Horsefly Bites: Symptoms, Treatments & Prevention

Horsefly is known for attacking animals and humans and feed on their blood. Horsefly bites can be itchy or painful and can contain infectious materials.

The horsefly is known under many different names in different countries, including deer flies, clags, gadflies or breeze flies. These flies are known for attacking animals including humans and biting them to feed on their blood. This can be dangerous because the horsefly can transmit parasites or diseases amongst its victims. Like mosquitos, female horseflies must ingest a protein based blood meal before they can reproduce. Some varieties of horseflies are very large in size, while others more resemble the average housefly. While these flies are typically seen as pests, they also serve a role as pollinators for flower.

Symptoms of Horsefly Bites

Horseflies use long mandibles to rip open the skin in order to gain access to the blood. This allows easier access to the blood than what would be received by a needle-like mouth like a mosquito has, and it makes it possible to make a successful bite through fur or clothing. It also has an evolutionary advantage because the bite will be much more painful, forcing the victim to focus on the wound rather than killing the fly. Because of this, the fly will typically get away after biting and it will then return to drink the blood as necessary. Typical symptoms of a bite by horsefly include:

Pain. The area where you were bitten will be torn, and will become sore. Red lumps will typically develop around the area where the bite occurred as your body exhibits a histamine reaction. This may cause the bite to become inflamed or itchy as your body attempts to remove any infectious materials from the area.
Allergic Reaction. Those who experience an allergic reaction to a horsefly bite may develop a body rash after being bitten. They may break out in hives or develop wheezing as the body continues to react to the infection. The skin may become pink or swollen. The area around the eyes and lips may begin to swell and the patient may become dizzy or weak.
Infection. If the fly was carrying parasites or bacteria that cause an infection, the bite can become extremely painful. Pain may radiate from the area where you were bitten and you may notice pus oozing from the wound. This is a serious reaction that will need to be taken seriously to avoid further medical complications.

Treatments for Horsefly Bites

The bites may take longer time to heal than other insect bites which generally take two to three days. Here are some helpful tips to fasten the healing.

Use saliva. Once you realize you have been bitten, cover the wound with saliva. The saliva is filled with Histatin protein that contains healing properties which will allow you to limit the negative effects of the bite and protect it until you can get indoors and treat it properly.

Clean the wound. Mild bites by horsefly will usually heal in a few days without medical intervention. Simply clean the wound with soap and water and pat it dry to protect the wound from becoming infected. You may also use an antiseptic to clean and sterilize the wound.

Avoid scratching the bite when it begins to itch. This will prolong the healing time and increase the risk that you will get infectious materials on your hands that you could spread to others.

A hot compress can also provide relief. Soak a clean towel such as a tea towel in hot salt water and apply this to the wound. Take care not to scald the infected skin or apply too much pressure to the damaged area as this could increase your discomfort.

Different remedies. Many cultures have developed remedies to assist with the pain of a horsefly bite. Icing the wound will help reduce swelling and dull the pain associated with a bite. You can also apply aloe vera, vinegar, Epsom salt, raw onion, mud, honey, a paste made from baking soda or vinegar to the bite to help reduce swelling and discomfort. Once you have applied a topical agent to provide relief, cover the area with a loose bandage to protect the wound and to help keep the remedy in place.

Apply Drugs. If the bite is particularly itchy or swollen, apply Benadryl or hydrocortisone cream to the area to help eliminate these symptoms. Oral antihistamines can also be taken to help avoid an allergic reaction.

See a doctor. If you begin to suffer an allergic reaction, the bite is very painful or the bite appears to be infected seek medical attention immediately. Bites by horsefly can cause severe reactions which can be life threatening if the histamine reaction is very severe. They can also contain bacteria or parasites that can cause a dangerous infection throughout the body. Your doctor will evaluate your symptoms and provide the necessary course of treatment to eliminate these symptoms.

