Agronomic Crop Insects – June 3, 2011

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

Alfalfa
Continue to sample for potato leafhoppers on a weekly basis. Although adults are the main life stage present, we are starting to see the first nymphs. 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. More information on potato leafhopper, including how alfalfa growing conditions might affect the threshold, is available at http://ohioline.osu.edu/ent-fact/pdf/0033.pdf.”

Soybeans
Be sure to sample fields starting at emergence for bean leaf beetles and grasshoppers. In the earliest planted and emerged fields, we have started to see an increase in activity for both insects. As barley is harvested and soybeans are planted, these fields will be especially susceptible to attack from grasshoppers and feeding can often cause stand loss. 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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