By KEVIN WALKER
LANSING, Mich. — State officials have imposed an interior quarantine on the movement of hemlock trees because of increasing cases of hemlock woolly adelgid (HWA), an insect found on the trees. HWA is a tiny insect that forms small white masses on the stems and twigs of hemlock trees and, if left untreated, will kill them. Michigan has 170 million hemlocks.
The insect pest has been found in four counties on the western side of the state in the Lower Peninsula, including Allegan, Muskegon, Oceana and Ottawa counties. There have been rare instances of HWA showing up in nursery stock in other areas of the state. HWA is well established in Pennsylvania and other eastern states.
According to the Michigan Department of Agriculture and Rural Development (MDARD), the quarantine follows an increasing number of detections of this exotic pest in these four counties. MDARD believes the infestations there are attributable to nursery stock that was brought into the area prior to 2002, when the state’s exterior quarantine was first put into effect.
“Michigan’s hemlock trees are a keystone species that plays a critical role in many of the state’s forests,” said Gina Alessandri, MDARD’s Pesticide and Plant Pest Management Division director. “We need everyone’s help to slow the spread and protect this natural resource.”
The HWA secretes white wax as it feeds on sap from hemlock shoots and branches. HWA feeding can kill needles, shoots and branches. Over time, growth slows as trees become less vigorous and they may take on a grayish-green appearance. Infested hemlocks, especially large, old trees, are often killed when other stress factors – such as drought – affect trees. Michigan has more than 100 million mature hemlocks; if the pest becomes established, most of those trees will be killed.
HWA eggs can be carried by birds and can be moved on hemlock nursery trees, logs or firewood. Hemlock trees are important as wildlife habitat, but they are also commonly used as landscaping trees or bushes. As such, nurseries commonly sell them.
The interior quarantine regulates the shipment of hemlock nursery stock within and out of the four counties mentioned. The quarantine document includes details on what materials can and cannot be moved out of the affected counties – for example, treated posts made of hemlock are exempt from the quarantine.
There is also an exterior quarantine on hemlock materials that applies to 21 states, mostly on the eastern side of the country, plus British Columbia, Canada. As with the interior quarantine, there are exemptions to it, such as certain forms of treated wood, logs and the like. The exterior quarantine was last updated in 2014.
More about HWA, as well as the quarantines, can be found online at http://michigan.gov/exoticpests