Parasitoid wasp collapse lets cereal leaf beetle rebound in Ontario wheat

Cereal leaf beetle (CLB) parasitism by its key biocontrol wasp has crashed from 95 per cent in 1970s Ontario surveys to 2.4 per cent or lower in 2021 and 2022, and a three-year project launched in 2024 wants to find out why. The beetle, an invasive pest from Eurasia that chews wheat, oats and barley, has been rebounding across southern Ontario for roughly a decade after decades of effective natural suppression.

"This project's ultimate aim is to reduce the impact of cereal leaf beetle but in a variety of ways that are sustainable. We are looking at the natural enemies angle, the crop resistance angle, and the scouting and spraying guideline angle," says Lauren Des Marteaux, a field crop entomologist with Agriculture and Agri-Food Canada (AAFC) in Harrow, Ont., and the project's principal investigator. Her co-principal investigator is AAFC Lethbridge entomologist Haley Catton. The Sustainable Canadian Agricultural Partnership (SCAP) project is funded by Grain Farmers of Ontario and AAFC.

Why the wasp shift matters

Adult beetles chew straight through leaves; larvae leave a windowpane effect. Yield losses in winter wheat range from 0.5 to 50 per cent depending on infestation level, timing, plant stage and cultivar. If action thresholds are hit, eastern Canadian growers' 2025 options are Malathion or Decis 100EC (deltamethrin) — broad-spectrum products that harm parasitoids, pollinators and aquatic life, and carry dollar and greenhouse gas costs.

The historical workhorse was Tetrastichus julis, a tiny wasp that lays three to nine eggs per CLB larva and kills the pest after it spins a cocoon in the soil. Released in the U.S. in the 1960s, it spread to Canada in the 1970s and delivered up to 95 per cent parasitism. Catton's recent surveys found 0–52 per cent in 2019, then near-zero in 2021–22.

The surveys also turned up two wasps never before recorded in Canada — Lemophagus curtus and Diaparsis temporalis, both released in U.S. biocontrol programs in the 1960s–70s. L. curtus now appears to be Ontario's predominant CLB parasitoid, "completely overshadowing T. julis", Des Marteaux says. Each lays a single egg per larva, and European research suggests they may wound or kill T. julis larvae when competing.

The team rears collected larvae in labs at Harrow and Lethbridge, with AAFC taxonomist Andrew Bennett identifying wasps by physical traits and DNA barcodes. Des Marteaux suspects tillage is part of the story: T. julis overwinters inside cocoons in the soil, so tillage kills it, while L. curtus emerges within weeks and flies to safety. The team has applied for funding to survey more fields and test whether management practices shape wasp communities.

Which wheat varieties resist?

The resistance work traces to a heavy CLB infestation around 2018 in AAFC breeder Gavin Humphreys' winter wheat yield tests near Woodslee. Damage scoring showed most registered varieties were susceptible, but 25W31, Adrianus and Marker took less damage, and two lines from Lethbridge breeder Rob Graf showed much lower damage still. Since 2024, the team has screened 16 registered varieties and advanced lines over three years at Harrow and Ridgetown, scouting weekly from early May to mid-June for eggs, larvae, adults and damage on the top three leaves.

Low CLB pressure, especially at Harrow, has made results hard to read. "Whether that is statistically significant, I'm not sure yet," Des Marteaux says. She has, however, built open-source software that measures leaf area and CLB damage at 95 per cent-plus accuracy, processing thousands of images in minutes, and is working with Andrew Frewin at the University of Guelph to make it customizable for other crops and pests.

Timing sprays and pricing damage

Meghan Vankosky and Dylan Sjolie in Saskatoon are adapting a Prairie phenology model to tell southern Ontario growers when to scout and spray. OMAFA's Tracey Baute and Guelph-Ridgetown's Jocelyn Smith feed it data from Great Lakes Yield Enhancement Network winter wheat sites and supply larvae for the parasitoid work.

A greenhouse trial comparing paired stems of Ava and LT294 found heavy flag leaf defoliation by two larvae caused a 23 per cent yield loss — in line with the literature. Tellingly, one variety lost kernel weight, the other lost kernels per head.

Growers should watch for the project's final analyses, publications and presentations over the coming months, plus funding news on the wider Ontario wasp survey. As Des Marteaux puts it: better-timed decisions, fewer unnecessary sprays, lower costs — and more natural enemies to protect than anyone thought.