Forest Insect and Disease Newsletter

Tamarack: Tree with a troubled past

By Mike Albers, NE Region Forest Health Specialist

I would have expected the larch beetle to have been involved in killing the tamarack trees repeatedly stressed by e sawfly defoliation, but there is only passing reference to it a couple papers. Whether this is a case of the researchers focusing on the sawfly or really a case of the larch beetle not being an important agent in the mortality is a question. There is a report in a 1942 State of Minnesota Department of Conservation Statistical Report that there was a serious infestation of eastern larch beetle in the Pine Island State Forest and other infestations near Grand Rapids, Lake Minnetonka and Cambridge. It states that this is the first known serious.

At the time of the Public Land Survey in the late 1800's, tamarack was the most abundant tree in Minnesota with lots of it on upland sites. Today it ranks as the 6th or 7th most abundant tree. So what happened?

With over 120,000 acres with tamarack mortality in the last 11 years, you might think eastern larch beetle was the likely culprit but it was actually the larch sawfly. See Box. Larch sawfly, Pristiophora erichsonii, was first reported in North America in 1880. The first record of larch sawfly in Minnesota was in 1909. It's reported that between 1910 and 1926 an outbreak killed one billion board feet of tamarack in MN. There is another report claiming that between the late 1940's and 1970's, 40 perent of the tamarack in MN were killed by the sawfly. The outbreaks in North America during the twentieth century occurred across the continent wherever tamarack grew with similar losses reported. For example an Ontario report on tree decay in the 1950's stated that no studies were carried out on stem decay in larch because mature trees were so rare because of severe decimation by the larch sawfly and that some doubt that the species would ever be of commercial importance in the future.

image: Larva of larch sawfly Larch sawflies occur around the world wherever Larix species naturally grow. There has long been debate about whether larch sawfly was native to North America or not. Most researchers now agree that there are a number of strains in North America and some of these are native and other were introduced.

Larch sawfly has one generation per year. They overwinter in cocoons in the ground. Adults emerge in the spring and summer. The female lays eggs in slits she cuts into the growing shoots. Larvae emerge and eat the tamarack needles. When full grown in late summer they drop to the ground and spin silken cocoons in the litter and overwinter. Since tamarack are deciduous and drop all their needles each fall they are able to withstand defoliation better than other conifers, however repeated years of defoliation eventually results in mortality.

image: New shoot curled because of slits cut during egg layin A lot of research on larch sawfly has been done by many different researchers in North America. Studies in the early 1900's indicated that a paucity of parasites might be why large and damaging outbreaks were occurring. Because of this idea, the ichneumon wasp, Mesoleius tenthredinis, was collected in England and released in Canada in 1913. The wasp spread and is thought to have been important in ending the outbreaks in the late 1920's. However the sawfly populations soon became resistant to this parasite resulting in more outbreaks. It's believed that a resistant strain of the sawfly was accidentally introduced along with parasite from England. Parasitism declined into the early 1940's resulting in more outbreaks and mortality. Arnold Drooz reported 65% parasitism of sawfly in MN in 1935, but low levels by 1952 indicating the parasite could no longer be depended on as an important control factor because the sawfly developed immunity to it.

image: Map of defoliation by larch sawfly In the 1957, Canadian entomologists conducted additional parasite introductions using a Bavarian strain of M. tenthredini. The North American sawflies were not resistant to this strain of M tenthredini. In 1961 a second parasite from Europe, Olesicampe benefactor, was introduced into central and then eastern Canada. In 1970 and 1972, the University of Minnesota collected these two parasites in Manitoba and released them in Lake of the Woods, Beltrami, Koochiching and Itasca Counties. Spread and effectiveness of these parasites were studied into the late 1970's but not more recently.

With the lack of widespread outbreaks of larch sawfly in Canada or Minnesota in the past 35 years it would appear that the latest parasite releases have been successful, whether this relationship will continue remains to be seen.