Northeastern Colorado Pest Update

What’s bugging you?

by Assefa Gebre-Amlak, Extension Specialist, Colorado State University Extension

Grasshoppers: we are seeing some nymphs (immature grasshoppers) in Northeastern Colorado but the populations are much smaller than the economic threshold for the pest. USDA APHIS counts this week also show lower numbers (1 to 8 nymphs) per square yard in most of the area except two sites south of Wray (Yuma County) with 9-14 nymphs. In addition, the nymphs are at 1st and 2nd instar stages. There appears to be a delayed hatching and emergence of grasshoppers in most of the area.

However, landowners in high risk areas should continue monitoring grasshopper populations in rangeland. Early scouting is important because treatments are most effective when grasshoppers are small. The goal of scouting is to get an estimate of grasshoppers per square yard, as well as their stage of development.

Higher risk of grasshopper infestation is predicted for Yuma, Logan, Morgan and Washington counties in northeastern Colorado. Other counties with more localized spots of high risk include Kit Carson, Weld, Phillips, Sedgwick, Lincoln, Cheyenne, Kiowa, Prowers, Bent and Otero.

Weather conditions will determine how much of the outbreak potential will be realized. For example, cool wet conditions after hatch can result in enough mortality in immature grasshoppers to prevent an outbreak. In addition, if adequate moisture is available, forage regrowth will offset much of the grasshopper damage.

Most grasshopper outbreaks occur when drought conditions are prevalent.

There are over 100 different species of grasshoppers in Colorado but only about a dozen of these are considered important on rangeland, and five species of these cause most problems on crops.

Economic threshold for grasshoppers on rangeland: The simple economic threshold for grasshoppers in rangeland is 15-20 grasshopper nymphs per square yard. This number should result in eight to ten adult grasshoppers per square yard.  For details of grasshopper management go our website at:

Sunflower Stem Weevil: on the 15th of June 2011, 986-1182 degree days have been accumulated in northeastern Colorado using 41 degree Fahrenheit as a threshold for sunflower stem weevil development. According to CSU prediction model, 75% of adult weevil may emerge from old sunflower stalks in the field early this week (see degree days at:

Sunflower fields planted before June first are much more likely to be infested according to CSU field studies on this insect. Therefore, special attention should be given to early planted sunflowers 8 -14 leaf stage which is very attractive for egg laying by adult weevils. Late planted (after June 1st) sunflower fields will have less infestation of the insect.

The adults are difficult to see and have habit of dropping to the ground and “playing dead” when disturbed. Insecticides are applied to prevent adults from depositing their eggs in the stalk. A treatment is justified when one or more adults are found per three plants from V-8 through R-1.

Western corn rootworm: the WCR larvae feed on the underground root systems of corn plants. Peak feeding usually occurs from late June to mid July. Lodging (goose necking) of corn plants due to larval root feeding is a typical symptom of damage. Early planted fields will have relatively larger root systems when rootworm feeding starts and this makes them somewhat more tolerant to rootworm damage.

Damage from corn rootworm larvae is most likely in continuous corn. Chemical applications to first year corn are not recommended. Incorporation of soil insecticides into the soil protects wildlife.

If corn is planted prior to May 15, post emergent treatments are preferable. Performance of soil insecticides should be checked by leaving an untreated strip in each field.

1st Generation European corn borer: we are seeing European corn borer moth flight in some trap locations. However, there is delayed moth emergence as compared to historic flight data in northeastern Colorado. More moths are expected the next two weeks.

Corn crop has not reached the stage that is attractive for 1st generation egg-laying in most of the cornfields at the moment. More information on this pest management will come the next week.

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