Agronomic Crop Insects – August 20, 2010

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

Alfalfa
Be sure to watch for fall armyworm, beet armyworm, webworms and corn earworm, which can quickly defoliate alfalfa. Mixed populations of larvae can be found in fields and controls should be applied before significant defoliation occurs. Also, larvae must be small to achieve effective control.

Defoliators can be destructive in last cuttings, especially during drought conditions. When defoliators are present, early harvest may eliminate the problem. Although there are no specific thresholds, as a general guideline if the crop is more than 2 weeks from cutting and 25 to 30 percent of the terminals are damaged, treatment is suggested.

Soybean
Be sure to continue to scout carefully for earworms during the next few weeks. Local trap catches as well as traps to our south continue to have high moth activity.

Economic levels and hot spots of high levels continue to be found in fields throughout the state but they are not present in every field. In addition they are being found in both full season and double crop fields so the only way to know if you have an economic level will be to scout. 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). However, 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. With the recent rains, I have been asked if it will help to reduce/and or crash populations. Although extremely small larvae may be susceptible to the rains, we have not seen any indication of disease in worms so it is too early to decide if weather will play a role in moderating populations.

As far as defoliators, grasshoppers and bean leaf beetles are starting to cause economic levels of defoliation in some full season fields so be sure to watch for these two insects as well as corn earworm. Remember, that in addition to defoliation both can feed on and/or scar pods. There are also beet armyworms present in some fields. It also appears that in some cases they may be confused with yellow striped armyworms. The following links have pictures of both larvae :

Yellow Striped Armyworm
http://www.ent.iastate.edu/imagegal/lepidoptera/armyworm/nystripedarmyworm.html

Beet Armyworm
http://www.utextension.utk.edu/fieldcrops/cotton/cotton_insects/images/BAW-larva-close.jpg

Kentucky Pest News – Aug 10 newsletter – has good pictures of defoliators in soybeans –http://www.ca.uky.edu/agcollege/plantpathology/extension/KPN%20Site%20Files/kpn_10/pn_100810.html

Although population have been lower this season in our area as well as in a number of Midwestern states, you should continue to scout for soybeans aphids, especially in later planted fields. This aphid can increase if the temperature turns cooler. Remember the threshold is 250 aphids per plant with the populations rising up until the R-5/ and in some cases R-6 stage of plant development. You should also watch for beneficial insect activity that can help control populations.

Lastly, although populations have been moderate this season, there are reports of an increase in populations of green stinkbugs. You will need to continue 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. 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. We are currently following the same guidelines that are being used in Virginia. Thresholds are based on numbers of large nymphs and adults (green and/or brown stink bugs), as those are the stages most capable of damaging pods. As a general guideline, current thresholds 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.

For more information on what is occurring in Virginia, you will want to look at the Virginina Ag Pest Advisory (http://www.sripmc.org/Virginia/).

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