Agronomic Crop Insects – August 14, 2009

Joanne Whalen, Extension IPM Specialist;

Alfalfa, Grain Sorghum, Late Planted Field Corn and Grass Hay Crops
Over the past week, we have received reports from consultants in Maryland and Delaware regarding fall armyworm damage to all of these crops. In corn and grain sorghum, control will be difficult since armyworms feed deep in the whorls of plants. For the most effective control, materials must be directed into the whorls and at least 25 gallons of water used per acre to get a reduction in populations. Also, in many cases one application is often not enough to get satisfactory control – especially if larvae are large and feeding deep in the whorls. For alfalfa, field corn and grain sorghum, a number of pyrethroids, as well as Lorsban and Lannate are labeled for armyworm control. Labeled pyrethroids often indicate that larvae should be small (first and second instar only) and/or the highest labeled rate is needed, especially if larvae are larger.

In grass hay crops, fields should be watched closely after cutting for armyworm damage to the regrowth. Baythroid XL, Mustang MAX, and Warrior II are all labeled for armyworm control on grass hay crops. Insects must be small at the time of treatment to achieve control.

Before treatment, be sure to check all labels for the rate; comments on control under high populations and size of larvae; days to harvest, forage/silage restrictions, as well as other use restrictions.

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 double crop soybeans, be sure to watch for an increase in defoliators, especially green cloverworm. Remember, double crop soybeans can not tolerate as much defoliation as full season soybeans. Unfortunately, we do not have a threshold for the number of green cloverworms per sweep. A treatment maybe needed if defoliation has increased from one scouting visit to the next, especially if other defoliators are present at the time of treatment. Diseases can help to crash populations; however, we have only found a few diseased worms so far this week.

Continue to scout all soybeans (full season and double crop) for soybean aphids. We continue to find aphids in fields throughout the state and in some locations populations are increasing. We have heard of hot spots on the eastern shore of Maryland as well. When scouting for aphids, you need to look at the entire plant – not just the stems. Often times you will the first aphids on the newest emerging trifoliate that is not fully expanded. 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 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. Although we are not seeing high levels of beneficial insect activity, we are starting to see an increase in lady beetle number as well as parasitized aphids. You can also consider using speed scouting to make a treatment decision. Information on how to use speed scouting can be founds at:

We continue to find sporadic and low levels of corn earworms in fields throughout the state. 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. States to our south, including Virginia, have reported control failures with pyrethroids in soybeans in 2007 and 2008. Up until 2008, poor control in our area has been the result of treating too late, treating large worms or using too low of a rate. If you use a pyrethroid for earworm control, you should be using the highest labeled rate. In addition to the pyrethoids, Steward or Lorsban should also be considered, especially if armyworms are in the mix. In the past, we have used the treatment threshold of 3 corn earworms per 25 sweeps in narrow row fields and 5 corn earworms per 25 sweeps in wide row fields (20 inches or greater). These are static thresholds that were calculated for a 10-year average soybean bushel value of $6.28. A better approach to determining a threshold is to access the Corn Earworm Calculator ( which estimates a threshold based on the actual treatment cost and bushel value you enter.