Soybean aphids have been found in the region since early June, fortunately at below threshold levels. As soybeans begin flowering, it is time to think in earnest about monitoring fields, particularly those at greatest risk. Fields at greatest risk have been smaller fields with wooded areas bordering them. Checking field margins, particularly near wooded sites, should provide an early indication of a whether a field has aphids present.

A field scouting method for making soybean aphid treatment decisions was introduced in 2004. The procedure is referred to as “Speed Scouting”. The method is designed to speed up decisions on whether to treat or not by using information learned about the aphid’s distribution throughout the field.

When using the widely accepted treatment threshold of 250 aphids per plant and 80% of the plants with these levels, the response from people who will do the field scouting is less than enthusiastic. No one wants to count all those aphids. Probably what will happen is that to streamline the process, people will do some counting and eventually “train” themselves to recognize what 250 soybean aphids per plant looks like.

This approach is used in some of our insect scouting programs already. In wheat, we recommend treatment when 85+% of the stems have at least one aphid present. In this case, it has been demonstrated mathematically that under those conditions, if you were to actually count aphids, you would find an average of 13 aphids per stem which is the treatment threshold. These sampling methods are referred to as binomial sequential sampling plans. The binomial refers to two outcomes or classifications. The sequential sampling aspect refers to the process of keeping track of what is and isn’t infested and then being able to conclude, often times quickly, that the number of plants needed to make a decision with high levels of reliability have been sampled.

The protocol for a binomial sequential sampling plan for soybean aphid was developed by researchers at the University of Minnesota. A discussion about the procedure can be found at:

A ‘binomial’ plan refers to two choices; sometimes it means a presence/absence count or, as in this case, a pre-set cut-off number where counting can be stopped. For this procedure, the binomial sampling cut-off point is 40 aphids per plant. If a plant has less than 40 aphids, consider it non-infested; however, if the plant has 40 or more aphids (remember, counting additional aphids is not necessary after 40), consider the plant infested.

The field scout keeps track of the plants and how they are classified. A worksheet really is a must in order to keep track. Eventually, enough plants (the minimum number of plants is 11) will have been checked to start making some decisions. The statistical models these decisions are based on will allow you to make one of three decisions with a high degree of reliability (the model has been set at 75% reliability). Those decisions are:

1. Do not treat that field,
2. Treat that field, and
3. Resample that field in 3 to 4 days

The binomial sampling plan can improve the cost (especially in time commitment) of sampling because every insect no longer needs to be counted. Though not perfect, the model has attempted to balance reliability with cost of scouting. When fields are close to threshold levels, more plants are typically required to make a decision. However, when fields are clearly not at threshold or easily over threshold, decisions are made quickly and reliably.

An copy of the worksheet and directions for the procedure are provided by the links below. In addition, there are sample worksheets to illustrate decisions with the three different outcomes.

The model still can use some field validation. Try it out and see if you aren’t making quicker decisions with which you are comfortable.

   Here's the worksheet

                               . . . and some examples of how to fill it out