Research is ongoing in the field of invasive forest pests, as there are still a lot of scientific questions to be answered. Some invasive pests have existed in Canada for decades, and researchers have a good understanding about their basic biology and behavior. For example, the mountain pine beetle has always existed in Canada. When the current mountain pine beetle outbreak began in the 1990’s in BC, and eastward into Alberta in 2006, researchers were already aware of the potential devastating impacts to pine forests, and could focus their efforts on control and management of the population. Conversely, when emerald ash borer was discovered for the first time in North America in 2002, very little was known about the beetle or what it was capable of. In fact, it took two months, and experienced entomologists from multiple research institutions to even identify the species. Finally the beetle was identified as emerald ash borer, or Agrilus planipennis, by Eduard Jendek, an expert at the Institute of Zoology at the Slovak Academy of Sciences. With the identity of the pest confirmed, Michigan researchers wanted to know more. However, all the information they could find was one and a half pages in a Chinese book on forest pests. In the years following, researchers learned a great deal about the insects’ biology, physiology, and behaviour, and can now focus efforts on management and control.
Biology, physiology, and behavior
In order to understand how a pest or pathogen will impact a forest ecosystem, it is vital to understand the biology and physiology of both the invasive pest, and the trees it attacks. One of the major factors that determine whether a pest will become established in Canada is the ability of the pest to survive a cold Canadian winter. Scientists from the universities of Western Ontario, Waterloo, and the CFS have been working to understand the overwintering physiology of emerald ash borer, and its potential survival and distribution in Canada. Prepupae, the overwintering stage of emerald ash borer, can withstand average minimum temperatures of –30°C by employing antifreeze compounds (NRCan, 2014).
Recent advances in genomic research can also be useful for biosurveillance and invasive species management. Through the Genomic Applications Partnership Program (GAPP), Natural Resources Canada is developing ways to implement genomic tools into invasive species detection and identification. DNA detection tools target multiple genome regions of a species in question to tell researchers the identity of the species and provide information on its likely source (where it came from). The ability to rapidly and accurately identify an invasive pest, whether as an adult or an egg, a fragment or a full specimen, will allow for early detection and rapid response to control and eradicate the pest before it becomes established. This technology is currently being developed for identification of Asian gypsy moth and sudden oak death in British Columbia. The USDA forest service has also used molecular genetics technology to pinpoint the origin of hemlock woolly adelgid, and identify several distinct populations worldwide. Molecular genetics has also been invaluable for the biological control program by providing scientists with an efficient means to identify morphologically similar species of predatory beetles and flies (HWA Steering Committee, 2013).
Ability and rate of spread is another factor influencing the potential impacts caused by an invading pest or pathogen. For example, the mountain pine beetle has been spreading across the western mountain forests in British Columbia and Alberta. However, as the beetle starts to enter the boreal forest, the potential for spread is unknown. The boreal forest is a novel environment for this beetle and many questions—such as how quickly populations will spread and what their forest ecological, economic and social impacts will be —have yet to be answered. The Canadian Forest Service points out how the pine stands in the boreal forest are typically less dense and have smaller trees than British Columbia’s lodgepole pine forests. Such stand characteristics may not necessarily be optimal for beetle spread; however, new evidence suggests they may be less of an impediment to the spread and establishment of the beetle in boreal stands than previously believed. Research being conducted by the CFS and other agencies focuses on gaining greater understanding of the ecology and population dynamics of MPB in the insect’s new environment (NRCan, 2014).
The genetic makeup of all unique individuals within a taxonomic group can vary slightly, and trees are no exception. Some individual trees may be more susceptible to infestation by an invasive pest, while other trees may be more resistant due to inherent biological, chemical, or physical traits. It is possible, under the conditions imposed by an invasive species, the susceptible trees in a population will die out, and the trees with the resistant genotype will be able to propagate and spread across the landscape. This population shift is a form of natural (directional) selection; the allele providing resistance will become advantageous under the pressures of an invasive species infestation, and will improve the fitness of trees with that genotype. If researchers can identify trees with a resistant genotype on the landscape, they can develop breeding and replanting programs to augment the spread of these trees throughout the environment.
Emerald ash borer is reported to kill up to 99.9% of all ash trees they infest; however, there is a very small fraction of ash trees that have survived the invasion. Specifically, the survival rate of blue ash (Fraxinus quadrangulata) seems to be greater than any other native species of ash in Ontario. In a study conducted by researchers at Michigan State University, 60-70% of blue ash were found to be alive, and most of them healthy, in sites that had long been infested with EAB. The white ash in these forest sites had reached 100% mortality, apart from a few live saplings. Scientists want to know what makes blue ash partially resistant to EAB, and are investigating the genetic makeup of these trees (Tanis and McCullough, 2012)
Similarly, resistant individual trees exist in the hemlock and beech populations. Resistant hemlock genotypes are identified from devastated landscapes where all susceptible trees have died off. Breeding programs are being developed with these trees, and other identified resistant species, in an attempt to repopulate environments with resistant hemlock. Resistance breeding and planting programs are also being developed for beech bark disease affected beech trees. However, research is being done to determine how effective these programs will be; some scientists are questioning the survival success of the newly planted generation of beech trees, given that it may be competing for resources with dense root sprouts from non-resistant trees found on BBD devastated landscapes (HWA Steering Committee, 2013, McLaughlin and Greifenhagen 2012)
The loss of a dominant species in any forest environment can lead to a cascade of ecological impacts. Some potential invasive species induced ecological impacts are as follows: changes to forest structure, altered canopy gaps, reduced coarse woody debris, altered biogeochemical cycling, and altered ecological interactions among organisms (both aquatic and terrestrial). Specifically, populations of native species that have specialized interactions with the threatened host, such as terrestrial arthropod species with a high level of association with ash, might be at increased risk (Gandhi and Herms 2010).
