Joanne Whalen, Extension IPM Specialist;jwhalen@udel.edu
Soybeans
Once again we are finding a few fields with whiteflies; however, populations are not as high as past years when soybeans were under heat and drought stress. Although we have limited experience with whiteflies in our area, they have generally not been a problem in the past, especially if moisture is adequate. They are related to aphids (that is they are in the same order of insects) and so can cause yellowing on the leaves if populations are high enough. The following links provides pictures of whiteflies and some additional comments regarding whiteflies in soybeans.
http://bulletin.ipm.illinois.edu/article.php?id=832
http://ipcm.wisc.edu/blog/2012/07/questions-about-whiteflies-in-soybean/
We continue to find a variety of defoliating caterpillars as well as grasshoppers in full season and double crop fields but in general populations remain low. In some fields, fungal pathogens have helped to crash caterpillar populations. Continue to use defoliation thresholds to make treatment decisions for these insects. Remember, that in addition to defoliation, grasshoppers can feed on and/or scar pods.
Corn earworm populations still remain low and spotty in fields throughout the state. Since population levels will vary from field to field, the only way to know if you have an economic level will be to scout all fields. 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.
We found our first adult Kudzu Bugs just yesterday in a kudzu patch in Sussex County. We have not found any in soybeans so far this year. The treatment threshold is 1 nymph per sweep (one sweep = one “swoosh” of the net) during and after July.
Field Corn
During the past week, we received a number of calls about aphids in field corn. In many cases, populations are spotty within fields or are only being found on field edges. Currently, there are no treatment thresholds for aphids in corn past tasseling. In many cases, fields are beyond the point of considering a treatment due to the maturity of the crop and the presence of beneficial insects and/or parasitized and diseased aphids.
Although we have no thresholds for aphids in corn in our area, here are some considerations developed by entomologist in the Midwest that can help to make a treatment decision:
1. Are 80 percent of the plants infested with aphids?
2. Do most of the ears have aphids? What about the ear leaf and above?
3. How long has the field been infested and is the density increasing?
4. Do you see honeydew or sooty mold on the stalk, leaves or ear?
5. Are you seeing winged aphids or nymphs with wing pads? That may be a sign of migration out of the field.
6. Is the field under drought stress?
7. Do you see any bloated, off-color aphids? Natural fungi can quickly wipe out aphids. In addition are beneficial insects/parasitized aphids present.
8. What is the corn growth stage? Fields reaching hard dent should be past the point of justifying a treatment.
9. Some insecticides have a long pre-harvest interval so be sure to check the label.