The gypsy moth, Lymantria dispar (L.), is an invasive polyphagous folivore that feeds on >300 host tree species and has caused defoliation on more than 369,000 km2 since 1924. Following its introduction to North America in 1869 outside of Boston, MA, the gypsy moth has relentlessly spread and currently occupies a range in the U.S. that extends from northern Minnesota to North Carolina to Maine. Despite the number of potential gypsy moth host species, there is variation in the suitability of host plants that could affect rates of gypsy moth establishment and spread. Gypsy moth hosts can be broadly classified as primary or secondary host species; larval feeding on the latter hosts generally result in reduced larval growth and development, and reduced fecundity. We investigated the relationship between primary and secondary host tree density and fragmentation on nascent gypsy moth population growth rates across a latitudinal gradient from Wisconsin to North Carolina. We used annual trapping records from a national gypsy moth management program collected from 1999 to 2015. We estimated gypsy moth growth rates and waiting times to population thresholds within 5´5 km cells, and coupled these estimates with host plant fragmentation metrics and temperature data.
Gypsy moth growth rates were affected by both primary and secondary host fragmentation. Average growth rates varied as a function of mean primary and secondary host plant fragmentation. We observed that overall, average growth rates increased with decreasing primary host fragmentation. In contrast, mean secondary host fragmentation had a differential effect on growth rates depending on the gypsy moth population threshold. Wait times (e.g., the time in years between initial establishment and obtaining a specified threshold) also varied as a function of both primary and secondary host plant fragmentation, highlighting the role that secondary host plants can play in invading gypsy moth population dynamics. This study furthers our understanding of the composition of not only primary but also secondary host species on an invasive insect species, which could be used in future efforts to better predict establishment in novel environments.