Improved performance of the eastern spruce budworm on black spruce as warming temperatures disrupt phenological defences

Abstract

Phenological shifts, induced by global warming, can lead to mismatch between closely interacting species. The eastern spruce budworm, Choristoneura fumiferana, an important outbreaking insect defoliator in North America, mainly feeds on balsam fir, Abies balsamea, which has historically been well synchronized with the insect. But as climate change pushes the northern range limit of the budworm further north into the boreal forest, the highly valuable black spruce, Picea mariana, historically protected against the budworm by its late budburst phenology, is suffering increased defoliation during the current outbreak. We tested the hypothesis that rising temperatures can lead, not to a mismatch, but to an improved match between the budworm and black spruce through differential phenological advancement. For 3 years, eastern spruce budworm larvae were reared from instar 2 to pupae, on both black spruce and balsam fir, in a temperature free‐air controlled enhancement experiment (T‐FACE) consisting in 24 field plots, half of which were heated at +2°C from March to October. Our results show that every year, larval development was faster on heated trees and pupation was earlier than on unheated trees. Bud development was also accelerated in heated trees of both species. However, there was no difference in mass between pupae that developed at +2°C and controls at the end of the season. Finally, we found no difference either in development rate or pupal mass between larvae reared on black spruce and those reared on balsam fir. This suggests that under higher temperature regimes, eastern spruce budworm will be as successful on black spruce as on balsam fir, as black spruce budburst becomes better synchronized with the insect's emergence from diapause. This could lead to critical changes in outbreak dynamics and severity with important ecological state shifts at the landscape level.

Greater tree species diversity and lower intraspecific competition attenuate impacts from temperature increases and insect epidemics in boreal forests of western Quebec, Canada

“…Moreover, the 1970s generally coincided with warmer than average temperatures during the growing season in the region (Environment Canada 2022). These warmer temperatures may have induced an earlier budburst for black spruce during this decade, making them more susceptible to budworm attack (Bellemin-Noël et al 2021). During the two forest tent caterpillar epidemics, nonhost black spruce and jack pine may have benefited from severe reductions of growth in surrounding host trembling aspen in the study area (Chavardès et al 2021).…”

“…During the 1970s spruce budworm epidemic, black spruce growth was limited in cells with greater intraspecific competition, an indication that increased presence of surrounding host spruce with crowns more likely to be in contact facilitated budworm diffusion (Régnière and Fletcher 1983). The effects of such contact could amplify the severity of budworm epidemics should spruce budburst and budworm emergence synchronize (Bellemin-Noël et al 2021). During the budworm epidemic, higher jack pine growth was predicted by lower surrounding trembling aspen density.…”

A critical review of successional dynamics in boreal forests of North America

“…Climate change effects on insect-forest dynamics will vary considerably with species and geography. However, some effects on insect outbreak dynamics are emerging and should be monitored (i) increasing population growth associated with warming and enhancing the phenological synchrony with host species, leading to larger outbreaks (Fuentealba et al 2017;Bellemin-Noël et al 2021); (ii) decreasing precipitation and increasing drought events, which will increase tree vulnerability and facilitate insect outbreaks; and (iii) warmer winter temperatures, which will extend outbreaks (Weed et al 2013). Climate change is projected to increase the frequency of budworm outbreaks by 15% and increase outbreak duration by 6 years (Gray 2008).…”

Outbreak regulation mechanisms play different roles at different stages of an Eastern spruce budworm (Choristoneura fumiferana) outbreak.

“…Stand scale defoliation tends to follow the feeding preferences of the SBW, but this is altered by the presence of other tree species. In most cases, fir trees (Abies balsamea) and white spruce (Picea glauca) are more heavily defoliated than black spruce (Picea mariana) (Bellemin-Noël et al, 2021;Thomas, 1989). While black spruce is a host species for the SBW, the relationship is complex and its quality as a host for the SBW is marginal due to leaf toughness and mismatched timing in the phenology of budburst and larval emergence; both of which reduce SBW population growth (Fuentealba et al, 2020;Greenbank, 1963).…”

“…Cox PH models are widely used to assess morbidity risk of health interventions and lifestyle factors to disease outcomes (Kumar and Klefsjö, 1994), and the technique has been used to assess the risk of fire spread in landscapes (Cyr et al, 2016;Morin et al, 2015;Tremblay et al, 2018), but to our knowledge, this is the first application of the method to studying the spread of an insect epidemic. A landscape composition dominated by the preferred host should increase the growth rate of the SBW population through increased food supply and overwintering habitat, hence increasing mortality (Bellemin-Noël et al, 2021).…”