Erin Hodgson and Matt O'Neal, Department of Entomology
As soybean fields enter seed set throughout the state, we continue to get questions about how long to scout for soybean aphid. This year, aphid populations have been extremely variable with a limited number of fields exceeding the economic threshold (250 aphids per plant with increasing populations on 80 percent of the plants) in late July. In August, however, there have been more reports of increased aphid activity throughout the state (i.e., some fields were clean until last week). Many of the extension field agronomists also indicate winged aphids are showing up in fields. This correlates well with current aphid captures in the Regional Aphid Trapping Network (Figure 1).
Figure 1. Summary of winged aphids collected at the four Iowa suction traps in 2009. In addition to soybean aphid, there have been several other common species identified.
How do the winged soybean aphid numbers compare to other years? So far the actual number of soybean aphid trapped in Iowa is quite low and is similar to 2006 (Figure 2). However, other states are reporting high soybean aphid captures. Does this mean aphids recently found in Iowa fields came from other places? Winged aphids are capable of long distance migration via jet streams, but we aren't going to blame other states.
Figure 2. Summary of winged soybean aphid trapped from all four Iowa locations.
General guidelines recommend scouting for soybean aphid every 7-10 days through seed set (R5-R6), regardless of insecticide treatments applied during the year. Do not assume a seed treatment or preventative application will control aphid populations. In rare cases, fields treated at the economic threshold may need to be treated twice if aphids flare again during seed set.
We encourage growers and consultants to continue scouting because of all the winged aphids moving right now. Late-season infestations are possible and aphids can cause economic loss during seed development. As plants mature, they become less attractive to aphids. However, soybean aphids strongly prefer to feed on soybean and aphids will continue to feed as plants enter R7 (beginning maturity). In 2008, some fields had aphid populations increasing into September. What are the treatment recommendations for late-season infestations? Refer to ICM News article on this topic.
Erin Hodgson is an assistant professor of entomology with extension and research responsibilities. She can be contacted by email at firstname.lastname@example.org or phone (515) 294-2847. Matt O'Neal is an assistant professor of entomology with research and teaching responsibilities. He can be contacted by email at email@example.com or phone (515) 294-8622.