Wheat Midge

• Orange Wheat Blossom Midge

Introduction

The wheat midge (Sitodiplosis mosellana) is found around the world wherever wheat is grown. In recent years, significant damage to wheat crops has been reported in Alberta, Saskatchewan, Manitoba, Minnesota, North Dakota and several regions of British Columbia.

All wheat varieties are currently susceptible to wheat midge but some are more seriously affected than others. Although the midge also attacks other members of the grass family including barley, couch grass, intermediate wheat grass and rye, infestations on these plants are usually not serious enough to warrant control.

Damage

Infestations of wheat midge can reduce crop yields and lower the grade of the harvested grain.

The midge may exist at low population levels for several years before it becomes a significant problem. But if conditions become favourable, populations can reach epidemic proportions quickly. Producers often mistake the symptoms of an infestation and report that frost or drought has reduced wheat yields or grain quality.

Crop damage occurs during the larval stage. After hatching, the midge larva feeds on the developing wheat kernel, causing it to shrivel, crack and become deformed. The damage to the crop is not readily apparent because there are no visible changes in colour, size, or shape of the affected wheat head. Damage can only be detected by inspecting the developing seed itself.

Damage to wheat kernels will vary within a single head. A few kernels may be aborted entirely. Others will not fully develop and will be so small and light, they will pass through the combine with the chaff during harvest. Still others may be only slightly damaged. Some may not be affected at all.

The loss of individual kernels will lower yield, whereas damaged kernels will reduce the grade of the harvested wheat. Standards established by the Canadian Grain Commission limit midge damage in No. 1 CWRS and No. 2 CWRS to two per cent and five per cent respectively before grade is affected.

Life Cycle and Identification

The insect passes through a four-stage life cycle.

Adult

The adult midge is a very small, fragile orange fly about half the size of a mosquito, approximately 2-3 mm long (1/8 inch). Two black eyes cover much of its brown head. The midge has three pairs of legs which are long relative to its body size. Its wings are oval shaped, transparent and fringed with fine hairs.

Adult midge emerge from the pupal stage in late June or early July. During the day, adults remain within the crop canopy where conditions are humid. In the evening, females become active at the top of the wheat canopy, laying their eggs on the newly emerged heads of wheat. Female midge live for less than seven days and deposit an average of 80 eggs.

Eggs

Egg laying takes place after 8:30 p.m. when wind speeds are less than 10 km/h (6mph) and the air temperature is greater than 15°C (59°F). Eggs are laid singly or in clusters of three or four eggs on the florets or in grooves on the florets. The egg stage lasts four to seven days. In the following image, canola seeds are pictured at the top and wheat midge cocoons at the bottom.

 Figure 1
 Canola seeds (top) and wheat midge cocoons (bottom)
 

Larvae

Upon hatching, the small orange larvae feed on the surface of developing kernels. Frequently, there are three to four larvae per floret but in severe infestations, there may be as many as 26 larvae feeding on a single kernel.

Larvae feed and develop for about two to three weeks, by which time they grow to about 2 - 3 mm long (1/8 inch).

In dry conditions, larvae do not shed their last larval skin but shrink back inside it and stop developing. In this state, the larvae appear to be enclosed within a transparent envelope. The larvae can survive in this protected state for up to two months. When moisture conditions improve, the larvae become active, crawl off the wheat heads, find their way to the ground and bury themselves in the soil.

Most larvae remain within the top 5 cm (2 inches) of soil but some may burrow 10 cm (4 inches) below the soil surface. The larvae spin round cocoons which are about haft the size of a Polish-type canola seed. Overwintering larvae may remain dormant until conditions are favourable for development, whether the following spring or several years later.

Figure 2
Wheat midge larvae feeding on developing wheat kernel
 

Pupae

Once temperature and soil conditions end the overwintering period, the larvae become active and move to the soil surface to pupate. Depending on the conditions, the larvae will pupate with or without a cocoon. Adult flies begin to emerge from the pupae in late June or early July. Emergence may continue for about six weeks. Weather conditions affect the wheat midge life cycle significantly. Moist soil conditions during May and June are required for larval development and pupation. Dry conditions may result in the larva remaining dormant for extended periods of time. Warm calm evenings during July are favourable for egg laying.

Monitoring

Careful, close monitoring of the wheat field is necessary in order to identify a wheat midge infestation and to take the appropriate action. Recent studies indicate that wheat heads are more susceptible to damage when egg laying occurs during heading. Kernel damage is reduced by 20 to 25 fold between heading and early flowering. Therefore, fields should be inspected daily from the time wheat heads emerge from the boot leaf until anthers are visible on the heads.

Exception: Glenlea wheat remains very susceptible to damage after flowering. Monitor this variety carefully throughout the heading and flowering stages.

Field inspection should be carried out between 8:30 and 10:00 p.m. when female midge are most active. Females are more active when the temperature is above 15°C (59°F) and wind speed is less than 10 km/h (6 mph).

Midge populations can be estimated by counting the number of adults present on four or five wheat heads. One midge adult for each four or five heads usually warrants control measures.

Inspect the field in at least three or four locations. Midge densities and plant growth stages at the edge and centre of fields may be very different. The highest densities are often next to fields where wheat was grown in previous years or in low spots where soil moisture is favourable to midge development.

