Western bean cutworms and soybean aphids are on the decline

Western bean cutworm populations have dropped dramatically in the last week, but continue to check corn and dry bean fields. Also, we’re seeing the beginning of the end of the aphid season.

Western bean cutworms

The western bean cutworm trap network shows that moth catch peaked the last week of July and has dropped rapidly since. Many of the traps we check are now catching nothing. You can see a graph of the catch comparing 2009, 2010, and 2011 at the MSU Field Crops Entomology website. The 2011 peak was the same week as last season and also 2008. The last week of July is shaping up to be our average peak-week, although peak was as late as the second week of August in 2009, a record cool season. Total moth catch is less this year, but that is partly because there are fewer traps this season.
In corn, larvae are already in the ear zone and there is nothing that can be done at this point but to check fields before harvest for damage or to compare efficacy of Cry1F Bt to a refuge or conventional corn. Larvae in dry bean fields can still be killed by insecticides because they crawl around in the canopy to feed, but we hope that at-risk fields in central and northern Michigan were already treated. Stay tuned in for the winter Extension meetings for data from Bt corn efficacy and yield trials and dry bean biology studies.

Soybean aphids

Aphid numbers were creeping up the last couple of weeks (most reports from southwest Michigan), but this week numbers are trending back down dramatically. Natural enemies and disease are the likely cause. There are plenty of both by mid-August, which is why it is an iffy-call to spray soybeans this late for aphids. Also, at this point in the season, with the change in temperature and day length, the soybean aphid population is preparing to move back to buckthorn for egglaying and overwintering. Indeed, the first winged soybean aphids were caught last week in suction traps in Illinois, Indiana, Iowa and Michigan. This marks the beginning of the end of the aphid season. See aphid suction trap counts at the North Central IPM Center website.

Dr. DiFonzo’s work is funded in part by MSU‘s AgBioResearch.

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