Researchers at the CFS and the University of Guelph are currently investigating impacts that result from the loss of ash trees in ecologically sensitive areas. Some of these impacts include; changing ground vegetation communities, reduced input of ash leaf litter into terrestrial and aquatic ecosystems, altered nutrient cycling in the soil, and impacts on local invertebrate communities.
Similar questions are being asked about forests facing the aftermath of beech bark disease. Certain forests can be dominated by beech, and can undergo significant structural and functional changes once beech is removed. New research is focused on determining the effects of BBD on stand and ecosystem traits, such as resulting tree species composition and biodiversity of plants and animals in stands where beech was once a primary species. BBD-killed trees have been observed to produce dense root sprouting after the main tree has been killed. Researchers are questioning if these sprouts will develop into beech thickets, and how the high density of sprouting might impact the regeneration on other tree species in the forest by competition for resources (McLaughlin and Greifenhagen 2012).
Improved detection methods
If prevention fails and an invasive species crosses our borders, it is essential to detect and identify them as soon as possible. We need to know that an invasive has arrived so that we can respond quickly.
For example, emerald ash borer is currently well established and widespread throughout southern Ontario, parts of Quebec, and many midwestern U.S. states. Scientists want to determine the most effective way to detect EAB when it arrives to new areas of North America. Early detection is vitally important, so appropriate management strategies can be implemented before the population becomes locally established. To detect the presence of EAB, researchers have developed methods of using green prism traps and branch sampling (link to videos). CFS researchers have also demonstrated the attraction of male emerald ash borers to a female-produced pheromone. The pheromone has been shown to increase both trap captures and trap detection rates, and is now used commercially as a lure on the green prism detection traps. Researchers are now investigating the use of the pheromone to disorient males and thereby reduce successful mating (NRCan, 2014).
Similar to EAB, pheromone lured traps can be used to detect early populations of Asian longhorned beetle. Several research projects have been initiated to improve survey and detection methods that enhance the chance of successful eradication (University of Vermont, 2005).
Detection for mountain pine beetle is not performed using detection traps, but with landscape scale surveys. Usually aerial surveys are performed from fixed wing or rotary aircraft, where technicians use a combination of sketch mapping, GPS coordinates, and aerial photography to record the areas where MPB has attacked the trees. Usually the aerial surveys are followed by localized ground surveys, which provide a more detailed account of the level of infestation. (Wulder et al., 2004) Research is ongoing to track the spread of MPB into new areas in western Canada, so control measures can be used appropriately.
One of the biggest areas of current research is biological control. Scientists want to fight bugs with bugs, and are working to find natural predators or pathogens to combat invasive species. In some cases, biological control species can be native to Canada or the United States, and sometimes they can be non-native species, introduced from the home range of the invasive pest.
Some natural enemies may help reduce emerald ash borer populations. Native species that attack EAB include species of woodpecker, parasitoid wasps (insect parasites that kill their hosts), and pathogenic fungi. CFS researchers are studying levels of mortality caused by these pathogens and parasitoids, in order to quantify their potential impact, as well as methods of increasing or augmenting their populations. Scientists in Michigan are continuing to study non-native parasitoid wasps imported from China to combat EAB. Controlled experimental releases of Oobius agrili, Tetrastichus planipennisi, and Spathius agrili have been approved at study sites in the U.S. since 2007 with some promising results. Controlled release of two of these wasp species, T. planipennisi and S. agrili, was approved in Ontario in 2013. Research is currently being done to evaluate the effectiveness of these species for controlling the spread of EAB (NRCan, 2014, USDA Forest Service, 2014).
Similar efforts are in place for Asian long-horned beetle. Several kinds of entomopathogenic (insect killing) nematodes and fungi are being investigated as natural enemies of ALB to act as agents of biological control. Researchers are also searching for parasitoids that attack eggs and larvae of ALB in their native range of China (Centre for Invasive Species Research, 2014).
Hemlock woolly adelgid has been present in North America since the 1950’s, and biological control efforts have developed over time. The USDA forest service (2013) reported that two species of predatory beetle, Laricobius nigrinus (native to the U.S. Pacific Northwest), and Sasajiscymnus tsugae (native to Japan), have been successfully released and established across the United States. Additional natural enemies have been identified and researchers are progressing toward release for additional biological control. The USDA Forest Service (2013) identifies these species as; 2 species of Scymnus lady beetles (one from China and one from the Pacific Northwest), Leucopis flies from the Pacific Northwest, and a more voracious Laricobius species from Japan (HWA Steering Committee, 2013).