I recently had a conversation with a young farmer who had been scouting his fields for soybean aphids. Threshold levels had not been reached but his neighbors on two sides had all sprayed for aphids. He was concerned and wondering if he should also spray. After looking at the calendar, I asked him, “What growth stage are your beans?” His response was simple: “Growth stage? Why should I worry about growth stage?”
It’s early September, pods are developing and seeds are filling. The current recommended threshold of 250 aphids per plant on 80% of the aphid population only applies up to growth stage 5.5. A more mature soybean field is likely not to be economically affected by aphids. This brings up a common problem faced by growers this time of year: location of scouting. The soybean plant has this wonderful ability to produce both vegetative and reproductive growth, simultaneously, throughout much of its life (Figure 1). As you monitor the growth stages, the focus is in the upper four nodes from R4 though R6. However, one must remember the bulk of your yield is in the lower nodes. Sacrificing beans in the bottom nodes to make pods in the upper nodes is a money-losing proposition.
Figure 1. Development and timing of vegetative growth, flowering, pod development and seed filling. Adapted from: https://crops.extension.iastate.edu/files/article/SoybeanGrowthandDeveloment 0.pdf
Growth Stage Condition
R4 at least one ¾ inch long pod found in upper four nodes
R5 at least one 1/8 long seed found in a pod in upper four nodes
R6 at least one filled pod found in upper four nodes
R7 One mature pod found anywhere on plant
Time spent at each growth stage varies with environmental condition. To be effective at aphid scouting, growth stage must be evaluated while monitoring/counting aphids. The decision process is confounded with the scouting focused at the top of the crop, while yield is immensely affected by the size of the beans at the bottom of the plant. The decision is also confounded by the movement of aphids within the plant. As the plant growth rate decreases, and the vegetation in the upper four nodes becomes less desirable, the aphids tend to move down the plant and attack the filled pods in the bottom of the plant.
Current Minnesota Extension recommendations are to scout weekly through growth stage R5, use the economic threshold of an average of 250 aphids per plant AND more than 80% of plants having aphids AND aphid populations increasing. While an economic threshold has not been developed for R6, infestations in early R6 may require treatment if aphid populations are very large and plants are experiencing other stresses. Aphid insecticide applications are not recommended at, or after, growth stage R7.
What does all this mean? Well, in my book, it means concentrate on making beans. This means knowing where your crop is physiologically. Once you do that, open the calendar, polish off the crystal ball and ask yourself; “Is there enough time between now and harvest to make more beans?” Most of the bean fields I have examined in August would require the insecticide to be flown on, making this an even more expensive proposition. At $9.50/bushel (CBOT), you won’t have a lot of margin left in that bean. In other words, choose wisely.
At the end of the day, you gotta know your nodes!
David Kee is director of research with the Minnesota Soybean Research & Promotion Council. Email him at david@agmgmtsolutions.com.
