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Larch sawfly

Approximately forty acres of larch sawfly defoliation occurred in S 17- T 52N- R 21W just west of Floodwood along Highway #2. The larch (tamarack) trees, twelve to thirty feet tall were entirely defoliated by the sawfly larvae. Feeding was completed by the first week of August and the trees were beginning to refoliate. Many dead larvae were seen hanging from branches and shoots. This is characteristic of larvae killed by the parasitic fungus Entomophthora.

Larch sawfly overwinters as pupae in the ground. Adults emerge starting in May. The female cuts a double row of slits along one side of an elongating shoot and deposits eggs. The egg slits damage the shoot and cause it to curl. This curling of the shoots is a characteristic of larch sawfly damage. The larvae are grayish green with black heads. They feed on the needles through the summer. Most defoliation is evident in July. There is one generation per year.

In the past two to three decades larch sawfly populations have been very low and scattered in Minnesota. Occasionally a small area will be defoliated for one or two years before the population dies out.

The larch sawfly is considered a very serious threat to larch and in the past it has been very destructive with large outbreaks across the US and Canada. Between 1910 and 1926, it is estimated that 1 billion board feet ( 2 million cords) of larch were killed by the sawfly in Minnesota. Another large outbreak started in Minnesota in 1939 and extended into the mid- 1950's or later.

The origins of the larch sawfly has been debated but it is now thought that there are a number of strains in North America. Some are native and others introduced from Europe and Asia. The sawfly was thought to have only a few natural parasites in North America. So a parasite from Europe Mesoleius tenthredinis was released in Manitoba in 1913 and is credited with reducing outbreaks during the 1920's and 30's. However a resistant strain of the sawfly, developed leading to the large outbreaks in the 1940's and 50's. The resistant strain of the sawfly is able to encapsulate the egg of the parasite and is not killed by the parasite.

Two European ichneumon parasites of the larch sawfly and a Bavarian strain of Mesoleiustenthredinis, one that the sawfly is not able to encapsulate, were subsequently released in Manitoba. These same parasites were also released in Koochiching, Lake of the Woods, Beltrami and Itasca Counties in Minnesota in 1971 and 1972. The parasites have spread and become established throughout Minnesota and Manitoba.

Since there have been no large outbreaks after the introductions were made, it appears that the introduced parasites along with the native parasites, fungal diseases and flooding of cocoon sites have been effective in keeping larch sawfly populations in check. This doesn't mean that there will not be any outbreaks of the larch sawfly in the future, but hopefully, any outbreaks will continue to be of short duration and not as destructive as in the past.