Agronomic Crop Insects – August 3, 2012

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

Alfalfa and Grass Hay Crops
We are starting to hear reports of the first occurrence of defoliators in hay crops. Fall armyworm can cause significant damage in grass hay so be sure to watch carefully since early detection is important to achieve effective control with labeled products. In alfalfa, a number of defoliators can cause problems including corn earworm, fall armyworm, beet armyworm and webworms. No thresholds are available; however, controls should be applied before significant defoliation occurs.

Soybeans
We continue to find fields with economic levels of spider mites. In areas that have received rain, it generally has helped beans to grow; however, if economic populations were present before the rain they can still be found. As a reminder, under heavy mite pressure and extended hot, dry weather, it often takes an extended period of free moisture on leaves, high humidity during the day and cool evening temperatures to get an increase in the fungal pathogens that can significantly reduce exploded mite populations. In many years, populations can start to decline by mid-August and we have started to find a few diseased mites. However, we are not seeing a significant decline in populations so it is important to continue to scout and apply controls if economic populations are present.

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).

Corn earworm moth trap catches continue to increase and larvae can be found in fields in Kent and Sussex counties. Be sure to scout both full season and double crop soybeans for this insect pest. Only time will tell if we will have a corn earworm outbreak in soybeans in our area. As we have seen in past years, trap catches can give an indication of the potential for a problem; however, only scouting on a routine basis will tell you if you have an economic problem. In the past, we have used the treatment threshold of 3 corn earworms per 25 sweeps in narrow fields and 5 corn earworms per 25 sweeps in wide row fields (20 inches or greater). These are static thresholds that were calculated for a 10-year average soybean bushel value of $6.28. A better approach to determining a threshold is to access the Corn Earworm Calculator (http://www.ipm.vt.edu/cew/) which estimates a threshold based on the actual treatment cost and bushel value you enter.

Consultants have also found a significant increase in beet armyworm (BAW) populations and defoliation. 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 need to be applied.

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