Prevention of Horsefly Bites

Traps can be purchased and placed in areas that are known to have swarms of horseflies. This will allow you to catch the flies so they cannot do damage to you or your animals. If you know that you will be working in an area that has horseflies flying about, wear long sleeves and pants to protect your skin. This will not eliminate the chance that you will suffer a bite by horsefly, but it will lessen the severity of the bites you receive.

Most horseflies will avoid people who are using insect repellent. Spray a high quality insecticide on yourself or around the area where you will be working to ward off these insects. Using a repellent with diethyltoluamide or DEET is particularly effective in keeping horseflies at bay. Some also believe that taking garlic supplements will allow you to develop a natural odor that helps to ward off horseflies when you sweat.


Horsefly — This is a large insect that will bite you as soon as possible. They have a length of 1.3 to 2.5 cm, triangular and carnivorous. When they bite, they take out a piece of meat and inject poison. The area around the horsefly bite will hurt for about five days. Gadflies are also important carriers of diseases such as leukocytosan turkey disease.

Origin of view and description

Horsefly is a representative of the family of horsefly insects (the order is diptera), or rather, a representative of the genus horseflies. These are full flies, the size of a housefly or the size of a bumblebee, which are sometimes called green-headed monsters. Their metal or rainbow eyes are found dorsally in the male and separately in the female.

Their mouth resembles a wedge-shaped miner tool. Other names for the insect are bat and ear. One of the most common species (Tabanus lineola) has bright green eyes and is known as a green head. The genus of lacewing, commonly known as the deer fly, is slightly smaller than horseflies and has dark marks on the wings.

Numerous painful bites of large populations of these flies can reduce milk production in dairy and beef cattle and prevent grazing of cattle and horses, as the attacked animals will come together. Animals can even get injured while fleeing these flies. In this case, blood loss can be very significant.

These large, strong-body flies are powerful and dexterous, circling or pursuing their goal with humiliating insistence to make painful injections into the skin and suck blood. The flies remain in contact with the host for only a few minutes, and then they leave until they again need to eat, which happens every 3-4 days.

A serious allergy to horsefly bites is not common, but it can be signaled by additional symptoms:

  • feeling of dizziness and weakness;
  • dyspnea;
  • temporarily swollen skin, for example, around the eyes and lips.

A more severe allergy is rare, but is urgent.

You need to call an ambulance for any signs of anaphylaxis, which include:

  • swelling, itching, or rash;
  • the face, lips, arms and legs are most likely swollen;
  • swelling of the throat and tongue are dangerous symptoms;
  • nausea, vomiting, or diarrhea;
  • difficulty swallowing or breathing.

Appearance and features

Horsefly is a fly of dark gray color, with gray-brown speckled wings and freakish striped iridescent eyes. Adult flies are brownish, hairy, strong, about 1.7 cm long, resembling honey bees, except that they have only one pair of wings. There are faint smoky spots on the wings of horseflies.

Fully grown larvae have a length of 0.6 to 1.27 cm and have dense yellowish-white or pinkish thick skin. They are obtuse at one (back) end and taper towards the other (front) end, on which there is a pair of strong hook-shaped mouth parts. Each body segment is surrounded by strong spikes. Horsefly tendrils have five segments and are thick at the base, becoming thinner with each segment. These antennae are long and thin. Horsefly wings are usually completely dark or completely transparent.

Interesting fact: The easiest way to detect horsefly is to look at its total size. The insect tends to be large compared to other biting flies. The males have eyes so large that they touch the crown of the head.

Not all horseflies depend on water, but many species lay their eggs on plants growing near ponds, rivers, and streams. Some species have aquatic larvae, while others live in moist soil. All feed on other invertebrates until they are ready to pupate and become adults. This means that you are more likely to meet larvae around ponds. Farms are often a hot spot for these flies, as they are attracted to livestock and horses.

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Now you know what happens when bitten by horsefly. Let’s see where this insect is found.

Where does horsefly live?

