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Scouting crop fields for pests?
Colorado There are a number of pests, namely, spider mites, western bean cutworm, sunflower stem weevils, European corn borer and grasshoppers (some areas) that should be included in scouting activity in our region. Banks grass mite Infestations have been detected in both northeastern Colorado and Front Range counties. Infestations of up to 8 leaves were observed in Eaton, weld County. With increasing hot and dry conditions, mite problems may increase in corn the next couple of weeks. Banks grass mites are minute, greenish colored arthropods with eight legs and a rounded body. BGM has dark pigmentation along both edges of the body near the rear and along the sides. Webbing on leaves and discoloration are often the first signs of an infestation. Initially, BGM are most abundant on the lower third of the plant and density declines as the infestation moves up in the plant. Mites damage corn and small grains by piercing plant cells with their mouthparts and sucking the plant juices. Banks grass mite builds up on the plant from the bottom up. 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. If the mites build up early and hot, dry weather persists, a second application may prove necessary. Areas of stressed plants are often the main source of infestation for the rest of the field. Control with Comite may not last for the full season. In the areas where both Banks grass mite and two-spotted mite species occur, the only effective chemical treatment seems to be an application of Comite II and bifenthrin 2ER as described for Banks grass mite control. Other materials registered, but not recommended, for this situation include dimethoate. Grasshoppers Grasshopper populations have been increasing in the Eckley and Wray areas and special attention should be paid to crop fields including corn, alfalfa and beans in these locations. Common grasshopper pest species include: Differential Grasshoppers, Redlegged grasshoppers, Migratory grasshoppers, and Twostripped Grasshoppers. Often grasshoppers develop in uncultivated areas and move into cultivated fields. Most grasshoppers overwinter as eggs in the form of a pod laid in the soil. Eggs hatch into nymphs in late May and June, maturing in two to three months. They build up in the field margins, which should be scouted during the season. If small hoppers reach 20 per square yard in margins, a chemical control may be justified. Once they disperse through the field, eight per square yard is considered to be economical. Sunflower stem weevil The adult female stem weevil prefers 8-14 leaf (about six to eight inches tall) sunflower plants for infestation and egg laying. Sunflower fields planted during last week of May 2003 are currently close to 8-14 leaf stage and may be infested. CSU studies show that fields planted before June first 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. Scouting for stem weevil is extremely difficult because of the adult weevil's tendency to drop to the ground at the slightest disturbance. When scouting for adult weevil, approach the plant with your face to the sun. Your shadow cast over the plant will cause the weevil to drop off the plant and go undetected. Next, look in the leaf axils, the undersides of leaves, and along the stem for the weevil. When scouting, sample sites should be 75 to 100 feet from the edge of the field. Use an X-pattern and examine five plants per sampling site for a total of 25 plants at eight to 14 leaf stages (development stages V-8 to R-1) in late June to early July. Determine the average number of adults per plant. Careful examination in several locations throughout the field will help determine stem weevil population present. A treatment is probably justified when one adult is found per three plants. Chemical applications are made to prevent the adults from depositing their eggs in stalks. Controlling the sunflower stem weevil can be accomplished at planting or with foliar applications of insecticide. CSU studies show that Furadan 4F treatment at planting or cultivation is more effective against sunflower stem weevil larvae than other insecticides. Foliar applications of other chemicals used in a timely manner may also be effective. Registered chemicals for foliar applications on sunflowers include: Asana XL, Baythroid, Sevin, Furadan 4F, Lorsban 4E, Superacide 2E, and Warrior. Onion thrips Thrips on almost all plants are way, way down from last year but not on onions. The common species present is almost 100 percent onion thrips (Thrips tabaci), unlike last year when many plantings were affected primarily by various Frankliniella spp. Onion thrips may infest a wide range of host plants including alfalfa and dry beans. In onions, onion thrips populations can be best determined by counting all the thrips on the plants. The great majority of the thrips are present at the base of the youngest pair of leaves. Thrips nymphs are pale yellow and the winged adults vary from light brown to dark brown. Knowledge of the relative susceptibility of a variety can be used to modify action thresholds. Since higher numbers of thrips are needed on tolerant varieties to benefit from insecticide treatment the action thresholds will be raised. As an interim recommendation an action threshold of 30 or more thrips/plant or higher is suggested for more thrips tolerant varieties. Action thresholds for varieties that are susceptible would be lower, perhaps in the range of 15-30 thrips. Some of the effective products for onion thrips management include Mustang Warrior, Pounce and Lannate. Western bean cutworm moth flying Western bean cutworm moth emergence and flight was detected between June 19 and 24 in two light trap locations in northeastern Colorado. Last week, all traps had western bean cutworm moths and the highest population was recorded in Haxtun and followed by Wauneta. First generation corn borer European corn borer moth numbers have declined in all trap locations between June 26 and July 3. The young first generation larvae should be feeding on the leaf near where they hatched. As the larvae grow, they move to the whorl or leaf sheath arena and feed. Larval damage is noticed as feeding scars and "shot holes" in the leaves. Symptomatic plants may be seen in cornfields starting this week. Larval damage is noticed as feeding scars and shot holes in the plants leaves. These damage symptoms are used in scouting of the first generation European corn borer. To determine infestation levels, check 25 plants in four locations or 20 plants in 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. 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. Date: 7/26/07
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