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Time to start scouting fields
By Assefa Gebre-Amlak Regional Pest Management Specialist, Colorado State University Extension We are seeing western corn rootworm t beetles in four out of five locations in northeastern Colorado. The highest numbers of beetles were observed in Wauneta (Yuma County). Adults often feed on corn silks. Severe silk pruning may result in yield reduction due to poor pollination. However, most damage is due to larval root feeding. Control of rootworm adults is intended either to protect silks during pollination or to prevent egg laying and damage to roots in next year's crop. Adults rarely become numerous enough to interfere with pollination. Control may be justifiable if there are more than 10 beetles per ear zone during the wet silk stage. If treatments are intended to prevent egg laying then treatment is recommended when beetle counts exceed 18,000 beetles per acre (three beetles in four plants at 24,000 plants per acre). This threshold also can be used for determining the need for a soil insecticide in the following year if adult control is not used. In first year corn, lower this to 12,000 beetles per acre because a higher proportion of the population will be egg-laying females. Adult treatments applied too early, that is, before 10 percent of the females are carrying fertile eggs, may not have much effect on egg laying. Determining the percentage of females with fertile eggs can be difficult, but generally the proper time for the application of adult treatments occurs two to three weeks after the first adult emerges. A second adult treatment should be considered if beetle densities rebound to above 12,000 per acre (one beetle in two plants at 24,000 plant per acre). Second generation European corn borer moth flight has also started at some light locations (Burlington and Eckley) in the Golden Plains Area. We have seen the second generation egg masses in the Eckley area. Corn borer egg masses are generally found on the underside of the leaf usually within three leaves above or below the ear leaf. These egg masses appear flat and white with black spots appearing as the eggs mature. Generally, late maturing cornfields are more attractive to 2nd generation egg laying moths than early maturing fields. Second generation flight can last for four weeks. Damage due to second brood in corn may contribute to lodging and ear drop losses. The simple method used to determine economic threshold is based on accumulation of the percentage of plants with egg masses during two consecutive weeks of scouting following the early portion of the egg laying period. If the first and the second scouting show 25 percent of cumulative plants with egg masses, treatments should be made. This percentage should be raised to at least 50 percent after pollination. Chemical treatment should be made prior to larval entry into the ear. If egg laying continues after treatment, a second application may be justified. Another more detailed method based on treatment cost, predicted yield, larval survival, and corn prices can be found in the High Plains IPM Guide, which is available at your local extension office. Spider mites: Low banks grass mite infestations were observed at three of our five locations in northeastern Colorado. These included Burlington, Kirk and Wauneta. Preventive miticide treatments with Comite II or Oberon should be considered now, as they will not be effective once infestations are well established. Once infestations are well established the most effective treatment in CSU studies has been a combination of bifenthrin and dimethoate (both of these are available under several trade names). Treat when there is visible damage in the lower third of the plant and small colonies are present in the middle third of the plant before hard dough stage. Webbing on leaves and discoloration are often the first signs of an infestation. Sunflower Insects: The three major pests of sunflower namely, sunflower head moth, banded moth and sunflower seed weevil are still commonly seen in the sunflower fields. Insecticide applications made at early bloom (R5.1) to prevent head as well as banded moths from laying eggs. Scouting in the early morning or early evening will provide the most accurate counts, since moths are most active at these times. Pheromone traps can be used to determine when scouting should be started, but a pheromone-based treatment threshold is not available for banded moth. Pheromone traps may be used to monitor head moth activity (see the High Plains Sunflower Production Handbook, www.oznet.ksu.edu/library/crpsl2/MF2384.pdf, for details). Less than one sunflower moth trapped per night is considered low risk, while more than four moths trapped per night is considered high risk and justification for treatment. Adults head moths migrate from southern areas in late June and July and are attracted to sunflowers beginning to bloom. Larvae begin to tunnel into seeds and other head tissue from July to August and each larva can destroy four to six seeds during development. Younger sunflower moth larvae feed primarily on florets and pollen. Older larvae tunnel through immature seeds and other parts of the head. As the larvae feed and spin silken threads, the dying florets and grass give the sunflower head a trashy appearance. When scouting, sample sites should be 75 to 100 feet from the edge of the field. Use an X-pattern, counting moths on 20 heads per sampling site for a total of 100 heads. One moth per two plants is the currently accepted economic threshold level. If visual scouting is used instead of pheromone traps, consider treatment if more than two sunflower head moths per five plants are observed while scouting during early bloom. Sunflower seed weevils (red and gray) are both small weevils found in sunflower heads, although the gray sunflower weevil is larger. Egg laying begins at the outer edge of the head and progresses inward, following seed development. Adults may be found from June to September. If seed weevil infestations are encountered late in the year, harvest may be delayed to avoid bringing infested seed into storage. Larvae emerging in storage will not damage additional seeds, but their bodies will remain in the storage. Insecticide applications are made to prevent adults from laying their eggs. Treat red sunflower weevil on oilseed sunflower when about 30 percent of the plants have reached the R 5.1 stage. The economic threshold ranges from 5 to 15 weevils per head, depending on plant population and market conditions (see the High Plains IPM guide for details). Confection sunflower should be treated to avoid quality penalties if less than 10 to 15 percent of the plants have reached R 5.1 and one or more red sunflower seed weevil can be found per head. Gray sunflower seed weevil is thought to be economically insignificant under most conditions. Scouting for red sunflower seed weevil can be difficult because of its distribution in the field and because of its habit of hiding in the heads. Start scouting when the yellow ray petals are first visible and stop when the majority of the plants in the field have passed 70 percent pollen shed (R 5.7), or when the action threshold has been exceeded. Avoid taking seed weevil counts from plants in field margins as they tend to congregate in these areas and counts will not be representative of the entire field. Count five sets of five plants, distributed across the field in an X-pattern. Date: 8/23/07
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