Infection by Nosema fumiferanae (Microsporidia: Nosematidae) and feeding on previous-year’s foliage do not interact in affecting late instars of the spruce budworm (Lepidoptera: Tortricidae)

Infection by Nosema fumiferanae (Thomson) (Microsporidia: Nosematidae) and feeding by late-instars on previous year's foliage due to depletion of current year's growth (also known as back-feeding) are likely to co-occur in high-density outbreak populations of the spruce budworm (Choristoneura fumiferana (Clemens) (Lepidoptera: Tortricidae)). We tested the hypothesis that effects of the two factors on larval fitness are exacerbated by their interaction. Twigs of balsam fir (Abies balsamea Linnaeus; Pinaceae) with or without new-growth shoots were fed to sixth-instar offspring from uninfected, moderately infected, or highly infected females in two laboratory experiments. Pupal mass and time to reach the pupal stage were significantly affected by infection and back-feeding but not by their interaction. Pupal mass decreased and development time increased with increasing spore burden in a linear fashion. Back-feeding had a significant effect on survival to the pupal stage, but infection did not, nor did their interaction. The relatively small changes in survival, pupal mass, and development time caused by infection and back-feeding were additive. The results thus refute our hypothesis that infection by N. fumiferanae and forced feeding on last year's foliage by late instars are likely to interact in affecting spruce budworm recruitment in older outbreak populations.