applications right on time because being too early or late can have a detrimental effect on crops, Jensen said. He encouraged farmers to scout in the fields, which can help determine the right schedule for their individual fields.

For soybean aphids and twospotted spider mites, populations only experienced a small uptick compared to the average, Jensen said. He recommended farmers be more aggressive and "adventurous" with their pest management practices regarding soybean aphids, while they can afford to be more conservative with twospotted spider mites. However, if you catch a spider mite infection too late, it can be hard to catch up, Jensen said.

"Always monitor populations of soybean aphids and spider mites at the same time, at least initially to see what's going on, and to practice the spirit of the soybean aphid economic threshold," Jensen said. "Understand that the economic threshold is only 250 (aphids) per plant. That's an arbitrary number – you can expect to get economic loss when we reach the ... economic injury level that is about 600-700 soybean aphids per plant."

Jensen added that the brown marmorated stinkbug is making splashes in urban areas this year, which means it's not far behind from doing the same thing in rural areas. He said both the stinkbugs and grasshoppers, which are usually prolific during drought seasons, can both pose issues next year and in the coming years, especially if dry weather continues next growing season.

As soybean gall midge continues to create problems in Nebraska, Iowa, Minnesota, South Dakota and Missouri, Jensen reported that the bug has not been detected yet in Wisconsin, according to a survey performed by the Department of Agriculture, Trade and Consumer Protection.

But corn rootworms could pose additional issues in the coming years as one southeastern Wisconsin farm was found this year to have a population of the rootworms that was resistant to Cry34/35Ab1 proteins in Bt hybrids, according to the EPA. Jensen said the EPA will help mitigate resistance issues as they become more widespread. He explained that resistance is more common than reported.

"Is there widespread resistance in Wisconsin? Certainly, we have local issues, especially where there's a high incidence of continuous corn and other areas of the state where we've had a history of continuous use of the same or similar Bt protein," Jensen said. "It's not always taught and it's rarely reported."

Jensen called for farmers to diversify their integrated pest management practices due to influxes in some pest populations, saying that there will be "no silver bullet." Farmers should scout for adults during the egg-laying period in order to keep all options on the table, he said. Other practices include rotation (biannual or annual), soil-applied insecticides on non-Bt hybrids and seed treatments.

"Don't assume there will be a silver bullet out there. I'm sure if there was right now, companies would have it available," Jensen said. "I know there are some things in the pipeline that EPA is looking at, but until they are available, I'm going to assume we only have what we got."

Damon Smith, extension specialist in plant pathology at Madison, also joined the webinar to give updates on soybean diseases and fungicides. He said there are not many new fungicide products on the market for 2021, except Delaro will be ready for distribution by the end of next year pending a label approval. Delaro, a Bayer product, will have a three-way mode of action and will be usable on corn, soybeans and cereals.

The tar spot fungus produces raised, black fungal structures called
ascomata on the surface of corn leaves.

Smith said gray leaf spot and tar spot were major 2020 soybean diseases, with both of them starting early in the season and moving slowly due to hot temperatures. Tar spot did begin to move more quickly in late summer as temperatures cooled down in some areas, especially those with irrigation. He added that tar spot has also been found in Ontario, Canada and an isolated area of Pennsylvania.

"Overall (we had) pretty reasonable disease management here in corn. Things could have been a lot worse," Smith said. "I'd say we didn't have situations like we had in the past couple of years, and we can really see that when we start to look at some of the yields."

In tar spot fungicide trials, Smith said Miravis, Delaro, Revytek and Lucento did exceptionally well with keeping the disease at bay. He said he is also continuing to study the accumulation of Fusarium graminearum, or fusarium head blight, in corn silage using Proline and Headline AMP. Some correlations are found between high amounts of fusarium head blight and deoxynivalenol (DON, or vomitoxin) if they are both located in the same part of the plant.

"DON is not moving in the plants, we actually have to have those infections on those specific parts in order to get the accumulation of vomitoxin," Smith said. "That means in some years we might get a root stock infection, other years we might get an ear infection, and we need to control those depending on the weather in those years. The weather does seem to play a significant role in what type of epidemic we're going to see."

Despite the good prognosis overall, Smith noted that frogeye leaf spot was more widespread and severe than ever in 2020 and has developed new levels of resistance to certain fungicides. A trial conducted in Arlington, Wis. found that using Delaro led to a higher level of severity, while Topguard, Lucento and Revytek helped reduce severity of frogeye leaf spot.