Effects of simulated climate warming and population source on synchrony of forest tent caterpillar (Malacosoma disstria Hübner) egg hatch and host leaf phenology

The effects of warming temperatures on phenological synchrony between herbivores and their host plants are poorly understood. Controlled temperature manipulations, followed by measures of insect and plant development, can help address this knowledge gap. However, the ability of many insects to disperse substantial distances over a few generations, in contrast to the immobility and long lifespans of trees, adds significant complexity to our interpretations, and impairs our predictive abilities. We investigated how interactions among four population sources of forest tent caterpillar (Malacosoma disstria Hübner), two of its major hosts, trembling aspen (Populus tremuloides) and paper birch (Betula papyrifera), and three temperature regimes influence the relative timing of egg eclosion, budbreak and leaf development. Egg masses were collected across a 572 km latitudinal gradient, and subjected to two overwintering and three spring temperature regimes. Timing of egg hatch varied among population source, overwintering location, and spring temperature regime. Insect mortality was very low, and unrelated to any of the above factors. As expected, all plants and insects developed more rapidly under warmer conditions, but aspen and birch advanced much more rapidly than did forest tent caterpillar. Under temperature regimes simulating future conditions, some of the more southern current insect populations became synchronous. We are currently constructing accumulated degree-day models to improve our estimates of temperature thresholds for forest tent caterpillar, and are also allowing us to overlay both plant and insect response manifolds.  These findings will provide useful information for predicting and managing migrating forest insect defoliators in the midst of a warming climate.