Agronomic Crop Insects – August 7, 2015

Joanne Whalen, Extension IPM Specialist;

Alfalfa and Grass Hay Crops
Be sure 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.

Field Corn
In the past couple of years, we have received calls about aphids in field corn by mid-August. In most cases, populations have been spotty within fields or are only found on field edges. Currently, there are no treatment thresholds for aphids in corn past tasseling. In many cases, fields have been beyond the point of considering a treatment due to the maturity of the crop and the presence of beneficial insects and/or parasitized and diseased aphids.

Although we have no thresholds for aphids in corn in our area, here are some considerations developed by entomologist in the Midwest that can help to make a treatment decision:

  1. Are 80 percent of the plants infested with aphids?
  2. Do most of the ears have aphids? What about the ear leaf and above?
  3. How long has the field been infested and is the density increasing?
  4. Do you see honeydew or sooty mold on the stalk, leaves or ear?
  5. Are you seeing winged aphids or nymphs with wing pads? That may be a sign of migration out of the field.
  6. Is the field under drought stress?
  7. Do you see any bloated, off-color aphids? Natural fungi can quickly wipe out aphids.In addition are beneficial insects/parasitized aphids present.
  8. What is the corn growth stage? Fields reaching hard dent should be past the point of justifying a treatment.
  9. Some insecticides have a long pre-harvest interval so be sure to check the label.

We continue to find low levels of defoliators (Japanese beetles, grasshoppers, silver spotted skipper, green cloverworm and isolated spots of fall armyworm and yellow striped armyworm) in both full season and double crop soybeans. 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.

Another defoliator that is showing up earlier in states to our south is the soybean looper. 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

In drought stressed areas of the state, we can find spider mites on field edges and within fields. Be sure to watch for hot spots of activity in field interiors. Early detection and control is needed for spider mite management.

Continue to watch for an increase in stink bug populations. Economic damage from stink bugs is most likely to occur during the pod development and pod fill stages. Brown Marmorated stink bug populations still remain extremely low and are only being found along field edges that border woods in New Castle County.

We continue to survey for Kudzu Bug but have not found any in soybeans or kudzu. In Virginia, kudzu bugs have been found in soybean fields in 21 southern/eastern counties but in all cases, these have been adults only, and at very low numbers ( Be sure to scout soybeans for this insect and follow the Kudzu Bug website – — for identification and treatment information. The treatment threshold is still one nymph per sweep.

We have also started to find an occasional soybean aphid in a few fields throughout the state. Cooler weather patterns favor an increase in populations. The economic threshold for soybean aphid established in the Midwest is 250 aphids per plant. Populations should be increasing and most of the plants should be infested (>80 percent) in order to justify an application. This threshold is appropriate until plants reach mid-seed set (R5.5). Spraying at full seed set (R6) has not produced a consistent yield response in the Mid-west. You should also consider beneficial insect activity before making a treatment decision. Most products labeled for soybean aphid will provide effective control

As far as corn earworm, we continue to find low levels of small larvae, mainly in double crop fields. The results of the annual corn earworm survey in field corn in Virginia, which has been used as an indicator of the potential for corn earworm in soybeans, indicates that statewide, approximately 17.5% of ears were infested with corn earworm. This is even lower than the numbers reported in 2014 (20%) and 2013 (18%) ( However, our trap catches just spiked this past week, especially our pheromone trap catches, so it will be important to scout all fields for earworms in the next week to 10 days. In making a treatment decision, the use of the Corn Earworm Calculator – developed in VA and NC ( will provide the best decision making information since it estimates a threshold based on the actual treatment cost and bushel value you enter.