Needle mining by the spruce budworm provides sustenance in the midst of privation

Nitrogen (N) content and mass of sections of whole needles and mined needles created by spruce budworm, Choristoneura fumiferana, feeding on balsam fir, Abies balsamea, and white spruce, Picea glauca, were measured, permitting estimation of the N content of budworm-eaten tissues. Whole-needle N concentration was 47-95% greater than that of the hollowed-out needles. N concentration of consumed tissue was 27-43% higher than that of whole needles. Relying on whole-leaf N concentrations to gauge the quality of food mined by insects will underestimate it. An electron micrograph of a typical cross section of mined white spruce needles shows that the insect can avoid needle terpenoids by mining around resin ducts. Results suggest that early-instar budworms discovered a behavioral solution for exploiting an abundant but usually suboptimal food - i.e., accessing the most N-rich tissues sandwiched between resin ducts and the low-N hull. Moreover, needle mining allows the hush-feeding budworm to emerge very early in the spring before buds open to begin their growth. This positions them to be strategically poised (among the earliest arrivals) to exploit the highly ephemeral, N-rich tissues that occur during early shoot elongation.