Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Soybeans
As full season and double crop beans mature, I continue to get questions regarding the need to treat fields for corn earworm, defoliators and stinkbugs. As my colleague Ames Herbert from Virginia Tech indicated in his Pest Advisory last week, if economic populations levels of corn earworm are present, late planted soybean fields that still have susceptible pods (earlier than R7 growth stage) would still be at risk from pod damage. If economic levels of defoliators (i.e. worm defoliators including soybean looper, beet armyworm and green cloverworm) are present, you will also need to consider the maturity of the crop as well as the health of the leaf canopy to make a treatment decision. In an article printed in 2010 regarding defoliation from soybean loopers, entomologists and agronomists in the south suggested that if economic levels are present, “fields will need to be protected as long as the pods are still green and until the lower leaves are just beginning to yellow. This should correspond, more or less, with the R6.5 stage (10 days after R6.0 = full green seed). If leaves are beginning to yellow up the stem, not from drought but from the maturity process, and there are any pods on the plant that are beginning to yellow, the field should be safe, that is no need to treat. Next you have to determine the health of the leaf canopy: is it robust, average, or thin. Each can tolerate different amounts of leaf loss before reducing yield potential. Robust fields (mid chest or higher) can tolerate a lot of feeding. Average fields (upper thigh to mid chest) can tolerate normal amounts of feeding. Thin canopy fields (mid thigh or below) cannot tolerate additional leaf loss. Also in this canopy assessment, you need to take a stab at estimating the current percent defoliation. This is not an exact measure, but your best estimate looking over the entire canopy top to bottom. The eyes tend to focus on those badly defoliated top leaves. Look beyond those and try to come up with an overall average.” When it comes to stinkbugs, you should continue scouting until the latest planted fields reach the R7 growth stage (a few studies in the south even say through the R-7 stage) when beans should no longer be susceptible to stink bug feeding.
In past newsletters, I have also written about the potential for grasshoppers and bean leaf beetles to feed on pods. Although bean leaf beetle populations have not been as high as past seasons, there are still some hot spots of activity so you will need to examine pods for feeding damage. During the last wet fall, we did see significant pod scarring late in the season that resulted in moldy beans. The follow link from Ohio State University provides good information on when treatment is needed for pod-feeding. In general, they state that a “treatment is usually indicated when pod feeding reaches 10-15% and beetles are still present and actively feeding. In fields where the pods have started turning yellow and brown, the adults will be leaving in search of greener pastures”. (http://corn.osu.edu/c.o.r.n.-newsletter#1).
If you do need to treat, be sure to check the label for the pre-harvest interval (time needed between last application and harvest) as well as other restrictions, including rotational restrictions.