Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa
Continue to sample for potato leafhoppers on a weekly basis. We can find both adults and the first nymphs in fields. Both life stages can damage alfalfa but the nymphs can cause damage very quickly. 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.
If you have planted a glandular haired variety, we do not have any local data but here is some information from Ohio State regarding treatment thresholds on these varieties:
“If the alfalfa is one of the glandular-haired, leafhopper-resistant varieties of alfalfa, the economic threshold is three leafhoppers per inch of growth (24 leafhoppers for 8” tall alfalfa, for example). However, if the resistant alfalfa is a new planting this spring, growers should use thresholds meant for regular alfalfa during the first growth from seeding. Because resistance improves as the seedling stand develops, research suggests that the threshold for a resistant variety can be increased after the first cutting.”
Field Corn
Economic levels of cutworms continue to be found in fields that did not receive a treatment at planting. If threshold levels are present and the weather remains dry, cutworms will tend to feed below the soil surface. If cutworms are feeding below the soil surface, it will be important to treat as late in the day as possible, direct sprays to the base of the plants and use at least 30 gallons of water per acre. Fields planted next to barley should be scouted for armyworms moving from barley into adjacent corn fields. Control will be difficult once larvae move deep in whorls. Remember, worms must be less than 1 inch long – some labels indicate that larvae need to be even smaller – to achieve effective control. The treatment threshold for true armyworms in corn is 25% infested plants with larvae less than one-inch long.
Small Grains
We are still finding economic levels of armyworms, grass sawflies and cereal leaf beetles in wheat and barley that has not been treated so be sure you continue scouting for all three pests. As far as cereal leaf beetle, research from Virginia and North Carolina indicates that the greatest damage can occur between flowering and the soft dough stage. Before making an application of an insecticide, be sure to check all labels for the number of days between last application and harvest.
As discussed during the winter meeting season, you should be aware that EPA is cancelling the use of methomyl (Lannate, Nudrin and all generics) on barley, oats, and rye and limiting its use on wheat to Idaho, Oregon, and Washington. It is my understanding based on information received from the registrants that they will be able to sell and distribute old labeled product for 18 months after the new label is approved. In addition, persons others than registrants , that is retailers as well as end users, will be able to use/sell all product in their possession bearing the old label until their supply is exhausted. However, as a reminder the label is the law; therefore, be sure to check the label on the container for up to date information before making a treatment decision. For more information, please check the following link
http://www.regulations.gov/#!documentDetail;D=EPA-HQ-OPP-2010-0751-0018.
Soybeans
Be sure to sample fields starting at emergence for bean leaf beetles and grasshoppers. If stand reductions are occurring from plant emergence to the second trifoliate, a treatment should be applied. Although no precise thresholds are available, a treatment may be needed if you find one grasshopper per sweep and 30% defoliation from plant emergence through the pre-bloom stage. As a general guideline, a treatment may be needed for bean leaf beetle if you observe a 20 – 25% stand reduction and/or 2 beetles per plant from cotyledon to the second trifoliate stages.