Agronomic Crop Insects – August 16, 2013

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

Alfalfa and Grass Hay Crops
Continue to watch for defoliators in grass hay crops and alfalfa. It is important to catch populations before significant damage has occurred and when larvae are small. In addition to checking labels for rates, be sure to check for all restrictions including but not limited to comments on control under high populations and size of larvae; days to harvest and forage/silage restrictions. Be sure to continue to scout alfalfa for leafhoppers. Once yellowing has occurred, significant in-season damage and long-term stand damage has already occurred.

Soybeans
We are starting to hear about economic levels of earworms in fields in states to our south. In general, economic populations are reported as spotty. They are also finding areas where earworms are causing economic levels of defoliation. In past years, we have also observed corn earworms feeding on foliage and blossoms as well as the pods. During the last week, we have seen an increase in corn earworm pheromone trap catches. So far, low levels of larvae have been found in full season and double crop fields in Kent and Sussex counties. Since population levels vary from field to field, the only way to know if you have an economic level will be to scout all fields. Although there is no threshold for corn earworm feeding on flowers or leaves, the data from North Carolina has indicated that feeding on flowers can result in reduced yields by delaying pod set. When looking at foliage feeding by corn earworm, you will need to use defoliation thresholds to make a treatment decision – the defoliation level during bloom to pod fill is 15% defoliation. 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.

Be sure 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 and a combination of species can be found in fields throughout the state. You will need to sample for both adults and nymphs when making a treatment decision. Available thresholds are based on beans that are in the pod development and fill stages. As a general guideline, current thresholds for stink bugs are set at 1 large nymph/adult (either brown or green stink bug) per row foot if using a beat sheet, or, 2.5 per 15 sweeps in narrow-row beans, or 3.5 per 15 sweeps in wide-row beans.

We are also seeing an increase in soybean aphid populations in fields throughout the state. The current cooler weather pattern will be favorable to population increase. As a reminder, there is both a conventional method and a speed scouting method that can be used to make a decision on management for soybean aphid. Please see the following link for more information on scouting and treatment thresholds. http://cropwatch.unl.edu/web/cropwatch/archive?articleID=5278671

During the past week, we have also observed an increase in soybean looper populations. It is important to identify soybean looper correctly because they can quickly defoliate fields and they can be difficult to control. Identification can be difficult because although there is a “black footed” phase of the soybean looper there is also a “green phase” that can be confused with cabbage looper – which is easier to control. One characteristic that might help is the presence of microspines on soybean loopers that are not present on cabbage loopers; however, you will need high magnification to see the microspines. Soybean loopers are a migratory pest, difficult to control and pyrethroid resistance has been documented in states to our south. Be sure to select a material that lists soybean looper control on the label. Belt, Besiege, Blackhawk and Steward all list soybean looper on the label. In most cases, higher labeled rates will be needed so be sure to read all labels for rates and restrictions.

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