Effect of Temperatures, and Natural Enemies, on Soybean Aphids

Effect of temperature on soybean aphids: As with other insects, soybean aphids have a temperature range where development is optimum, with development being slower below and above the optimum range. For soybean aphid the temperature for optimum growth is around 27C. At lower temperatures development will be slower. At the other extreme, in experiments where soybean aphids were kept at high temperatures, nymphs did not complete development at 35C and died within 11 days (J. Econ. Entomol. 2004: 854-861).
Soybean aphid consumption by natural enemies: High levels of natural enemies have been reported by some monitoring levels of soybean aphid. Predaceous insects are the easiest natural enemies to recognize; levels of parasitism and insect pathogens are not often as obvious but with training can be estimated as well. Lady beetles, lacewings, and larvae of hover flies are some of the common and easily recognizable predators in soybean fields. When checking soybean fields, consider whether the aphid populations are increasing or decreasing, and the level of natural enemies present.
Common species of lady beetle that may be noted in soybean fields include the sevenspotted lady beetle and the multicoloured asian lady beetle (Figure 1).

Figure 1. Larva of multicoloured asian lady beetle

A study looking at predation rates by these two species of lady beetles on soybean aphids found that for sevenspotted lady beetle third instar larvae could each consume up to 105 aphids per day, adult females could each consume up to 115 soybean aphids per day, and each adult male up to 78 aphids per day when soybean aphids are high. (Environmental Entomology: 2009. 708-714). For multicoloured asian lady beetle, third instar larvae could each consume up to 112 aphids per day, adult females could each consume up to 95 soybean aphids per day, and each adult male up to 53 aphids per day when soybean aphids are high.
Some have been sending in photos of hover fly larvae, which may be found in many crops where aphids are present, wondering what they are. Although the adults, which are good bee mimics, are easy to recognize, the larvae may not be as well known. Note that hover fly larvae (Figure 2) are legless, and narrow towards the head. There are different species that can vary in colour, often brown or green.

Figure 2. Larvae of 2 species of hover flies and soybean aphids

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-2015-08-17.html

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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

Have a follow-up question?