Soybean Aphids Present in Most Soybean Fields

Published: September 3, 2009

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.

In summary:

• Multiple generations of aphids (20+) may be produced through the growing season in more northern states where aphids are present throughout the growing season.
• In Missouri, just a few generations are produced in most years after winged aphids typically migrate into the state during summer.
• Soybean aphids reproduce parthenogenically which means that female aphids give birth to only female aphids which are already pregnant when born. No males are needed for most generations.
• Populations of soybean aphid can double every 5-7 days when conditions are favorable.
• Soybean aphids damage plants by sucking plant juices from stems and leaf foliage.
• This insect prefers cool conditions and reproduces at an optimal rate when temperatures are around 78 degrees F.
• Numbers of this pest vary from field to field so each field should be individually scouted to determine average aphid numbers per plant and plant growth stage.
• Treatment is justified when the economic threshold of 250 or more aphids per plant is reached and the soybean plants are in the R1 to R5 stages of growth.

The following insecticides are recommended for use on soybean aphid infestations in soybean: