Agronomic Crop Insects – September 9, 2016

Joanne Whalen, Extension IPM Specialist;

Alfalfa and Grass Hay
Continue to watch for defoliators in grass hay crops and alfalfa including earworm, webworms and all armyworm species. If a field was cut recently and has started to regrow be sure to also watch for damage to the re-growth. In addition to checking labels for rates and pre-harvest interval (time needed between last application and harvest), be sure to check for all restrictions including, but not limited to, comments on control under high populations and size of larvae; days to harvest and forage/silage restrictions. No thresholds are available; however, controls should be applied before significant defoliation occurs in a field.

Corn Earworm – We have received reports of an increase in corn earworm larval levels in a few areas this past week, with some fields approaching and/or at threshold levels. This is mainly occurring in late planted fields and in fields planted behind vegetables. With the sustained trap catches, it will be important to continue to watch these fields since they still have susceptible pods that are at risk from pod damage.

Defoliators – If economic levels are present, you will 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 back 2010 by southern entomologists and agronomists, they provided the following guidance regarding late season defoliation:

“If economic levels of defoliation 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 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. You will also 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 you need to estimate defoliation. Be sure to look at the entire canopy from top to bottom not just the more affected top leaves to come up with an overall average. “

Stinkbugs – You should continue scouting until the latest planted fields reach the mid R-6 stage, when beans should no longer be susceptible to direct loss from stink bug feeding. Once soybeans reach mid R-6 to R-7 (beginning seed maturity) , studies from the south say that scouting may still be needed to avoid quality damage from stinkbugs which can include underdeveloped or aborted seeds, green stem syndrome, reductions in seed vigor and viability, and a reduction in the storage stability of harvested seeds

Pod Scarring – You also need to consider the potential for grasshoppers and bean leaf beetles to feed on pods. During the last wet fall, we did see significant pod scarring from bean leaf beetles late in the season that resulted in moldy beans. The following link from Purdue provides information on decision making regarding the number of bean leaf beetles per sweep, percent pod injury and when you need to consider a treatment.

Reminder — If you do need to treat for any of the above insects, 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.