Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa
Be sure to watch for fall armyworm, beet armyworm, webworms and corn earworm which can quickly defoliate alfalfa. Mixed populations of larvae can be found in fields and controls should be applied before significant defoliation occurs. Defoliators can be destructive in last cuttings, especially during drought conditions. When defoliators are present, early harvest may eliminate the problem. Although there are no specific thresholds, as a general guideline if the crop is more than 2 weeks from cutting and 25 to 30% of the terminals are damaged, treatment is suggested.
Soybeans
As the potential for late season insect control increases, be sure to check all labels for the rates, days from last application to harvest as well as other restrictions.
Corn Earworm Alert
The potential for corn earworm pressure in soybeans is high statewide. Trap catches remain high throughout the state, moths can be found laying eggs in double crop fields statewide and larvae are being found. With the continued high trap catches throughout the state, be sure to check all fields for earworms. Although open canopy blooming fields will be the most attractive to egg laying moths you should check all fields to be sure you do not miss an infestation. As a general guideline, a treatment should be considered if you find 3 podworms per 25 sweeps in narrow row fields and 5 podworms per 25 sweeps in wide row fields (20 inches or greater). However, these static thresholds were calculated for a 10-year average soybean bushel value of $6.28. The best approach 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. As reported in previous newsletters, states to our south, including Virginia, have reported control failures with pyrethroids in soybeans in 2007 and 2008. As of the 2008 season, poor control with pyrethroids in our area has been the result of treating too late, treating large worms or using too low of a rate. If a pyrethroid is used for earworm control, you need to be using the highest labeled rate. In addition to the pyrethoids, Steward, Lorsban or Larvin should also be considered, especially if armyworms are in the mix. In some fields, fall armyworm and beet armyworm can also be found.
You will also need to continue to scout for soybean aphids. With the recent cooler night temperatures, we are seeing an increase in populations in all three counties, especially in later planted fields and economic levels have been found in fields in Kent County. As a general guideline, treatment is needed through the R5 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 R6 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 R6 but in some years and some situations there has been an economic return. Spraying after R6 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. This number should provide a 5 to 7-day lead time for treatment to avoid economic loss.
Although populations have been generally lower this season, we are also starting to see an increase in populations of stinkbugs. 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 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, as those are the stages most capable of damaging pods. As a general guideline, current thresholds 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.