Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa
Potato leafhoppers are now present in fields so be sure to sample on a weekly basis after the first cutting. 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
As small grain dries down, be sure to watch for armyworms moving out of small grain and into adjacent corn fields. You should also scout corn for armyworms in fields that were planted into a small grain cover. Remember, worms must be less than 1 inch long to achieve effective control. The treatment threshold for armyworms in corn is 25% infested plants with larvae less than one-inch long. Large larvae feeding deep in the whorls will be difficult to control.
Small Grains
As small grains dry down, be sure to watch for head clipping from armyworm and sawfly. In fields that did not receive an insecticide spray, we have started to find an occasional field with economic levels of armyworms, as well as old head clipping from grass sawfly larvae. On barley, significant head clipping from armyworms can quickly occur. As a general guideline, the threshold for armyworms in barley is one per foot of row and for wheat 1 to 2 per foot of row. As a guideline, a treatment should be applied for sawflies when you find 2 larvae per 5 foot of row innerspace or 0.4 larvae per foot of row. However, remember if the number of clipped heads is twice the worm count for sawflies then it is generally too late to treat for them.
We also can find an occasional wheat field with economic levels of cereal leaf beetle, especially later maturing fields. Research from North Carolina and Virginia indicates that the greatest amount of damage occurs between flowering and the soft dough stage of plant development. Before making an application of an insecticide, be sure to check all labels for the number of days between last application and harvest.
Soybeans
Continue to sample for slugs, bean leaf beetles and grasshoppers. As a general guideline, the treatment threshold for grasshoppers is 1 per sweep and 30% defoliation. Sprays may be needed sooner if stand loss is occurring. Early detection and control of small grasshoppers is necessary to achieve control. We have started to see an increase in bean leaf beetle (BLB) activity in the earliest planted and emerged fields. A new fact sheet from Ohio provides good information on bean leaf beetle biology as well as provides pictures of adult beetles and leaf damage (http://ohioline.osu.edu/ent-fact/pdf/0023.pdf).