Life history trade-offs in host choice for a specialist folivore, spruce budworm (Choristoneura fumiferana)
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Date
2019
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University of New Brunswick
Abstract
Spruce budworm, Choristoneura fumiferana Clem., is an important outbreaking pest in Atlantic Canada. It defoliates several coniferous tree species, including balsam fir, Abies balsamea (L.) Mill, and black spruce, Picea mariana (Mill.) Britton, Sterns & Poggenb.. Budworm is a flush feeder, meaning it feeds on the newest foliage as soon as the buds burst, and times its emergence from diapause accordingly. This emergence often lines up with that of balsam fir, but is asynchronous to its secondary host, black spruce, which bursts much later. This asynchrony with black spruce begs the question of why budworm would choose black spruce as a host if/when balsam fir is available. To better understand the consequences of host plant selection by budworm in the spring for feeding, as well as prior to overwintering, I completed field experiments to determine how host tree species influences the budworm performance during both the overwintering and feeding periods. I established weekly cohorts to put budworm on two host species, simulating a broad window of emergence from diapause, beginning a few weeks before expected budburst of balsam fir to a few weeks after budburst of black spruce. After pupation, I removed the budworm from branches they were on. I assessed bud stage of these host trees for each cohort and various performance measures of surviving budworm. Budworm on balsam fir had higher survival than those on black spruce. I also investigated budworm overwintering mortality on these same trees. In northern New Brunswick, I felled and collected various sections of the host trees including branches and trunks from the upper and middle crown in the spring prior to emergence from diapause. I collected emerging budworm from these tree sections and reared them so I could assess parasitism. Unless budworm emerge from diapause perfectly synchronous to balsam fir’s budburst, black spruce is just as good of a host for budworm to feed and overwinter on. Budworm overwintering on balsam fir had higher parasitism, likely due to its evolution with that host. There are ultimately trade-offs between overwintering mortality and having a superior quality food host. This study is the first to look at how these tri-trophic interactions may affect budworm host selection.