Agronomic Crop Insects – June 24, 2011

Joanne Whalen, Extension IPM Specialist;

Continue to sample for potato leafhoppers on a weekly basis. We are starting to see a significant increase in populations. 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 recent years, we have seen an increase in primarily brown stinkbug damage to developing corn ears — especially when fields are adjacent to wheat fields. Last year, we also saw damage from Brown Marmorated Stink bug (kernel damage not distorted ears) in areas north of the canal in New Castle County. We are continuing to survey fields to evaluate the extent of the damage from all species this season. The following is a summary of information from the University of Georgia on stink bug damage in corn as well as pictures of damage.

“Corn is most susceptible to stink bug injury during ear formation before tasseling stage (VT). Bugs will 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) also will kill kernels on the ear. Once the ear has elongated, stink bug feeding during the blister and milk stages blasts individual kernels usually causing them to abort.”

Although we have not developed thresholds for our area, the following thresholds are used in Georgia : 25% infested plants (1 bug per 4 plants) as a threshold during ear elongation to pollen shed and 50% infested plants (1 bug per 2 plants) during the later part of pollen shed and blister/milk stage.

We also see that initially stink bugs tend to be more prevalent on the field edge, so only a perimeter spray may be needed.

Be sure to sample fields for bean leaf beetles, potato leafhoppers, grasshoppers, green cloverworm and spider mites. Grasshopper populations have increased significantly, especially in no-till fields. As barley and wheat are harvested and soybeans are planted, these fields will be susceptible to attack and grasshopper 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. Numerous products are labeled for grasshopper control including a number of pyrethroids, dimethoate, Lorsban (chlorpyrifos), Orthene 97 (acephate) and Sevin XLR (carbaryl)i. Be sure to check all labels carefully before combining insecticides and herbicides since there are a number of restrictions on the labels.

We are also seeing an increase in defoliation from green cloverworm, especially in drought-stressed fields. The best way to make a treatment decision in full season soybeans is to estimate defoliation. Before bloom, the defoliation threshold is 30%. As full season beans enter the reproductive stages, the threshold drops to 15% defoliation.

Continue to watch carefully for spider mites. The recent rains in some areas of the state are helping to soybeans to grow ahead of damage. However, in other areas we are finding fields with economic levels of mites, both on field edges and in some cases in field interiors – so be sure to scout the entire field to make a treatment decision. Labeled materials include dimethoate, Lorsban, Hero (zeta-cypermethrin + bifenthrin) as well as a number of stand-alone bifenthrin products. All of these products need to be applied before mites explode. Be sure to read the labels for use rates and restrictions – including but not limited to combinations with herbicides, number of applications as well as the time between applications