Preingestive Detection of Tannins byChoristoneura fumiferana(Lepidoptera: Tortricidae)

The spruce budworm, Choristoneura fumiferana (Clemens) (Lepidoptera: Tortricidae), is considered the most important pest threatening coniferous forests of eastern North America. Its major hosts are balsam fir [Abies balsamea (L.)], white spruce [Picea glauca (Moench) Voss], red spruce (Picea rubens Sarg.), and black spruce [Picea mariana (Mill.) Britton, Sterns and Poggenb.], all of which contain antiherbivore defensive compounds, including tannins. Tannins are known to act as feeding deterrents, digestibility reducers, and toxins in numerous insects and specifically to reduce growth and survival in the spruce budworm. Using two-choice feeding assays, we show that tannic acid deters spruce budworm feeding, whereas tannins extracted from white spruce and containing 97.8% condensed tannins stimulate it. Moreover, a dose-response relationship was demonstrated for both types of tannin. To our knowledge, this is the first report of a phagostimulatory role of condensed tannins in any insect. Tannic acid only deterred feeding when in solution with sucrose but remained undetected when in solution alone, whereas the condensed spruce tannins produced a positive response alone, and this response was increased by the addition of sucrose. Based on these results, we speculate that both types of tannins produce their respective effects through different mechanisms: tannic acid seems to interfere with sucrose detection, whereas spruce tannins can evidently stimulate feeding directly in this insect. For both types of tannins, the insects do not seem to be able to detect the 0.75% tannin concentration, suggesting a possible response threshold. Rearing insects on diets containing tannic acid revealed possible effects of aversion learning and of induction when tested in feeding assays with tannic acid and spruce tannins, respectively.