Agronomic Crop Insects – July 29, 2016

Joanne Whalen, Extension IPM Specialist;

Alfalfa and Grass Hay Crops
During the upcoming week, be sure to start sampling fields on a weekly basis for defoliators in grass hay crops and alfalfa. True armyworm, yellow striped armyworm and fall armyworm can all attack hay crops. We have heard reports of low levels of fall armyworm in fields. Populations are also starting to increase in southern states so we could also see migratory populations over the next month. Significant damage can occur in grass hay fields from these two insects so 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 in our area but areas to the south indicate that a treatment should be considered if you find three to four larvae per square foot. Controls should be applied before significant defoliation occurs. If fields are ready to harvest, harvesting is suggested rather than applying an insecticide.

Field Corn
It is time to begin looking for aphids in field corn, especially in the earliest planted fields. Often times, populations will be spotty within fields or are only found on field edges. Although there are no treatment thresholds for aphids in corn past tasseling here are some considerations developed by entomologist in the Midwest that can help you 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.

Although NOT KNOWN to occur in Delaware, you should scout sorghum for a new potential aphid species, the Sugar Cane Aphid (SCA). The following information was included in last week’s Virginia Pest Alert: “This insect has moved north faster this year compared to last and has been found in Virginia. Fields need to be scouted once a week once SCA has been detected in a region. Research from the south has shown yields can be drastically reduced if insecticide application is delayed for several days once threshold levels are reached. States and regions in the south vary slightly in their recommended threshold levels, but in general, an insecticide application is justified when 50 aphids per leaf are present on 25 percent of the plants.” Sugarcane aphids are yellow and can be distinguished from other aphids in sorghum by the presence of black tailpipes on the tail (cornicles) and black feet below their yellow legs.

We continue to see a mix of defoliators (grasshoppers, Japanese beetles, bean leaf beetles, yellow striped armyworm and green cloverworm) in full season and double crop soybeans. It is also time to start checking for corn earworms in soybeans. Although our corn earworm trap catches have been lower so far this season, we are starting to see an increase in a few locations. When populations are high, corn earworm larvae also feed on soybean leaves so you should include them in the mix when scouting for defoliators. The same defoliation thresholds used for other insects pre-bloom and during pod set would apply to corn earworm. We continue to see an increase in native stinkbug populations (native green and brown) in the earliest planted fields. Economic damage from stinkbugs occurs 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.