Agronomic Crop Insects – September 13

Joanne Whalen, Extension IPM Specialist;

Continue to sample fields on a weekly basis for defoliators including earworm, webworms and all armyworm species. We received a few reports this past week of fields with economic levels of defoliation. Although we have limited experience at this time of year with damage to re-growth, be sure to check re-growth if worms were present at harvest to determine if larvae are still present and holding back the re-growth.

Corn Earworm
– Although corn earworm levels still remain low, late planted soybean fields that still have susceptible pods are still at risk from pod damage. There have been reports of newly hatched larvae in a few fields.

Defoliators – This past week the predominant defoliators we found were soybean looper, grasshoppers and newly hatched green cloverworm. If economic levels 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 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. 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 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 – When it comes to stinkbugs, you should continue scouting until the latest planted fields reach the R7 growth stage, when beans should no longer be susceptible to stink bug feeding. Once soybeans reach mid R-6 and R-7 (beginning seed maturity) , studies from the south say that scouting is still needed to avoid quality damage from stinkbugs which can include underdeveloped or aborted seeds, green stem syndrome, reductions in pod fill, seed vigor and viability, yield loss 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. 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”.

Soybean Aphid – We continue to find and here reports of economic levels. To review treatment decision making (information developed in the Midwest):

(a) The current economic threshold for aphids is still set at 250 aphids per plant through the R5 growth stage (3 mm long seed in the pod at one of the four uppermost nodes on the main stem) with an increasing population.

(b) You should check fields at least twice before making a treatment decision. If you find 250 per plant you need to re-check in 3-4 days to see if the population is increasing.

(c) When beans reach the R6 and later stages, the thresholds increase to 1,000 aphids per plant. This insect can be controlled by beneficial insects so be sure to watch for natural enemies including lady beetles, parasitized aphids and fungal pathogens that can help to crash populations.

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.