Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa and Grass Hay Crops
Continue to watch for defoliators in grass hay crops and alfalfa. True armyworm, yellow striped armyworm and fall armyworm can all attack hay crops. We continue to hear about low levels of fall armyworm in fields, both locally and in states to our south. 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
As expected, we are starting to see an increase in aphid populations, especially in the earliest planted fields. In many cases, populations are spotty within fields or are only found on field edges. Although there are no treatment thresholds for aphids in corn past tasseling, as a general guideline, you may consider a foliar application when most of the plants are infested (>80 percent), and aphids are have exceeded 500-1,000 per plant. An insecticide may be warranted if aphid honeydew and sooty mold are evident above the ear leaf and plants have not reached hard dent (R5). Here are some additional considerations developed by entomologist in the Midwest that can help to make a treatment decision:
- Are 80 percent of the plants infested with aphids?
- Do most of the ears have aphids? What about the ear leaf and above?
- How long has the field been infested and is the density increasing?
- Do you see honeydew or sooty mold on the stalk, leaves or ear?
- Are you seeing winged aphids or nymphs with wing pads? That may be a sign of migration out of the field.
- Is the field under drought stress?
- Do you see any bloated, off-color aphids? Natural fungi can quickly wipe out aphids. In addition are beneficial insects/parasitized aphids present.
- What is the corn growth stage? Fields reaching hard dent should be past the point of justifying a treatment.
- Some insecticides have a long pre-harvest interval so be sure to check the label.
Soybeans
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. The first small larvae have been detected in fields in Sussex County. Although our local overwintering population was lower again this year, there are reports from the south of heavy populations that can make it to our area as migratory moths on weather fronts. Before pods develop, corn earworm larvae can 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.
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
You will also need to continue to watch for spider mites in full season and double crop fields. With the hot, dry weather last week, populations were starting to increase. With the recent rains and increased humidity, we did see populations crash in some but not all fields. You should be aware that we now have 2 new miticides labelled on soybeans in Delaware: Agri-Mek SC and Zeal SC (NOTE: these are the only formulations registered for use on soybeans). Be sure to read the labels for all restrictions, including but not limited to, rates, adjuvants, number of applications, and growth stage restrictions.
Zeal SC – http://www.cdms.net/ldat/ldCCK003.pdf
Agri-Mek SC – http://www.cdms.net/ldat/ld9NL020.pdf