First soybean aphid update of the year – they are out there, but not many of them. Many of you will recall that soybean aphid has a complex life cycle that requires another host plant – buckthorn. This woody shrub is a native of Asia, as are soybean aphid and soybeans. When soybeans are not present, soybean aphids require this plant to reproduce upon. Most importantly, this period includes the overwintering egg stage.
Many of you may recall previous issues of Pest&Crop that have chronicled aphid sampling trips through Indiana, Illinois, Ohio and Michigan in spring and fall to assess how many aphids made it to buckthorn, how many eggs they laid, numbers of predators, winter survival etc. These trips, led by Dr. David Voegtlin, emeritus of the Illinois Natural History Survey, consist of visits to known aphid overwintering sites and can be a useful indicator of what to expect in terms of soybean aphid pressure later. For example, a visit to these areas in fall of 2009, (when we had extremely high numbers of winged aphids throughout the Midwest), revealed that many of the aphids that reached buckthorn were killed by pathogenic fungi and natural enemies. Counter-intuitively, very few eggs went into overwintering that year with a result of very low pressure in 2010. Here is Dr. Voegtlin’s report from last week’s survey:
“Last Wednesday through Friday I made my annual spring trip to survey for the soybean aphid on buckthorn. After the very low numbers recorded in the suction trap network last fall I was not expecting to find much, if anything. General locations visited on these trips are the Rome City area in N.E. Indiana, Toledo area, Irish Hills in S. E. Michigan, Kellogg Forest near Battle Creek, Michigan, Calumet area S. of Chicago, Joliet, IL and Quad Cities area. A number of colonies were found at the Kellogg Forest area and Quad Cities sites. After extensive searching in the Rome City area only one colony was found. No soybean aphids were found at any other locations. Given the cool spring weather the development has apparently been slow. Colonies had only wingless adults and nymphs. No winged aphids were observed and I did not see any natural enemies associated with any of the colonies. This is more than I expected to find. I think that in years with very low fall flight the aphids that do make it to buckthorn are in such low numbers that the predators do not find them. A few egg layers on one plant is not enough to attract the predators and they can successfully deposit eggs. The opposite seems to happen in years of high fall flight. Predators, particularly Asian lady beetle, flock to heavily infested trees in such high numbers that there are no female aphids that reach maturity and deposit eggs.”
What does all this mean for Indiana? Probably only that the risk of spring/early summer treatable aphid infestations is quite low. Typically, we get most of our pressure later in the season (early to mid-August), probably from winged migrants that arrive from other states (WI and MN, for example). Those states have more buckthorn and therefore a greater risk of early season aphid problems. What follows are soybean fields that are infested sooner in the growing season, which can lead to large aerial aphid migrations in search of soybean that are relatively aphid-free. This is where Indiana and surrounding states come in. This is also one of the reasons that seed treatments are generally not the best option for soybean aphid management in Indiana – the insecticides are usually no longer present at levels sufficient to manage aphids by the time they show up in large numbers.
With many areas of Indiana just planted, or being planted now, moths may have found these weedy fields as an ideal egg-laying site. Tillage at, or just before, planting will provide little control of the eggs or newly hatched larvae – it will mostly serve to move them around a bit. Since black cutworm has been a minor pest the past several years, producers may have a false sense of security with the seed-applied insecticides and/or Bt-corn. The lack of damage during these past years has been more due to the record early dates for corn planting combined with low black cutworm arrivals. This combination has been ideal for limiting black cutworm risk.