Agronomic Crop Insects – August 24, 2012

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

Soybeans
Be sure to continue to scout carefully for earworms as well as defoliators during the next few weeks. As we get closer to harvest be sure to check all labels for the days between last application and harvest.

Threshold levels of corn earworm continue to be found in fields in Kent and Sussex counties but they are not present in every field. Although we can find various sizes of larvae, in most cases they are still relatively small. In addition they are being found in some full season (especially where canopy is not closed) 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, we are starting to see a new hatch of small larvae.

Since last week’s report, we also are starting to find an increase in stinkbugs, especially in full season fields. The population levels as well as species vary from field to field depending on your location in the state. In Sussex County, the predominant species are native green and brown stink bugs. From the Milford and Harrington areas in Kent County through New Castle County, the brown marmorated stink bug (BMSB) can be found in the mix. We are just starting to find a few hot spots of BMSB with the highest populations generally still along woods edges. 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 (native 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 2.5 per 15 sweeps in narrow-row beans, or 3.5 per 15 sweeps in wide-row beans.

Once again we are finding a few fields with high levels of whiteflies. Although we have limited experience with whiteflies in our area, as far as we know, whiteflies 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. Damage is most likely to occur when beans are stressed. 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 significant number of defoliators including beet armyworm (BAW), fall armyworm, yellow striped armyworm, green cloverworm, soybean loopers and grasshopper in double crop and a few full season fields. All of these insects are defoliators and you will need to use percent defoliation to make a treatment decision. There are no available thresholds for the number of the above insects per sweep. Remember, that in addition to defoliation, grasshoppers can feed on and/or scar pods. 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. As a reminder, the pyrethroids have not provided effective control of beet armyworm or soybean loopers so a product labeled for these 2 species in soybeans will be needed if defoliation is present.

Since many of our pests in soybeans migrate to us from the south, the following two links provide information on what is occurring in Virginia and North Carolina:
http://www.sripmc.org/Virginia/
http://www.nccrops.com/.

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