Agronomic Crop Insects – May 13, 2011

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

Field Corn
Be sure to continue sampling all fields for cutworms and slugs. Although we no longer have the resources for a black cutworm trapping network, there are a few traps out at the research station and trap catches have been very consistent in recent weeks. Information from the Midwest indicates that there trap catches have been higher than normal this year so be sure to scout fields even if a preventative insecticide or a Bt-variety was used for cutworm control. For cutworms, fields should be sampled through the 5-leaf stage for damage. We are finding leaf feeding as well as cut plants caused by both cutworms and slugs in no-till corn – sometimes in the same field. If slugs are damaging plants, you will be able to see “slime trails” on the leaves. As a general guideline for cutworms, a treatment should be considered in 1-2 leaf stage corn if you find 3 percent cut plants or 10% leaf feeding. If cutworms are feeding below the soil surface, it will be important to treat as late in the day as possible, direct sprays to the base of the plants and use at least 30 gallons of water per acre.

In no-till field in Kent and New Castle counties, we are starting to see any increase in slug damage. Newly hatched juvenile slugs can be found under residue in no-till fields. The use of Deadline M-Ps (or other available metaldehyde baits) should be considered if the weather remains cool and damage is increasing. There is also a 2ee recommendation for Lannate LV for slug management that was issued for corn last season (http://www.cdms.net/LDat/ld183004.pdf).

You should also sample no-till fields for true armyworms, especially where a grass-cover or volunteer small grains were burned down at planting. As a general guideline, a treatment may be needed for armyworms if 25% of the plants are infested plants with larvae less than one-inch long.

We are also seeing a few fields with bird damage that is sometimes confused with cutworm damage. You can distinguish bird damage from cutworm damage by the pattern in the field. Generally longer strips of damaged plants, plants pulled out of the ground, and/or plants cut high that are compressed at the base of the stems, all indicate bird damage. Although birds can cut plants off at the soil surface, they tend to pull plants out of the ground. In addition, if you look closely you will see “bird prints” near the missing plants or holes where birds have pulled plants out of the ground.

Small Grains
Continue to scout fields for cereal leaf beetles, armyworms and sawflies as well as aphids feeding in the heads of small grains. In unsprayed fields that have a history of cereal leaf beetle, we have observed a significant increase in activity this week. Cereal leaf beetle can cause the greatest economic loss from flowering through the soft dough stage. Once wheat reaches the hard dough stage, cereal leaf beetle feeding damage generally has little effect on yield. It is important that you scout fields on a weekly basis until harvest for armyworm and sawfly larvae. We continue to find larvae in fields that have not been sprayed. Although sawflies and armyworm can attack and cause economic losses in both wheat and barley, in outbreak years the damage often occurs quicker in barley. Since populations of all of these insects vary from field to field, fields should be scouted to determine if economic levels are present. As a general guideline, if multiple insects are present, the threshold for each insect should be reduced by 1/3 to ½. 

 

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