Horseflies, as a rule, live in forests. Species usually feed in the daytime and are most noticeable on calm, hot, sunny days. They are usually found in both suburban and rural areas near ponds that serve as breeding sites, and where mammalian hosts are most numerous.

Larvae develop in the gastrointestinal tract of animal hosts in winter. In late winter and early spring months, adult larvae are found in the feces of the host. From there, they dig into the soil and form a puparia from the skin of their larvae of the last stage (age). They turn into adult flies inside the puparia and appear after 3-10 weeks.

Adults are active from mid-summer to autumn. Adult females glue eggs on the hair of horses, especially on the hair on the forelegs, as well as on the stomach, shoulders and hind legs. The eggs hatch after 10-140 days with the proper irritant (moisture, heat and friction) caused by the horse licking or biting the hair infected with the egg.

Tiny larvae of the first stage (age) enter the mouth and dig into the tongue for approximately 28 days before they molt and move into the stomach, where they remain for 9-10 months, developing into the third stage after about 5 weeks. One generation of horseflies grows in a year.

What does horsefly eat?

Adult horseflies usually feed on nectar, but females require blood before they can multiply efficiently. The bites of female horseflies, especially large individuals, can be quite painful because their mouth parts are used for tearing and grinding, unlike mosquitoes, which simply pierce the skin and suck blood. They have jagged, saw-like teeth that cut open skin, then they release an anticoagulant to stop blood clotting while they enjoy food.

An interesting fact: Female horseflies require up to 0.5 ml of blood for reproduction, which is a large amount compared to their size. They can take about 200 mg of blood in a few minutes.

Horsefly bites can develop into large, red, itchy, swollen bumps within minutes. Some people also report feeling fever, weakness, and nausea. For most, they are absolutely harmless, but extremely inconvenient. In exceptional cases, some people may suffer from an allergic reaction with symptoms such as dizziness, wheezing, shortness of breath, spotty skin rash, and severe swelling that may be visible on the lips or tongue.

Horseflies are intermittent feeders. Their painful bites usually cause the victim to respond, so the fly is forced to move to another host. Therefore, they can be mechanical carriers of certain diseases of animals and humans. The horsefly females are also constant and, as a rule, will continue to bite the host until they either succeed in acquiring their blood meal or are killed. It is even known that they pursue their intended goals for short periods of time. Some species are carriers of pathogens, but in most diseases carried by flies, they are associated only with livestock.

During outdoor activities, wear light colored clothing and insect repellent to prevent horsefly bites. If they are included in structures, the best method of control is to exclude, including checking all doors and windows.

Features of character and lifestyle

Adult horseflies are fast, strong pilots capable of flying more than 48 km, although they usually do not spread widely. Most often they attack moving and dark objects. Horseflies often rest on paths and roads, especially in forest areas where potential owners are waiting for them. Flies attract light and sometimes gather in the windows. Horseflies are more common in hot sunny weather with light wind, for example, during the daytime in mid-summer. They can become more pests when thunder accompanies hot weather.

Horseflies are daily circuits, that is, they are active during the day. They prefer to feed on the blood of cattle such as cows and horses. This can be problematic because horseflies carry pathogens that can cause disease in some livestock species, which can lead to potential economic losses. And, unfortunately, horseflies do not have problems when they feast on people or pets, if they are given such an opportunity.

Interesting fact: Like other blood-sucking insects, such as, for example, mosquitoes, horsefly females use both chemical and visual signals to locate the hosts. Carbon dioxide emitted by warm-blooded animals provides a distant signal to attract flies from a distance, while visual signals such as movement, size, shape and dark color serve to attract horseflies over shorter distances.

Social structure and reproduction

Horseflies undergo a complete metamorphosis, which includes passing through 4 full stages of life. This is an egg, a larva, a chrysalis and an adult stage. Females lay lots from 25 to 1000 eggs on vegetation that stands above water or in moist areas. The larvae that hatch from these eggs fall to the ground and feed on decaying organic matter or small organisms in the soil or water.

