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Eastern hemlock, Tsuga canadensis (L.) Carriere, is an important tree species in the forests of eastern North America. It is also among the most widely grown evergreens in ornamental landscapes. A small aphid-like insect, the hemlock woolly adelgid, Adelges tsugae Annand, which is native to Japan, is a destructive pest of forest and ornamental eastern hemlock in at least 11 eastern states from North Carolina to southern New England.
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Hemlock woolly adelgid feeds on the young hemlock twigs near where the needles are attached. The insect not only sucks valuable fluids from the tree but it also probably injects a toxic saliva during feeding. Feeding causes needles to discolor and drop prematurely, it causes branches and limbs, particularly in the lower part of the crown to die back, and it ultimately kills the tree, usually within four to six years. Unfortunately, hemlock trees of all sizes and ages are susceptible, none has shown resistance, and none of the native natural enemies that now inhabit our hemlock forests are effective control agents. Hemlock woolly adelgid was first discovered in southern Connecticut in 1985. It now occurs in 155 of the state's 169 towns; only 14 towns in Litchfield and Windham counties are free of the adelgid. Eggs and small immatures of the adelgid called nymphs can be carried by wind, birds, white-tailed deer, and humans during logging, nursery and recreational activities. Hemlock woolly adelgid has been spreading at the rate of about 25 miles each year. Because the adelgid can survive cold winter temperatures, it will most likely continue its relentless spread northward. Therefore, it threatens to eventually eliminate hemlock throughout much of its natural range in the northeastern United States and southeastern Canada. Fortunately it is possible to manage hemlock adelgid on hemlocks in nurseries and in ornamental landscapes by carefully inspecting trees for adelgid infestation and by employing various cultural and chemical control practices. Last year I authored Station Bulletin #925 which describes in detail how ornamental hemlocks can be kept healthy in the face of the adelgid threat. Single copies of this bulletin are available free upon request to: Publications, CAES, Box 1106, New Haven, CT 06504. |
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Adelgid management has been unsuccessful in forests where trees cannot be thoroughly treated with chemical pesticides. The persistence of hemlock in the forests of eastern North America may ultimately depend upon biological control, a process whereby natural enemies successfully regulate pest numbers. Because none of the native natural enemies that inhabit adelgid-infested hemlock forests in eastern North America are effective, biological control will probably require the establishment of an effective enemy from Japan, the homeland of hemlock woolly adelgid.
In 1992 I traveled to Japan to study the adelgid there and to explore for natural enemies. I discovered that populations of hemlock woolly adelgid in Japan are being controlled by several natural enemies, the most effective of which are the coccinellid beetle, Pseudoscymnus tsugae sp. nov., a previously unknown species, and the oribatid mite, Diapterobates humeralis (Hermann). The coccinellid beetle, also known as a ladybird beetle or a ladybug, feeds voraciously on all adelgid life stages. The mite on the other hand does not feed on the adelgid, but rather feeds on the woolly material surrounding its eggs. However, in so doing mites kill adelgid eggs by dislodging them from the tree. My studies in Japan convinced me that these two natural enemies would be good biological control candidates for introduction into Connecticut hemlock forests.
The importation of any living organisms into the United States must be approved by the USDA, Animal and Plant Health Inspection Service (APHIS). Approval requires submission of a report which details all known information on the organism, its potential impact in the environment, and the risks and benefits of its release. Oribatid mites such as the one I found in Japan are not known to be pests anywhere in the world. They feed on algae and fungus that is associated with decaying litter in the forest. Likewise, the ladybugs are a group of insects which have an excellent track record. They are predators of numerous insect pests including scales and aphids, and few ladybugs are harmful. Furthermore, our studies with P. tsugae determined that this ladybug is highly specific to adelgids, all of which are considered pests, and that releasing the beetle would have no adverse impact on the environment. On the basis of my research and reports to APHIS, I received permission to receive shipments of both natural enemies from Japan and to release them in Connecticut.
