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Extension > Minnesota Crop News > Treating late-season soybean aphids

Thursday, August 13, 2015

Treating late-season soybean aphids


by Robert Koch (Extension Entomologist), Bruce Potter (IPM Specialist) and Ian MacRae (Extension Entomologist)

Late-season management of soybean aphids can be challenging. There have been many winged aphids colonizing fields over the past couple of weeks. Furthermore, the forecasted weather conditions will be excellent for aphid population growth.  Fields that were seed treated with insecticide and those treated early with insecticides tank-mixed with herbicide or fungicide are not immune to infestation. Those early-season prophylactic applications of insecticide do little to prevent soybean aphid populations from reaching damaging levels later in the season. Here, we provide an overview of late-season scouting and management recommendations for soybean aphid.

Continue regular scouting of soybean fields through the early R6 growth stage. As you count aphids, do not include the white cast skins of aphids, but do include the small light-colored live aphids, called "white dwarfs." White dwarfs will produce the more typical larger, green aphids in late R5. Our recommendation is to treat if the field is at the treatment threshold of an average of 250 aphids/plant with more than 80% of the plants having aphids until the field is at R6. Fields may need to be treated to avoid yield loss in the late R5 and early R6 stages. Although a treatment threshold for early R6 is not known, indications are that much higher populations are required to cause economic loss.

Considering cumulative aphid days may also help in making late-season treatment decisions. Before R6, it takes over 5,000 cumulative aphid days to cause economic yield loss. If there’s an average of 500 aphids on a plant over the last 10 days of R5, for example, that field will hit the economic loss level.

If a field is below threshold, continue to keep an eye on it (especially with the current weather conditions!), but hold off treating until threshold; you never know - heavy rains with wind, fungal disease, predation and parasitism can all knock aphid populations back. Aphids may also begin to move to buckthorn (the overwintering host) in the next couple of weeks as soybean beans mature and temperatures drop. However, explosive aphid population growth can occur in the late season, especially in late-planted fields or fields with iron deficiency chlorosis (IDC).

Treatment decisions should be made field by field, because variety and planting date among other factors can affect yield impact by aphids. Even though we've had a long haul with this insect this year, keep scouting your fields. Regular scouting and timely application of insecticides based on the threshold through the R5 growth stage should eliminate difficult decisions related to treatment of large infestations and preharvest intervals (PHI) in the R6 growth stage.

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