It is time to be soybean aphid scouting and be aware of current population densities in farmers' fields to determine if they are increasing. Soybean aphids made an early appearance in Wisconsin soybean fields this year. Infestations are fairly widespread, although still low to moderate in density on a field average basis. However, individual plants with high numbers (up to 168 aphids per plant) have been reported by WI DATCP, UW researchers and crop consultant calls.
The economic threshold is 250 aphids per plant and is based on 20 to 30 plants sampled from throughout the field. Finding one or two plants with high numbers does not necessarily mean a field is at threshold. Additionally, if you find many plants (widespread infestation) throughout a field with low numbers, watch the field for population increase. Natural enemy abundance (predators and parasitoids) and temperature (soybean aphids develop more slowly at sustained high temperatures in the 90°F + range) will, in part, determine whether and when economic populations develop.
Soybean fields without insecticidal seed treatment are likely to be infested first. For fields with insecticidal seed treatment, much of the insecticidal activity will diminish after approximately 46 to 55 days. Therefore, depending on planting date, be sure to include seed-treated soybean fields in your soybean aphid scouting plan after mid-July.
Summary of 2013 Soybean Aphid Management Recommendations:
Scout weekly beginning in late June or early July, and no later than R1 beginning bloom soybean growth stage.
Check 20 to 30 plants per field, covering 80% of the field in your sampled area.
Use an economic threshold (action threshold) of 250 aphids per plant if populations are actively increasing. Regular field visits are required to determine if soybean aphid populations are increasing.
For more information, visit the North Central Soybean Research Program Plant Health Initiative soybean aphid website: http://www.planthealth.info/aphids_basics.htm