Egg: oblong and amber in colour (Canadian Food Inspection Agency 2016). Eggs which develop into sistentes (singular sistens) are approximately 0.36 mm long and 0.23 mm wide, while those which produce progredientes (singular progrediens) and sexuparae are approximately 0.35 mm long and 0.21 mm wide.
Adult sistentes can lay a single batch of up to 300 eggs, creating a spherical "woolly" ovisac made of white waxy threads. Progrediens adults also lay their eggs (up to 250) in similar, but smaller, cottony ovisacs.
Sexuparae deposit up to 15 eggs, which are 0.37 mm long and 0.25 mm wide, beneath their wings. However, the sexuales that emerge from these eggs fail to develop any further.
Photos: Michael Montgomery, USDA Forest Service, bugwood,org
Adult HWA and eggs inside an ovisac
Nymph: Crawlers (first-instars) are brownish-orange in colour and are about 0.44 mm long and 0.27 mm wide. Second-instars have short, thick legs and are approximately 0.57 mm long and 0.34 mm wide. Third-instars are approximately 0.67 mm long and 0.43 mm wide. Fourth-instars are 0.74 mm long and by 0.47 mm wide.
For sexuparae, first-instar nymphs are similar in appearance to those of sistentes and progredientes. Second-instars are approximately 0.60 mm long and 0.35 mm wide. Third-instars are approximately 0.77 mm long by 0.47 mm wide. Fourth-instars are 0.89 mm long and 0.49 mm wide.
Adult: Adult sistentes are about 1.41 mm long by 1.05 mm wide and are engulfed by a heavy waxy-coating (Canadian Food Inspection Agency 2016).
Adult progredientes also have a waxy-coat and are approximately 0.87 mm long and 0.63 mm wide.
Adult sexuparae are approximately 1.09 mm long and 0.51 mm wide. These adults are dark brown and have long antennae (five-segmented), compound eyes, and four textured wings.
There are two generations and three forms of hemlock woolly adelgid in North America. The two generations that occur on hemlock are known as sistens and progrediens, all of which are female and reproduce asexually. The second generation of HWA can produce two forms: progrediens adults (which are wingless and stay on hemlock) or sexuparae (which have wings and migrate to spruce). Sexuparae are not known to survive on native North American spruces or on spruce trees from their native range that have been transplanted in North America. As the sexuparae form is not important to the biology of HWA in North America, we will focus on the sistens and progrediens forms that attack hemlock.
The sistens generation, sometimes referred to as the overwintering generation, is present from June until March of the following year; whereas the progrediens generation (spring generation) is present from March until June (Fidgen & Turgeon 2016). Both generations have six stages of development: egg, four nymphal instars, and adult (Cheah et al. 2004). Once the progrediens oviposit (lay eggs) on hemlock, the sistens hatch in June-July. The newly hatched first instar nymph is known as a crawler, which moves around the hemlock looking for a place to start feeding (Canadian Food Inspection Agency 2016). It then feeds briefly before becoming inactive for the remainder of the summer. In mid-October, the crawler will resume feeding and will continue to develop through the four nymphal instar stages. Following this, the sistens matures into a full adult in early May. These adult sistens each produce a single ovisac containing up to 300 eggs, which when hatched will produce progrediens. Because there is no diapause or overwintering requirement for the progrediens generation, the life cycle occurs rapidly, usually from spring to early summer.
Figure: Reardon et al. In Biological Control of Hemlock Woolly Adelgid. Burlington: USDA Forest Service Forest Health Technology Enterprise Team, 2004, 3
Hemlock woolly adelgid annual life cycle on hemlock in North America
Signs of infestation by HWA include (Havill et al., 2014):
White “woolly” sacs at the base of hemlock needles on most recent twigs
Premature bud and shoot dieback
Premature needle loss
Thinner, greyish-green crown
Dieback of twigs and branches
Discolouration of foliage
Death within 4-15 years
Photo: Chris Evans, Illinois Wildlife Action Plan, Bugwood.org
Photo: USDA Forest Service- Region 8 - Southern Archive, USDA Forest Service, Bugwood.org
Photo: Chris Evans, Illinois Wildlife Action Plan, Bugwood.org
The Hemlock Woolly Adelgid is a native species in Asia. Populations are found in the Pacific NW of the USA and Canada, where its population is thought to be controlled by a combination of natural enemies and host resistance. HWA made its way to eastern North America, likely on infested planting stock and was discovered in Richmond, Virginia in 1951. HWA can be found in every state from Georgia northeast to Maine and six of those states share a border with eastern Canada (see photo below). Although HWA has been eradicated in Ontario, new introductions are almost certain to occur in provinces bordering infested US states. HWA can survive in plant hardiness zone 5a and higher (Kanoti et al. 2015), which puts most of Canada’s hemlock trees at risk. Furthermore, it is believed that cold tolerance is an adaptive trait, meaning that HWA could evolve to tolerate colder climates as it spreads northward into Canada.
Dispersal of HWA occurs by wind, animals, and human movement of nursery stock, logs, and other wood products (Canadian Food Inspection Agency, 2015). Spread rates have been estimated at up to 20-30 km per year (Ontario HWA Working Group 2014).
Map: U.S. Department of Agriculture, Forest Service. 2016 (January 26). http://na.fs.fed.us/fhp/hwa/maps/2015_HWA_Infestation_Map_20160502.pdf
The map below is the EDDMapS (Early Detection & Distribution Mapping System) Ontario distribution map for the hemlock woolly adelgid as of May 2017. To see the current EDDMapS distribution map, click on the map below.
