Populations of wheat stem sawfly are increasing in parts of southern Alberta, the latest provincial survey has found.
The survey, conducted in the fall, found increased levels of sawfly damage in Forty Mile County. However, sawfly numbers seem 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, with damage ratings based on 85 fields in 19 municipalities. In each field, the number of wheat stem sawfly cuts and the number of uncut stems are determined in a one-metre area of stubble in four locations.
The per cent 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. Alternatively non-host broadleaf crops or oats can be grown to avoid sawfly damage.
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 tunnelling can result in hidden yield losses of 10 to 15 per cent in each stem affected with additional losses because of lodging at harvest.
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.
It is possible that population hot spots still exist in areas of lower risk, but the overall sawfly risk remains lower than the outbreak levels of the early 2000s. However, the increased sawfly damage in many spots of southern Alberta signals a resurgence due to the drier conditions over the past few years.