Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa and Grass Hay Crops
Continue to watch for defoliators in grass hay crops and alfalfa. Significant damage can occur in grass hay fields from true armyworm and fall armyworm. 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. No thresholds are available; however, controls should be applied before significant defoliation occurs.
Soybeans
We continue to find defoliators (Japanese beetles, grasshoppers, green cloverworm, soybean loopers, fall armyworm and yellow striped armyworm) in both full season and double crop soybeans. In some fields, we have seen a significant increase in green cloverworm and soybean looper populations. As a general guideline, treatment decisions for defoliators should be based on the following defoliation thresholds:
(a) Full Season Plantings – 30% defoliation pre-bloom; 15% defoliation from bloom through the end of pod fill; 35% – once fully developed seeds are present
(b) Double Crop Plantings (especially if growth is poor) – 20% defoliation pre-bloom, 10% defoliation from bloom through pod fill; 15% defoliation – once fully developed seeds are present.
As far as soybean loopers, this insect is a migratory pest and in past years we have seen it cause significant defoliation in outbreak years. It is often a problem in dry years. Since resistance to pyrethroids has been documented in states to our south, a non-pyrethroid option will need to be selected if they become a problem. We also have other looper species in our fields so proper identification is important. The following link from Virginia includes pictures to help with identification
http://blogs.ext.vt.edu/ag-pest-advisory/soybean-loopers-are-infesting-soybeans-in-north-carolina/
We are also seeing an increase in native stink bug populations, especially in fields that have reached the R-5 stage (beginning seed – seed is 1/8 inch long (3 mm) long in the pod at one of the four uppermost nodes on the main stem). Brown marmorated stink bug populations remain low and are only being found along field edges that border woods in New Castle and Kent Counties. Economic damage from stink bugs is most likely to occur during the pod development and pod fill 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. As a general guideline, we are using a new threshold in the Mid-Atlantic Region — 5 stink bugs per 15 sweeps. This is the threshold for soybeans produced for grain. If you are producing soybeans for seed, the threshold is still 2.5 per 15 sweeps.
We continue to find low levels of small corn earworm larvae, mainly in double crop fields. Moth catches in both black light traps and pheromone traps continue to increase so it is important to watch for corn earworm in fields throughout the state. When populations are high, corn earworm larvae also feed on soybean leaves, so you should include them in the mix when scouting for defoliators. Once pods are present, a treatment decision should be made for corn earworm using the Corn Earworm Calculator – developed in VA and NC (http://www.ipm.vt.edu/cew/) – since it will provide the best decision making information based on the actual treatment cost and bushel value you enter.