Agronomic Crop Insects

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

Alfalfa
Potato leafhoppers are now present in fields so be sure to sample on a weekly basis after the first cutting. Once the damage is found, 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
During the past week, we have seen an increase in the number of no-till fields with economic damage from cutworms. Damage has mainly occurred in fields that were not treated with a cutworm product at planting. Be sure to scout fields carefully for cutworms – in some cases you will need to check fields twice a week to be sure you do not miss an economic population. In addition to cut plants, be sure to watch for leaf feeding which can be an indication of the potential for significant cutting damage and yield loss.

Slugs continue to be a problem in later planted fields. In fields where Deadline MPs have been applied at a rate of 10 lbs. per acre and the distribution of pellets is at 5 per square foot, control has been very good. The best control with the Deadline M-Ps (up to 2 weeks in some cases) has been observed when applications were made and there was at least one day of sunny weather after an application.

Small Grains
We continue to find armyworms and cereal leaf beetles in barley and wheat fields that were not treated. Population levels remain variable throughout the state so scouting fields will be the only way to determine if an economic level is present. Although armyworm can attack both wheat and barley, they can quickly cause significant losses in barley. Heavy defoliation of the flag leaf can result in significant economic loss. Armyworms generally begin head clipping when all vegetation is consumed and the last succulent part of the plant is the stem just below the grain head. Larvae can feed on the kernel tips of the wheat, resulting in premature ripening and lower test weight.

As barley and wheat approach harvest, the treatment options change due to the pre-harvest interval (the waiting period between application and harvest). In addition, not all materials are labeled on both crops so be sure to carefully read all labels – remember the label is the law.

Soybeans
As the earliest beans emerge, be sure to watch carefully for slug damage. Remember, if you had a problem in past years, the slugs may still be present in fields and can quickly damage soybeans if present as plants emerge. Be sure to also watch fields carefully for bean leaf beetles and grasshoppers. Small grasshoppers have already been detected in fields before planting. Early detection and control of small grasshoppers is necessary to achieve control. As a reminder, OP insecticides (examples – dimethoate or Lorsban) cannot be combined with SU/ALS herbicides. Since other materials may also state restrictions regarding combinations of insecticide and herbicides, you should be sure to check all labels carefully before combining insecticides and herbicides.

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