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Bug of the Month – Wheat Stem Sawfly

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Sawfly on the rise

male sawfly
A male sawfly. Credit Henri Goulet

Over the last few years, wheat stem sawfly made a comeback. This pest has plagued wheat, and to a lesser extent other cereals, ever since settlers ploughed the Prairies.

Weather factors, natural enemies, cultural practices and, particularly, planting solid stem cultivars reduce populations to terminate outbreaks. The last outbreak lasted about a decade starting around 1999.

Dry and hot weather in the last few years likely released the pest from natural enemies. Also growers tend to plant vulnerable cultivars when pest populations are low. Surveys conducted by Shelley Barkley (Alberta Agriculture and Forestry) this fall indicate that in some areas south of Lethbridge and closer to the border, cutting damage by the sawfly exceeded 50%.

Sawfly adults emerging from cut wheat this year has the potential to infest 80% of neighbouring fields the next summer.

You can scout wheat stem sawfly at the same time as wheat midge; meaning you can get two bugs with one stone! Scouting post-harvest will give you a better understanding of how next season populations may affect your fields.

Wheat Stem Sawfly prefer to eat spring & durum wheat crops. Winter wheat, rye, grain corn, and barley are second in preference but still make a delicious meal for the pest. Additionally, they enjoy eating native grass species that may line the sides of your fields.

Thankfully, for oat producers, the Wheat Stem Sawfly is not a problem as oat crops are toxic to them.

wheat stem sawfly
Wheat stem sawfly developmental stages (Art Cushman, Bugwood)

What do Wheat Stem Sawfly do?

Larvae will feed on the pith of the crop stem; feasting on the soft, cylindrical tissue inside the plant. This reduces the strength of the stem & makes it easier to fall over or break.

Sawfly larvae will continue down the stem to the base of the crop. Once there, they’ll cut themselves out; rendering the plant vulnerable to disease & increasing the likelihood of the crop toppling over. Crops that fall are unable to be swathed and harvested and impact the yield producers will see at the end of the season.

While they’re a terror in the field, their presence in native grass only adds to the economic pressure they bring. By feeding and killing the grass species around the fields, they increase the strain & pressure put into the soil & crops in the surrounding area. This applies extra pressure & economic damage in areas where the margin crossovers are greater.

What can you do?

Post-harvest scouting is the best way to get information on the pest. Take a 1-metre measuring stick & gardening trowel out to the field. Sample 4 sites, roughly 50 metres apart, along the edge of the field. Ideally, you’ll want to find sites 4-5 rows in.

Once you’ve found a site, count the number of stems that are cut by the swather and the number of short tops with closed tops. These can be easier to find if you dig up the clusters. so remember to bring your trowel! Additionally, you can feel the difference between the stalks. The affected stems are more solid & the cuts are much rougher.

By identifying how many crops in these areas were affected, you’ll have a better understanding of what you can do to repel them from your fields.

A number of strategies can mitigate sawfly damage

A group of researchers presented the following summary in publication Prairie Soils and Crops and available online:

Beres, B. L., Cárcamo, H. A., Weaver, D. K., Dosdall, L. M., Evenden, M. L., Hill, B. D., McKenzie, R. H., Yang, R. C., & Spaner, D. M. (2011). Integrating the building blocks of agronomy and biocontrol into an IPM strategy for wheat stem sawfly. Prairie Soils and Crops https://www.prairiesoilsandcrops.ca, 4, 54-65. (https://prairiesoilsandcrops.ca/articles/volume-4-7-screen.pdf)

From the article:

  • diligent pest surveillance
  • planting solid-stemmed cultivars
  • continuous cropping with appropriate pre-seed residue management
  • seeding rates no greater than 300 seeds m-2
  • 30 to 60 kg N ha-1
  • harvest cutting heights of at least 15 cm to conserve parasitoids

Other ideas

  • Learn how to identify pest damage; a change in the firmness of the stalk can help identify the presence of a pest or disease 
  • Consider your rotation times; increasing or decreasing the number of rotations between your crops can help with starving out the pest/disease 
  • Harvest earlier; an early harvest can increase your chances of catching the larvae before it has a chance to move below cutting heights  

For more information, visit:  
https://www.alberta.ca/wheat-stem-sawfly-overview.aspx