Experimental Evidence for Branch-to-Bird Transfer as a Mechanism for Avian Dispersal of the Hemlock Woolly Adelgid (Hemiptera: Adelgidae)

Abstract

Birds have long been hypothesized as primary dispersal agents of the hemlock woolly adelgid (Adelges tsugae Annand). Although A. tsugae eggs and mobile first instars (crawlers) have been collected from wild birds, key mechanistic elements necessary for avian dispersal have never been examined. To evaluate the mechanisms of bird-mediated A. tsugae dispersal, we conducted both stationary (i.e., where crawlers must actively disperse) and disturbance (i.e., where crawlers may transfer from substrates due to mechanical abrasion) dispersal trials. For stationary trials, we tested the role of perching duration, ovisac density, and seasonal timing on the rate of crawler transfer to immobile preserved bird mounts at a single site in Connecticut. For disturbance trials, we explored if transfer rates were different when branches were actively brushed against birds. Both stationary and disturbance trials resulted in successful transfers of A. tsugae to bird mounts, with disturbance trials having significantly higher rates of transfers. Crawler counts from stationary trials increased significantly with local ovisac density. Additionally, we found a nonlinear relationship between crawler transfer and experimental week, with crawler transfer highest at the beginning of sampling in May, coinciding with avian spring migration in Connecticut and the emergence of progrediens crawlers, and spiking again near 14 June, when sistens generation crawlers began to emerge. While many aspects of potential avian dispersal of A. tsugae remain unknown, these results suggest that crawler transfer to birds may occur most often when peak crawler emergence coincides with the northward migration of many small passerine bird species.

Crapemyrtle Bark Scale: A New Threat for Crapemyrtles, a Popular Landscape Plant in the U.S.

“…Morphological characters of crawlers could facilitate its dispersal by wind including flat and small body, relatively long legs, and lateral wax filaments on the body fringe [20]. Under experimental conditions, birds were capable of transferring nymphs of the hemlock woolly adelgid, Adelges tsugae (Annand) (Hemiptera: Adelgidae) by touching infested branches [35]. Crawlers of four armored scales, including Aspidiotus nerii Bouche (Hemiptera: Diaspididae), Abgrallaspis aguacatae Evans, Watson & Miller, Hemiberlesia lataniae (Signoret), and Diaspidiotus perniciosus (Comstock), were found possessing a suction cup-like structure on hairs at the end of each leg, which can help them latch on larger insects to disperse [36].…”

Sensitive environmental DNA (eDNA) methods to detect hemlock woolly adelgid and its biological control predators Leucotaraxis silver flies and a Laricobius beetle

“…eDNA may be transported over short distances through wind or from higher branches through rainfall (Valentin et al, 2021). Just like the spread of A. tsugae themselves, their eDNA can be transported over long distances via birds and other insect vectors (Russo et al, 2016; Russo et al, 2019). For these reasons, it is important to acknowledge that eDNA methods may result in false positives and may not indicate localized infestations.…”

Evaluating a potential area-wide IPM strategy for managing hemlock woolly adelgid in the eastern United States

“…This introduced population has been tracked back to a clade originating in the southern region of the island of Honshu, Japan (Havill et al 2006). Since its introduction to Virginia, HWA has managed to spread at a rate of roughly 12 km/year mainly via wind dispersal and indirect animal activity (McClure 1989a, Evans and Gregoire 2007, Russo et al 2016. Currently in the eastern U.S., HWA can be found around the Great Lakes, northeast to New England, south as far as Georgia, and as far west as Kentucky and Tennessee (Ward et al 2004, Havill et al 2014.…”

“…Infested hemlock stands deteriorate depending on the level of infestation (McClure 1991, Eschtruth et al 2013. HWA spreads to new trees primarily through windblown dispersal and secondarily by incidental transport via birds and other animals (McClure 1989a, Russo et al 2016). Currently, HWA is established throughout the eastern seaboard; as far north as Maine and south to Georgia and, more recently, as far west as the Great Lakes (USFS 2015).…”