Watching for wheat stem sawfly in southern Alberta
According to the latest survey (2021) the wheat stem sawfly was going to be an issue in southern Alberta in 2022.
Experts in the pest insect field, say the final results won’t be known until farmers get out there and look come August.
Wheat Stem Sawfly (Cephus cinctus) is native to North America, historically surviving on native grasses. First damages to wheat were reported in 1895 near Moose Jaw, Sask. and Souris, Manitoba.
According to www.alberta.ca/wheat-stem-sawflyoverview.aspx, “Changes in farming practices have affected the abundance of wheat stem sawfly. Tractor farming increased the relative abundance of wheat and decreased the proportion of oats grown. In addition, as strip farming gained acceptance, sawflies spread easily from stubble and native grasses to wheat. As stubble farming of wheat on wheat stubble increased, so did wheat stem sawfly.”
2021 saw some losses due to wheat stem sawfly in Alberta, with higher incidences in the southern region of the province closer to the MD of Taber and County of 40-Mile areas.
Shane Janzen, Agronomy Lead with Independent Crop Inputs Inc. in Taber, explained, “for wheat stem sawfly, you want to look at the year prior. Did you have damage? and that gets you looking for it this year. Sweeping for adults in June/early July helps too.”
Janzen saw several fields in the dryland acres around the Turin to Vauxhall area with sawfly.
“I’ve had my eyes out for them and have been seeing a few here and there. The tricky thing with wheat stem sawfly is there aren’t any chemical options to take out the pest. If you see them there out in your field, it’s too late and you need to start thinking about next year how to manage the problem.”
Crop rotation is one off the best methods of dealing with sawfly. Oats are immune to the pest, and they do not survive in any broad-leafed crops. Larvae can survive in barley, but rarely thrive. Solid stem varieties of wheat are more tolerant to sawfly damage, although not resistant. Hollow stemmed wheat and durum are most susceptible, and it is inadvisable to plant them year after year.
Tillage control and harvest management is another option. “You can swath your crop instead of straight cutting it, which isn’t always ideal in this area as the wind picks it up and your swath goes for a ride,” said Janzen.
www.alberta.ca/wheat-stem-sawfly-overview.aspx, explains the summer, overwintering and spring appearance of wheat stem sawfly as follows:
The sawfly larva feeds within the stem and burrows down to or below ground level by the time the wheat heads begin to ripen. The larva then turns around, heads upwards and cuts most of the way through the stem at a point somewhere between soil level and about 2 cm above the ground, seals the end above itself, spins a cocoon in the stem and passes the winter as a larva in diapause (hibernation).
Overwintering larvae pupate within their cocoons in May; adults begin to emerge in early summer from stubble fields and native grasses. As is common for many insects, males start to emerge first followed within a few days by females. In Alberta, sawfly adults appear from late June to early July. They are rather inactive insects that drift from plant to plant and spend most of their time resting on grass stems.
The best control are parasitic wasps, particularly Bracon cephi, a native braconid wasp. “There are a bunch of parasitic wasps that kill the wheat stem sawfly. You have to hope they are out there,” said Janzen. He recommends leaving lots of stubble. “it’s not the easiest management but you want to leave enough stubble so hopefully these parasitic wasps overwinter well and then you have a battle in the field where the good bugs are trying to eat the bad ones.”
Parasitic wasps can be very small in size and are the most beneficial insect control out there explained Janzen. “On a broad spectrum, parasitic wasps are awesome. The tricky thing is conventional insecticides kill those too. Ensure you are following economic thresholds and not spraying just because you think you have a problem. You need to do the checks to determine if there is a reason to spray.”
With sawfly, spraying wouldn’t do any good as there isn’t a chemical option that kills it. Spraying would only kill whatever parasitic wasps are present. “It’s just good management,” said Janzen. “Realizing you have the problem. Be out in the field come August and September to see if you had lots of plants fall over.”
If sawfly is present, a sawdust powder will be visible in a cut open stem, indicating where the sawfly has been chewing.