Agronomic Crop Insects – September 14, 2012

Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu

Alfalfa
Continue to sample fields on a weekly basis for defoliators including earworm, webworms and all armyworm species. We continue to get reports of fields with economic levels of defoliators. Although we have limited experience at this time of year with damage to re-growth, it will be important to check for the presence of larvae to determine if they are still present and holding back the re-growth.

Soybeans
Where economic populations levels of corn earworm are still present, late planted soybean fields that still have susceptible pods will 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 related to 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.

You will still need to consider the potential for grasshoppers and bean leaf beetles to feed on pods. Although bean leaf beetle populations have been generally low this past season, 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. Information from Ohio indicates 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”.

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.

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