Crop & Pest Report


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Hot Temperatures will Impact Soybean Aphids and Insecticide Control (7/21/11)

Soybean aphid populations continue to slowly increase in major soybean producing counties in southeast ND.

See NDSU IPM maps below. So, please be vigilant scouting through R5 (early seed).

Percentage of soybean plants infested with more than one aphid

Average number of aphids per soybean plant


Temperatures have been hot in the high 90s F with a heat index over 105 F this past weekend and into this week. Hot temperatures affect the longevity (or life expectancy) and reproduction of soybean aphids. Laboratory research at constant temperatures indicates soybean aphids are temperate insects that do better at moderate temperatures in low 80s F. As temperature increases, longevity and reproduction decline. The cooler the temperature, the longer aphids survive, and reproduction is also better. At a constant temperature of 90s F in the laboratory, soybean aphid’s longevity dropped dramatically and reproduction was almost nil.

However, in the field temperatures fluctuate more with nightly lows about 20 degrees less than the high temperature for the day. Transitory high temperatures have little effect on soybean aphid development as long as nightly lows drop below into the 70s F. Brief periods of hot temperatures don't affect the rate at which females produce nymphs, only the survival of the nymphs. Nymphs born during the heat of the day don’t survive well when temperatures exceed the 90s F. As a result, hot temperatures will slow aphid population growth, but will not stop or control aphid populations.

Development of the soybean canopy also moderates the survival conditions for aphids. As the canopy closes, the highest temperatures occur near the top of the canopy. So, soybean aphids may move to the lower mid-canopy leaves to avoid the hot temperatures near the top of the plant. Proper insecticide pressure (40 psi) and coverage (20 gpa by ground, 3-5 gpa by air) is important to ensure canopy penetration and control of aphids that are moving away from upper canopy.

Higher temperatures and low humidity can affect insecticide performance in several ways.

  1. The metabolic activity of insects increases as temperature increases. Enhanced metabolism may either enhance the toxic effects of the insecticide or increase insect ability to detoxify or tolerate insecticides. The toxicity of organophosphate insecticides, such as chlorpyrifos (Lorsban), dimethoate, or methyl parathion (Penncap-M), increases as temperature increases. In contrast, the toxicity of pyrethroid insecticides, such as Asana XL (esfenvalerate), Baythroid XL (beta-cyfluthrin), Delta Gold (deltamethrin), Mustang Max (zeta-cypermethrin), Ambush or Arctic (permethrin), or Warrior II (lambda-cyhalothrin), decreases as temperature increases.
  2. High temperatures and low humidity may cause small insecticide droplets to evaporate before reaching and penetrating the canopy. Evaporation may reduce insecticide control. Evaporative effects can be reduced by avoiding application during the heat of the day, increasing water volume, increasing droplet size by changing nozzles, or by adding crop oils or non-ionic surfactants (consult labels or companies for specific recommendations).

(Source: Ken Ostlie, UMN, Minnesota Crop News)

Janet J. Knodel - Extension Entomologist







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