Thursday, July 20, 2017

Soybean Aphid Speed Scouting: An option for people who don't like to count aphids

With the focus on soybean aphid scouting, I still like to promote the scouting procedure called “Speed Scouting”. The method speeds up making decisions on whether to treat or not based on the aphid’s population distribution in the field when the treatment threshold of 250 aphids per plant and 80% of the plants with these levels is reached. 

Who wants to count aphids on soybean plants? If you do, fine. Most people don’t like the idea. They want a faster process.
  
It is strongly recommended you look in the field to observe how pest populations are developing. Personally, if  “Speed Scouting” will get someone into the field who otherwise wouldn’t, there is a benefit. Observing pest population growth from its beginning, through its peak, and then the decline, is very educational. It also builds confidence in a person’s ability to make a correct treatment decision.

Speed Scouting is easy to learn. It is designed to be quick, so a person isn’t dedicating excessive time scouting fields. The method provides a statistical level of reliability, even for the beginner.  

The method is formally referred to as a binomial sequential sampling plan. The binomial refers to two possible outcomes (e.g., Yes or No; Infested or Not Infested) when you look at a randomly selected plant. The sequential sampling aspect refers to the process of keeping track of what plants are and aren’t infested (based on the model’s definition of “infested”) and being able to conclude, often times quickly, whether you need to treat or not. 

The protocol for a binomial sequential sampling plan for soybean aphid was developed by researchers at the University of Minnesota. You can go here for general information about soybean aphid scouting, which includes comments related to "speed scouting".

Here is a quick overview:
After collecting data from commercial soybean in southern and central Minnesota, entomologists at the University of Minnesota developed a binomial sampling plan, called Speed Scouting for Soybean Aphid. The recommendation is to use this sampling plan through the pod set stage (R4).

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 =   not-infested
  • 40 or more aphids =   infested
(remember, counting additional aphids is not necessary after 40)

The field scout keeps track of the plants and how they are classified. A worksheet (or, a smarthphone app developed by U of Nebraska) 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 upon 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 the field,
  2. Treat the field, or
  3. Resample the 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 (Your Time!). When fields are close to threshold levels, more plant samples 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.

A link to the worksheet and directions for the procedure are provided below. There is also a worksheet that can be printed and cut to make pocket-sized cards for field recording.

Modifications: 
It has been recommended that treat decisions be confirmed several days after scouting to ensure the population is actually increasing by re-scouting if there are doubts about your decisions. 


“Speed Scouting” pocket card 
A *PDF document for printing 8 pocket-sized cards for speed scouting can be downloaded from the newsletter webpage. The 2-sided card can be used to record other aphid related information that might be helpful. Print on card stock paper, cut, and easily carry a record of your aphid scouting results.


  
Reference
Hodgson, E. W., E. C. Burkness, W. D. Hutchison, and D. W. Ragsdale. 2004. Enumerative and binomial sequential sampling plans for soybean aphid (Homoptera: Aphididae) in soybean. J of Econ. Entomol. 97(6): 2127-2136.

Hodgson, E. W., McCornack, B. P., Koch, K. A., Ragsdale, D. W., Johnson, K. D., O’Neal, M. E., Cullen, E. M., Kraiss, H. J., DiFonzo, C. D., and Behnken, L. M. 2007. Field validation of Speed Scouting for soybean aphid. Online. Crop Management doi:10.1094/CM-2007-0511-01-RS.