Agronomic Crop Insects – May 11, 2012

Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu

Alfalfa
In addition to checking for weevils feeding on re-growth, be sure to begin checking all fields for leafhoppers within one week of cutting. Spring planted fields should also be sampled since they are very susceptible to damage. Once the damage is found, 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
In addition to slugs and cutworms, be sure to sample fields for true armyworm larvae, especially where a grass cover or volunteer small grains were burned down at planting. As a general guideline, a treatment may be needed for armyworms if 25% of the plants are infested with larvae less than one-inch long.

Small Grains
Continue to scout fields for cereal leaf beetles, armyworms and sawflies. In unsprayed fields that have a history of cereal leaf beetle, we have observed a significant increase in activity this week. Cereal leaf beetle can cause the greatest economic loss from flowering through the soft dough stage. Once wheat reaches the hard dough stage, the beetle feeding damage generally has little effect on yield. It is important that you scout fields on a weekly basis until harvest for armyworm and sawfly larvae. We continue to find larvae in fields that have not been sprayed yet. Although sawflies and armyworm can attack and cause economic losses in both wheat and barley, in outbreak years the damage often occurs quicker in barley. Since populations of all of these insects vary from field to field, fields should be scouted to determine if economic levels are present. As a general guideline, if multiple insects are present, the threshold for each insect should be reduced by one third to one half. 

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