Spruce budworm impact, abundance and parasitism rate in a patchy landscape

Abstract

The hypothesis that vegetational diversity may lessen the impact of forest insect pests by favoring natural enemies is appealing to those who seek ecologically sound solutions to pest problems. We investigated the effect of forest diversity on the impact of the spruce budworm Choristoneurafumiferana following the last outbreak, as well as the budworm's current abundance and parasitism rate, in the boreal forest of northwestern Québec. Mortality of balsam fir caused by the budworm was greater in extensive conifer stands than either in "habitat islands" of fir surrounded by deciduous forest or on true islands in the middle of a lake. Adult spruce budworm abundance, assessed by pheromone traps, did not differ significantly between the three types of sites. Larval and pupal parasitism rates were examined by transferring cohorts of laboratory-reared larvae and pupae to trees in the three site types and later collecting and rearing them. The tachinid Actiainterrupta, a parasitoid of fifth and sixth instar larvae, as well as the ichneumonid pupal parasitoids Itoplectesconquisitor, Ephialtesontario and Phaeogenesmaculicornis, caused higher mortality in the habitat islands than on true islands or in extensive stands. Exochusnigripalpistectulum, an ichneumonid that attacks the larvae and emerges from the pupae, caused greater mortality in the extensive stands of conifers.

Comparing modern and presettlement forest dynamics of a subboreal wilderness: Does spruce budworm enhance fire risk?

“…Neighborhood influence on site vulnerability was limited to adjacent cells, consistent with studies from New Brunswick (Su et al 1996, but see Cappuccino et al 1998). Modifications to the biological disturbance agent extension included more precise host preference and susceptibility parameters (Table 6) and modification of mortality rules to emulate cell-scale gaps in budworm host (Kneeshaw and Bergeron 1998), but sparing 10-year-old balsam fir cohorts as advance regeneration (Morin 1994, Frelich 2002) (Appendix A).…”

“…Mature host trees older than 50 years are also more susceptible to damage than young trees (MacLean 1980). Mixed stands containing deciduous tree species often sustain less damage to fir and spruce than pure stands of host species (Su et al 1996, Cappuccino et al 1998. Forests with high mortality from recent budworm damage burn with high intensity, particularly in spring before the leafout of deciduous shrubs (Stocks 1987).…”

Plant neighbour identity and invasive pathogen infection affect associational resistance to an invasive gall wasp

“…The ''enemies hypothesis'' (Elton 1958;Root 1973) posits that plant communities with higher species richness provide more resources and habitats and thus can shelter more diverse predator or parasitoid communities (Wilby and Thomas 2002;Schuldt et al 2011;Castagneyrol and Jactel 2012), which could in turn provide a better control of herbivore populations (Riihimäki et al 2005;Leles et al 2017). A spill-over of natural enemies from associated to target trees is expected if associated and target trees share common or alternative prey or hosts (Cappuccino et al 1998). However, a greater abundance or diversity of herbivores' enemies does not necessarily result in higher predation rates as there is a wide range of enemy-enemy interactions that can be positive, negative or neutral (reviewed by Letourneau et al 2009).…”

Gap dynamics in Balsam Fir, <em>Abies balsamea</em> - Yellow Birch, <em>Betula alleghaniensis</em>, forests of Québec

“…The other deciduous trees in the canopy take advantage of the small gap by simply lengthening the branches in their crown, thus rapidly closing the opening (Runkle and Yetter 1987). The second reason why insect outbreaks do not play a larger role is that mixed stands are less susceptible to Budworm outbreak than pure Balsam Fir stands (Su et al 1996;Cappucino et al 1998). Finally, the third reason is that the study was not conducted in a period of severe outbreak.…”