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Eastern New York Field Crops Pest Report, 2004

This is a seasonal scouting report giving growers in the Eastern New York area information on the presence of agricultural pests. The report is written by Ken Wise, IPM Extension Area Educator for Livestock and Field Crops.

June 4, 2004

Alfalfa Weevil and Parasitoids: Alfalfa weevils have many natural enemies. One group of these are tiny beneficial wasps called parasitoids. Some parasitoid species attack weevils in their egg stage, other types attack larvae, and still other types attack adults. A common weevil parasite (Bathyplectes spp.) lays itís eggs in late instar larvae just before they pupate. The parasitoid egg hatches and the young wasp larva feeds on the developing pupa effectively killing the weevil before it completes itís development. In a healthy weevil cocoon the weevil pupa looks a bit like a green weevil beetle caught in a state of suspended animation. A weevil parasitized by a Bathyplectes wasp would be missing but replacing it in the cocoon would be a small dark brown egg-shaped structure about 1/8 inch long. The wasp cocoon can be a mahogany brown color with or without a narrow white band around itís middle. For more information check out our on-line publication: IPM for Alfalfa Weevil.

Fungus Killing Alfalfa Weevil?

During wet springs with high relative humidity a fungus aids in reducing alfalfa weevil infestations. I observed alfalfa weevil larvae infected with the fungus Zoophthora phytonomi. Infected larvae slowly turn a pale cream color and dies. When conditions favor Zoophthora phytonomi it can kill about 60% of the larval population.

Potato Leafhopper
We have seen potato leafhopper in several areas of Eastern NYS in low numbers. As the summer progresses and the weather starts to warm the populations of potato leafhopper have the potential to increase rapidly. For management information on potato leafhopper check our on-line guide: Potato Leafhopper on Alfalfa Management Guide

Field Corn
Slugs Slime the Corn
I observed slug damage on several corn plants this week. Slugs can be a minor problem in cool wet springs like we have been having this year. Slugs do windowing type damage (leaf tissue eaten with one clear layer of cells left, that looks like a window), and leave a slime trail (a shiny trail near its feeding damage). Here is a picture of SLUG damage on corn.

Other parts of the US Armyworms are present

In the Mid-West common armyworm has damaged corn and some wheat fields. We have not seen any damage here in NYS as of this week. It is always good to keep a keen eye on your corn, wheat, oats and grass fields for armyworm. True armyworm larvae appear smooth, cylindrical, pale green to brownish when they are still small. Mature larvae are smooth and marked with two orange, white-bordered strips on each side. True armyworms range in size from 1/8 inch to 1.5 inches long. It is important to detect armyworm areas early, while larvae are still small, since large larvae do most of the feeding and quickly destroy whole stands of corn, grasses and small grains. Because armyworms feed at night look for chewed leaves, cut stems, lodged plants, pellet-like frass on the ground, and larvae hidden under plant canopy and surface residue. You will need to be aware that armyworms can move from field to field every quickly. Start scouting for the armyworms in May and repeat scouting every 3 days to 5 days. Monitor fields for armyworm larvae when they are less than 1/2 inch long. If there are sufficient numbers and damage is present, an insecticide could be justified. Treat only the affected area and a 20 to 40-foot border around the infestation. Rarely does a whole field need to be treated for armyworm. Larger armyworm larvae, greater than 1/2 inch long, are much more difficult to control. These large larvae are more tolerant of insecticides, reducing the effectiveness and economic viability of this option.

Do Defoliators Do Damage?

Japanese beetle and Mexican bean beetle are the main defoliators of soybeans in NYS. While they are minor pests, defoliation of soybeans sends up many red flags by growers. The question normally is how much leaf defoliation is too much in soybeans? The good thing is that soybeans can withstand much defoliation without losing yield. The soybean defoliation threshold is 35 percent of leaf eaten or missing V1 to just before bloom. During blooming through pod-filling stages, the threshold is 20 percent defoliation. The following pictures are a guide that depict 10, 20, 30 and 40 percent defoliation:

10 percent defoliation   

    20 percent defoliation

30 percent defoliation    

40 percent defoliation

(Source: Purdue University Field Crops Pest Management Manual, 1/92)

For more information on managing insect pests on soybeans, check out our on-line publication: Soybean Insect Pest Management Guide

What are the early season diseases that can infect soybeans? See next weekís report for enlightenment!

Small Grains
More on Cereal Leaf Beetle

While at the Cornell Musgrave Research Farm last week we observed cereal leaf beetle on wheat. Even if we have not seen this pest in Eastern NYS yet it is important to know what it looks like. Here is a photograph Keith Waldron snapped of eggs and a larva of cereal leaf beetle:

Thanks to: Jeff Miller (Oneida County) Keith Waldron (NYS IPM) and Julie Stavisky (NYS IPM) for contributing to this week's report.

Ken Wise