Don't Confuse Wheat Midge With Lauxanids

Not every small fly in the crop will be a wheat midge. The wheat midge may be mistaken for another small fly that is common in wheat - the lauxanid.

At 2.5- 4 mm in length (1/10-1/6 inches), the lauxanid is a little larger than the midge. It is yellowish-brown in colour compared to the predominantly orange colour of the midge. The lauxanid may be observed during the day and early evening resting on the wheat leaves or on the awns. When disturbed during the day, it will fly above the crop canopy. At rest, its body will be oriented in the horizontal position or with its head pointed towards the ground.

In contrast, the midge is not active during the day. Wheat midge tends to flutter from plant to plant and assumes a vertical position with its head pointed skyward when resting on the plants.

Figure 3
 Lauxanid, Camtoprosopella borealis (left) and wheat midge (right)
 

Pest Management

Biological, cultural and chemical controls affect wheat midge populations.

Biological Control

Wheat midge populations are often held in check by a small, 1-2 mm (1/25-1/12 inch) long parasitic wasp called Macroglenes penetrans (Kirby). This small wasp emerges from its pupa about the same time as its host and lays its eggs inside those of the wheat midge. The wasp egg and the midge egg hatch about the same time and the tiny wasp grows slowly inside the midge larva. It remains dormant within the midge larva over winter.

In the spring, the parasite grows rapidly, destroying the midge larva. Under natural conditions, this parasitic wasp controls about 40 percent of the over-wintering population of wheat midge each year.

The effectiveness of two additional parasites - one from Europe and one from British Columbia - is currently being researched in Saskatchewan. Meanwhile, in Winnipeg, wheat varieties resistant to wheat midge are being examined. If these two research initiatives are successful, the opportunities for biological and cultural control of wheat midge will be greatly expanded.

Figure 4
Macroglenes penetrans

Cultural Control

Continuous wheat cropping should be avoided because this practice favours the buildup of midge populations.

When wheat midge populations are high in the soil of a particular field, it is best to switch from wheat and plant crops that are not susceptible to midge, such as oilseeds and pulse crops. Cereals crops such as barley, oats and annual canary grass can also be grown with little or no risk of damage.

For spring wheat varieties, damage from a wheat midge infestation can be minimized by selecting early maturing varieties, increasing seeding rates to two bushels per acre and seeding as early as possible. By seeding early maturing varieties early, the crop may head and flower before peak adult midge populations occur.
 

Chemical Control

An insecticide application is recommended if the action threshold has been reached before the crop has flowered. The timing of the application will vary with the insecticide. Consult the specific recommendations for the product being used. All insecticides should be applied in the evening when female midge are most active at the top of the crop canopy. However, early morning applications may also produce acceptable results.

Application during the advanced stages of flowering is discouraged because plants in this growth stage are no longer susceptible to attack and the insecticide will have a negative impact on midge parasites.

Dimethoate (Lagon, Cygon 480) and Chlorpyrifos (Lorsban  4E, Pyrinex, Nufos, and Clorex) are registered for the control of wheat midge on wheat in Canada.

Dimethoate is applied with ground or aerial equipment provides effective contact control of adults and some residual control. It does not control eggs. Application should be made within 24 hours of reaching the action threshold - while the adults are still active. If adult midge persist, a second application may be required, provided the crop has not started to flower. Dimethoate; should not be applied within 21 days of harvest.

Chlorpyrifos effectively control both adults and eggs. Because these insecticides control eggs, they do not have to be applied within 24 hours of having reached the action threshold as is the case with dimethoate. In fact, application should be delayed up to four days after the recommended action threshold has been reached to allow the emergence of the maximum number of wheat heads from the boot.

Application rates for Chlorpyrifos are lower for ground sprayers (336-405 mL/acre or 830-1000 mL/ha) than for aircraft (405 mL/acre or 1000 mL/ha). Uniform insecticide coverage of wheat heads is essential for the control of eggs. Evening application is recommended although early morning applications can provide acceptable control.

Chlorpyrifos should not be used within 60 days of harvest. Do not re-enter the treated field for at least 48 hours.

Ground Application

Field sprayers equipped with flat fan (F) nozzles, oriented at a 45 degree angle forward, provide the best coverage. Boom height should be adjusted to comply with recommendations of the nozzle manufacturer. The insecticide should be applied at 240- 275 kPa (35- 40 psi) in the highest recommended water volumes. High water volumes (75 - 100L/ha) provide better protection than low water volumes (25- 50 L/ha).

Aerial Application

Insecticides should be applied in the evening using water volumes of 18.7- 37.4 L/ha. Coverage and kernel protection improve with higher water volumes.
 

Acknowledgements

This publication has been produced as a combined effort of the following agencies:

• Manitoba Agriculture and Food
• Saskatchewan Agriculture & Food
• Alberta Agriculture, Food and Rural Development
• Agriculture and Agri-Food Canada
• North Dakota State Agricultural Extension Service
• Minnesota State Agricultural Extension Service
   
• Canada-Manitoba Agreement on Agricultural Sustainability
• Canada-Saskatchewan Agriculture Green Plan Agreement
• Canada-Alberta Environmentally Sustainable Agricultural Agreement

For further information, contact your GO representative.