Agronomic Crop Insects – June 10, 2011

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

Alfalfa
Continue to sample for potato leafhoppers on a weekly basis. We are now finding adults and nymphs in fields. Although both life stages can damage alfalfa, 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.

Field Corn
We have received reports of cereal leaf beetle adults moving out of small grains and feeding on the edge of corn fields. Although we do not have any firm thresholds for this insect on corn, as a general guideline controls may be needed on corn for feeding damage if you find an average of 10 beetles per plant and 50% of the plants exhibit feeding damage. In the Midwest, it has been reported that the adult beetle is a vector of maize chlorotic mottle virus (MCMV) that causes corn lethal necrosis disease. Thresholds would be much lower if this disease is an issue. To date, I am not aware of this occurring in Delaware; however, be sure to let us know if you find potential problems.

Soybeans
Be sure to sample seedling stage beans for bean leaf beetles, grasshoppers, thrips and spider mites. We are seeing an increase in the population levels of all three insects in soybeans.

(a) Grasshoppers: We can find significant numbers in many fields but the highest populations are being found in full season no-till soybeans. As barley is harvested and soybeans are planted, these fields will be especially susceptible to attack by grasshoppers which 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.

(b) Bean Leaf Beetle: 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. These treatment thresholds should be reduced if bean pod mottle virus is present or you suspected this virus was present in previous seasons.

(c) Thrips: Populations levels have increased on soybeans throughout the region and Ames Herbert at Virginia Tech is currently doing research on this insect in soybeans. In the past, information from North Carolina indicated that “soybean thrips and other thrips species can feed and reproduce on the leaves and buds of soybean seedlings. Their feeding creates bleached-out lesions along the leaf veins and gives a silvery/bronzed appearance to the leaf surface when damage is severe. These insects are very small (less than 1/10 inch) and are torpedo shaped. While thrips always occur on soybean seedlings, it is only during outbreak years that they cause concern. In particular, during dry weather and on earlier planted full-season soybeans, thrips populations can explode when plants are growing slowly. Under these circumstances thrips injury will occasionally kill seedlings. Other stressors, such as nutrient deficiencies and herbicide injury, can add to thrips damage and cause plant loss.” Yellowing can occur from thrips but there are also a number of other factors that can cause yellowing so it is important to scout fields to identify what is causing the yellowing. Although no precise thresholds are available, as a general guideline, treatment may be needed if you find 4-8 thrips per leaflet and plant damage is observed.

(d) Spider Mites: With the continued hot, dry weather, it comes as no surprise that we are starting to find the first spider mites in seedling stage soybeans. Early detection and control is needed to achieve spider mite suppression. Dimethoate, Lorsban (chlorpyrifos), Hero ( zeta-cypermethrin + bifenthrin) as well as a number of bifenthrin products (such as Sniper and Bifenture) are available for spider mite control in soybeans. All of these products need to be applied before mites explode. Be sure to read the labels for use rates and restrictions – there is a limit on the number of applications as well as the time between applications on all of the materials labeled for spider mite control.

 

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