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Which insect pest and plant diseases to be monitored?
By Assefa Gebre-Amlak
Entomology Agent
Colorado State University Cooperative Extension
It is time to scout alfalfa and Wheat fields for insects and diseases. We are seeing two insect pests species, alfalfa weevil and pea aphid in alfalfa. Small colonies of Russian Wheat aphids were observed in the eastern part of Washington County. With the current dry situation, brown Wheat mite is another pest to be watchful of. There are also reports on these pests from western Kansas (Kansas State University).
Alfalfa weevil
Alfalfa weevil is the most destructive insect of alfalfa hay in the intermountain western region of the United States. Both larvae and adults feed on alfalfa; the larval stage is the damaging stage, lowering yield and quality.
When temperatures warm to about 48 F (9 C) in the spring, the weevils become active. First and second instars feed in the tightly folded leaves of stem buds. When half to full grown, the larvae tend to move onto open leaves near the terminals. Larval development is completed in about three to four weeks, with the peak damaging larval populations often coinciding with the first cutting of the crop. Fully-grown larvae move into the plant crowns and soil debris to pupate. The larvae spin loosely woven, net-like cocoons, in which they pupate.
Timing of sampling for weevils: Estimation of the weevil instars present in the field can be calculated using degree-days. Alfalfa weevil development increases at a nearly constant rate as the temperature rises above 48 F. (9 C.). For the alfalfa weevil, degree-days are accumulated after March 1 for each 24-hour period in which temperatures exceed 48 F (9 C).
When 425 degree-days have accumulated, fields should be inspected for second instar larvae, paying special attention to fields where the first instar larvae were detected. In the Golden Plains area, so far we have not accumulated the amount of degree-days indicated above. Our future updates will include accumulated degree-days.
The third and fourth larval instars cause most of the economic damage; therefore, it is very important to estimate weevil density before the damaging instars develop. Tip damage is readily visible at this time if a heavy infestation is present. Folded leaves must be opened to detect lighter infestations that still may develop into an economically important infestation. If larvae are detected during a minimum of a 10-minute initial search in any of your fields, sample surveys should be conducted in each field to determine if the infestation is likely to cause economic damage.
Cultural Control: 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. For effective products, check High Plains IPM Guide (www.highplainsipm.org).
Russian Wheat aphid
In 2003, a new biotype of the Russian Wheat aphid was observed. It is virulent to all of the resistant varieties. It is not known what the distribution of the new biotype will be, but currently it is assumed that it will occur wherever the original aphid was found. The original aphid is still the most common of the two types, so resistant varieties still have some value and are still recommend in areas with consistent Russian Wheat aphid problems.
Chemical control of Russian Wheat aphid is recommended for the new biotype as well as the original biotype on susceptible varieties. Use the economic thresholds below for chemical treatment at different crop stages. For effective products, check High Plains IPM Guide (www.highplainsipm.org).
Brown Wheat Mite
The mite infestation is associated with drought stress. Affected plants have finely mottled leaves that appear yellowed or bronzed at a distance, but lack the webbing produced by the Banks grass mite. Populations are usually limited to continuous Wheat or where volunteer was present during the previous spring.
Management: Because this pest problem is so sporadic, chemical control is the only effective management practice. The economic threshold for this pest is not well defined, but it is at least several hundred mites per row-foot in the early spring. The decision to treat is difficult since the mite is associated with drought stress. If it rains, mite levels will be significantly reduced regardless of the use of insecticides, while if it does not rain the crop yield may be so reduced by drought that it may not be worth treating. Also, if white eggs are present and red eggs are mostly hatched, the population is in natural decline, and treatment is not economically sustainable.
Effective products include Dimethoate, Lorsban and Dy-syston. If both Brown Wheat Mite and Banks Grass Mite infestations occur at the same time, use only Dimethoate, other products are not recommended for this situation. Before using these products, check High Plains IPM website (www.highplainsipm.org) for more information.
Date: 5/12/04
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