Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa
Continue to sample for potato leafhoppers on a weekly basis. We are starting to see a significant increase in populations. Once plants are yellow, yield loss has already occurred. The treatment thresholds are 20 per 100 sweeps on alfalfa 3 inches or less in height, 50 per 100 sweeps in 4-6 inch tall alfalfa and 100 per 100 sweeps in 7-11 inch tall alfalfa.
Field Corn
Over the past couple of weeks, we have observed the movement of cereal leaf beetle adults from adjacent small grains into corn fields. Although they are generally found on field edges, there have been recent reports of field wide infestations. Although we do not have any firm thresholds for this insect on corn, as a general guideline controls may be needed on corn for adult feeding damage if you find an average of 10 beetles per plant and 50% of the plants exhibit feeding damage. In the Midwest, it has been reported that the adult beetle is a vector of maize chlorotic mottle virus (MCMV) that causes corn lethal necrosis disease. Thresholds would be much lower if this disease is an issue. We still have not found this virus in Delaware; however, be sure to let us know if you find potential problems.
Soybeans
Although insect activity remains generally light on early planted soybeans, you should continue to sample fields on a weekly basis for bean leaf beetles, potato leafhoppers, grasshoppers, green cloverworm and spider mites. As barley and wheat are harvested and soybeans are planted, these fields will be susceptible to attack and grasshopper feeding can often cause stand loss. The following link provides a guideline for decision making on the above insect pests:
http://extension.udel.edu/ag/2012/06/05/ipm-soybean-insect-thresholds/