By: James Tansey, PhD AAg, Provincial Insect/Pest Management Specialist, Regina
One of the more important pests of wheat in Saskatchewan is the wheat midge, Sitodiplosis mosellana. This small (2-3 mm) fly lays eggs under the glumes or floret grooves of newly emerged heads. Eggs are laid late-June to early-July in the evening, near sunset, individually, or in clusters of three to four. Adult activity is greatest around 8:30 p.m. and can last for up to six weeks.
The adult midge is very delicate insect, so windy conditions can reduce activity or drive egg laying to lower portions of the plant. Adult midges are short lived, usually less than week. Eggs hatch in under a week. Newly hatched larvae begin to feed on developing kernels. Numbers within individual florets can be as high as 26. Larval feeding continues for up to three weeks. When larvae are mature, they fall off the plant and burrow into the soil where they pupate and emerge as adults the following year. Dry conditions reduce midge pressure. Emergence can be erratic and reduced if rainfall is less than 25 mm in May. Emergence can actually be delayed by a year if conditions are unfavourable.
There are options for managing wheat midge. If spring wheat is planned as part of a rotation, there are midge-tolerant wheat varieties available as varietal blends (VB). These blends include 10 per cent susceptible wheat. This may initially seem like a waste, but are essential to preserving the hesitance trait. You need to promote members of the midge population that have not been selected for resistance. Their genetic trait for susceptibility helps to dilute emergent resistance. For 2020, VBs are available in CWRS, CPSR, CWSP, CWSWS, CNHR, CWES, and Durum wheat classes. Visit the Midge Tolerant Wheat Stewardship Team's website for information on midge-tolerant wheat and VBs. Refer to the 2020 Saskatchewan Seed Guide for more information.
Chemical control can also be achieved when adults are active; insecticides registered for control include products with the active ingredients chlorpyrifos or dimethoate. Monitor wheat in July when the crop emerges from boot stage until flowering. Check crop canopy at dusk for signs of wheat midge adult activity. At each stop, examine 10 heads and count midge adults on or near heads. You'll likely need to crouch down to do this. To protect yield and grade, economic thresholds of one midge per four to five heads, and one adult per eight to10 heads, respectively, are recommended.
The province monitors wheat midge populations on approximately 420 sites, and produces a map of local pressures anticipated for the following year. These numbers represent the local counts of viable (unparasitized) pupae. Wheat midge populations are often well-controlled by an one to two mm long parasitic wasp, Macroglenes penetrans. These maps are presented at grower meetings and are available on the wheat midge page, industry sites, and on the Prairie Pest Monitoring Blogsite beginning in early winter.