April's Ag Advice by April Borders We are now entering the early reproductive growth stages (R1-2) for some of our soybeans. This is the time that soybeans are the most sensitive to defoliation. Scouting now becomes a very important tool to ensure that our crops will be safe and protected.
If you haven't seen them already, we will soon be seeing the first generation bean leaf beetle occurring in our soybean fields and we can expect feeding over the next month. Bean leaf beetles are most important during the "green bean" reproductive stage of soybeans. At this stage, bean leaf beetles may continue feeding on the leaves, but they may start clipping the developing pods from the stem or they may be directly feeding on the seed pods. Pod feeding may eventually result in discolored, wrinkled, or moldy seeds during harvest.
Defoliation levels during these stages must be monitored by the grower. If grasshoppers are also present in the field, monitoring is even more important as defoliation levels could raise to serious levels from pressure by both insects.
We advise growers to initiate scouting procedures to prevent defoliation from reaching the 15 to 20 percent defoliation threshold during the early reproductive growth stages, R1-R5, later defoliation thresholds can raise to 20 to 25 percent during growth stage R6.
There is special interest in scouting for bean leaf beetle populations. Evidence suggests that the population size of the second generation of bean leaf beetles that cause pod injury in late August and early September is relative to the size of this upcoming first generation. Monitoring this first generation can give us insight as to the potential size of the second generation and possible pod injury.
If the first generation is large, evident either through sampling large numbers of bean leaf beetle or through the presence of significant leaf feeding, it will be imperative that the second generation and subsequent pod injury be watched more closely later this summer. If the first generation is relatively low or non-existent, the probability of later problems is lower. Although we still recommend scouting all fields the entire summer.
Remember when out scouting your fields be on the look out for the soybean aphid. The aphid has been found in several counties already this year.
Make sure that you are looking at the newest growing point as this is where the aphids will most likely be found. Although aphids will usually be found on the top, young leaves of the plant, the entire plant should be checked. If and when aphids appear, use the economic injury level (EIL) calculations to determine threshold levels.
There is also an EIL calculation available for threshold levels for the bean leaf beetle. For more information on the bean leaf beetle or the soybean aphid or their threshold calculations, call the Clay County Extension Office at 677-7111.