Agronomic Crop Insects

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.”

This past week we received a few reports of high thrips populations in alfalfa. In past years, we have seen increases in thrips during hot, dry weather conditions. Reports from other areas of the country indicate that thrips feeding on developing leaf tissue can cause the leaves to distort as they emerge. Leaves may also be curled, with a cupped or puckered appearance. Since there are no thresholds for thrips in alfalfa, the following information from other areas of the country may be helpful when considering the need for thrips management: (a) high populations of bean or onion thrips may cause damage, especially in dryland conditions and (b) if a thrips treatment is contemplated, it is best to cut as soon as possible and treat the regrowth if the infestation persists. Thrips are very difficult to control in alfalfa, so excellent coverage is important and two applications may be required for satisfactory results

Field Corn
Since last week, we have had a number of reports of true armyworm moving from barley into adjacent corn fields. In many cases, larvae are large and 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 1 inch long.

There have also been a number of reports of thrips feeding on field edges. As small grains dry down, it is not unusual to see thrips populations increase along the edge of a field. In the past, no controls have been needed but it always warrants watching fields to be sure populations and leaf damage does not increase. No thresholds are available.

Although most fields are past the seedling stage, there have been a number of questions this spring about damage caused by seedling stage insect pests. The following posting by Dominic Reisig, North Carolina State University Extension Entomologist, does an excellent job of describing seedling insect pests and their damage: http://www.nccrops.com/2012/05/23/distinguishing-among-insect-injury-types-in-seedling-corn/. One of the insects he describes is the sugar cane beetle. Although I have never encountered sugar can beetle attacking field corn in Delaware, we did receive one question about this insect from a field in southern Maryland. It continues to be a very different year so be on the lookout for unusual insect problems this year.

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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