Iowa Field Reports: Rising Pest Numbers in Soybeans

Soybean gall midge larva. Photo: Iowa State University

Limited rain fell across the state this past week, and approximately 36% of the state is considered to be abnormally dry according to the U.S. Drought Monitor as of last Tuesday, August 6. In addition to the dry conditions and moisture stress, other issues ISU Extension and Outreach field agronomists saw this past week included soybean gall midges, soybean aphids, the start of a third generation of thistle caterpillars, growth regulator injury in soybeans, and Tar Spot in corn. Read on to see what’s happening in specific areas across the state.

Northwest Iowa

Joel DeJong (Region 1): “Last week was pretty dry, but most fields are doing OK. Sandy soil areas are showing the stress, but those are the only areas showing significant stress right now. Corn ranges from just finished with pollinating for the late corn to dough stage for the earlier corn. Some gray leaf spot is present and scattered rust, but most fields are not showing a lot of disease currently.

“Soybeans appear to be mostly late R3 to R4, with the early fields approaching R5. Some frogeye is present, you can find gall midge damage on field borders quite easily in some counties, and soybean aphids are beginning to show up. Aphids might be close to the threshold at the NW Research Farm this week. A good rain would be welcome. Calls regarding off-target movement of dicamba in soybeans continue to come in.”

Paul Kassel (Region 2): “An analysis of two planting dates using the same 99-day corn hybrid planted in Clay County shows that the May 5 planting date should reach black layer development on September 17. The same 99-day hybrid planted on June 5 in Clay county predicts an October 20 black layer date. The actual black layer in this example will likely be a few days before October 20.

“Corn hybrids require about seven Growing Degree Days (GDD) less to mature for every day of planting delay (about 140 GDD less in this example). There is a 56% chance of a 28oF freeze event before October 20. A few farmers are reporting low numbers of soybean aphids. A few days ago soybean aphids were about nonexistent, so take time to scout your fields. There have been lots of painted lady butterflies flying around.

“It remains to be seen if we will see a third generation of thistle caterpillars in our fields. Regardless, farmers are encouraged to check their soybean fields for insect damage and defoliation through mid-August.”

Central Iowa

Meaghan Anderson (Region 7): “Most of Central Iowa went another week with little to no rainfall, but some areas received a well-needed shower to keep grain fill going. Some corn and soybean fields are showing stress from the warm temperatures and rainfall deficit, especially when you get out in (corn) fields and look at things more closely. Most corn is in the R4 stage, with some of the earlier planted fields beginning to dent already.

“Gray leaf spot is still the most prevalent disease, but I’ve received numerous photos of Physoderma brown spot as well. Insect pressure is low overall, but some fields have had issues with Japanese beetles along field edges. I noticed a pocket of high northern corn rootworm beetle pressure in SW Dallas and NW Madison counties this last week.

“Soybeans are mostly in the R4 to R5 growth stage and are dominating my phone calls. In the last week, I started noticing thistle caterpillar eggs on the upper side of soybean leaves again; these will take about a week to develop into caterpillars. We also confirmed the presence of soybean gall midge in Dallas County, so please be out scouting for it. I’ve also started to get reports of soybean aphids; however there does appear to be a lot of variability in populations so also be scouting for aphids.”

East Central, Southeast, and South Central:

Rebecca Vittetoe (Region 8): “Last week was another dry week across most of my area. The crops, hayfields, and pastures are showing signs of stress. Corn ranges from VT to R4 and soybeans are mainly R3 to R4. From a disease perspective, gray leaf spot and common rust tend to be the common diseases found in corn, and soybeans continue to have little to no disease pressure.

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“Tar spot has been found in a couple of my more northern counties again this year. There have been plenty of painted lady butterflies flying around the area. I suspect that this week we could start to see the third-generation larvae in soybean fields. I continue to get calls regarding growth regulator injury in soybeans. Other calls and field visits have been on pasture management, fall seeding of forages, and late-season weed escapes.”

Virgil Schmitt (Region 9): “Rainfall last week was mostly between 0.5 and 1.0 inch, with western Lee receiving less than 0.5 inch, a little streak through Cedar, Muscatine, and Scott Counties receiving between 2.0 and 3.0 inches, and most of Jackson County receiving between 1.0 and 2.0 inches. Corn ranges from R2 (June planted corn) to R4 (April planted corn). Some of the R4 corn is starting to show some early signs of denting. There is a little rust and gray leaf spot. I’m receiving reports of more tar spot in Jackson County.

“Soybeans are from R4 to R5. There is little disease pressure at this point. Group 4 herbicide injury calls are continuing. Oats are mostly harvested. Pastures are turning brown. Potato leafhoppers continue to be an issue in hay. Inquiries about and Group 4 herbicide injury on soybeans and fungicides on corn and soybean dominated calls last week. Other calls included potato leafhoppers, weed escapes (mostly waterhemp), drift, and anthracnose leaf blight on corn.”

Josh Michel (Region 11): “While some areas in the eastern part of my region did receive up to a half inch of rain, many areas in south central Iowa remained dry over the last week. Fields and pastures have been showing signs of stress with the lack of moisture. Reports of grasshoppers and potato leafhoppers continue in hayfields. April planted corn is mostly around R3 to R4 with the late planted June corn being at VT to R1.

“Early planted soybeans are mostly around R4, with a few fields possibly reaching R5 later this week. Common questions this past week included foliar diseases in corn (common rust and gray leaf spot), Japanese beetles feeding on silks and causing defoliation in soybeans, thistle caterpillars, late season weed escapes, and herbicide injury in soybeans.”


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