Last year was the fifth hottest year in recorded history. This year followed suit, with June, July and August of 2023 being the hottest months ever recorded globally, according to the European Union Climate Change Service. Here at home in the United States, if you feel like your town was toastier than normal, you are probably correct, as almost 10,000 cities and towns tied or broke daily heat records this year, according to the National Weather Service.
Concerns over global warming and climate change are not new. In her 1951 bestselling book, “The Sea Around Us,” famed environmentalist Rachel Carson warned of an impending crisis in the seas due to warming temperatures around the world.
In this article, we explore direct and indirect effects of global warming on insect and mite pests of trees and shrubs. By direct effects, we mean the developmental, reproductive and behavioral responses of pests to temperature. Indirect effects include changes in biotic interactions between insect and mite pests and other organisms in the community. These interactions include changes in host-plant quality and susceptibility to pests and interactions with natural enemies – beneficial predators, parasitoids and pathogens that help mitigate problems caused by pests.
Direct effects of temperature on insects and mites
All organisms live within confines of temperature extremes. For every animal, plant, fungus and microbe, there exists a thermal limit to how hot or cold it can be before it perishes. For warm-blooded organisms like ourselves, these boundaries are fairly narrow. But for insects, which are cold blooded, thermal boundaries may be broad, enabling pests to survive months of sub-freezing temperatures in winter and temperatures exceeding 100 degrees Fahrenheit in summer.
Within these broad temperature bands, there will be slimmer thermal limits in which pests thrive. Beyond these limits, heat and cold become lethal. Many insects have behaviors enabling them to regulate their body temperature, like basking in the sun to warm, moving to shade to cool or vibrating muscles to generate body heat. But most of the warmth to power metabolic processes comes from the ambient temperature of the environment around them.
Within this range of benign temperatures, there is usually a direct relationship between ambient temperature and the rate at which an insect develops. (Figure 1) The warmer the temperature, the faster it develops. Of course, other factors come into play here, including the quality and quantity of food available and other environmental conditions such as moisture. But temperature is a primary driver.
Shifting ranges of pests
The aforementioned limiting effect of lethal cold temperatures in winter historically constrained many insect pests to parts of the country warm enough to allow them to survive. Well-known examples include key pests such as hemlock woolly adelgid (HWA, Photos 1 and 2) in the eastern United States, where its distribution is limited to locations where winter temperatures remain above minus 20 F. However, as winters moderate in the eastern half of the U.S., the adelgid has expanded its range. Over the last several decades, it has spread from its introduction point in Virginia to several northern states in New England and the Midwest.
If temperatures continue to warm at their current rate, scientists predict HWA will expand its range throughout New England, further north in several Midwestern states and into southern Canada, slaughtering stands of Canadian hemlock along the way.
Mountain pine beetle
In western states, a similar devastating range expansion is underway for mountain pine beetles (MPB). Vast expanses of MPB’s favored pines, such as lodgepole and ponderosa, were once protected from beetle attack by lethal cold temperatures found at high elevations and latitudes. As regional temperatures warmed, beetles expanded their range, colonizing and killing massive tracts of forests at higher elevations that were previously unavailable to MPB due to frigid temperatures.
A second and perhaps more disturbing geographic shift in MPB is also underway. (Photos 3 and 4) Cold mountaintops that once served as a barrier to an eastern range expansion of these beetles have been breached due to warmer temperatures at high elevations. This provided a gateway for MPB to move eastward, and they are doing so. Recent studies show that novel pines not found in the western historic range of the MPB, including eastern white pine, Scots pine, jack pine and red pine, are now known to be suitable hosts for MPB. This does not bode well for pine forests in central and eastern North America, where novel-pine hosts may be attacked and killed by spreading MPB.
Southern pine beetle
Southern pine beetle (SPB) has undergone a similarly destructive range expansion in the eastern U.S. Southern pine beetle is known to attack and kill several species of pines throughout its range, and a lack of food resources is not thought to have limited its geographic range.
Laboratory and field studies revealed that ambient temperatures near zero degrees Fahrenheit were lethal to SPB. Pines growing at higher altitudes and latitudes in colder locations were thermally protected from the beetle. As minimum winter temperatures gradually increased during the last century, the range of SPB has increased dramatically. Historically, SPB was an outbreak pest of pine forests primarily in southern and southeastern states ranging from Arizona to Florida. By the beginning of the 2000s, SPB had reached the pine barrens of New Jersey.
