Debate the forces behind it, if you will, but there is no arguing that many species of animals are expanding their geographical ranges, edging northward — or upward in altitude — while others are reoccupying areas that they had vacated in times past. Some of the new arrivals are welcome, such as the cardinal, the beloved backyard bird that has stretched its range from the Southeastern United States north to Canada.
Others, notably several mosquitoes and ticks that can carry illnesses such as Lyme disease, West Nile, Zika, dengue and yellow fever, are distinctly non grata and pose new challenges for pest control managers. Several forest and agricultural pests, such as the mountain pine beetle, also are invading new realms.
“Expanding pest ranges and increasing population numbers will need to be included in the selection of suitable crops for production, and better integrated pest management can be developed for those pests likely to cause the greatest losses in yield,” said a Department of Agriculture (USDA) advisory on expanding insect ranges that was issued two years ago.
No better example exists of the alarming spread of a pest species than that of the blacklegged tick (Ixodes scapularis), which harbors more disease pathogens, including the Lyme disease bacterium, than any other vector. Once reported mostly from New England and the upper Midwest, it now lurks in most of the nation’s eastern half. The number of United States counties in which the tick is considered to be established has more than doubled since 1998, according to the Journal of Medical Entomology.
Some of the increase may be attributable to increased surveillance — but not all of it. The tick is showing up more and more where it once was too cold for it to prosper, increasing in density in other places and returning to old haunts because reforestation increases the number of its hosts, especially white-tailed deer.
Blacklegged ticks, like Goldilocks, prefer it not too hot, not too cold. “Tick survival is contingent on a temperature range between 10-35 degrees Celsius with increased mortality associated with temperatures at either extreme,” said a research team headed by Dr. Megan A. Linske from the Connecticut Agricultural Experiment Station in New Haven, which has pioneered studies of Lyme ticks.
The ticks — or, at least, their nymphs — like it soggy, too. Nymphs need 80 percent humidity for winter survival, said the research team headed by Linske in the journal Insects. In areas where thickets of invasive Japanese barberry have grabbed hold, its thorny branches shelter a humid microclimate that amounts to tick heaven, so eliminating this plant invader also cuts down on ticks.
So does the removal of leaf litter and snow cover from tick habitat in winter, according to a study in New England headed by Linske. “Ticks in plots where leaf litter and snow accumulation were unmanipulated had significantly greater survival compared to those where leaf litter was removed and where both leaf litter and snow were removed,” she said. “The findings of this research can be utilized in both small- and large-scale management of blacklegged ticks. Residential landscape modifications can effectively reduce tick overwintering survival by strategically removing the protective barrier caused by leaf litter presence.”
Warmer winters and profound changes in habitat play key roles in range expansion and its consequences, but, increasingly, research suggests the overall picture is much more complex.
Dr. Philip M. Armstrong, virologist and medical entomologist at the Connecticut Agricultural Experiment Station, has been monitoring the risk of mosquito-borne viruses in Connecticut for more than two decades.
“We found that overall mosquito abundance has increased by 60 percent, and there have been numerous introductions of U.S. native and exotic mosquito species into the state during the previous 20 years,” he said. “We detected a south-to-north trend of increasing species richness indicating that many species are moving northward, possibly in response to changes in land use and climate. Mosquitoes have a boom and bust life cycle and can readily exploit new environments created by human activity. Globalization, landscape changes and climate change all provide opportunities for mosquitoes to expand into new habitats that we create for them.”
Given the subtle ecological changes that instigate species expansion, one size, in effect, does not fit all when it comes to responsive control strategies. The news is not all bad, however, because understanding the intricacies of what is going on can be turned to a PMP’s advantage.Consider, for example, a recent report in the Journal of Medical Entomology by Ohio State University scientists describing how, as warm weather lasts longer in fall, the genetically controlled switch that turns off the activity of mosquitoes and sends them into life-extending dormancy activates later and later in the season. Mosquitoes in this state — called diapause — are, in effect, frozen in time. The timing of diapause is a genetically based survival mechanism that halts a mosquito’s development, enabling them to live months longer than their non-diapausing counterparts. Without diapause, mosquitoes would not be able to survive harsh winters in temperate regions and could not start the next generation for the coming spring.
Consequently, not only do more mosquitoes survive to spring, but they extend their period of fall activity and thus the need for control measures. Not a good trend, but the other side of the coin, according to researchers, is that pest managers take advantage of the new findings by factoring the timing of the shutdown into their strategies of monitoring and control. One way is to extend control measures into the period before the suspended development occurs, according to a paper by the Ohio State scientists published in the Journal of Medical Entomology.
The day length, or photoperiod, that triggers half or more of a population to enter diapause at a certain point in autumn is called the critical photoperiod (CPP). It is specific to species. Scientists believe that the introduced Asian tiger mosquito (Aedes albopictus), vector of chikungunya and dengue viruses, has spread as far north as New England because it adapted its diapause to North American photoperiods.
Besides diurnal light, other environmental cues such as falling temperatures are in the mix that triggers diapause. Extended fall warmth has edged the CPP response of some mosquitoes toward shorter, more southern day lengths as northern growing seasons have become longer. Figuring out the CPP of arthropod vectors could improve and enable pest managers to match monitoring and control to the newly extended activity.
Pest species that are climbing mountains testify to the fact that warming is prompting a vertical as well as horizontal expansion of range by some species. Not only are they moving into new territory, but they’re increasing their density. According to a study by the USDA, mountain pine beetle populations in high-elevation, five-needle pine stands have been rocketing to higher levels than previously recorded.
“Short-term tactics can be used to reduce ongoing infestations by directly manipulating beetle populations, although due to cost, these are limited to use in high value areas such as campgrounds and the wildland-urban interface,” said a USDA advisory. “Semiochemicals, communication compounds released by beetles and trees, can be used to attract and repel beetles of some species.”
Other tactics include a combination of insecticides sprayed directly on tree boles and sanitation harvests whereby infested trees are removed. These types of controls can have a significant influence on non-target species such as other invertebrates, fish and birds, however, which must be considered.
Mountain pine beetles and other bark beetles, and fire, both can disturb and change the structure and composition of a forest ecosystem. Moreover, they can interact reciprocally. Bark beetles attack and kill trees that otherwise might survive fire damage. In turn, trees killed by beetles burn easily, and this contributes to fires. Removal of beetle-killed trees, especially in the wildland-urban interface where homes and forests are close together and near recreation areas, reduces the risk of catastrophic fires.
If research described last year by Western Carolina University is any indication, not all range expansion upwards is propelled by warming temperatures, although rising mercury could aggravate the situation. Red imported fire ants (Solenopsis invicta) are adapting to the colder temperatures in the Great Smoky Mountains of North Carolina.
First detected in Brunswick County in 1957, the ants have moved across the lowlands and increasingly are found in the state’s western mountains. Although experts in the past have noted that cold weather should act as a buffer to the invading ants, “The ants are adapting to the winters here,” said Jim Costa, executive director of the school’s Highlands Biological Station. “Colonies were found to persist at elevations over 4,000 feet, and our study showed in laboratory tests that their ability to withstand cold was directly related to where they were collected from, with higher elevation ants proving to be more cold hardy than their lowland cousins.”
Whatever the causes behind the invasions, it is a brave new world out there as far as the array of pests that require management.
“Pest managers can navigate this new terrain by keeping up on the latest science that examines the changing environment,” said Phillip Armstrong. “For pest managers, it’s important to know your enemy and stay alert to new species in your area. Prompt identification is an important first step to countering the threat posed by new insect species.”