By Mitchell Japp, MSc, PAg, Provincial Specialist, Cereal Crops and James Tansey, PhD, AAg, Provincial Specialist, Insect/Vertebrate Pest Management
“Green is the colour…” We think that’s how the song or chorus starts off, and goes on something about football. In 2019, green is the colour of the wheat midge risk map – mostly. Does that mean you’re off the hook for scouting? Not exactly.
The wheat midge forecast map is based on a survey of soil samples taken from more than 400 sites across the wheat-producing region of the province every fall. The soil samples are inspected for wheat midge cocoons. Those cocoons that are still viable are counted, while those that have been parasitized are not. The risk ratings in the map are based on the number of viable midge found in the soil samples. When the conditions for midge are ideal, even those areas rated yellow, and possibly some of the green, could lead to yield loss and reduced quality. It is also important to remember that while the map is a valuable tool for crop planning and budgeting, there may be considerable variation between sample points that cannot be represented on the map. We can’t sample every field, so the risk map is a good guideline, but there’s no substitute for scouting. More on this in a bit.
What we know now about wheat midge risk for 2019
Wheat midge are less tolerant of dry conditions than crops. While crops are good at making use of stored soil moisture, wheat midge need spring precipitation before they start to emerge. Basically, if there hasn’t been 25 mm of precipitation by the end of May, wheat midge emergence is delayed and erratic.
When their emergence is delayed and erratic, often the wheat will be past the susceptible stage by the time sufficient numbers have emerged to do damage.
Whether or not you need to scout
Scouting for wheat midge is labour intensive and generally not an experience that farmers and agronomists speak of fondly, so it’s understandable if you want to avoid it. To combat high pressure from wheat midge, many growers have opted to choose wheat midge-tolerant varieties, with more than 30 per cent of seeded area in Saskatchewan planted to midge-tolerant varieties over the past few years. It doesn’t hurt that those varieties also tend to be high-yielding. There is no recommended economic threshold for spraying for wheat midge with midge-tolerant varieties. In fact, keeping some midge going in these blends helps to sustain resistance. And, if you’ve got midge-tolerant varieties, the wheat midge parasites can be quite effective at managing the population. #fieldheroes.
But, if you don’t have a midge-tolerant wheat, for peace of mind, go out and scout. Scouting is ideally done in the evening, when the wind is calm and the light is failing. Wheat midge are weak fliers, so they don’t tend to be out when it is windy. Female wheat midges are most active after 8:30 pm and at temperatures above 15 C. Egg laying may still occur on tillers below the canopy when wind speeds are too high at the top of the canopy.
Get down below the canopy and watch for wheat midge. There are also harmless lauxanids that look similar, so make sure to ID them correctly. Although lauxanids are also small and orange, they have the look of a house fly, whereas wheat midge looks more like a tiny orange mosquito. Wheat is susceptible to wheat midge from the time the boot splits until mid-flowering.
For susceptible wheat varieties, the economic threshold is one midge per four to five wheat heads for yield and one midge for every eight to 10 heads for grade. Consider the potential yield of the crop, the potential loss and the cost of spraying.
For more information, visit our wheat midge webpage.