LONG TERM CONTROL OF THE WOOLY HEMLOCK ADELGID (Adelges Tsuga)
Toward IPM Strategies in the Absence of Biological Control
Tom Burgess, LA, NREMT-P
Catherine A. McDonald, Ph.D.
copyright (c)1999 by Tom Burgess
ABOUT THE AUTHORS:
Tom Burgess is a licensed arborist with over thirty years experience in all aspects of tree care. His business, Residential Foresters, specializing in organic and natural arboriculture, also provides large tree moving and aftercare. He is a member of the International Society of Arboriculture, the Connecticut Tree Protective Association, and the National Arborist Association. He is a Nationally Registered EMT-Paramedic and a National Ski Patrol Certified Patroller. Tom and his canine partner, German Shepherd Dog Willie, a certified Avalanche Dog, are currently preparing for FEMA certification. Willie’s daughter Beep is athletic coach for the pair.
Catherine A. McDonald, medical sociologist, is an associate arborist with Residential Foresters. Her M.A. is in the field of industrial sociology; her Ph.D. emphasized research design and natural medical therapies. Catherine’s four German Shepherd Dogs teach her the art of inter-species cooperation. She utilizes this knowledge to counsel other humans and dogs toward harmony.
The beautiful eastern Hemlock (Tsuga Canadensis) is being threatened by the Wooly Hemlock Adelgid (Adelges Tsuga). Horticultural oil spraying has been the primary weapon of control. Its low degree of toxicity and low risk of developing resistance have fostered its application.
Spraying, however may not be appropriate in all situations and the lack of residual effect can be deleterious in others. This paper describes a case where micro-injection was employed, discusses the results, and explores the possibilities of systemics in adelgid control.
The eastern hemlock (Tsuga Canadensis) is a graceful tree valued by homeowners as a specimen or more commonly as a hedge. It is also plentiful in the forests of eastern and north eastern United States. It has some value as a commercial wood and provides food and shelter for deer and birds. (1)
Large stands of hemlocks are being defoliated by the wooly adelgid (Adelges Tsuga), a small insect, native to Japan and lacking natural predators in its new environment. The adelgid, first found in Virginia in 1950, was transported to southern New England by hurricane Gloria in 1985. In our work we commonly see weakened and partially defoliated tress. However, severe damage and dead trees are evident from the highways. Widespread damage occurs in state forests and campgrounds and at some institutional campuses.
Control of the wooly adelgid has been dependent on dormant (horticultural) oil. Immediate knockdown, low residual, and no resistance build up have been advantages. But there are drawbacks. Five years ago, one spraying per season sufficed. Currently untreated trees and heavily forested areas serve as a reservoir for continued infestation. Infested trees border treated sites. Three applications per season are required in many sites. The cost is prohibitive for some homeowners.
Additionally, spraying is inappropriate in some locals. Neighborhood design, local wind conditions, adjacent high volume businesses, day care facilities, and open water or pools are some of the factors that make spraying difficult or impossible.
THE CASE STUDY
The wooly adelgid is an insidious problem. Often the homeowner is unaware that their trees are infested until significant defoliation has occurred. We were called by a homeowner to a situation where over 150 feet of thirty foot hemlocks suffered severe defoliation. The hemlocks served as a wind screen and privacy barrier on a property boundary. The neighbor’s house was ten feet adjacent to the trees. They objected strongly to any spraying.
In our judgment defoliation had proceeded to the point where the effect of spraying was dubious. The homeowner agreed. The trees were micro-injected with the systemic insecticide Imidacloprid as a last hope treatment. Imidacloprid (Trade name “Pointer”) was micro-injected using the Wedgle Technique at the rate of 1 ml per inch diameter breast height (DBH).
We checked on the trees later that season and saw no visible results. After several seasons of growth the situation was reversed. The trees appear normal although a few adelgid are present. Interestingly one untreated tree has died.
It should be noted that the trees in this study were on a lake shore and were therefore well hydrated, possibly enhancing the distribution of the insecticide throughout tree vasculature. Further study is needed to ascertain the efficiency and longevity of this treatment modality in drier soils.(5)
We believe this case demonstrates the value of employing systemic strategies in the control of the wooly adelgid. Consistent with the goal of reduced pesticide use, micro injection not only salvaged a row of trees, but also eliminated the need for several seasons of treatment.
Although experiments are underway with biological controls, it will be some time before they are available commercially.(3,4) In the meantime we are exploring the value of other injectable systemics and ground dispersion methods. An expanded armamentarium is needed to preserve the hemlock.
(1) Harlow, William H., Trees of the Eastern United States and Canada, Dover Publications, NY 1957
(2) Norwich Bulletin, May 28, 1999, A6
(3) Dr. Mark McClure, entomologist, Valley Experimental Station, South Windsor, CT is developing a predator species.
(4) Norwich Bulletin, op cit.
(5) This is consistent with the advice of Ed Marrotte, Consumer Horticulturist, University of Connecticut Extension Service, who recommends that trees be adequately hydrated prior to any use of systemic insecticide (personal communication).
WOOLY HEMLOCK ADELGID NEWS — August 1999
Residential Foresters is pleased to announce another treatment in the control of the hemlock adelgid. As many of you are now aware this pest is defoliating and killing hemlocks to the point where it is now an epidemic. Our spraying of horticultural oil can keep the adelgid in check. But in some instances two or three seasonal applications are required.
We recently completed a case study application of a systemic pesticide, Imidacloprid, that when injected into the tree or the ground can give coverage for up to several seasons. As it is absorbed into the tree, no surface residual is left on the foliage. Additionally pesticide drift is never a concern with this technique. In our study thirty severely defoliated trees recovered but an untreated tree died.
We will be recommending this treatment when appropriate for the care of your hemlocks. Please call us with questions concerning the treatment of your trees.
Tree Ark, LLC
Tom Burgess, Licensed Arborist, EMT – Paramedic
860 429-9972 • Fax 860 429-9973
Connecticut Arborist License # 62028
DEP # B8-0282
Member, Connecticut Tree Protective Assoc.
CT DEP Business Registration #8-0282