Map: EDDMapS. 2017. Early Detection & Distribution Mapping System. The University of Georgia - Center for Invasive Species and Ecosystem Health. Available online at http://www.eddmaps.org/; last accessed May 18, 2017.
There are import and domestic movement requirements for all hemlock (Tsuga spp.), yeddo spruce (Picea jezoensis) and tiger-tail spruce (Picea polita) trees (and many of these species’ tree products) to prevent the introduction and spread of the hemlock woolly adelgid (HWA) (Canadian Food Inspection Agency 2015).
The following Canadian Food Inspection Agency (CFIA) policy relates to HWA:
The economic value of hemlock to the forest industry is not as high as other trees species, however, eastern hemlock can be processed for use in general construction or as pulp (Canadian Food Inspection Agency 2015).
Photo: River Valley Woodworks
Several animal species require hemlock forest for part, or all, of the year (Chowdhury 2002). Many birds use hemlock trees as a food source and for nesting and roosting sites. Hemlock trees are also commonly found in riparian areas, where they play an important role in regulating and maintaining water temperatures. For example, the canopies of hemlock trees can shade streams, cooling the water and making it habitable for brook trout. In winter, hemlock provides a valuable thermal cover to deer and birds during cold weather.
HWA will cause significant changes to hemlock ecosystems: biotic communities and ecosystem services could be significantly disturbed. Loss of eastern hemlock could result in changes in energy inputs, nitrogen cycling, microclimate environment, and physical environment, all of which could negatively affect the health of vegetation, birds, aquatic organisms and mammals (Snyder et al. 1998).
Photo: Gabriel Popkin
Hemlocks lining a stream
One social impact of HWA is the resulting loss of aesthetic value of hemlock trees, since they are commonly used as ornamental trees; a diseased hemlock tree, infested with woolly masses can be especially displeasing to look at. Hemlock trees also provide shade and thermal protection in residential areas. Loss of hemlock trees will result in property-value losses (Li et al. 2014). HWA infestation will also affect recreation experiences. Loss of hemlock trees will affect stream temperatures which could affect fishing quality. The defoliation and loss of trees could also affect the quality of recreational hiking activities.
Hemlock Woolly Adelgid, and the resulting loss of hemlock trees, has the potential to cause major ecological impacts in Canada. In many forests, hemlock serves as a foundation tree in the environment, with a disproportionate control of ecological function. In this photo, the canopy thinning caused by HWA can lead to more light and water reaching the forest floor, and can impact nutrient and water cycling, wildlife habitat, and micro-climate temperature.
Since HWA is passively spread by wind, birds, animals and humans, this pest can move long distances and poses a significant threat of establishment in provinces adjacent to the infested eastern US states. It is important to continually monitor for establishing HWA populations.
Visual surveys to detect HWA are conducted annually in Canada at high risk sites. These sites include current and historic importers of hemlock nursery stock and areas at risk of natural spread, including green spaces/urban parks and hemlock forest stands within 100km of the Canada/United States border (Ontario HWA working group 2014).
Visual surveys are only able to examine a small portion (within 6 m of ground level) of the potential HWA habitat as hemlock crowns can reach heights of 30 m or more (Farrar 1995). Research by the Canadian Forest Service is attempting to improve the reach of ground based detection surveys; hemlock crowns are being sampled for the wool of HWA using VELCRO®-covered racquetballs launched with a slingshot and 1st instar crawlers are being intercepted with sticky traps (Fidgen, Whitmore & Turgeon 2015). To learn more about these techniques, click here.
Photos: Jeffrey Fidgen
A) Modified racquetball with HWA wool (red arrows)
B) Launching balls into the crown of hemlock
Respond & Control
In Canada, outside of the regulated area, all new finds of HWA are eradicated using mechanical control only. Infested trees are cut down and burned on-site in an effort to eradicate HWA. Delimitation surveys are used to monitor the success of the eradication treatment (Ontario HWA working group 2014).
Insecticide formulations with the active ingredients imidacloprid or dinotefuran are widely used in the United States. Methods of applying these active ingredients include: soil applied treatments (eg. time-released tablets or drenches), basal bark sprays and tree injection (see photo below). Initial studies suggest that a tank mix of imidacloprid and dinotefuran applied as a basal bark spray may be the most optimal chemical control tactic (Invasive Species Centre 2015).
Photo: Great Smoky Mountains National Park Resource Management, USDI National Park Service, Bugwood.org
It is generally agreed that the best strategy to achieve long term protection of hemlock over the landscape is with biological control agents of HWA. The USA has heavily invested in the rearing and release of several predators of HWA from parts of the world where HWA is native (e.g., Pacific NW, China, Japan). Before being released into a new habitat, the prospective biological control agents undergo rigorous laboratory testing in quarantine to make sure their environmental impact is restricted to the targeted prey, in this case HWA.
The tooth-necked fungus beetle (Laricobius nigrinus) and two silver flies (Leucopis piniperda, Leucopis argenticollis), native to the northwest USA and British Columbia, Canada, are voracious predators of the adelgid and are being actively released across the eastern USA. Several other predators, mostly from the far east, are in various stages of quarantine evaluation, release, and monitoring of their efficacy in the wild (Invasive Species Centre 2015). In addition, at least two fungal pathogens as biological control agents of HWA seem effective (Havill, Vieira & Salom 2014).
The tooth-necked fungus beetle (Laricobius nigrinus Fender), pictured below, is a natural predator of HWA.
Photo: Pennsylvania Department of Conservation and Natural Resources - Forestry, Bugwood.org
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