Some of the pests we currently see in north and northeastern Colorado include first generation European corn borer, grasshoppers, western bean cutworm, spider mites and banded sunflower moth. We recommend special attention should be given to western bean cutworm and spider mites the next two to three weeks.
Western bean cutworm moth flight activity has increased in all pheromone trap locations in Colorado (www.NoCopestalert.org). Historic moth flight data from light traps in northeastern Colorado show that the moth flight peaks at most of the locations between July 12 and July 27. We are using pheromone traps, this year, to monitor WBC.
Eggs are laid shortly after the moths emerge. The eggs are deposited in clusters of four to 200 on the top surface of upper leaves. When first laid, the eggs are white with a thin red ring around the top. As they age, they change to brown, then immediately prior to hatching, they are purple to black in color. The eggs hatch in five to seven days. The majority of the western bean cutworms feed until mid-September.
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. Destruction of the kernels may reduce corn yields by as much as 30 to 40 percent.
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 that hybrids containing the Herculex corn borer trait have moderate resistance to western bean cutworm.
Western bean cutworm is also a pest of dry beans. Egg masses and larvae are easier to scout in corn than in dry beans. If corn is infested, adjacent beans are likely to be infested, also.
Damage by young western bean cutworm larvae is not very noticeable in dry beans, as they feed primarily on tender leaves and flower parts. As pods form, worms chew holes in the pod walls and feed on developing seeds. Pod feeding generally begins about three weeks after the peak pheromone trap catch. Feeding occurs primarily at night and during cloudy days. 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.
If you are using pheromone traps to monitor western bean cutworm in dry beans, watch potential for economic loss, and apply insecticides 10 to 20 days after the peak flight occurred. On the date of peak catch if the cumulative totaled across the two traps for a field is:
1. Between 0 to 700 moths per trap, the risk of significant damage by WBC is low.
2. Between 700 and 1000 moths per trap, the damage is low to medium:
--Presence of substantial WBC activity in adjacent corn fields. These fields should be thoroughly scouted to decide whether to treat either crop.
--Presence of pod feeding approximately three weeks after peak flight. If pod feeding is noticeable, a treatment should be considered.
3. Above 1000 moths per trap, the risk is high. Treatment for WBC may be warranted. The greater the trap catch, the greater the risk for damage. Adjacent corn fields and pod feeding in the beans can be checked to further verify the need for treatment.
There are two potential species of mites that may infest your corn (two spotted mite and banks grass mite). Banks grass mite infestations up to five leaves were observed in northeastern Colorado and Front Range counties. Banks grass mite builds up on the plant from the bottom up. With increasing hot and dry conditions, mite problems can be expected to increase. Scout your corn fields for this pest along with western bean cutworm and 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.
Grasshoppers are still abundant along edges of crop fields and the rangeland/pastures. If small hoppers reach 20 per square yard in field margin, a chemical control may be justified. Once they disperse through the field, eight hoppers per square yard are considered to be economical. Use of higher rates of effective insecticides is recommended when adults are present.
In rangeland/pastures, 15 to 20 grasshopper nymphs per square yard is considered the economic threshold. 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.
Since grasshoppers typically breed outside the yard or garden, controlling grasshoppers in these areas is most productive. As grasshoppers move over your fence, insecticide treatments in the yard and garden are of little effect. On the other hand, sprays of several insecticides kill grasshoppers on rangeland, pastures and crops. Various baits can also be used against these pests.
For details of effective pesticides and rates of application for management of the pests mentioned, check the High Plains IPM guide at http://highplainsipm.org.