In 2014, pitch pines on Long Island, New York, were mobbed, and shortly thereafter SPB, was detected on Martha’s Vineyard, Massachusetts, killing native pines. As temperatures continue to warm, SPB is expected to continue its march northward along the Eastern Seaboard.
Wood-boring beetles are not the only clan of key landscape pests enjoying warmer temperatures in northern states. Southern caterpillars are on the move, too. Back in 2011, an extension bulletin from New England stated, “In fact, bagworms are only a continuous problem starting at the latitude that includes Maryland and ranging south to the Gulf of Mexico.” (Photo 5) Bagworms now regularly pillage evergreens and deciduous trees and shrubs as far north as Maine, Massachusetts, Michigan, New York and Wisconsin.
Too hot for some
The previous examples show how warming temperatures allow some cold-intolerant species to expand their ranges to higher elevations and latitudes. Are there cases where warming results in retraction of pests from areas where they once thrived but now have become simply too hot for them to live? Maybe so.
In the coastal plain of Virginia, populations of spongy moths (SM, formerly called gypsy moths) have declined from historical levels. (Photo 6) In this region, temperatures have reached levels no longer optimal for the survival and development of eggs and caterpillars of spongy moths. The southern range of SM in this region is retracting northward.
A similar range shift could be underway in northern Georgia for the previously mentioned HWA. In southernmost parts of its range, where temperatures are the warmest, immature stages of HWA had the lowest survival, with mortality reaching almost 90%. In a warming world, pests like HWA may be able to expand their range northward as lethal cold temperatures wane, but their southern range may contract where it is simply too warm to survive.
Development and changing patterns of voltinism
In entomology-speak, voltinism refers to the number of generations a pest has each year in a certain location. Many key pests of trees and shrubs, like SM, Japanese beetle (JB) and spotted lanternfly (SLF), have but one generation each year. These pests are called univoltine, meaning one generation annually. Irrespective of their geographic location in North America, they are hard-wired to go through a single reproductive cycle each year. This is not the case for all insects or mites.
Several insects and mites have the ability to complete more than one generation each year. Their development and, hence, number of generations will be linked to temperature. In temperate regions, in a warming world, spring arrives earlier each year and winter arrives later. This extension of benign temperature allows pests to become active earlier each year and remain active longer.
For multivoltine pests, like spider mites, warmer temperatures allow populations to build rapidly as the season progresses. At temperatures around 60 F, twospotted spider mites (TSSM) require about 36 days to develop from egg to egg-laying adult. When temperatures hit the upper 80s, this transformation takes only seven days. This translates into five times as many TSSM in the same amount of time at 80 degrees compared to 60 degrees.
This is part of the reason why populations of spider mites explode and their associated damage escalates as temperatures rise during hot summer months. A similar scenario unfolds when spider-mite-susceptible plants are planted in hot, sunny exposures in landscapes or in the hardscape-dominated precincts of urban heat islands.
This pattern holds for many other insects as well. Many sucking insects, such as aphids and scales, have differing numbers of generations depending on their geographic location. For example, crape myrtle bark scale (CBS), the new invasive scourge of crape myrtles, has two generations at the latitude of Maryland, but as many as four generations in hot places like Texas. (Photo 7)
This phenomenon occurs in other tree pests as well, including wood-boring beetles. Southern pine beetles may complete as many as nine generations annually in southernmost portions of its range and as few as three generations in northern states. A similar pattern holds for emerald ash borer (EAB). In warmer parts of its range and at lower elevations, EAB completes one generation per year, but at cooler, higher elevations and northern latitudes, it may take two years for juvenile stages to complete development.
Warming, drought and pest outbreaks
In this year of fire, a tragic and sobering lesson is unfolding regarding the relationships among heat, drought and fire, as unprecedented wildfires ravaged tinder-dry forests in North America. Climate change is predicted to increase the frequency and intensity of droughts. Not only will drought directly affect the survival and growth of trees, but physiological changes in trees associated with water deficits may indirectly and negatively affect their survival by altering their susceptibility to pest attack.
Responses of insect pests to drought-stressed trees are variable. Drought stress may lower the ability of conifers and deciduous trees to defend themselves from attack by insects such as bark beetles, flat-headed borers and wood-boring caterpillars. Drought also may increase the nutritional content of plant sap, which can favor sap-feeding insects like scale insects and aphids.
