Agronomic Crop Insects – August 10, 2012

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

Alfalfa and Grass Hay Crops
As indicated in last week’s newsletter, you need to continue to watch for defoliators in grass hay crops and alfalfa. Significant damage has occurred in a number of grass hay fields from true armyworm and fall armyworm. It is important to catch populations before significant damage has occurred and when larvae are small. In addition to checking labels for rates, be sure to check for all restrictions including but limited to comments on control under high populations and size of larvae; days to harvest and forage/silage restrictions.

Soybeans
Economic levels of corn earworms in Kent and Sussex counties in Delaware and on the lower eastern shore of Maryland. Trap catches continue to increase throughout the state and moths can be found laying eggs in fields. It appears that we could see an extended moth flight and egg laying period in soybeans. Since population levels vary from field to field, the only way to know if you have an economic level will be to scout all fields. Remember, corn earworms can feed on the foliage and blossoms as well as the pods. Although there is no threshold for corn earworm feeding on flowers or leaves, data from North Carolina has indicated that feeding on flowers can result in reduced yields by delaying pod set. When looking at foliage feeding by corn earworm, you will need to use the defoliation threshold as well as the presence of worms to make a decision (again – there is no worm threshold available for leaf and/or blossom feeding). Once pods are present, the best approach to making a decision on what threshold to use for corn earworm is to access the Corn Earworm Calculator developed at Virginia Tech (http://www.ipm.vt.edu/cew/) which estimates a threshold based on the actual treatment cost and bushel value you enter.

Be sure to scout for stinkbugs in fields that are in the pod development and pod fill stages. Economic damage is most likely to occur during these stages and a combination of species can be found in fields throughout the state. You will need to sample for both adults and nymphs when making a treatment decision. Available thresholds are based on beans that are in the pod development and fill stages. As a general guideline, current thresholds for native stink bugs are set at 1 large nymph/adult (either brown or green stink bug) per row foot if using a beat sheet, or, 2.5 per 15 sweeps in narrow-row beans, or 3.5 per 15 sweeps in wide-row beans. We do not have a threshold for brown marmorated stink bugs (BMSB).

We continue to find beet armyworm (BAW) in fields in Kent and Sussex counties. Since this insect is primarily a defoliator, you should use the defoliation thresholds to make a treatment decision. As a reminder, the pyrethroids will not provide effective control so a beet armyworm product labeled for soybeans will be needed if the defoliation threshold is reached.

We continue to sample fields statewide for both Brown Marmorated Stink Bugs (BMSB) and Kudzu Bug, projects funded by the Delaware Soybean Board. We are finding very low levels of BMSB and so far no Kudzu Bugs have been detected. In speaking with my colleague Ames Herbert from Virginia Tech, he indicated that they have found Kudzu Bug in 17 counties but they are still only finding adults. These adults are very mobile so we will need to remain vigilant. Thresholds developed in the south are based on nymphal counts. Please view the following link for pictures of adults and nymphs.

http://www.nccrops.com/2011/06/24/kudzu-bug-confirmed-in-34-north-carolina-counties/

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