Horsefly larvae develop in mud along the edges of a pond or the banks of streams, wetlands or percolation areas. Some of them are watery, and some develop in relatively dry soil. The larval stage usually lasts from one to three years, depending on the species. Mature larvae creep into drier places to pupate, and eventually adults appear. The duration of the pupal stage depends on the species and temperature, but may vary from 6 to 12 days.

It is difficult or almost impossible to find and eliminate the breeding site of horseflies. They breed in environmentally sensitive wetlands, which is why the effects of drainage or insecticides on non-target organisms or water supply are of concern. In addition, these insects are strong fliers that can move from a certain distance. Breeding sites can be very extensive or at some distance from the location of the problems.

Fortunately, horseflies are sporadic problems for a particular time of the year. Some adaptation in behavior or the use of repellents may allow you to enjoy the outdoors.

Natural enemies of horseflies

Along with many other flying insects, horseflies are also a key food source for many other animals higher up the food chain. They help support other species, such as bats and birds, while aquatic insect larvae feed on fish.

Birds that feed on horseflies:

  • black-headed cardinals are songbirds with large, conical, thick beaks. Their color depends on the gender of the bird: the fiery male has an orange cinnamon body with a black head and black and white wings, and immature males and females are brown in color with an orange spot on the chest. They prey on various insects, including horseflies and caterpillars. Black-headed cardinals can be found mainly in the western United States in thickets and forest edges, as well as in yards and gardens;
  • Sparrows are one of the most common birds in North America and can be seen mainly in flocks. It is known that if there are insects in the garden, including horseflies, then sparrows can become a nuisance for your house if they are overpopulated. They build their nests in the walls of the house, destroying the forest. Their feces can also be harmful to human health. Despite this, they can go a long way in reducing horsefly populations around homes;
  • swallows feed mainly on insects, as well as grain, seeds and fruits and live near fields and areas with an abundance of flying spaces and a natural supply of water. These are fast-flying songbirds that range in color from pale brown to blue-white and live in much of North America. Flying insects such as horseflies are the main food source for swallows;
  • Warblers are insectivorous birds that feed on spruce buds and horseflies. Their population often fluctuates in proportion to the population of insects that they eat. There are about 50 different types of warblers. These are small songbirds with white lower parts, green backs and white lines in their eyes. Young warblers are dark green with a characteristic pale line of eyes and pale yellow lower parts.

Population and species status

The horsefly population is growing in stuffy weather. Mostly in warm, humid and calm weather, they become a real plague for horses and their owners. In the world there are more than 8000 different types of horseflies, related to each other. I use various methods of fighting against horseflies.

Unfortunately, there are few methods to control horseflies and minimize their bite. The risk of bites can be reduced, but there are currently no known ways to completely eliminate it. As with most other types of insect infections, preventive measures are the first line of defense against horseflies at home. Good sanitation and house cleaning can prevent horsefly infection, as their larvae tend to develop in decaying organic matter. Installing a screen on doorways and windows can also prevent flies from entering the room and populating the house.

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There are traps for horseflies, but their effectiveness varies. Traps consist of a large dark sphere moving back and forth, often sprayed by some kind of animal musk or a similar attractive aroma. This sphere is located below a bucket or similar container containing a sticky flytrap — horseflies attracted to the sphere take off and, ideally, fall on the tape. Draining any standing ponds around the property can also help to minimize the risk of horsefly infection.

If you have already discovered an infection by horse flies in your home, preventative measures will be of little help. Natural horsefly control methods include fly paper and fans. The horsefly is disturbed by smoke, so burning candles can also induce them to leave the house in which they settled. However, these measures show at best marginal efficacy in removing horsefly infection. Pesticides can also be moderately successful in controlling horsefly populations.

