Feature Article

Recent Happenings in Biological Control of Forest Insect Pests

Forest ecosystems provide production systems that are much longer lived, more diverse, and generally more stable, than most agroecosytems. Thus, many of the most long lasting and effective biological control programs have been established against forest pests. Combined with the facts that applying insecticides over such large areas and over such a tall canopy is rarely economical, and that society demands other uses of forests, such as recreation, watershed, wildlife, and aesthetics, in addition to fiber production, the reason for a strong historical emphasis on biological control becomes even more clear.

Some recent developments in the biological control of forest pests may be of interest to readers of Midwest Biological Control News.

Westward Migration of the Gypsy Moth. The gypsy moth is the most damaging pest of deciduous trees in North America. It affects production forestry, but also interferes with recreation, reduces home values in urban and suburban settings, and alters environmental quality. The gypsy moth was deliberately brought into New England by a French entrepreneur, escaped, and has been progressing westward and southward ever since. It is now established west into most of Wisconsin, and south into North Carolina. Female European strain gypsy moths are flightless, so the major means of dispersal is by human transport. A number of biological control agents were introduced against the gypsy moth, including predaceous beetles and parasites. These helped somewhat, by reducing the frequency, duration, and impact of gypsy moth outbreaks, but the outbreaks persist. The most spectacular recent development has been the emergence of the fungal pathogen, Entomophaga maimaiga. This pathogen was introduced during the early part of this century, presumably became established, but subsequently had little impact. During 1989, the gypsy moth erupted into one of its most extensive and potentially damaging outbreaks ever. But the populations collapsed, due to what appeared to be an epizootic caused by nuclear polyhedrosis virus. Closer examination, however, revealed it was due to E. maimaiga, which had gone largely unreported for 80 years. The reasons for this sudden success remain a mystery, and we may never know if it was due to a mutation, subsequent accidental introduction, or some other factor. E. maimaiga has been recently introduced into a number of states along the gypsy moth's leading edge, with dramatic results. Some of the most successful releases have been in Michigan. This pathogen is almost entirely specific to the gypsy moth, yet it can be effective at lower gypsy moth densities than are typically needed for virus to exert effects.

At the University of Wisconsin, we have just completed a two year survey of the natural enemies of the gypsy moth. The good news is that E. maimaiga, the larval parasitoid Cotesia melanoscela, the egg parasitoid Ooencyrtus kuvanae, some parasitic flies, and NPV were found to be present. Unfortunately, they were here in extremely low numbers-only 1 larva out of over 1000 larval and soil samples had E. maimaiga, and only a few larvae were parasitized. Based on these results, the Department of Natural Resources conducted releases of E. maimaiga and C. melanoscela in the autumn of 1997. They also disseminate O. kuvanae whenever possible. We will conduct follow-up surveys of these releases over the next two summers.

Some additional projects are actively underway on biological control of the gypsy moth throughout the the Midwest. Many of these are aimed at determining how the tree species on which this polyphagous insect feeds will affect various natural enemies. We now have a fairly good appreciation of how tree species affects the performance of Bacillus thuringiensis kurstaki, two strains of C. melanoscela, O. kuvanae, and others. There has also been substantial work, particularly at Michigan State University, on how various biological control agents, including Btk, might affect endangered species and nontarget insects. Work at the USDA APHIS laboratory in Niles, Michigan, is evaluating some new parasitic flies that seem to be effective at relatively low gypsy moth densities, yet are host specific. Work in Illinois and Indiana continues to develop new insect pathogens such as microsporidia, and Minnesota is experimenting with various types of anticipatory biological control. Efforts among the various states are coordinated through a couple of groups, such as the North Central Regional Committee on Gypsy Moth, NC-212, the USDA Forest Service "Slow the Spread" program, and the various state agencies.

Chemical Ecology of Bark Beetles. Bark beetles are the most damaging group of insects affecting North American forests. All of the major pests in North America are indigenous, with the exception of the smaller European elm bark beetle, the vector of Dutch elm disease pathogen. Most bark beetles colonize previously killed trees, but a few attack live trees, especially conifers. They kill trees by collectively exhausting the trees' defense systems, through mass attacks. These mass attacks are coordinated by aggregation pheromones.

Entomologists have known for some time that predators of bark beetles, such as clerid beetles and hister beetles, locate their prey by cueing in on the bark beetles' pheromones. This improves the efficiency of these natural enemies, but also poses some management challenges. For example, one tactic that has been tried repeatedly throughout the world is using aggregation pheromones to "trap out" the bark beetle population. This method results in lots of trapped beetles, but insignificant tree protection, largely because it also reduces predator populations. In some cases it has even been reported to extend outbreaks.

