Soil sample tests in North Dakota indicate a decrease in levels of overwintering wheat midge larvae (cocoons) for the 2012 season.
A total of 142 soil samples were collected from 20 counties to estimate the regional risk. The distribution of wheat midge in the 2012 forecast map is based on unparasitized cocoons found in the soil samples collected in the fall of 2011. With only 12 percent of the samples statewide being moderate to high risk for wheat midge infestation, most wheat producers should get a break from insecticide costs.
Although the wheat midge populations fell from last year's high, there still are a few pockets of moderate to high risk that need to be monitored closely in the northwestern and north-central regions of North Dakota. In 2011, wheat midge populations ranged from zero to 1,879 midge larvae per square meter, with an average of 171 larvae per square meter. In contrast, wheat midge populations ranged from zero to 3,750 midge larvae per square meter, with an average of 417 larvae per square meter in 2010.
The decrease in the wheat midge population can be attributed to the wet spring of 2011, which delayed wheat planting until June 5 to June 12 in the northern tier. This was later than the typically early to mid-May planting date and, based on the planting model for susceptibility to wheat midge, was just within the 600 growing degree days, which is the end of the susceptible period of wheat to wheat midge. Late planting dates, after 600 growing degree days, can mitigate midge damage. In addition, a large amount of agricultural acreage went into prevent planting, especially in the northwestern region of North Dakota. This reduced the availability of host crops to wheat midge.
Areas where populations of cocoons exceed 1,200 per square meter are at a high risk for wheat midge infestation in 2012, but accounted for only 2 percent of the samples. These areas include isolated pockets in central Burke and southwestern McHenry counties.
If wheat is planted in these high-risk areas, producers must be prepared to monitor the fields closely for wheat midge infestations and include the cost of an insecticide in their wheat production budget. Otherwise, undetected and uncontrolled infestations may result in significant yield losses. Fortunately, the price of wheat is high, which should make pest management decisions easier for producers.
Wheat midge populations of 501 to 1,200 midge larvae per square meter (moderate risk) account for only 9 percent of the samples. Areas at moderate risk include central Divide, central and southeastern Bottineau, eastern Ward, west-central McLean, southwestern McHenry, northern and southern Pierce, western Rolette and southern Cavalier counties. Areas where populations are above 500 larvae per square meter also will require close monitoring by wheat producers (see scouting section below).
In most of the remaining counties, 42 percent of the samples had 201 to 500 larvae per square meter (low risk) and 33 percent had 200 or fewer larvae per square meter. Although these areas with 201 to 500 midge larvae per square meter are considered low risk, it is good insurance to scout fields to determine if an action threshold population level exists.
Wheat midge larvae feed on the kernel and negatively affect yield, grade and quality. Early planting and selecting an early maturing variety of hard red spring wheat is one of the best preventive strategies to mitigate wheat midge damage. Early planting (prior to 200 midge growing degree days using a base of 40°F) can reduce midge damage because the wheat will flower before peak midge emergence. Wheat is most susceptible from heading to 80 percent of the primary heads with anthers. Planting between 200 and 600 midge degree days is the high-risk window for planting wheat because wheat midge will be emerging during heading. According to NDSU Extension Service agents and USDA NASS, most of the 2012 wheat crop was planted during this high risk period. Late-planted wheat (after 600 midge degree days) will miss the peak emergence of wheat midge but runs the risk of lower yields, frost damage or even greater losses due to barley yellow dwarf virus, which is a virus transmitted by cereal aphids.
To aid in scouting and risk evaluation, a degree-day model has been developed to predict the emergence of adult wheat midge and is available on the North Dakota Agricultural Weather Network at http://ndawn.ndsu.nodak.edu/wheat-midgedd-form.html. Select the nearest NDAWN weather station and enter your planting date to get a table that lists accumulated midge degree days and indicates whether your wheat is in the susceptible stage (heading) when wheat midge is emerging. Observations indicate the following degree day accumulations for events in the wheat midge population (base temperature 40°F):
The map (below) indicates that wheat midge adults are starting to emerge in areas with higher risks based on larval soil samples.
SCOUTING should be conducted at night when temperatures are greater than 59 degrees and winds are calm (less than 6 miles per hour) during the heading to early flowering crop stages. With the higher prices for hard red spring wheat, we recommend using the ECONOMIC THRESHOLD for durum when the adult midge density reaches one midge per seven to eight heads. The critical spray timing is from late heading to early flowering. Most insecticides labeled for wheat midge control can be tank-mixed with a fungicide if scab is a potential problem.
The adult wheat midge is an orange-colored, fragile, very small insect approximately half the size of a mosquito. It is about 0.08 to 0.12 inch (2 to 3 millimeters) long with three pairs of long legs. It has a pair of wings, which are oval, transparent and fringed with fine hairs. Two eyes are conspicuous and black. Be careful not to confuse the lauxaniid fly with wheat midge. The lauxaniid fly is yellowish brown, larger and more robust - about 1/10 to 1/6 inch (2 to 4 mm) in length - than the wheat midge. It also actively flies above the wheat canopy during the day and early evening. In contrast, the wheat midge flutters from plant to plant only in the evening. At night, the lauxaniid can be observed resting in the wheat canopy in a horizontal position with its head pointing down in contrast to the wheat midge, which rests with its head pointing upward.
The parasitic wasp, Macroglenes penetrans, plays an important role in keeping wheat midge in check naturally most years by killing the wheat midge larvae. The average parasitism rate for the wasp was 19 percent in 2011, compared with 17 percent in 2010 and 13 percent in 2009. Parasitism ranged from 0 percent to 100 percent across the state, with the higher rates occurring in areas where midge populations have been high the past year. More than half of the samples in 2011 (52 percent) had zero parasitism, which could cause wheat midge populations to increase in future years. Parasitic wasp populations should be conserved by spraying insecticides only when necessary, and avoiding any late insecticide applications (after 50% flowering) to minimize any negative impacts on the parasitic wasps, which are active at that time.
The soil samples were collected by NDSU Extension Service agents in the fall of 2011. The wheat midge soil survey is supported by the North Dakota Wheat Commission.