Agronomic Crop Insects – May 30, 2014

Joanne Whalen, Extension IPM Specialist;

Continue to sample for potato leafhoppers on a weekly basis. We can find both adults and nymphs in fields. 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.”

Field Corn
Fields planted next to barley should be scouted for armyworms moving from barley into adjacent corn fields. Control will be difficult once larvae move deep in whorls. Remember, worms must be less than 1 inch long – some labels indicate that larvae need to be even smaller – to achieve effective control. The treatment threshold for true armyworms in corn is 25% infested plants with larvae less than one-inch long.

At this time of year we often hear reports of sugar cane beetle adults attacking seedling stage corn in North Carolina and a few counties on the lower eastern shore of Maryland. The damage can be confused in some cases with wireworm and/or below ground cutworm feeding, so be sure to learn about this insect and the type of damage it can cause when you are sampling corn fields. So far, I am not aware of any fields being attacked in Delaware but it does appear this insect could be expanding its range. Here is a link from North Carolina with more information on this pest ( Unfortunately, there are no rescue treatments to control this insect pest.

Small Grains
We are still finding pockets of armyworms, grass sawflies and cereal leaf beetles in wheat and barley that has not been treated so be sure you continue scouting for all three pests. As far as cereal leaf beetle, research from Virginia and North Carolina indicates that the greatest damage can occur between flowering and the soft dough stage.

Be sure to sample fields starting at emergence for bean leaf beetles and grasshoppers. 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.