Agronomic Crop Insects – September 11, 2009

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

Soybeans
We continue to hear reports of newly hatched corn earworm larvae in fields that were not treated in some areas of Sussex County and on the lower eastern shore of Maryland. If you have not checked your fields, be sure to check them for corn earworm.

With the continued cool temperatures, soybean aphids are still a problem in a number of double crop fields, especially in the western areas of the state and in a number of counties in Maryland. With the continued cool weather, populations are increasing in some fields and populations are well above the threshold of 250 aphids per plant. After talking to a few entomologists in the Midwest about soybean aphids, they still emphasize that treatment is needed through the R-5 stage (seed is 1/8 inch long in the pod of one of the four uppermost nodes on the main stem) of soybean development if economic levels are present. As far as spraying through R-6 stage (pods containing a green seed that fills the pod cavity at one of the four uppermost nodes on the main stem) – their recent data continues to vary as to the benefit of spraying once plants reach the R-6 but in some years and some situations there has been an economic return. Spraying after R-6 stage has not been documented to increase yield in the Midwest. The suggested treatment threshold from the Midwest is still 250 aphids per plant with an increasing population. The following link from Wisconsin provides additional information on aphid management (http://www.plantpath.wisc.edu/soyhealth/aphids/aphid_management.htm). Information from the Midwest indicates that if the majority of aphids are winged or developing wings, it is an indication that the aphids will soon leave the field.

There are also a number of other insects still present in double crop fields including stinkbugs, bean leaf beetles, grasshoppers, and green cloverworms. The threshold of all may need to be reduced if a mixed population is present. As a reminder, both bean leaf beetles and grasshoppers will also feed on pods.

Small Grains
As you make plans to plant small grains, you need to remember that Hessian fly can still be a problem. Although we have not seen major infestations for the last few years, we did see fields with isolated infestations this spring. Since the fly survives as puparia (“flax seeds”) in wheat stubble through the summer, you should still consider this pest as you make plans to plant small grains. In our area, damage has been the result of spring infestations. Plants attacked in the spring have shortened and weakened stems that may eventually break just above the first or second node, causing plants to lodge near harvest. Warm fall weather conditions can extend fly emergence and egg-laying beyond the fly-free dates, but these dates should still be used as a guideline for planting. Since we rarely see plants stunted in the fall, we still feel that most of the damage we see is occurring from spring infestations. Plants attacked in the fall at the one-leaf stage may be killed outright. Wheat attacked later in the fall will be severely stunted, with the first tillers killed and plant growth delayed. Plants infested in the fall can easily be recognized by their darker than normal bluish coloration and leaves with unusually broad blades. Combinations of strategies are needed to reduce problems from Hessian fly:

● Be sure to completely plow under infested wheat stubble to prevent flies from emerging.

● Avoid planting wheat into last season’s wheat stubble, especially if it was infested with Hessian fly.

● Avoid planting wheat next to last season’s wheat fields – the most serious infestations can occur when wheat is early planted into wheat stubble or into fields next to wheat stubble.

● Eliminate volunteer wheat before planting to prevent early egg-laying.

● Do not use wheat as a fall cover crop near fields with infestations.

● When possible, plant after the fly-free date. (Oct 3 – New Castle County; Oct 8 – Kent County; Oct 10 – Sussex County).

● Plant resistant varieties. You should look for varieties that have resistance to Biotype L. You will need to check with your seed dealers to identify varieties that are adapted our area.

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