Issue No. 19, Article 2/July 30, 2004
Despite the Relative Lack of Economic Infestations, Soybean Aphids Still Deserve Attention
A lot of people continue to monitor soybean fields for the presence of soybean aphids, and scrutiny for this pest is a good thing. We wish sometimes that scouting for other pests would be as diligent. Fortunately, scouting efforts thus far in 2004 have revealed few soybean aphids in fields throughout the state. In last week's issue of the Bulletin (issue no. 18, July 23, 2004), we noted that some people have observed "hot spots" of high densities of soybean aphids in some fields, and we encouraged everyone to keep their eyes on the development of these localized infestations. The recent weather conditions (cooler than usual) have been beneficial for development of soybean aphids, so a full-blown, fieldwide infestation could evolve from a hot spot. We learned firsthand last year how quickly aphid populations could become threatening.
We have read one unconfirmed report that soybean aphid densities have reached or exceeded the economic threshold established for the Midwest--250 aphids per plant during the R1 through R4 stages--in northern Illinois. If this report is accurate, people in northern counties should ratchet up their scouting efforts. Obviously, soybean aphid densities have not been as high in 2004 as they were at this same time in 2003, so it is unlikely that economic infestations will develop in most fields in Illinois this year.
Most people will agree that one of the most challenging aspects of managing soybean aphids is scouting for and counting the pests. Counting aphids and determining an average density for a field are particularly difficult when numbers of aphids are low. To address this concern, entomologists at the University of Minnesota have developed a more efficient scouting process, which they refer to as "speed scouting."
The scouting plan is a binomial sampling plan, or a modified sequential sampling plan. The objective is to enable scouts to sample for aphids more efficiently and to make decisions about control more quickly. The plan is explained thoroughly on the University of Minnesota "Soybean Aphid in Minnesota" Web site. Most helpful are the example forms that are completed for a "do not treat decision," a "treat decision," and a "resample the field decision." In these examples, mock numbers of soybean aphids are used to explain how decisions to treat or not to treat can be made quickly, and how continued sampling is required under certain circumstances. Marlin Rice, extension entomologist at Iowa State University, also recently wrote an article about speed scouting for soybean aphids for Integrated Crop Management, which includes yet another magnificent photograph.
Because the key to the speed scouting procedure is efficiency, the binomial sampling cutoff point is 40 aphids per plant. In other words, you need not count more than 40 aphids on any given plant. If you quickly determine that there are more than 40 aphids on a given plant, you record your observation ("+") and move on to the next plant, per instructions. The fewest plants you will sample with this procedure is 11, and the most you will sample is 31.
Because the speed sampling procedure is explained so thoroughly and so well in the aforementioned articles, we will not repeat the articles in their entirety in this issue of the Bulletin. However, it is important to note that this procedure does not create a new threshold of 40 aphids per plant. This sampling plan uses the percentage of infested plants (at least 40 aphids on a plant is considered infested) as an indicator of damaging soybean aphid populations. There is a statistical relationship between the economic threshold (250 aphids per plant) and the proportion of plants that are infested (i.e., 40 aphids per plant).
As we have frequently stated, new information will derive from research conducted on soybean aphids and from our annual experiences with this invasive species. As this new information accumulates, we will make you aware of it in as timely a fashion as possible. We will try to stay one or more steps ahead of the soybean aphid and other insect pests so that you can be prepared with the most current information.--Kevin Steffey