The scale and parasitoid community on native hemlocks in Japan

Abstract

Elongate hemlock scale (EHS), Fiorinia externa (Hemiptera: Diaspididae), is an invasive species in the United States of Japanese origin that was first detected in New York in 1908. In the United States, EHS attacks eastern hemlock (Tsuga canadensis) and Carolina hemlock (Tsuga caroliniana), and has established in most states where these trees occur. Elongate hemlock scale density can be several orders of magnitude higher in the United States than in its native Japan despite the presence of the parasitoid Encarsia citrina (Hymenoptera: Aphelinidae) in both locations. We postulated three possible reasons for this: (1) cryptic diversity of E. citrina attacking EHS exists in Japan, but not in the United States, (2) additional parasitoid species attack EHS in Japan at significant rates, or (3) alternate armored scale hosts on hemlock in Japan enhance rates of parasitism of EHS by E. citrina or other parasitoid species. We surveyed 88 sites (or subsites) throughout Japan for all armored scales on hemlock to explore the scale community and the genetic diversity of the associated parasitoids, as well as the relationship between scale community species richness and percent parasitism of EHS. Six armored scale species were found on hemlock in Japan. Using two gene loci (28S-D2 and COI), 18 identified or putative species of parasitoids were reared from these six armored scale species. Only five of the 18 parasitoid species were found in more than one scale host species. Eleven of the 18 parasitoid species were found in EHS, and six of these were exclusive to EHS. Encarsia citrina was recovered from all six armored scale species, and there was no evidence for cryptic species of E. citrina or that it differed genetically from populations of this species found in the eastern United States. There was no relationship between the number of armored scale species present in a given location and percent parasitism of EHS by all parasitoid species. Our phylogenetic analysis corroborated the morphological identification of the parasitoid species, including those newly described, published in an earlier study. The results from this study suggest that the high densities and low parasitism rates of EHS in the United States may be due to a depauperate scale-parasitoid community compared to Japan. In particular, this study identified ten new parasitoid species attacking EHS in Japan, which suggests strong parasitoid pressure on EHS populations there. Despite the diverse parasitoid community associated with EHS, parasitism rates were often low. This may be indicative of natural variation in spatial/temporal host-parasitoid population dynamics, scale suppression from plant-mediated factors, or a combination of both. The hemlock scale parasitoids found in this study provide many potential biological control candidates should management of EHS or other hemlock scales become necessary in the future.