The sawfly survey conducted in fall 2020 clearly shows an increasing population in various parts of southern Alberta. Increased levels of sawfly damage were found in Forty Mile. Sawfly seems to have declined in Willow Creek and Vulcan Counties, but that might just be a reflection of the randomness of the survey and not of the region. There may be fields in the region counties that had higher cutting levels.
The wheat stem sawfly map is based on cut stem counts conducted in the fall of 2020 and the damage ratings are based on 85 fields in 19 municipalities. In each field, the number of wheat stem sawfly cut and the number of uncut stems are determined in a one meter of stubble in four locations.
Overall, the sawfly risk remains lower than the outbreak levels of the early 2000s but the increase sawfly damage in many spots of southern Alberta signals a resurgence due to the drier conditions over the past few years. The random nature of the survey means that individual fields may still have higher wheat stem sawfly populations than are indicated in the survey map. Field locations denoted by a black dot had zero sawfly found in the survey.
Wheat stem sawfly 2020 survey (PDF, 808 KB)
The percent of stems cut by sawfly gives an indication of the number of reproductive adult sawflies that will emerge in late June through early July. Winter conditions have very little impact on sawfly populations and a high proportion of wheat stems cut in the fall will produce adults. Producers in areas with moderate to high levels of cutting should consider using solid stem wheat as a control strategy.
Female sawflies lay eggs inside grass and grassy crop stems; the eggs hatch and tunnel inside stems until the crop starts to dry down near harvest. As the crop starts to ripen the sawfly larva migrates to the stem base and cuts a notch most of the way through the stem, wind and/or wet weather cause the cut stems to break and the heads to fall to the ground. Feeding damage from the tunneling can result in hidden yield losses of 10 to 15% in each stem affected. Further yield losses can occur from lodging at harvest.
It is possible that population hot spots still exist in areas of lower risk, producers need to be aware of the potential risks in their own fields. Cutting levels higher than 10 to 15% or higher in the previous crop year indicate the need to consider seeding solid stem wheat to reduce sawfly losses. Alternatively non host broadleaf crops or oats can be grown to avoid sawfly damage. It is important farmers evaluate their individual situations in making their variety choices. When populations are low it is typical to have small localized populations of sawfly that affect only one field or even just a portion of one field. At lower populations, wheat stem sawfly also tends to have a very strong edge effect where they migrate into the current year crop from the previous year stubble.
Parasitism can reduce populations and reduce the level of cutting. A parasitic wasp, Bracon cephi, has been shown to have significant impact on sawfly populations. The use of solid stem wheat varieties and the increase in parasitism are the major factors that lower sawfly populations in Alberta.
The 2020 wheat stem sawfly survey was carried out by Alberta Agriculture and Forestry with support from Farming Smarter and Chinook Applied Research Association – thank you for your contribution. Thank you Jan Lepp for managing the data from the field sheet to spread sheet. Thank you David Giffen, Agriculture and Agri-Food Canada, for building the map.