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What's bugging you?By Assefa Gebre-Amlak CSU Extension, Regional Pest Management Specialist Colorado European corn borer moth begins to fly European corn borer moths are flying in most of the pheromone trap locations in Colorado. Corn borer larval damage is not expected in corn plants that are below 16 inches long (with extended leaves) due to presence of a chemical substance called DIMBOA. However, as corn develops, most varieties lose their resistance to larval feeding and should be scouted. First generation corn borers prefer taller plants for egg laying; therefore, the earliest planted fields are more likely to have higher egg populations. Non-Bt corn fields need to be scouted if the corn is above 16 inches in extended leaf height. Scouting for egg masses on first generation corn borer is impractical, and infestation is best determined by examining plants for feeding symptoms and live larvae. Larval damage is noticed as feeding scars and shot holes in the plants leaves. To determine infestation levels, check 50 plants in four to five locations in the field. Be sure to move into the field at least 100 feet from the border. To determine the number of live larvae, pull the whorl from each symptomatic plant and carefully unwrap the leaves. Chemical control of first generation is justified when 25 percent of the plants have feeding damage and live larvae. Control measures must be taken before the larvae bore into the stalk. A more complex formula which incorporates the number of borers per plant, control cost, expected percent control, and the value of the crop is available in the High Plains Pest Management Guide or can be obtained from the Cooperative Extension Service. Please see Table 1 for weekly first generation corn borer. Timely and accurate scouting is the key to managing European corn borer in standard (non-Bt) corn hybrids. Remember that conditions are localized and each field should be scouted to make accurate decisions of its management. Western corn rootworm Western corn rootworm damage symptoms are being observed in cornfields in many locations and fields need to be scouted. 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. 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. Sunflower stem weevil emerging According to the Colorado State University field study, accumulated degree-days can be used to predict emergence of adult weevils from old sunflower stalks in northeastern Colorado. We believe, at the moment, 90 percent adult emergence has already occurred either this week or last week in northeastern Colorado. For the actual accumulated degree-days in your area, check our web site: www.NoCopestalert.org. Degree day accumulation for your location can be calculated by taking average of daily maximum and minimum temperatures and subtracting 42.8 degree F. (threshold for this species) and summing that up for dates and months desired. The sunflower stem weevils infest native sunflowers and other wild host plants when first they emerge, before they move to new sunflower crops. CSU studies also showed that sunflower fields planted before June 1 are much more likely to be infested. Adult stem weevils are gray-brown in color with white spots on their upper body and are only one-eighth of an inch long. Plant feeding by the adult sunflower stem weevil causes very little damage. Larval feeding activity can cause two types of damage: (1) introduction of disease through feeding sites and (2) lodging of plants prior to harvest. Scouting and management techniques of sunflower stem weevil will be covered in the next pest alert. For more pest management information, check High Plains IPM Guide (http://highplainsipm.org) and www.NoCopestalert.org or contact your county Extension office. 6/30/08 Date: 6/25/08
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