Soybean Aphids & Importance of Economic Thresholds

Soybean aphids have been showing up in many fields, but so far populations have stayed below the economic thresholds. A few things to note regarding economic thresholds for soybean aphid:

  • The economic threshold for aphids in soybeans is 250 aphids per plant, and the population is increasing, and the plants are in the R1 (beginning bloom) to R5 (beginning seed) growth stages.
  • The reason that “and the population is increasing” is part of the threshold is because the actual economic injury level, where control costs will equal yield loss, is actually about 670 aphids per plant. The economic threshold, where control is suggested, has been set much lower than 670 to allow time for the spray to be applied before increasing populations could potentially reached 670 per plant. The population doubling time for soybean aphids can be as low as about 7 days if they are not being regulated well by natural enemies or weather.
  • Aphid populations do not always continue to increase. So we can not assume that because a certain number is present this week that even more will be present the next week. Populations can plateau or start decreasing due to natural enemies or weather conditions or events.
  • The above-mentioned economic threshold is based on the average of multiple plants selected from throughout the field and not just hotspots or field borders. Avoid making treatment decisions based on field borders. Small aphid hotspots often collapse from predation, parasitism or emigration.
  • When the value of the soybean crop is high, it is a mistake to try to lower the economic threshold below 250 per plant and the population increasing. There is already a large gap between the economic injury level and the economic threshold that is suggested. But what also needs to be considered is that the damage boundary, which is the lowest insect pressure where any yield loss can be detected, is well above the 250 economic threshold that used. So it would be illogical to try to reduce a threshold which is already below a level where yield loss can not be detected. Although economic injury levels (where control cost = yield loss) do change with commodity prices, damage boundaries do not. And given that aphid populations do not increase in linear fashions, there would be no advantage, and potential costs, to spraying at levels below the suggested economic threshold for soybean aphids. The research publication where the economic thresholds for soybean aphids are presented states “setting an economic threshold at lower aphid densities increases the risk to producers by treating an aphid population that is growing too slowly to exceed the economic injury level in 7 days, eliminates generalist predators, and exposes a large portion of the soybean aphid population to selection by insecticides, which could lead to development of insecticide resistance (Journal of Economic Entomology. 2007: 1258-1267). So to summarize, monitor fields and use the suggested economic thresholds.
  • When aphid levels are high, exact counts are not possible and would consume too much time. Estimates after a quick examination of the plant will be sufficient. Note in figure 1 below the soybean aphids, and the white shed skins from aphids that have molted. Avoid including the shed skins in your counts.
Soybean Aphids on Soybeans

Figure 1. Soybean aphids and shed skins.

The above information was submitted by John Gavloski, MAFRD Entomologist.  It is available in this week’s issue of the Manitoba Insect and Disease Update which is posted at: http://www.gov.mb.ca/agriculture/crops/seasonal-reports/insect-report-archive/insect-update-2015-08-04.html

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