Canadian Forest Service Publications

Influence of partial cutting on parasitism of endemic spruce budworm (Lepidoptera: Tortricidae) populations. 2014. Seehausen, M.L.; Bauce, É.; Régnière, J.; Berthiaume, R. Environ. Entomol. 43:626-631.

Year: 2014

Issued by: Laurentian Forestry Centre

Catalog ID: 35552

Language: English

Availability: PDF (request by e-mail)

Available from the Journal's Web site.
DOI: 10.1603/EN13326

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Silvicultural treatments such as thinning have been suggested as management tools against the spruce budworm, Choristoneura fumiferana (Clemens) (Lepidoptera: Tortricidae). Among other things, parasitoids are also proposed to be influenced by silvicultural procedures, but the effect of thinning on spruce budworm's natural enemies has not been tested yet. In this study, the influence of partial cutting on parasitism of endemic spruce budworm populations has been investigated in mature balsam fir-white birch forests. Two intensities of partial cutting (25 and 40% stand basal area reduced) were conducted in 2009 and parasitism of introduced spruce budworm larvae and pupae was determined during the 3 yr after these treatments. Pupal parasitism was too low for comparison between treatments. However, 2 yr after treatments, parasitism of the fourth- and fifth-instar larvae was significantly reduced in plots with both intensities of partial cutting, which was attributed to the parasitoid Tranosema rostrale (Brischke). Three years after treatments, no significant influence of partial cutting on parasitism of spruce budworm larvae was found. This study suggests that the influence of partial cutting on parasitism of endemic spruce budworm populations is not consistent, but that under certain circumstances parasitism is reduced by partial cutting.

Plain Language Summary

Spruce budworm (SBW) outbreaks are the most significant natural disturbance affecting balsam fir stands in Canada. Silivicultural practices, particularly partial cutting, have been proposed as a forest management tool for controlling SBW. In mature balsam fir–white birch stands, researchers studied the impact of partial cutting on parasitoids that attack SBW. The SBW populations were endemic and two types of selective cutting (25% and 40%) were carried out in 2009. The researchers introduced larvae and pupae into the stands over a three year period following the cutting in order to study the levels of parasitism. Their findings were as follows:

•The level of parasitism in pupae was too low to be able to establish a comparison between the two types of selective cutting;

•Two years after partial cutting, parasitism in larvae had been significantly reduced. The parasitism was caused by Tranosema rostrale, a small wasp that lays a single egg directly below the skin of SBW larvae;

•Three years after partial cutting, no significant difference was found in larvae parasitism.

Partial cutting therefore has no stable influence on parasitism in endemic SBW populations, but under certain conditions, it can reduce parasitism.