Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa
Continue 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. Virginia continues to report large movements of corn earworm moths throughout much of eastern Virginia.
Economic levels of corn earworm 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. If fields were already sprayed, be sure to watch for newly hatched larvae. With the sustained flights, there could be a new hatch of small larvae. As a reminder, if small larvae are present after you have sprayed the first time, this does not mean that the product applied failed – it only means that there is a new hatch that needs to be treated a second time if economic levels are present.
As far as defoliators, grasshoppers and bean leaf beetles continue 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.
Since last week’s report, some consultants have started to find a significant increase in stinkbugs, especially in full season fields. 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 large 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 stinkbugs), as those are the stages most capable of damaging pods. As a general guideline, current thresholds are set at 2.5 per 15 sweeps in narrow-row beans, or 3.5 per 15 sweeps in wide-row beans.
There have also been a number of questions over the last week about increases in whiteflies populations. Although we have limited experience with whiteflies in our area, as far as we know, whiteflies have not been a problem in the past even when we have seen what folks describe as “clouds of whiteflies”. They are related to aphids (that is in the same order of insects) and so can cause yellowing on the leaves if populations are high enough. Damage is most likely to occur when beans are stressed. The following link provides pictures of whiteflies and some additional comments regarding whiteflies in soybeans. (http://bulletin.ipm.illinois.edu/article.php?id=832 )
For more information on what is occurring in Virginia, you will want to look at the Virginia Ag Pest Advisory (http://www.sripmc.org/Virginia/).