Host tree age as a selective pressure leading to local adaptation of a population of a polyphagous Lepidoptera in virgin boreal forest

Abstract

We tested the hypothesis that host tree age may act as a selective factor and lead to local adaptation of the hemlock looper (Lambdina fiscellaria), a geometrid Lepidoptera that has a wide geographical distribution and has evolved in different eco-zones characterized by different levels of floristic composition, age structure and fragmentation level. Considering that hemlock looper outbreaks mainly occurred in old forests, we compared the biological performances of two populations. The first population was collected in the northern virgin boreal forest, which is dominated by mature and overmature coniferous stands that have not suffered from human disturbance. The other population was collected in the southern mixed-wood forest, which is more diversified and has been modified by forest harvesting. Larvae were reared under controlled conditions on foliage from three age classes of balsam fir trees: juvenile, mature and overmature. Although we measured significant variations of biological performances between the two populations, no significant effect of the age of the balsam fir trees could be detected for males from both populations or for females from the southern population. However, northern females were strongly affected by the age of balsam fir trees on which they fed, as their pupal weight was 12% higher and their fecundity increased by 27% on overmature trees compared with juvenile ones. These results indicate that under the same selective pressure, females adapt their strategy to maximize their fitness, and thus they appear as the driving force of evolution through the local adaptation concept. Furthermore, the two populations evolved in distinct habitats and their adaptation reflects selective pressures occurring inside their original environment. This is the first report on local adaptation of an herbivore that is mediated by host tree age. Changes in forest age structure may have a considerable impact on insect local adaptation and presumably on their population dynamics.

Effect of simulated fall heat waves on cold hardiness and winter survival of hemlock looper, Lambdina fiscellaria (Lepidoptera: Geometridae)

“…Thus, the temperate population could be expected to be better adapted to fall heat waves than boreal ones but our results indicate the reverse. However, life-history traits of hemlock looper populations also vary along latitudinal gradient and boreal populations belong to a 4-instar ecotype compared with the 5-instar ecotype in the south (Berthiaume, 2007;Berthiaume et al, 2009). Females of the 4-instar ecotype lay larger eggs and this could explain the difference observed in egg survival when the two hemlock looper populations were submitted to fall heat waves.…”

“…The major outbreaks of the hemlock looper were reported in boreal balsam fir forests (Hébert and Jobin, 2001;Berthiaume et al, 2009). However, this species is largely distributed and does not belong only to boreal balsam fir forest.…”

“…habitat for the temperate populations (Berthiaume et al, 2009). Consequently, balsam fir is less frequent and abundant in temperate forests compared with boreal forests.…”

“…The species is native to North America (Martineau, 1984) and its larva is known for its defoliating outbreaks in northern balsam fir (Abies balsamea (L.) Mill.) forests (Otvos et al, 1979;Hébert and Jobin, 2001;Berthiaume et al, 2009). However, it is a polyphagous herbivore, with larvae feeding on a variety of coniferous and deciduous trees (Carroll, 1956;Hébert et al, 2006;Berthiaume, 2007).…”

“…This important difference may affect cold hardiness and winter survival of these two ecotypes because hatching probability increases with egg mass (Delisle et al, 2012), even if the two populations studied in Delisle's research belong to the boreal ecotype. Moreover, the boreal ecotype is better adapted to local ecological conditions that characterize virgin boreal forests of the north: it performs better on balsam fir and white birch (Berthiaume, 2007), as well as on mature and overmature balsam fir (Berthiaume et al, 2009) trees that dominate the northern boreal landscape.…”