Bill Cissel, Extension Agent – Integrated Pest Management; bcissel@udel.edu
Scout Alfalfa for Potato Leafhoppers
Continue to sample alfalfa for potato leafhoppers. Sample weekly starting seven days after the first cutting until final harvest. Ten sweep net samples should be taken in 10 random locations throughout the field when the alfalfa is dry. The threshold for alfalfa 3” or less is 20 leafhoppers per 100 sweeps, 4-6” tall is 50 per 100 sweeps, 7-10” tall is 100 per 100 sweeps and greater than 11” is 150 per 100 sweeps. If the field is more than 60 percent bud stage or if it has experienced “hopper burn”, the alfalfa should be cut instead of sprayed.
For more information on the identification, biology, and management of potato leafhoppers, please review our fact sheet: http://extension.udel.edu/factsheets/potato-leafhopper-control-in-alfalfa/
Here is a link to our Insect Control in Alfalfa Recommendations (pure stands only): https://cdn.extension.udel.edu/wp-content/uploads/2012/05/18063238/Insect-Control-in-Alfalfa-final-for-2017.pdf
Here is a Youtube video discussing how to sample for potato leafhoppers: https://youtu.be/7ybclcNu2rA
Soybeans Defoliating Insects
I am starting to find green clover worms in soybeans throughout the state. We rarely have to treat for green clover worms because soybeans can typically withstand substantial defoliation during vegetative growth stages with minimal impact on yield. Also, green clover worm infestations often crash due to disease or are parasitized before reaching threshold. However, if you also have other defoliating pests such as grasshoppers and bean leaf beetles feeding on soybeans, a treatment may be necessary. During vegetative growth stages, the threshold for defoliating insects is 30% defoliation. Once the plants begin blooming and during pod fill, the threshold is 15% defoliation. When several defoliating insect pests are present, (i.e. green clover worms, grasshoppers, and bean leaf beetles) combine the damage from all the pests when determining if your field is at threshold.
Here is a link to our Soybean Insect Management Recommendations for chemical control options:
Watch for Stink Bugs Moving into Corn
As discussed in last week’s article, watch for stink bugs moving into adjacent corn fields as wheat matures and is harvested. Brown stink bugs are the most common species found in wheat in Delaware. Corn is most sensitive to stink bug feeding injury when infestations occur during earlier plant growth stages (late vegetative-tasseling) compared to later growth stages (pollen shed-grain fill). Based on recommendations from NC State, the threshold for stink bugs in corn is one stink bug per four plants when the ear is forming, during ear elongation and beginning of pollen shed; and one stink bug per two plants after pollen shed (these thresholds have not been evaluated in Delaware).
Begin scouting corn fields for stink bugs, especially those fields adjacent to wheat and fields bordered by weedy ditches. The greatest yield loss potential from stink bug feeding occurs prior to pollination so waiting until tasseling may be too late.