Which insect pests are to be monitored now?
European corn borer moth numbers continue to increase in pheromone traps in most of the trap locations in northeastern Colorado. We should be able to see larva feeding damage in corn starting this week and continue scouting. Larval damage is noticed as feeding scars and shot holes in the plants leaves.
To determine infestation levels, check 25 plants in 4 locations or 20 plants in 5 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 of its management.
Once economic infestations of European corn borer are determined, there are a number of effective insecticide products available for its control (High Plains IPM Guide (www.highplainsipm.org).
We are seeing large numbers of grasshoppers (mostly immature stage) in Yuma and eastern part of Washington County. Continue monitoring for this pest both in crops and rangeland the next three weeks in the indicated counties.
In crop land, grasshoppers build up in the field margins which should be scouted during the season. If the small hoppers (nymphs) reach 20 per square yard in margins, a chemical treatment may be justified. Once they disperse through the field, eight per square yard is considered to be economic.
In grassland, 15 to 20 grasshopper nymphs per square yard is considered the economic threshold. It is important to estimate grasshopper population density to determine if economic threshold has been reached. When estimating grasshopper density, walk 50 to 100 feet away from the road, finding a typical rangeland. This number is considered to equate to eight to 10 adults. 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.
Cutworms and wireworms are the two groups of insects to keep an eye during the early stage of sunflower development. The cutworms include darksided cutworm, dingy cutworm complex, sandhill cutworms and pale western cutworms. Check your fields and treat if there is one cutworm per square foot or if stand losses are approaching the lower limits of optimum plant populations. Consider sieving the surface of 5 square feet of soil through coarse screening as way of improving detection of cutworm larvae.
Wire worms are most damaging to sunflowers when the crop planted in wheat stubble because the adult beetles are attracted to grasses to lay eggs. The larvae kill plants by feeding directly on the seed and preventing germination or feeding on the stem between seed and the soil line. There is nor rescue treatment for wireworm.