Tuesday, April 23, 2019

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The Hemlock Woolly Adelgid (HWA) is a destructive insect pest of forest and ornamental Hemlock trees in the eastern United States and is a serious threat to our native Hemlock resource. It was found for the first time in native Maine Hemlocks in 2003 in Kittery and York. HWA was first found on York Water District (YWD) property in May of 2006. As is often the case, the initial detection of HWA on the water district was by a concerned member of the public. Natural spread of the adelgid occurs by wind and phoresy (carried by other animals including birds and small mammals). It is likely that the majority of HWA infestations on York Water District arrived via natural spread.

Treatment options for HWA infestations in the forest are currently limited. The United States Forest Service (USFS) recommends biological control, using several natural controls, as the best option for managing HWA in the forest (USDA Forest Service 2005). The biological control agents currently available are all adelgid specializing predatory beetles. Two species, Laricobius nigrinus (Ln) and Sasajiscymnus tsugae (St) have been released on York Water District property by Maine Forest Service staff.

With current winter climate conditions HWA populations on District property will likely continue to cycle; alternately being driven down by cold winters, and allowed to rebuild by the absence of cold. Tree condition will also affect population levels, with poor tree health leading to localized crashes in adelgid populations. Hemlock tree decline and mortality brought about by HWA is likely to occur on YWD property. Decline, in the form of foliage thinning, bud starvation and branch dieback, can already be observed in some pockets of hemlocks. HWA-related mortality has not yet been seen, but is likely to occur. Both decline and mortality are likely to be most severe on sites with moisture limitation such as ledge or excessive moisture. Both these conditions limit root development, which can lead to stress in the trees.
More information about HWA in Maine can be found at www.maineforestservice.gov/HemlockWoollyAdelgid.htm.
York Water District 86 Woodbridge Road PO Box 447 York Maine 03909 207-363-2265