Bill Cissel, Extension Agent – Integrated Pest Management; bcissel@udel.edu
Alfalfa
Potato leafhopper populations can still be found at economic threshold throughout the region. Continue to sample alfalfa for potato leafhoppers weekly, starting seven days after 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
Scout for Two-spotted Spider Mites in Soybeans:
The recent rains and cooler temps have reduced plant stress and slowed down Two-spotted spider mite (TSM) reproduction. However, I continue to find mites and eggs in soybean fields. The weather over the next several weeks will ultimately determine if mite populations increase so be sure to scout your fields so that you can determine if populations are increasing or decreasing. Scout for TSM by examining the underside of 5 leaflets in 10 locations for mites, noting the presence of mite eggs and the amount of leaf damage. The threshold for TSM during bloom to podfill is 20-30 mites per leaflet and 10% of plants with one third or more leaf area damaged.
Concentrate scouting efforts on field edges for initial detection, especially edges bordered by grass and road ditches (it’s not unusual to also find hot spots in the interior portions of the field). TSM typically develop on grasses and other plants on field borders before ballooning into fields. Once TSM are detected, scout the interior portions of the field to determine if they have spread throughout the entire field. If only concentrated on field edges, spot treating may be an option. If spot treating on field edges, extend the treated area about 100 ft further into the field from the damaged area.
A hand lens is necessary when scouting for spider mites to see mite eggs and nymphs. Here is a short Youtube video demonstrating how to use a hand lens: https://youtu.be/lFz004Wl28E
Here is a link to our Soybean Insecticide Recommendations for chemical control options: https://cdn.extension.udel.edu/wp-content/uploads/2012/05/18063934/Insect-Control-in-Soybeans-2017-final.pdf
Also note that in 2016, two miticides were registered for use on soybeans; Zeal SC, (Valent U.S.A Corporation) and Agri-Mek SC, (Syngenta Crop Protection, LLC). These are the only labeled formulations of these products. Please consult the label for rates, additional restrictions, and adjuvant requirements.
Zeal SC Supplemental Label for use on soybean: http://www.cdms.net/ldat/ldCCK003.pdf
Agri-Mek SC Label: http://www.cdms.net/ldat/ld9NL020.pdf
Continue to Scout Soybeans for Defoliating Insects: Continue to scout soybeans for defoliators including grasshoppers, bean leaf beetles, Japanese beetles, and green clover worms. This week, bean leaf beetle populations have increased in full season and double crop fields. I also started finding newly hatched grasshopper in many fields.
During reproductive growth stages (Bloom-Podfill), the threshold is 15% defoliation. Prior to blooming, the threshold is 30% defoliation. When estimating defoliation, randomly select leaves from the entire plant, not just the newest growth or leaves with the greatest amount of defoliation.
Here is an image to help in estimating the amount of defoliation:
Are Your Fields at Risk to Lodging from Dectes Stem Borer? Dectes stem borers are present in soybean fields throughout the state and can be seen “hanging out” on leaves and easily captured with a sweep net. Knowing you have an infestation of Dectes stem borers in your soybean field can be used to schedule harvest to minimize lodging losses.
Check out week #16 Guess the Pest for more information on the Dectes stem borer: http://extension.udel.edu/weeklycropupdate/?p=10833