Bark beetle pheromones come in a variety of configurations, with different isomers and additives. Work at the University of Wisconsin has demonstrated that alterations in these secondary features can allow bark beetles partial escape from predators while maintaining intraspecific functionality. That is, by producing different versions of the same pheromone, beetles can maintain the communication needed to attack trees and attract mates, yet reduce their attractiveness to natural enemies. Presumably this "shell game" leads to counter-adaptations by the predators, followed by new innovations by the bark beetles, etc.

This coevolutionary race has some important implications to biological control. First, forest managers make control decisions based on the relative abundance of pests and predators captured in pheromone traps. But such lures were developed to maximize attraction by the target bark beetle. We now know that such estimates greatly underestimate the numbers of natural enemies, which prefer different forms of these chemicals. Based on these findings, it will soon be possible to make appropriate corrections in our estimates. Behavioral disparities between predators and prey also offer some opportunities to reduce the negative impacts of pheromonal trap out programs on beneficial insects. Depending on the combination of chemicals in the pheromone lure, the ratio of pest to beneficial insects in these traps can vary enormously. So we may soon be able to simultaneously maximize pest catch while excluding most predators. This approach might be applicable to a broad range of forest and agricultural pests. Augmentation of natural enemies to foci of bark beetle attack could also be improved by deploying the appropriate lures. Additional work is evaluating the roles of host tree chemistry, and volatiles emitted by bark beetle fungi, in the chemical signals attracting predators, and parasites. These projects are being conducted in collaboration with the University of California - Berkeley.

Biological Control of Chestnut Blight. Not all biological control in forestry is directed at insect pests. One of the worst disturbances to North American forests was the introduction of the chestnut blight fungus. Chestnut blight decimated what was a predominant species in the Northeast and portions of the Midwest. Native trees had no appreciable resistance against infection.

Recent work has demonstrated that a "hypovirulent" form of the fungus has a very different type of relationship with chestnut trees. Unlike the aggressive strains of the pathogen which can quickly colonize and kill trees, hypovirulent strains may form only limited, superficial lesions. However, infection with the hypovirulent strain does not provide systemic protection; these trees can still be infected and killed by the typical virulent fungi. The hypovirulent fungi are not a true strain, however, but rather this condition is induced by the presence of virus-like agents with the hypovirulent fungus. Thus, this biocontrol agent may play a key role in tipping the host-pathogen relationship in favor of the host tree. Workers at Michigan State University are currently among the most active groups studying the molecular biology of this system.

A major challenge lies in determining how hypovirulent fungi can be distributed and maintained in the wild. Current emphasis is largely on high value trees such as in parks, where inoculation is feasible. Some of the more futuristic ideas include the possibility of developing certain insects as vectors.

New Arrivals. Some recent introductions pose serious challenges to the forest products industry and forest health.

The pine shoot beetle, Tomicus piniperda, a native of Europe, entered the US through ports in Ohio. It attacks the tips of trees, causing disfigurement. It could especially impact Christmas tree production, a major industry in the Midwest. This insect is widely distributed in Michigan and northern Illinois, and was first detected in Wisconsin last summer.

The USDA organized a three-pronged management approach: determine the host range among North American trees, evaluate cultural controls, and consider releasing a European predator, a clerid beetle. The major cooperators were the APHIS laboratory in Niles, Michigan, the USDA-FS, and Michigan State University. The first line of research showed that this insect could build up populations in downed native tree species, but could only have appreciable effects on live Scots pine. The second line of research developed practical cultural methods that fit well into Christmas tree production, such as removing and destroying tree bases in which beetles overwinter. The third avenue considered evidence that this exotic clerid could conceivably interfere with the native clerids that are so important to the natural control of native bark beetles (described above). Thus, it was decided to not introduce this predator, at least for as long as conditions remain stable. Such decisions should be viewed as successes, not setbacks, for biological control. They demonstrate that critical evaluations precede releases, and that these evaluations take a broad view of pest management, environmental safety, and integrated approaches.

Similar analyses are underway for another introduced pest, the hemlock woolly adelgid. This represents an extremely serious threat to northeastern and midwestern hemlock forests, just when a return to old growth hemlock ecosystems are being promoted as a cornerstone of forest health. Native eastern hemlocks appear totally susceptible to this Chinese insect, which threatens to have an impact equivalent to the balsam woolly adelgid in the southeast. A number of predators from China are being evaluated by USDA Forest Service personnel.

Ongoing Successes. It's easy to become preocuppied with current and new threats to forest ecosystems. But it is also important to recognize that these cases represent unusual exceptions to the general pattern of natural control of most tree-feeding species. It is worth pointing out that a number of successful biological control projects are inconspicuous precisely because of their spectacular success. Some of the best examples include the conifer sawflies, several species of which were introduced, caused severe outbreaks, and were subsequently controlled by introduced parasitoids. Most of these species remain at innocuous levels during most years, particularly on the native tree species that comprise our commercial and unmanaged forests.

- Kenneth F. Raffa,University of Wisconsin - Madison


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