Classifying and Identifying Insects

Insects are the largest group in the animal kingdom. Scientists estimate there are over 1 million insect species on the planet, living in every conceivable environment from volcanoes to glaciers.

Insects help us by pollinating our food crops, decomposing organic matter, providing researchers with clues to a cancer cure, and even solving crimes. They can also harm us by spreading diseases and damaging plants and structures.

How Insects Are Classified

Insects are arthropods. All animals in the phylum Arthropoda have hard external skeletons called exoskeletons, segmented bodies, and at least three pairs of legs. Other classes that belong to the phylum Arthropoda include:

  • Arachnida (spiders)
  • Diplopoda (millipedes)
  • Chilopoda (centipedes)

The class Insecta encompasses all of the insects on the earth. It is most often divided into 29 orders. These 29 orders use the physical characteristics of the insects to group similar insect families.

Some insect taxonomists organize the insects differently, using evolutionary links instead of physical traits. For the purpose of identifying an insect, it makes more sense to use the system of 29 orders, since you can see the physical similarities and differences between insects you observe.

Here is an example of how an insect, the monarch butterfly, is classified:

  • Kingdom Animalia: the animal kingdom
  • Phylum Arthropoda: arthropods
  • Class Insects: insects
  • Order Lepidoptera: butterflies and moths
  • Family Nymphalidae: brush-footed butterflies
  • Genus Danaus
  • Species plexippus

The genus and species names are always italicized and used together to give the scientific name of the individual species. An insect species may occur in many regions and may have different common names in other languages and cultures.

The scientific name is a standard name that is used by entomologists around the world. This system of using two names (genus and species) is called binomial nomenclature.

Basic Insect Anatomy

As you may remember from elementary school, the most basic definition of an insect is an organism with three pairs of legs and three body regions: head, thorax, and abdomen.

Entomologists, scientists who study insects, might also add that insects have a pair of antennae and external mouthparts. As you learn more about insects, you will find there are some exceptions to these rules.

The Head Region

The head region is at the front of the insect’s body and contains the mouthparts, antennae, and eyes.

Insects have mouthparts designed to help them feed on different things. Some insects drink nectar and have mouthparts modified into a tube called a proboscis to suck up liquid. Other insects have chewing mouthparts and eat leaves or other plant matter. Some insects bite or pinch, and others pierce and suck blood or plant fluids.

The pair of antennae may have obvious segments or look like a feather. They come in different forms and are a clue to identifying the insect. Antennae are used to perceive sounds, vibrations, and other environmental factors.

Insects can have two types of eyes: compound or simple. Compound eyes are usually large with many lenses, giving the insect a complex image of its surroundings. A simple eye contains just a single lens. Some insects have both kinds of eyes.

The Thorax Region

The thorax, or middle region of an insect’s body, includes the wings and legs. All six legs are attached to the thorax. The thorax also contains the muscles that control movement.

All insect legs have five parts. Legs can be different shapes and have different adaptations to help the insect move within its unique habitat. Grasshoppers have legs designed for jumping, while honey bees have legs with special baskets to hold pollen as the bee moves from flower to flower.

Wings also come in different shapes and sizes and are another important clue to help you identify an insect. Butterflies and moths have wings made of overlapping scales, often in brilliant colors. Some insect wings appear transparent, with just a web of veins to identify their shape. When at rest, insects like beetles and praying mantids keep their wings folded flat against their bodies. Other insects hold their wings vertically, like butterflies and damselflies.

The Abdomen Region

The abdomen is the final region in the insect body and contains the insect’s vital organs. Insects have digestive organs, including a stomach and intestines, to absorb nutrients from their food and separate waste matter. The sexual organs of the insect are also in the abdomen. Glands that secrete pheromones for marking the insect’s trail or attracting a mate are in this region as well.

Take a Closer Look

The next time you observe a lady beetle or a moth in your yard, stop and take a closer look. See if you can distinguish the head, thorax, and abdomen. Look at the shape of the antennae, and watch how the insect holds its wings. These clues will help you identify a mystery insect, and provide information about how the insect lives, feeds and moves.

