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 25–30% of the terminals are damaged, treatment is suggested.
Soybeans
Be sure to continue to scout carefully for earworms during the next few weeks. Local trap catches continue to have high moth activity and we are getting reports of an increase in the number of locations throughout the state with economic numbers.
Economic levels and hot spots of high levels of corn earworm (CEW) continue to be found in soybean fields in Kent and Sussex counties but they are not present in every field. In general, most larvae are still small but we are hearing reports of a few more medium size larvae in the mix. Although they have been mainly found in dry land and irrigated double crop fields, they have also been found in a few full season fields. The only way to know if you have an economic level will be to scout all fields. The best way to calculate 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 CEW populations. Although extremely small larvae may be susceptible to the rains, we have not seen any indication of populations being reduced by the rains. It is too early to decide if weather will play a role in moderating populations.
As far as defoliators, grasshoppers and green cloverworm populations are starting to cause economic levels of defoliation in double crop and a few full season fields. Remember, that in addition to defoliation grasshoppers can feed on and/or scar pods. Beet armyworm and yellow striped armyworm can also be found, but so far generally at low to moderate levels. With all defoliators, you will need to estimate defoliation to make a treatment decision. 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. With the increase in diversity of caterpillars being found in fields, the following links have pictures that can help with identification:
http://www.ent.iastate.edu/imagegal/lepidoptera/
http://www.utextension.utk.edu/fieldcrops/cotton/cotton_insects/images/BAW-larva-close.jpg
http://www.ca.uky.edu/agcollege/plantpathology/extension/KPN%20Site%20Files/pdf/KPN1242.pdf
Soybean aphid populations remain very low in our area; however, spraying for this insect has occurred in the Midwest. 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 up until the R-5 stage 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 of native brown and green stink bugs have been moderate in general, we continue to find fields with economic populations throughout the state (2.5 per 15 sweeps). The highest levels of brown marmorated stink bug (BMSB) are still being found in soybean fields in New Castle County but they are also present in fields in Kent County. In many, but not all cases, the highest levels of BMSB can be found on field edges and when practical an edge treatment might be considered. You will still need to scout fields on a weekly basis after an edge treatment to be sure that economic levels have not moved into the field interiors.