Agronomic Crop Insects – May 28, 2010

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 Grain
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 armyworms. On barley, significant head clipping from armyworms can quickly occur. As a general guideline, the threshold for armyworms in barely is one per foot of row and for wheat one-two per foot of row.

We continue to find an occasional wheat fields 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.

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