Wheat midge reported as new wheat pest in Washington state

The wheat midge (Sitodiplosis mosellana) has arrived in Washington state, with the potential to damage spring wheat crops. However, WSU scientists do not recommend farmers apply insecticides unless they know they have an economic infestation.

The midge is also called the ‘orange wheat blossom midge’ because it is orange in color and it infests wheat at pollination or flowering time. It originated in Europe and has been an economic pest in Canada and the northern US, including most recently; Bonner, Boundary and Kootenai Counties in Idaho.

The insect has been captured in low numbers in pheromone traps in Spokane County (Peone Prairie), Lincoln County, and Garfield County. The pheromone is a sex hormone that attracts only the adult male midge. Because the pheromone draws in insects from across large fields, the trap is a sensitive indicator of the presence of the midge but it is not a reliable measure of its population density in the field.

“The wheat midge has been reported previously in Washington,” said Diana Roberts, WSU Extension agronomist. “We put out the pheromone traps because we suspected the midge would move in from Idaho, but we don’t know the background population levels over the past few years.”

Closer examination of wheat fields in the area has not revealed midge populations that will likely cause economic damage this season.

“We do not recommend that growers go out immediately and spray their crops because that kills beneficial insects such as ladybird beetles,” continued Roberts. “We are in contact with researchers in Canada who have biological controls for this pest. While the midge populations are still at low levels, we will take the opportunity to bring in the biocontrols.”

The adult midge is a fragile insect with a body type similar to a mosquito, but about half the size. It has an orange body, conspicuous black eyes, three pairs of long legs and one pair of wings. The female lays eggs on the awns and heads of wheat plants. The eggs hatch into larvae that crawl inside the floret and feed on the very young, developing grain and cause yield loss and shrunken kernels. The larvae are about the same size, shape and color as the anthers of the wheat floret.

Spring wheat crops are most susceptible to infestation (egg laying) by the midge from heading until the anthers hang out of the florets. After that time the midge causes little damage. Winter wheat and barley crops are seldom affected. In addition, the midge needs warm temperatures, calm weather and moisture for successful egg laying.

During the day the adult midges hide deep in the crop canopy. The best time to scout for the midge is on calm, clear evenings within one hour of sunset. The adults may be seen flying in the canopy or sitting on wheat awns with their heads facing upwards.

“Take care with identification,” cautioned Roberts. “There is a lookalike fly that has an orange, but fat body and it rests head downwards in the wheat. And I took photos of what I thought were midge eggs but they turned out to be tiny, orange aphids!”

There are no established economic thresholds for midge infestations in Washington. In North Dakota, the thresholds are one midge per five heads for hard red spring wheat or one midge per seven heads for durum wheat.

For further information contact Diana Roberts at WSU Spokane County Extension, phone 509-477-2167 or e-mail [email protected]. North Dakota State University has an on-line bulletin on the wheat midge at http://www.ag.ndsu.edu/pubs/plantsci/pests/e1330.pdf.