June 1999
By Tracy Sayler & Dorinda Anderson
Library
Prairie Grains is the official
publication of
the Minnesota
Association of
Wheat Growers,
North Dakota Grain Growers Association,
South Dakota Wheat, Inc., and the Minnesota Barley
Growers Association.
Prolonged periods of cloudy, wet weather can cause grasshopper disease, and bouts of heavy rains during emergence will kill young grasshoppers, by trapping them in the soil or physically washing them away, says Phil Glogoza, North Dakota State University extension entomologist.
Although the outlook for grasshoppers is down considerably from previous years, it will still be wise to anticipate localized hot spots, which will correspond with areas that had adult grasshoppers present last fall.
Unfortunately, weather that hurts grasshopper development favors development of the wheat midge. Moist soil conditions benefit midge emergence, and late planting makes wheat more susceptible to midge infestations.
Soil surveys taken last fall indicated a general increase in overwintering wheat midge populations throughout northern and central counties in ND. Further, dramatic population increases occurred in Divide and Burke counties, where wheat midge had not caused significant problems during the past three years.
"The population increases in an area from Pierce to Dickey counties in the central part of the state are particularly alarming, from the standpoint that past economic infestations were more localized," says Glogoza.
Areas of northwest Minnesota saw wheat midge activity last year, including Kittson, Marshall, and even some parts of Norman, which had not seen activity before. Although insect problems are cyclical with the weather, University of Minnesota extension entomologist Ian MacRae believes the wheat midge has joined the grasshopper as an insect that will need to be monitored every year in the Northern Plains.
Fortunately, wheat midge prediction, scouting, and control measures are improving. NDSU crop scientists now use a temperature-based prediction model that can be matched with wheat development to pinpoint when wheat plants are vulnerable to the midge, and to help determine when scouting is warranted.
Wheat is susceptible to midge infestation from the time the head emerges from the boot until 80 percent of the primary heads have anthers visible. Peak midge activity occurs about 9 p.m., on evenings when air temperatures exceed 59 degrees F and wind speed is less than 6 miles per hour. When temperatures are less than 59 degrees, or wind speed is greater than 6 mph, adults are not actively laying eggs on the primary wheat heads. Lorsban 4E-SG is the sole registered treatment, and should be applied when 75% of wheat heads have emerged from the boot. It’s too late to treat after 50% of wheat heads have flowered.
Whether to treat when prices are low can be a tough call. However, Glogoza believes that for some insects, including the wheat midge, the economic threshold can adjust with the price.
"The recommended treatment level is generally when one adult midge is observed for every four or five wheat heads. Treatment costs about $10 per acre. If you can recoup that cost, then follow the standard threshold. If not, consider moving the threshold from one adult midge per three wheat heads. I try to emphasize the best management information we have, and then have producers adjust it as they see fit, knowing what prices are," says Glogoza.
Some producers may wish to combine fungicide and insecticide applications. This can be done as long as the two chemicals don’t antagonize each other, and treatment timing is appropriate for both chemicals.
For example, if treatment for aphids is warranted, it needs to be done at the flag leaf stage to prevent most economic loss. However, applying a foliar fungicide to suppress scab needs to be done at grain flowering. Thus, the two treatments aren’t compatible.
Indeed, spraying a field as a type of insurance because insects might be present isn’t economical, and can even lead to insecticide resistance, says MacRae. Scouting, though time consuming, is important so spraying is done when needed, and at the right rates to tackle the problem.
See the following web sites for more online information on insect management in small grains and other crops: http://www.ndsu.nodak.edu/entupdates, and http://www.extension.umn.edu/Crops/.
Survey of Wheat Midge
Larvae in ND
Soil Shows Overwintering Midge Numbers Up for 1999*
No data 0-220 221-506 507-814 815-1188 >1188
Midge larvae per square meter
*Survey conducted by Department of Entomology at NDSU
June 1999