The recent arrival of additional soybean aphids migrating into Missouri from more northern locations, the successful reproduction of aphids which arrived earlier this summer, the lack of beneficial insects in soybean fields, and cool temperatures favorable for aphid reproduction have resulted in infestations and increased numbers of soybean aphids in most Missouri soybean fields. In a survey conducted this past week, aphid numbers varied by region with numbers in most fields below the economic threshold number of 250 aphids or more per plant on plants in the R1 (flowering) through R5 (partial pod fill) stages of growth. Aphid numbers above the economic threshold have been reported from some soybean fields in east central Missouri around Montgomery City, in central Missouri in Callaway and surrounding counties, and to a lesser extent in southwest Missouri south of Nevada. Fewer problems with this pest have been reported from more northern counties where a majority of fields are in R5 or later stages of plant growth. Fields with potassium deficit soils often support higher numbers of aphids than those with no potassium deficit.
Producers are encouraged to scout individual fields to determine aphid numbers and plant growth stages. As previously stated, the economic threshold the early warning population level at which treatment is justified) is 250 aphids or more per plant. The economic injury level (number of aphids per plant when the cost of control equals the yield loss from the aphid population) is 1006 aphids per plant. This difference in numbers allows for a 2-4 day period in which to treat an increasing population of aphids after the 250 level per plant has been reach and before economic loss has occurred. These threshold and economic injury levels are for soybean plants in the R1 to R5 stages of growth. Yields of 0 to 14 bushels/A may be protected when economic infestations of soybean aphids are treated at the R1 stage of plant growth and decreases with each increasing growth stage until a yield protection of 0 to 2.5 bushels/A is typically realized at the R5 stage of plant growth.
Questions often asked include (1) does it make sense to treat a soybean population before it reaches the 250 per plant threshold? The answer as reflected in numerous studies conducted in states north of Missouri is no. These multiyear studies show no yield advantage to treat soybean aphid populations on soybean before aphid numbers reaching or exceeding the established threshold of 250 aphids per plant in growth stages R1-R5.
Likewise, (2) producers often ask if aphids should be treated once soybean plants reach the R6 stage of growth (completion of pod fill)? Although some Canadian studies show a slight yield advantage to treat soybeans in the R6 stage of growth, no studies from the US show advantages to treating aphid infestations on R6 and later growth stage of soybean. A third question is (3) whether soil moisture conditions change the threshold. Soil moisture does affect yield loss from aphids with greater yield losses occurring in dry years and less yield loss occurring in wet years. Adequate soil moisture and adequate nutrient levels allow soybean plants to better compensate for the loss of plant juices which occur as aphid feed.
The following insecticides are recommended for use on soybean aphid infestations in soybean:
|Common Name||Trade Name||Rate of Formulated Material per acre||Additional Label Information|
|esfenvalerate||*Asana XL||5.8 to 9.6 fl. oz.||On foliage|
|cyfluthrin||*Baythroid XL||2.0 to 2.8 fl. oz.||On foliage|
|bifenthrin||*Brigade 2EC||2.1 to 6.4 fl. oz||On foliage|
|chlorpyrifos + gamma-cyhalothrin||*Cobalt||13 to 26 fl. oz.||On foliage|
|carbofuran||*Furadan 4F||1/2 pt (see note below**)||On foliage|
|zeta-cypermethrin||*Hero||4.0 to 10.3 fl. oz.||On foliage|
|chlorpyrifos||*Nufos 4E||1 to 2 pt.||On foliage|
|acephate||Orthene 97||3/4 to 1 lb.||On foliage|
|microencapsulated methyl parathion||*Penncap-M||1 to 3 pt.||On foliage|
|permethrin||*Pounce 3.2 EC||4.0 to 8.0 fl. oz.||On foliage|
|gamma-cyhalothrin||*Proaxis||1.92 to 3.2 fl. oz.||On foliage|
|lambda-cyhalothrin||*Warrior||1.92 to 3.2 fl. oz.||On foliage|
|*Designates a restricted-use pesticide. Use is restricted to certified applicators only. Regardless of the formulation selected, read the label to determine appropriated insecticide rates, directions, precautions, and restrictions.
**Furadan 4F produced and labeled before 2009 season may still be used until Dec. 31, 2009
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REVISED: May 10, 2012