Agronomic Crop Insects – August 19, 2016

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

Soybeans
We continue to find a variety of defoliating insects in soybeans. Over the past week, we saw a significant increase in defoliation from green cloverworm, cabbage looper and soybean looper in both full season and double crop fields. As a reminder, soybean loopers are not effectively controlled by the pyrethroids so materials labeled for soybean loopers including Besiege, Blackhawk, Radiant or Steward will be needed. The highest labeled rate is generally needed for soybean looper control.

Once again we are finding a few fields with whiteflies; however, populations are not as high as in past years. Problems generally occur in drought stressed fields so be sure to watch for increases in populations. Whiteflies 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/

Continue to watch for stink bugs in all fields during the pod development and pod fill stages. We continue to see an increase in populations, especially green stink bugs. You will need to sample for both adults and nymphs when making a treatment decision. As a general guideline, we are using a new threshold in the Mid-Atlantic Region — 5 stink bugs per 15 sweeps. This is the threshold for soybeans produced for grain. If you are producing soybeans for seed, the threshold is still 2.5 per 15 sweeps.

Corn earworm populations still remain low in most fields throughout the state. However, we are seeing a significant increase in moth catches in some of our pheromone traps and black light trap catches in states to our south are seeing a sharp increase. There are also reports of heavy populations in soybeans in some areas of VA. 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 have just started to find a few soybean aphids in fields throughout the state. Remember, this aphid is more of a problem when the weather remains cooler. The economic threshold for soybean aphid established in the Midwest is 250 aphids per plant. Populations should be increasing and most of the plants should be infested (>80 percent) in order to justify an application. This threshold is appropriate until plants reach mid-seed set (R5.5). Spraying at full seed set (R6) has not produced a consistent yield response in the Midwest.

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