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Alfalfa weevil, wheat leaf and stripe rust on riseBy Assefa Gebre-Amlak Regional Pest Management Specialist Wheat In early May the Colorado State University diagnostic lab received several wheat samples from southeastern Colorado with rust. wheat leaf rust was found in Prowers County on variety Ripper. Alfalfa We are seeing an increase in alfalfa weevil infestations in many fields in northeastern Colorado and Weld County. Larvae feeding in the folded leaves can heavily damage stem terminals, but initial damage is not always clearly visible. The closed, overlapping foliage of the stem terminals should be unfolded to detect feeding damage. Heavily infested stands have a grayish or frostlike appearance due to the dried defoliated leaves. Damage also may reduce hay quality due to loss of leaf tissue, leaving only the lower quality stems. Damage to regrowth buds may occur when the plants break dormancy and after first cutting. Monitoring alfalfa weevil: Two sampling techniques may be used. Sweep sampling using a standard sized 38 cm diameter net is the most efficient method for estimating larval population. Sampling should begin when 148 degree days have been accumulated, when the larvae are expected to be primarily second instars and when alfalfa hay has reached at least 10 inches in height. Ten, 180 degree sweeps are taken while the sampler is walking through the field. Count the number of larvae per sweep and repeat this sampling procedure, taking a minimum of three samples for fields up to 20 acres, four samples for fields up to 30 acres and five samples for larger fields. Weevil infestation may be patchy or uniform depending on terrain, weather and soil. Inspection for weevils in every distinctive section of the field will aid in determining the pattern of the infestation. Bucket method or stem count method is used to determine the number of weevil larvae per stem. Take three six-stem samples in fields one to 19 acres, four samples in fields 20 to 29 acres, and five samples in fields 30 acres. Decision making: the economic threshold for a sweep sample is 20 larvae per sweep. The economic threshold for the stem sampling method is 1 1/2 to 2 larvae per stem. A non-insecticide control measure for alfalfa weevils is an early first harvest if an economic infestation is not detected until late in the growth of the first cutting. Harvesting alfalfa in an immature stage provides good control of larvae for the first crop. Rapid removal of hay will accelerate larval mortality due to desiccation by direct sunlight. An early first cutting tends to cure more rapidly because lighter windrows dry quickly, and forage quality is enhanced by higher crude protein and lower fiber content. Additional steps should be taken to ensure that surviving larvae do not cause economic damage to the regrowth. If larval survival under the windrows is high and baling is delayed (e.g., due to rainfall), damage to regrowth may be exacerbated. Regrowth should be inspected at a height of one to two inches to determine larval density. Corn Two insects that may cause damage to your corn seeds and seedlings are seedcorn maggots and pale western cutworm. Pay attention to lack emergence and seedling damage. We will provide details on their damage and management next week. For more detailed management information on wheat diseases and alfalfa weevil check our IPM guide at highplainsipm.org. A 4 6/4/07 Date: 5/25/07
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