Synchrony of Winter Moth (Operophtera Brumata) Larval Eclosion With Bud-Break of Different Tree Species in New England and Its Effect on Defoliation

Abstract

Forest pests are particularly responsive to the influence of climate change, rapidly bringing about shifts in herbivory intensities, changing their distribution and outbreak frequency, altering their relationships with natural enemies, and generally reducing biodiversity. It is not uncommon for spring defoliating insects to cause the death of host trees or to cause a devastating loss for fruit crops as they consume leaves and flowers. One spring feeding insect, winter moth, defoliates newly emerged spring leaves and flowers in a wide variety of deciduous tree species in New England. It has been noted that phenological synchrony between bud-break and larval eclosion is critical for the fitness of many spring-feeding insect herbivores, yet it is currently not well understood if the synchrony of winter moth eclosion with host tree bud-break influences the feeding habits of the winter moth caterpillar.

To further knowledge about this synchrony, this study monitored egg hatch timing in the landscape with different host tree species and categorized the resulting defoliation damage. The primary hypothesis was that defoliation damage by winter moths to different tree species is largely determined by the synchrony of larval hatching with bud-break. A second hypotheses evaluated was that eggs laid on rough, dark colored bark will hatch earlier then eggs laid on smooth, light colored bark. Twelve different tree species located within the Arnold Arboretum’s woody plant collections in Boston, Massachusetts were selected to compare winter moth larval densities to defoliation severity. Refining the measure of egg hatch date variation by taking into account bark type and the directional aspect of the tree trunk (north or south) required a critical analysis with attention to tree species variation within this study. The analysis of winter moth larvae defoliation damage on different host trees in relation to the stage of bud-break was achieved by observing tree buds and recording budburst stage, number of feeding caterpillars, and percent defoliation throughout the spring season.

The egg hatch timing on different types of tree bark and in varying sun exposure levels was analyzed finding texture, color, directional aspect and sun exposure to play significant roles in timing of winter moth egg hatch. Tree species and bud-break stage played a significant role in the amount of defoliation received, suggesting that the synchrony of bud break with egg hatching is an indicator for the severity of defoliation levels. The two Malus species were some of the first trees to break bud. Malus prunifolia exhibited the highest levels of defoliation at 88%, while Quercus coccinea was one of the later species to bread bud and only ended up with 20% defoliation. Larval densities decreased swiftly once the tree leaf stage became fully developed, at bud-break stage 4, well before the larvae pupated. This strongly suggested that larvae disperse to other food sources as tree leaves expand, supporting the idea that larval defoliation damage to tree species has some dependency on the synchrony to tree bud-break.