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Which insect pest and plant diseases to be monitored early spriBy Assefa Gebre-Amlak Pest Management Specialist, Northern Region Colorado State University Cooperative Extension You may not see insect pests and/or diseases in your fields at the moment, but it is time of the year that the early pest problems may show up the next couple of weeks depending on the weather situations and scouting alfalfa and wheat fields for insects and diseases should be considered. 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 1 March for each 24-hour period in which temperatures exceed 48°F (9°C). Using this technique sampling should begin when 148 degree days have accumulated. This timing coincides with the peak occurrence of second instars. In years of unusually warm or cold springs, weevil development may occur earlier or later than normal. The alfalfa weevil population relative to the estimated date of first harvest may influence management choices. Third and fourth larval instars cause most of the economic damage, so initiating sampling at the peak occurrence of second instars should provide adequate sampling prior to economic weevil populations. In the northern region, 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. 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 that it will occur wherever the original aphid was found. The original aphid is still present, 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.
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. We have had no reports of leaf or stripe rust in Colorado. There are reports of light leaf rust in northern Texas. The weather patterns the last week may have increased its intensity. We need to keep an eye out. Remember that the critical time for control is when the flag leaf emerges, so we still have time to make decisions on whether to spray or not. Colorado State University will be surveying wheat fields this season in eastern Colorado for nematodes and specifically for cereal cyst nematode. According to Dr. Ned Tisserat, we currently do not have nematode problems in wheat in the eastern portion of the state, but we also have not done a systematic survey. For effective products and other management options for the pest problems mentioned, check High Plains IPM Guide (www.highplainsipm.org). A 8 5/7/07 None Date: 4/20/07
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