Agronomic Crop Insects – June 8, 2012

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

Alfalfa
Continue to sample for potato leafhoppers on a weekly basis. We continue to find 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
In addition to thrips feeding, we are finding a few fields with cereal leaf beetle adult feeding. Although cereal leaf beetle problems were low in most small grain fields this season, beetle adults can be found moving out of untreated 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 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 for beetle feeding would be much lower if this disease is an issue. As of last season, we have not seen this virus in Delaware corn fields. However, please let us know if you suspect a problem.

We are continuing our stink bug survey of corn fields next to wheat to evaluate movement of stink bugs from wheat into corn. Just this past week, we have found a few stink bugs in corn along field edges adjacent to wheat. Although we have not developed thresholds for our area, the following information developed in the South (North Carolina and Georgia) should provide guidance for management in our area:

(a) Until the V6 stage, the economic threshold is four stink bugs per plant.

(b) “Corn is most susceptible to stink bug injury during ear formation before the tassel stage (VT). Bugs feed through the sheath, causing a dead spot on the ear. As the ear expands it becomes distorted and curves, usually outward. Feeding during silking and pollen shed (R1) will also kill kernels on the ear. Once the ear has elongated, stink bug feeding during the blister and milk stages can blast individual kernels usually causing them to abort.”

(c) When the ear is forming, during ear elongation, and during pollen shed, the treatment threshold used in the South is one stink bug per four plants (25% infested plants).

(d) From the end of pollen shed to blister/milk stage, the threshold is one stink bug for every two plants (50% infested plants).

Soybeans
Be sure to sample seedling stage beans for bean leaf beetles, grasshoppers, thrips and spider mites.

Grasshoppers: Population levels are starting to increase, especially 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 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 maybe needed if you find one grasshopper per sweep and 30% defoliation from plant emergence through the pre-bloom stage.

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 in your area or you suspected virus the previous season.

Thrips: At this time, soybean thrips and other thrips species are present in seedling stage fields. Thrips 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 seedling stage soybeans, 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.

Spider Mites: The first spider mites have been found in seedling stage soybeans so be sure to scout fields as soon as plants emerge. Early detection and control is needed to achieve spider mite suppression. Dimethoate, Lorsban (chlorpyrifos), Hero (zeta-cypermethrin + bifenthrin) as well as a number of stand-alone bifenthrin products 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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