Changes in Yellowstone Climate

The Greater Yellowstone Ecosystem is a complex region, encompassing approximately 58,000 square miles and 14 mountain ranges. Weather varies greatly across steep elevational changes, bringing snowfall to some areas, and warm, dry conditions to others. This dynamic system has provoked the curiosity of researchers for a long time.

Across Space and Time

Space and time are critical to the evaluation of real-world data, and every study defines their parameters differently. This can make it difficult to get a sense of what is actually occurring. Climate summaries over longer periods of time and across larger areas tend to mask local extremes. Conversely, a continuously changing set of short-term reference averages (weather “normals”) could unintentionally obscure the long-term magnitude of change. It is important to look at climate information across many scales and to use available data and models to arrive at reasonable answers to our questions about how climate has changed, how those changes will affect the park, and what impacts we may be able to anticipate in the future.

Analyzing smaller areas within the Greater Yellowstone Ecosystem (GYE), say in Yellowstone National Park or on the Northern Range, poses specific challenges. Small regions have fewer actual monitoring stations to feed data to computer models and gridded weather data is often used to fill in the gaps. As a consequence, small-area analyses may not be as accurate. Local field observations from stream gauge and weather stations can be used to verify some of the observed trends, and to describe local conditions to which the ecological system may be responding. This “ground-truthing” allows researchers to arrive at reasonable conclusions about ecological activity.

Temperature and Precipitation

Global temperature is the master force affecting climate. Everything else that climate affects—sea level rise, growing season, drought, glacial melt, extreme storms—is driven by changes in temperature. Weather stations have been maintained within the GYE since 1894, resulting in some of the longest running records of temperature and precipitation anywhere in the United States. These days, increasingly sophisticated satellite technology as well as data sets yielded by the science of climate modeling, also help climate experts and park managers assess the current situation in the GYE across several scales.

There is evidence that climate has changed in the past century and will continue to change in the future. Researchers looking at annual average temperatures report an increase of 0.31°F/decade within the GYE, consistent with the continuing upward trend in global temperatures. Recent studies show mean annual minimum and maximum temperatures have been increasing at the same rate of 0.3°F/decade for the GYE. Conditions are becoming significantly drier at elevations below 6,500 ft. In fact, the rise in minimum temperatures in the last decade exceeds those of the 1930s Dust Bowl Era.

Future Temperature and Precipitation

All global climate models predict that temperatures in the GYE will continue to increase. Projections of future precipitation vary based on differing scenarios that account for future levels of greenhouse gas emissions, which depend upon economic, policy, and institutional improvements, or lack thereof. Any potential increases in precipitation that may or may not occur will be overwhelmed by temperature increases. Considering the most recent trends in which warmer temperatures have been exacerbating drought conditions during the summers, a warmer, drier future for the GYE appears likely in the coming decades. By the latter part of the 21st century, the hot, dry conditions that led to the fires of 1988 will likely be the norm, representing a significant shift from past norms in the GYE toward the type of climate conditions we currently see in the southwestern United States.

NPS / Neal Herbert

Snowpack and Snow Cover

Snowmelt in the alpine areas of the Rocky Mountains is critical to both the quality and quantity of water throughout the region, providing 60–80 percent of streamflow in the West. Throughout the GYE, snow often lingers into early summer at high elevations. Each year, a large spike in water flow occurs when snow starts to melt at lower elevations, usually in late February and early March. Peak flow is reached when the deep snow fields at mid and high elevations begin to melt more quickly, typically in June. Minimum flow occurs during winter when all the previous year’s snow has melted, temperatures have dropped, and precipitation comes down as snow instead of rain so only water flowing from underground sources can supply the streams. By contrast, the proportion of stream flow due to rain storms is significantly lower than the contributions of snow melt.

