Insect update for late August 2014

An update on corn rootworm and soybean aphid populations, as well information on when and if to spray for soybean aphids this late in the season.

It’s been a slow season for insects this year, so there wasn’t much to write about. It was a cold winter, but insects that overwinter in Michigan are prepared for such conditions, and the deep snow cover was probably a good insulating blanket. More likely it was spring weather conditions and lack of crop synchrony that played a larger role in reducing populations of certain species. However, I have had calls about a few insects in the last week.

Corn rootworm

Rootworm populations took a hit. In the St. Johns, Michigan area, fields that had a lot of beetles last year have very few in 2014. The wet May and late corn planting probably had a lot to do with this. After hatching, tiny larvae must find a root fairly quickly to survive. If corn planting is delayed, root masses are smaller, or the soil is waterlogged, many larvae will die before finding a host. This does not mean that rootworms are gone or that Bt corn resistance is no longer a problem. It simply means that there is a smaller population undergoing selection.

Soybean aphids

Soybean aphids are low or nonexistent in most locations, although I’ve had a few calls about individual fields in southwest Michigan, lower-central Michigan and central Michigan. The infested fields in southwest Michigan had symptoms of severe potassium (K) deficiency. K-deficient fields have higher aphid populations. The long-term solution for such fields is to manage fertility. The populations in central Michigan likely came from secondary flight from states to the west of Michigan. One caller from Clinton County said that aphids were found in a field along a tree line, a good sign that winged individuals were blown in. Both South Dakota and Minnesota report increases in aphids, and they could be a source.

It bears repeating that for the most part, it usually doesn’t pay to spray for aphids this late in the season. Why?

  • Remember the economic threshold (ET) of 250 aphids ET isn’t a magic number, it’s simply a convenient point that entomologists use to target a spray to prevent populations from going over a more important number, the economic injury level (EIL). The EIL is when damage actually starts to occur, i.e., you lose money. For soybean aphids, the EIL is  about 670 aphids per plant. This late in the season, it’s worth delaying a spray after 250 to see if you even reach 670 aphids per plant. It’s a good bet that you won’t.
  • The ET of 250 aphids per plant is valid through the R5 plant stage. After R5, the threshold goes up. (Entomologist estimate at least about 1,000 aphids per plant, but we aren’t sure how high.) So, at R5 and beyond, you can tolerate more aphids per plant, especially if you’ve had adequate rainfall.
  • Biological controls are taking over now, including predators and parasitoid wasps. With dewy mornings, fungal pathogens also begin to kill aphids under the canopy. Also, as day length decreases, the aphid population begins to change to different forms, including males, that leave soybeans and head back to the overwintering host.
  • At R5 and beyond, soybean plants are no longer a great host for aphids. Once seeds are forming in pods, the plant sap changes and aphids no longer get the best nutrition. Thus, you begin to see aphids on the bottom leaves instead of the active growing points. The aphids themselves are not fat and green, but wimpy, small and white.
  • Spraying often means running over plants, potentially reducing yield 1-2 bushels an acre.

If you don’t believe my explanation, believe the data. Below are yields from plots sprayed during the last aphid outbreak in Michigan. Sets of plots were sprayed on each date. Other sets of plots were sprayed three times (3x) or untreated (none). In Saginaw, Michigan, plots sprayed when aphid numbers were between the economic threshold of 250 (July 6 and 13) and the EIL of 670 (July 21) had the same yield as plots sprayed three separate times! But thereafter, yield stair-stepped down as aphid numbers increased to over 9000 per plant by early August. By the end of August, when plants were in the R5-R6 stages, spraying resulted in no-better yield than doing nothing at all.

At another location that year in Calhoun County, aphids went over threshold on July 26, but only got to 595 per plant in late August when plants were in the R5 stage. The populations never actually went over the EIL. At this site, yields did not differ, even if the plots were sprayed once, three times, or not at all. This data shows that the EIL is valid, and that late sprays don’t pay.

Saginaw 2005: Aphids reached over 9,000 per plant in the untreated plots

Aphid population in Saginaw 2005

Calhoun 2005: Aphids reached 595 per plant in untreated plots

Aphid population in Calhoun

When might it pay to spray this late? In late-planted fields. Plants that are behind in crop stage are still a good aphid host. So, if you want to determine where best to put your time and money, scout your latest-planted fields. If you find aphids, wait a week to see if the population crosses the 670 aphid per plant EIL. Otherwise, save your insecticide for another season.

Dr. DiFonzo’s work is funded in part by MSU‘s AgBioResearch.

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