This response may vary due to the intensity and duration of drought stress and the species of insects attacking the tree. Clever studies conducted in urban heat islands demonstrated that hot,
water-stressed maples were more favorable hosts for gloomy scales (GL), and water-stressed willow oaks supported higher densities of oak lecanium scale (OL) compared to cooler, less water-stressed trees. But one size doesn’t fit all, as water-stressed trees were less susceptible to attack by white peach scale (WPS).
Much remains to be learned about the complex relationships among heat, drought, plants and their susceptibility to pests.
Drought also may disrupt the relationship between insect pests and pathogens that helps to reduce pest populations. Throughout the eastern United States, the fungal pathogen Entomophaga maimaiga provides spectacular reductions of SM populations under conditions of ample rainfall. However, under conditions of drought, survival of the fungus in the soil declines dramatically, greatly reducing rates of spring infection in SM caterpillars following egg hatch. This has allowed prolonged outbreaks of SM, with severe defoliation and tree loss in several areas of New England beset by drought conditions. (Photo 6)
In a warming world, heat and attendant drought may uncouple pest and pathogen dynamics, which spells trouble for our trees.
Warming and interactions between pests and their natural enemies
Natural enemies, predators and parasitoids play critical roles in keeping insect and mite pests of trees and shrubs at bay. Recent studies suggest that warming may have stronger negative effects on parasites and predators than their hosts, herbivorous pests that feed on trees and shrubs.
Some parasitic wasps that help control outbreaks of scale insects are not able to survive and reproduce at high temperatures tolerated by their scale hosts. When attacked by parasitic wasps, many insects, including scales, employ a physiological mechanism to encapsulate and destroy the alien growing in their body. At warmer temperatures, this defense is more effective than at cooler temperatures. In concert, these factors may help pests escape their natural enemies in a warming world.
A third way warming temperatures may favor pests and disfavor natural enemies hinges on asymmetrical shifts in the phenology (the normal seasonal progression of biological events) of pests and their natural enemies. To complete their life cycle, parasitic insects must synchronize their attack and development with susceptible stages of their host.
Adult oak lecanium scales are attacked by tiny parasitic wasps that halt egg production of female scales. (Photo 8) In urban forests, scales develop more rapidly on the hottest oak trees, and females begin laying eggs earlier in the season compared to scales on cooler oak trees. Scientists discovered that on the warmest trees, female scales could produce and lay many more eggs before parasites arrived and ended their mischief, compared to scales on cooler oak trees. This phenological mismatch resulted in greater densities of scales on warmer trees in the city.
If these studies in urban heat islands are bellwethers for interactions between tree pests and their natural enemies, then a warming world may shift the balance between natural enemies and prey in favor of the pest.
In summary, we have tangible evidence that distributions of plants and pests are changing as the world warms.
- Higher temperatures facilitate development of insects and mites.
- Coupled with longer growing seasons, many pests will complete more generations each year.
- Higher temperatures and associated water stress may alter tree defenses and nutrition and make them more vulnerable to pests.
- Warming may disrupt activities of predators, parasites and pathogens, tipping the balance of nature in favor of pests.
This constellation of forces may result in greater pest damage and tree loss. These changes present new and ongoing challenges for tree care professionals striving to create and maintain sustainable urban and natural forests.
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K. J. Dodds et al. 2018. Expansion of Southern Pine Beetle into Northeastern Forests: Management and Impact of a Primary Bark Beetle in a New Region.
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S. D. Frank. 2020. Review of the direct and indirect effects of warming and drought on scale insect pests of forest systems.
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A. Mech et al. 2018. Increases in summer temperatures decrease the survival of an invasive forest insect.
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M. J. Raupp et al. 2012. Disasters by design: Outbreaks along urban gradients.
D.W. Rosenberger et al. 2018. Development of an aggressive bark beetle on novel hosts: Implications for outbreaks in an invaded range.
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Michael J. Raupp, Ph.D., is professor emeritus at the University of Maryland in College Park, Md. His writing, research and scientific outreach have received a dozen national and international awards. He is a regular guest on television and radio. His most recent book, “26 Things that Bug Me,” introduces youngsters to the wonders of insects and natural history, while “Managing Insects and Mites on Woody Plants,” published by the Tree Care Industry Association, is a standard for the arboricultural industry. Visit his websites at www.bugoftheweek.com and https://www.youtube.com/user/BugOfTheWeek.