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Climate change is expected to affect both snow accumulation and rate of spring melt. In some places, warmer temperatures will mean more moisture falling as rain during the cooler months and the snowpack melting earlier in the year. The reduction in snowpack is most pronounced in spring and summer, with an overall continued decline in snowfallprojected for Yellowstone over the coming decades. The Yellowstone, Snake, and Green rivers all have their headwaters in Yellowstone. As major tributaries for the Missouri, Columbia, and Colorado rivers, they are important sources of water for drinking, agriculture, recreation, and energy production throughout the region. A decrease in Yellowstone’s snow will affect millions of people beyond the boundaries of the GYE who depend this critical source of water.

Future Snowpack and Snow Cover

The interaction between snowpack, temperature, and precipitation involves a complex interchange between heat and light. Warming temperatures increase evaporation; increased moisture in the air could lead to more snowfall and cloud cover. The increased cloud cover could block additional heat from reaching the surface of the earth resulting in cooler temperatures below. However, increased temperature could possibly limit snowfall instead—by converting it to rain or by melting snow rapidly once it falls, thereby driving snowlines further up the mountains. Recently modeling work indicates that snowpack will almost certainly decline in the long-term.

Changes in the area covered by snow are especially important as snow reflects more solar radiation out to space (albedo) than bare ground and tends to keep the surface cool. When land is exposed, sunlight is absorbed by the surface of the earth. This raises the overall surface temperature, which leads to more melting and less snowcover.

Stream Flow and Water Temperature

Glaciers, snowpack, and rainfall produce water that flows through streams, lakes and rivers, and these waterways are critical to life. Analyses of streams during 1950–2010 in the Central Rocky Mountains, including those in the GYE, show an 89% decline in stream discharge. Reduced flows were most pronounced during the summer months, especially in the Yellowstone River. In addition, stream temperatures have changed across the range of the Yellowstone, with a warming of 1.8°F (-16.8°C) over the past century. Continued warming could have major implications to the management and preservation of the many aquatic resources we have today. Changes in volume and timing of spring runoff may disrupt native fish spawning and increase nonnative aquatic species expansion.

Growing Season

The Intergovernmental Panel on Climate Change (IPCC) predicts that overall forest growth in North America will likely increase 10–20% as a result of extended growing seasons and elevated CO2 during the next century but with important spatial and temporal variations. Forests in the Rocky Mountain/Columbia Basin region are expected to have less snow on the ground, a shorter snow season, a longer growing season due to an earlier spring start, earlier peak snowmelt, and about two months of additional drought. Despite a longer growing season, Yellowstone forests will likely be less dense, more patchy, and have more diverse age structure. In fact, experts project less tree cover in much of the park as well as potential migration of new species like Ponderosa pine. Complicating matters, increased drought stress and higher temperatures may increase the likelihood of widespread die-offs of some vegetation.

The integrated runoff response from the Yellowstone River has been toward earlier spring runoff peaks, which suggests that the majority of the park is experiencing shorter winters and longer summers as a result of snowpack changes. Changes in these seasonal patterns will likely disrupt vegetation growth and development, causing plants to bud, flower, fruit and die at different times of the year than they do now. Those changes, in turn, would alter or seriously disrupt wildlife migrations, one of the key resources for which Yellowstone National Park is globally treasured.

Extreme Events: Insect Activity

Although outbreak dynamics differ among species and various forests, climate change appears to be driving current insect outbreaks. Western spruce budworm outbreaks were more widespread and lasted longer in the 20th century than in the 19th century primarily because of fire suppression and increasing fir populations. However, patterns of spruce budworm outbreaks have been tied to climate nationwide.