Since 1993 we have received several shipments of mites and ladybugs from colleagues in Japan. With the help of post doctoral research scientist, Dr. Carole Cheah; research assistants Rob Ballinger, Beth Beebe and Mary Klepacki; and volunteers, Dr. Latta Guddera and Sarah Picard, we have been working feverishly to implement a biological control program to save our hemlock forests from the woolly adelgid.
Difficulties in rearing large numbers of oribatid mites in the laboratory have stymied our efforts to establish this enemy in Connecticut forests. Furthermore, for unknown reasons, the mite has not shown that same interest in the woolly egg masses of the adelgid here in Connecticut as it did in Japan. Accordingly our research during the past two years has focused on the ladybug which, unlike the oribatid mite, is much more amenable to mass rearing in the laboratory.
We have been rearing ladybugs year round in our Windsor laboratory. From a starting population of less than 50 adults, we have now reared more than 8,000 adult ladybugs for our studies on its biology and for our field experiments. During the past three years we have released nearly 7,000 adult ladybugs in adelgid-infested hemlock forests in Bloomfield, Cheshire, New Hartford and Windsor. Our studies there are evaluating the potential of the ladybug to be a biological control agent of hemlock woolly adelgid.
You may be wondering if P. tsugae is the same ladybug that has been showing up in large numbers on the sides of houses or other light colored surfaces in the fall. The answer is no! The ladybug which has this annoying aggregating behavior in the spring and fall is another Asian species, Harmonia axyridis Pallas, which is a much larger beetle, about the size of a small pea, and mainly orange in color with black spots. P. tsugae, the beetle that we are now releasing to combat hemlock woolly adelgid, is only about the size of a poppy seed and is jet black, and fortunately it does not display the annoying aggregating behavior of H. axyridis. Despite its being somewhat of a nuisance at times, H. axyridis is undoubtedly eating huge numbers of aphids and is a great benefit to farmers and gardeners. In fact, H. axyridis will even attack hemlock woolly adelgid, especially during the spring, although the adelgid is not a preferred host of this ladybug.
Attributes of Successful Biological Control Agents
Any natural enemy will need to possess a number of important attributes if biological control is to be successful. Our studies thus far have revealed that the ladybug, P. tsugae, possesses many important attributes of a successful biological control agent.
Ability to Establish: In order to be successful the biology of P. tsugae in the forest will need to be compatible with its adelgid host. Indeed, we have found that the ladybug's life cycle is very well synchronized with that of the adelgid. For example, both insects have two generations each year in the field. Spring egg hatch of ladybugs coincides perfectly with peak egg laying and hatching of adelgids; furthermore a second generation of ladybugs occurs in June at the very same time that the second generation of adelgids does. Also, when adelgids are inactive for a period of about 14 weeks during summer, adult ladybugs are able to survive by feeding on dormant young adelgids. Ladybugs resume egg laying in October when adelgids again become active. The resulting ladybug larvae complete their development in the fall by feeding on adelgid nymphs, and then overwinter as adults.
Successfully establishing P. tsugae in our forests may have the added benefit of controlling other adelgid pests as well. Our preliminary studies have revealed that this ladybug will also attack other important adelgid pests including the Balsam woolly adelgid, the Cooley spruce gall adelgid, and the white pine adelgid. These alternate hosts could serve to enhance the establishment and survival of P. tsugae in the conifer forests of eastern North America.
Ability to disperse: In order to be successful P. tsugae will need to disperse from the release site and spread throughout the adelgid-infested forest. Yellow sticky traps that are attractive to both male and female adult ladybugs were hung in the hemlock forest in Windsor at various distances from our release site. In addition, we periodically inspected hemlock branches on release trees and on nearby trees for ladybugs. We found that some adult ladybugs moved off of release trees onto nearby ones which indicates that they have begun to spread within the infested forest.
Ability to overwinter: In order to be successful P. tsugae will have to be able to survive the winter climate where hemlock grows. To evaluate the ladybug's ability to survive winter weather we used two types of traps to attempt to capture adult ladybugs that had overwintered. Virgin female adult ladybugs which are attractive to adult males were placed inside a funnel trap consisting of a wire frame around which was placed a fine mesh sleeve. Twenty such traps were hung in hemlock trees at the release site. Twenty yellow sticky traps were also hung in trees at the site.
