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Pests infesting summer crops in Colorado


We are seeing a significant increase in Western Bean cutworm moth populations in traps this week. Other current pests include first generation European corn borer, grasshoppers and sunflower stem weevils.

Western bean cutworm was originally considered to be a pest of dry beans but now is recognized as a serious pest of field corn. Following hatch, young western bean cutworms move to one of two places on the corn plant, depending on the stage of development of the corn. If corn has not tasseled, larvae feed on pollen in the developing tassel. If corn has tasseled, larvae feed on silk in the ear: this type of silk feeding may cause pollination to be poor. Once the ear has formed, larvae feed on developing kernels.

Growers should scout fields until 8 or more percent of the plants have egg masses or small larvae in the tassels, and the crop is at least 95 percent tasseled. Chemical control should be initiated after 95 percent tasseling but prior to larval entry into the ear. If tasseling is much less than 95 percent, the percentage plants infested should be raised, as fewer larvae are likely to reach the ear. Keep in mind those hybrids containing the Herculex corn borer trait.

Damage by young western bean cutworm larvae in dry beans is not very noticeable, as they feed primarily on tender leaves and flower parts. Pod feeding generally begins about three weeks after the peak pheromone trap catch. Feeding occurs primarily at night and during cloudy days. As pods form, worms chew holes in the pod walls and feed on developing seeds. Some mid-sized worms may remain in the pod during the day, but larger cutworms will hide in the soil at the base of the plants.

Two pheromone traps per field are used to monitor western bean cutworm moth in dry beans. On the date of peak pheromone trap catch, if the seasonal cumulative total catch averaged across the two traps for a field is: between 700 and 1000 moths per trap, the risk of significant damage the damage is low to medium and above 1000 moths per trap the risk is high.

If counts are high enough to warrant treatment, treatment should be applied 10 to 20 days after the peak flight. This allows for all eggs laid in beans to hatch, is early enough to avoid damage from large worms, and results in maximum control.

Grasshoppers: mixed populations of adult and immature grasshoppers are currently seen in a number of counties of the northern region including Logan, Morgan, Washington, Weld and Yuma. Large populations of grasshoppers are found in weedy areas of roadsides. The highest populations were observed in two locations, Highway 61 and County Road 57 in Washington County and between Yuma and Eckley on highway 34 in Yuma County where thousands of grasshoppers on highways.

In rangelands, 15 to 20 grasshopper nymphs per square yard is considered the economic threshold. This number is considered to equate to eight to 10 adults. When estimating grasshopper density, walk 50 to 100 ft away from the road, finding a typical rangeland. However, the economic threshold can be modified by climatic conditions. If moisture is adequate regrowth of the consumed or destroyed vegetation may offset the damage.

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