Summer and spring precipitation are positively correlated with increased frequency of outbreaks over regional scales and long time frames, but experimental evidence suggests that drought may promote infestations. Although bark beetle infestations are a force of natural change in forested ecosystems, several concurrent outbreaks across western North America are the largest and most severe in recorded history. From 2004 to 2008, the area of mountain pine beetle outbreaks increased across Wyoming from 1,000 to 100,000 acres. At the end of 2016, 26% of whitebark pine trees in the GYE had been killed as a result of mountain pine beetle, whitepine blister rust, wildland fire, and other factors. Since 1999, an eruption of mountain pine beetle events has been observed that exceed the frequencies, impacts, and ranges documented during the last 125 years. Aerial assessment of whitebark pine species populations within the GYE has indicated a 79% mortality rate of mature trees. These changes may be early indicators of how GYE vegetation communities will shift due to climate change.

These outbreaks of bark beetles in the West have coincided with increased temperatures and changes in precipitation patterns, suggesting a response to a changing climate. Warming temperatures and the loss of extreme cold days reduce winter overkills of insects, speed up life cycles, modify damage rates, and lead to range expansions, particularly in the north.

Future Insect Activity

Climate change, and particularly warming, will have a dramatic impact on pest insects, and the recent trends of increasing outbreaks are expected to worsen. The greatest increase in mountain pine beetle outbreaks is expected to occur at high elevations, where models predict warmer temperatures will increase winter survival. At low elevations, however, mountain pine beetle populations may decrease as warmer temperatures disrupt the insects’ seasonality. Climate change will also alter host susceptibility to infestation. Over the short-term, trees will likely increase in susceptibility to pests due to stress from fires, drought, and high temperatures; over the long-term, these stresses will cause tree ranges and distributions to change. Moreover, climate change and changes in CO2 and ozone may alter the conifers’ defensive mechanisms and susceptibility to beetles through their effects on the production of plant secondary compounds.

Insect infestations are damaging millions of acres of western forests and there is clear evidence that damage is increasing. Nonetheless, future predictions of the extent of infestations remain uncertain because our understanding of insect infestations is incomplete. Key uncertainties include the influence of drought and precipitation changes, how altered forest/host composition will alter outbreaks, the biochemical response of trees and evolution of defensive mechanisms, regional differences, and the interactive effects of fire, plant disease, and insect outbreaks.

Extreme Events: Fire Activity

The increasing frequency of warm spring and summer temperatures, reduced winter precipitation, and earlier snowmelt in the West during the last 20 years has led to an increase in the frequency of very large wildfires and total acres burned annually. The relative influence of climate on fire behavior varies regionally and by ecosystem type, but generally current-year drought, low winter precipitation, wind conditions, and high summer temperature are determining factors for area burned in the Rockies.

Fire dynamics have been altered by climate indirectly through its effects on insect infestations and forest health. By changing the forest environment, bark beetles can influence the probability, extent, and behavior of fire events, but despite the widely held belief that bark beetle outbreaks set the stage for severe wildfires, few scientifically and statistically sound studies have been published on this topic. That fire promotes beetle infestations is clearer; the fire-caused injury changes conifers’ volatile emissions, increasing their susceptibility to bark beetles.

Future Fires

Most evidence suggests that climate change will bring increases in the frequency, intensity, severity, and average annual extent of wildland fires. Models project that numerous aspects of fire behavior will change, including longer fire seasons, more days with high fire danger, increased natural ignition frequency and fire severity, more frequent large fires, and more episodes of extreme fire behavior. The best evidence is for increases in the average annual area burned. However, the charcoal in lake sediment cores is telling a different story in Yellowstone. These records extend back 17,000 years, and were taken from Cygnet Lake on the Central Plateau. Charcoal from 8,000 years ago, when temperature increases equal what we are now experiencing, shows more frequent but smaller fires than today.

Projecting the influences of climate change on future patterns of fire is extremely difficult. Fuels, along with fire weather, determine fire size and severity: the stand-replacing fires of today open up the forests where stands have been burned, limiting fuels for the next fire. As a result, areas with frequent fires also tend to have small fires. Other factors, such as increases in non-native, annual grass invasions, may alter fire dynamics, making predictions based on climate alone difficult.

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