Abstract
The hemlock woolly adelgid Adelges tsugae Annand (Hemiptera: Adelgidae), is a non-native invasive pest that has caused widespread decline and mortality of eastern hemlock (Tsuga canadensis (L.) Carr. (Pinales: Pinaceae)) and Carolina hemlock (T. caroliniana Engelm.) in the eastern United States. Our preliminary experiments evaluated the utility of a rain-down technique to induce artificial infestations of A. tsugae on hemlock seedlings en masse. Experiments were conducted in PVC (1 m3) cages topped with poultry wire for placement of A. tsugae-infested branches, and with 1 m(2) gridded glue sheets and/or hemlock seedlings placed below to capture adelgid abundance, distribution, and infestation rate data. In the March 2011 experiment, the density of progrediens crawlers (adelgid nymphs, first instars) that rained down inside the PVC cages was significantly higher in the high ovisac treatment compared to the low ovisac treatment, with an estimated 513,000 and 289,000 crawlers per m(2) falling beneath each treatment, respectively. Resulting A. tsugae infestation rates on Carolina hemlock seedlings placed inside the cages did not differ between the treatments but were at or above established damage threshold densities for the adelgid. Infestation rates on eastern hemlock seedlings that were placed in cages nine days after the experiment started were below damage threshold levels and did not differ between the treatments. In the May 2011 experiment, the density of sistens crawlers raining down was substantially lower, with 17,000 and 33,000 falling per m(2) in the low and high ovisac treatments, respectively. Resulting infestation rates on Carolina hemlock seedlings were extremely low and well below damage threshold levels. Although A. tsugae crawlers were well distributed across the 1 m(2) gridded glue sheets placed at the bottom of each cage, hot spots of unusually high crawler density did occur in both experiments. This rain-down technique shows potential for use in an operational tree-breeding program where screening large numbers of hemlock seedlings for resistance to A. tsugae is required.
Highlights
The hemlock woolly adelgid, Adelges tsugae Annand (Hemiptera: Adelgidae), is a serious invasive forest pest in the United States, where it poses a significant threat to the longterm sustainability of hemlock (Tsuga spp. (Pinales: Pinaceae)) ecosystems
Upon molting to the adult form, the sexuparae migrate away from hemlock for sexual reproduction on spruce (Piceae spp.); the native spruce flora of eastern North America have far been immune to A. tsugae infestation and successful sexual reproduction has never been observed in the introduced range
Even with a systematic and even distribution of branches, “hot spots” of unusually high crawler density occurred in both ovisac density treatments in the March and May experiments, and higher crawler densities were frequently found toward the center of the sheet as opposed to near the edges. This aspect of the rain down technique needs to be refined to promote a more homogeneous distribution of crawlers within infestation cages before this method can be deployed in an A. tsugae resistance screening and breeding program for eastern and Carolina hemlock. These preliminary studies on the development of a rain down technique for artificially infesting eastern and Carolina hemlock seedlings with A. tsugae are encouraging and suggest that this might be a suitable approach for mass inoculations for resistance screening
Summary
The hemlock woolly adelgid, Adelges tsugae Annand (Hemiptera: Adelgidae), is a serious invasive forest pest in the United States, where it poses a significant threat to the longterm sustainability of hemlock (Tsuga spp. (Pinales: Pinaceae)) ecosystems. The distribution of A. tsugae in this region covers a 19state area, ranging from southern Maine south along the Appalachian Mountain chain to northern Georgia and west into Ohio (USDA Forest Service 2011). Throughout much of this range, it has caused widespread decline and mortality among populations of eastern hemlock (T. canadensis (L.) Carr.) and Carolina hemlock (T. caroliniana Engelm.). Upon molting to the adult form, the sexuparae migrate away from hemlock for sexual reproduction on spruce (Piceae spp.); the native spruce flora of eastern North America have far been immune to A. tsugae infestation and successful sexual reproduction has never been observed in the introduced range
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