Agronomic Crop Insect Management – May 20, 2016

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

Alfalfa
The first potato leafhopper adults are now being found, so be sure to sample all fields on a weekly basis. Although adults and nymphs both damage alfalfa; it is the nymphs that can quickly cause damage. 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
Slugs and cutworms continue to be found in economic levels in fields throughout the state. In later planted fields, be sure to look for both as soon as corn is spiking out of the ground. As a general guideline, a treatment is recommended for cutworms if you find 10% leaf feeding or 3% cut plants. For fields next to barley fields that were not treated, be sure to watch for armyworm movement from barley into corn.

Small Grains
As soon as the earliest planted barley begins to dry down, be sure to watch for head clipping from armyworm. In fields that did not receive an insecticide treatment we continue to find armyworm 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. Before making an application of an insecticide, be sure to check all labels for the number of days between last application and harvest.

Soybeans
Be sure to sample fields starting at early emergence for slugs, bean leaf beetles and grasshoppers. Unfortunately, control of slugs in soybeans is very difficult to time, especially when slugs have already hatched, because the plant’s growing point is within the emerging cotyledons. If germination occurs when slugs are actively feeding, the slug can feed on the cotyledons and growing point, which can result in the death of the plant. 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.

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