Ag Weather Forum

Late July into early August is when we start ramping up our field scouting for soybean aphids on my family's farm in southwest Minnesota. Our soybeans are in the R2 and R3 stages and as the pods begin to fill, aphids pose a risk to our soybean yields. The weather pattern can play a significant role in whether these pests flourish.

During the past 30 days, rainfall has been plentiful across southern Minnesota, Iowa, northern Missouri and western Illinois. At least 6-8 inches of rain has been observed in some of these locations. Rainfall has been timely, but heavy in spots. Fields that have tile can handle the extra water and crops are mature enough now that flooding risks have been reduced.

Temperatures during the past month have been largely above average for much of the Corn Belt. Since July 1, Cedar Rapids, Iowa, has had 23 days where the high temperature reached above average. The average high temperature in Cedar Rapids for the month of July is 83 degrees Fahrenheit. For the month of July so far, DTN has estimated that temperatures have been 3-4 degrees F above average for much of Indiana and Ohio as well.

All those weather developments could make a difference in whether aphids are a problem in your fields. In a recent blog post, University of Minnesota Extension Entomologist Robert Koch noted that both heavy rainfall and high temperatures can slow aphid population growth. Temperatures exceeding 90 degrees F can be a speed bump to how fast their population grows. Additionally, heavy rainfall and high winds can knock aphids off soybean leaves. You can read more about what Koch has to say about soybean aphids here: https://blog-crop-news.extension.umn.edu/….

Even if you're in an area that has been receiving hot weather and consistent rainfall or thunderstorms, you may not be out of the woods completely from aphid infestations. Aphids are a pest known for reproducing quickly as female aphids clone themselves in the summer and can have a birth rate of 3-8 aphids per day for 30 days, according to Iowa State University's Extension and Outreach. It only takes a matter of a couple days for a population of aphids to double. To learn more about a soybean aphid's biology, you can visit: https://crops.extension.iastate.edu/….

While the weather may not have been favorable for aphid infestations during July, given the heavier rainfall in some states, we still have all of August to get through and this will be when soybeans reach their critical R5 and R6 stages of growth, especially in the northern Corn Belt.

Farmers who have been dealing with aphids since they were first introduced to the Corn Belt in the early 2000s know it is important to scout fields in July and August. As late summer approaches, watch for periods of southerly winds, dry conditions and temperatures below 90 degrees F. These will be the primary weather ingredients that help aphids flourish.

As field scouting continues, keep weather factors, economic thresholds and the presence of beneficial insects in mind. Koch's blog provides more details on how to scout. In general, the discovery of 250 aphids per plant, with most of the plants infested, and aphid populations increasing is the economic threshold or trigger point to begin lining up insecticide applications to prevent economically damaging levels, which is about 675 aphids per plant. If insecticide is applied below the economic threshold, it could end up hurting the populations of aphid's natural enemies. Iowa State University's Extension and Outreach listed that lady beetle adults and larvae, lacewing larvae, and pirate bug nymphs and adults feed on soybean aphids.

To find more weather conditions and your local forecast from DTN, head over to https://www.dtnpf.com/…

Teresa Wells can be reached at teresa.wells@dtn.com

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