Agronomic Crop Insects – August 17, 2012

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

Alfalfa and Grass Hay Crops
Continue to watch for defoliators in grass hay crops and alfalfa. 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 not limited to comments on control under high populations and size of larvae; days to harvest and forage/silage restrictions. Be sure to also scout alfalfa for leafhoppers. Once yellowing has occurred, significant in season damage and long term stand damage has already occurred

Soybeans
Economic levels and hot spots of high levels of corn earworm larvae continue to be found full season and double crop fields in Kent and Sussex counties but they are not present in every field. In general, most larvae are small to medium in size, and occasional large larvae can be found. 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 defoliation 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. Remember, this threshold calculator was developed only for corn earworm.

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 a significant number of defoliators including beet armyworm (BAW), fall armyworm, yellow striped armyworm, green cloverworm and grasshopper in double crop and a few full season fields. All of these insects are defoliators and you will need to use percent defoliation to make a treatment decision. There are no available thresholds for the number of the above insects per sweep. Remember, that in addition to defoliation, grasshoppers can feed on and/ or scar pods. In full season soybeans in the pod fill stage, the threshold is 10-15% defoliation. Remember, double crop soybeans cannot tolerate as much defoliation since they often do not reach the leaf area index needed for maximum yields. As a reminder, the pyrethroids will not provide effective control of beet armyworm so a product labeled for beet armyworm in soybeans will be needed if defoliation is present.

Soybean looper populations are also starting to increase. Identification can be difficult because although there is a “black footed” phase of the soybean looper there is also a “green phase” that can be confused with cabbage loopers. One characteristic that might help is the presence of microspines on soybean loopers that are not present on cabbage loopers; however, you will need high magnification to see the microspines. Soybean loopers are a migratory pest, difficult to control and pyrethroid resistance has been documented in states to our south.

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