The yellow sticky traps yielded adult ladybugs during the third week of May 1996, indicating that P. tsugae had successfully survived one of the most severe winters on record in Connecticut. Indeed, this was exciting news, because ability to overwinter is a major hurdle that any introduced natural enemy must overcome.
Ability to reproduce: In order to be successful P. tsugae must be able to reproduce and increase its numbers from generation to generation. To do this it must locate and consume a sufficient number of adelgids to mature; it must then successfully mate and find suitable egg laying sites. At each of our release sites, we have been able to document that the ladybug is in fact successfully developing, reproducing, and increasing its numbers. Furthermore, adult females have displayed an uncanny ability to economize their egg laying. They seek out hemlock branches with adelgids and then lay the number of eggs that can be supported by the number of adelgids on that branch.
Ability to control adelgids: In order to be successful P. tsugae must ultimately be able to control the density of the adelgid below the level at which hemlock trees become injured. Our early findings are encouraging in this regard, because the ladybug appears to be having a significant impact on densities of the adelgid. Last summer we released adult ladybugs on five infested hemlocks at our forest site in Windsor. At that time we placed some infested branches inside nylon sleeve cages to protect adelgids from the ladybugs. We left other infested branches uncaged and exposed to ladybugs. This past spring we compared the number of adelgid eggs that were present on caged and uncaged branches. Much to our delight, we found that adelgids were five times less numerous on branches that had been exposed to ladybugs which suggests that P. tsugae had significantly reduced adelgid numbers. Unfortunately, the experiment could not rule out the possibility that the cages themselves had an effect on adelgid survival. Therefore, we are now conducting another study at the Windsor site to determine the impact of ladybugs on adelgids without the use of exclusion cages. A new group of infested trees was selected in a different section of the hemlock forest. Adelgid eggs were counted on branches prior to the release of ladybugs and will be counted again at the end of this season for comparison. We are also monitoring the health and recovery of hemlock trees at the ladybug release sites.
Summary
Clearly, P. tsugae possesses many important attributes of a successful biological control agent for hemlock woolly adelgid. Our research findings thus far are encouraging and certainly justify our continuing efforts to establish this ladybug in the hemlock forests of the northeast. However, I emphasize that additional studies are needed to substantiate that P. tsugae is an effective biological control agent, and to justify the massive effort that will be needed to rear enough ladybugs for release throughout the adelgid-infested area. Furthermore, much more detailed and conclusive evidence is needed before we can hope to entice the private sector into developing a commercial mass-rearing operation for this ladybug. Therefore, even if P. tsugae proves to be effective, its availability to the general public may still be at least a few years away.
The continued presence of hemlock in the forests and wooded ornamental landscapes of eastern North America may well depend upon our success in establishing P. tsugae and other exotic natural enemies. Saving our hemlock trees from the plight of hemlock woolly adelgid will remain the focus of our ongoing research at the Experiment Station.
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Dr. Mark S McClure |
About the Author: Dr. Mark McClure is the Chief Scientist at the Valley Laboratory of the Connecticut Agricultural Experiment Station in Windsor, CT. He is a research entomologist who specializes in the ecology and control of piercing and sucking insects on trees and shrubs. Dr. McClure has investigated five introduced species of scale insects and adelgids which are pests of hemlock and pine. He conducted sabbatical leaves in China (1982) and Japan (1984 & 1992) to investigate the distribution, damage, and natural enemies of scales and adelgids in their native habitats. He then introduced and experimented with a lady beetle he found attacking red pine scale, a wasp he found parasitizing hemlock scales, and a mite and lady beetle he found attacking hemlock woolly adelgid.
Dr. McClure has also studied the efficacy of sprayed, soil-applied, stem-injected, and stem-implanted pesticides and fertilizers on the control of these insect pests in forests and ornamental landscapes. In addition, he has published more than 100 articles in scientific and practical journals, including an integrated pest management guide for hemlock woolly adelgid, and he has co-authored two books on plant feeding insects.
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