Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Grass Hay Crops
We have received reports from consultants in Maryland and Delaware regarding insect damage to grass hay crops that are close to cutting. In both cases, the insect causing the damage was fall armyworm. Although there are no thresholds for this insect in grass hay crops, fields should be watched closely after cutting for damage to the regrowth. Baythroid XL, Mustang MAX, and Warrior are all labeled for armyworm control on grass hay crops. Insects must be small at the time of treatment to achieve control.
Soybeans
As the potential for late season insect control increases, be sure to check all labels for the days from last application to harvest as well as other restrictions
In areas of the state with high bean leaf beetle counts, be sure to watch for both defoliation and pod feeding. Be sure to check the following link from the Midwest for the most recent decision making information for this insect pest (http://www.extension.iastate.edu/CropNews/2008/Issues/20080728.htm).
We just received approval of our Section 18 request for the use of Oberon 2SC (spiromesifin) for two spotted spider mite control on soybeans. You must have a copy of the label in your possession before applying this material. You can get a copy from Dave Pyne at the Delaware Department of Agriculture. A few details of the label are as follows: (a) One application per season by air or ground at a rate of 8-12 oz per acre; (b) Minimum volume of 10 gallons per acre for ground application and 5 gallons per acre for aerial application; (c) 21 day pre-harvest interval (PHI); (d) Do not apply when bees are actively foraging in and around the treatment area; (e) Section 18 expires on September 15, 2008
Continue to scout for soybean aphids. Since this is more of a cool season aphid, we could see an increase with the recent temperatures. As a general guideline, treatment is needed through the R-5 stage (seed is 1/8 inch long in the pod of one of the four uppermost nodes on the main stem) of soybean development if economic levels are present. It may also be beneficial to spray through R-6 stage (pods containing a green seed that fills the pod cavity at one of the four uppermost nodes on the main stem) — reports vary as to the benefit of spraying once plants reach the R-6 stage but in some years and some situations there has been an economic return. Spraying after R-6 stage has not been documented to increase yield in the Midwest. The suggested treatment threshold from the Midwest is still 250 aphids per plant with 80% of the plants infested with aphids. You can also consider using speed scouting to make a treatment decision. Information on how to use speed scouting can be founds at:
http://www.nwroc.umn.edu/Cropping_issues/2007/Issue9/07_17_07_no4.htm
or
http://breeze.ag.vt.edu/speedscouting
We continue to find sporadic and low levels of corn earworms in fields in Kent and Sussex counties. As corn dries down, moths emerging from larvae found in corn fields will lay eggs in soybeans. Remember, corn earworms will feed on the foliage and the pods. The only way to know if you have an economic level will be to scout. Therefore, be sure to scout all fields for podworms. Although states to our south reported control failures with pyrethroids in soybeans in 2007, we did not see this in Delaware in 2007. In many cases, poor control in our area was the result of treating too late, treating large worms or using too low of a rate. If using a pyrethroid, you should be using the mid to high range rate. In addition to the pyrethoids, Steward or Lorsban should also be considered, especially if armyworms are in the mix. The pyrethroids will not provide effective beet armyworm control. In the past, we have used the treatment threshold of 3 corn earworms per 25 sweeps in narrow fields and 5 corn earworms per 25 sweeps in wide row fields (20 inches or greater). However, these are static thresholds that were calculated for a 10-year average soybean bushel value of $6.28. With higher soybean prices, the best approach to determining a threshold is to access the Corn Earworm Calculator (http://www.ipm.vt.edu/cew/) which estimates a threshold based on the actual treatment cost and bushel value you enter.