Agronomic Crop Insects

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

Alfalfa
Continue to sample fields on a weekly basis for leafhopper adults and nymphs as well as defoliators, including earworm, webworms and fall armyworms.

Soybeans
We continue to hear reports of newly hatched corn earworm larvae in fields that were not treated. However, in many cases there is a mixed population – i.e. small, medium and large larvae. Fields that were sprayed with a pyrethroid when the worms were small are still clean. If worms were larger at treatment time, then there are still larger worms present in fields. We have started to see parasitized large larvae so hopefully that is a sign that the populations are starting to crash. However, we have not seen any diseased worms yet. If a mixed population size is present at treatment time, then a material like Steward or Larvin should be used.

There are a number of other insects 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 did not see major infestations this year, we did see fields with isolated infestations. 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 to our area.

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