Populations of vectors will increase in response to extended seasons suitable for development and less severe winters. Epidemics of insect-borne diseases such as West Nile virus and dengue also could become more common in response to an increase in the frequency of extreme climatic events.
Global climate change (a.k.a., global warming) has been the subject of considerable debate for the past several years. Who to believe?
“The whole (global warming) thing is created to destroy America’s free enterprise system and our economic stability.” — Jerry Falwell
“I’d say the chances are about 50-50 that humanity will be extinct or nearly extinct within 50 years. Weapons of mass destruction, disease, I mean this global warming is scaring the living daylights out of me.” — Ted Turner
“If you asked me to name the three scariest threats facing the human race, I would give the same answer that most people would: nuclear war, global warming and Windows.” — Dave Barry
Despite early skepticism about the phenomenon of global climate change, it is now widely accepted by the scientific community, governments and most people. The main questions, going forward, are the speed at which change will occur and the specific environmental consequences. Climate change in the temperate zone of the United States will likely have three major components:
- Milder winters
- Longer growth seasons
- Increased frequency of weather extremes (e.g., tornados, droughts, floods, heat and cold)
What does this mean for PMPs? Should they revise their businesses in anticipation of new conditions and new pests? The real answer is “it depends.” You can expect some of the following:
- Pests will become active earlier in the year and there will be more pest insect/spider generations per season.
- Population sizes of pests will tend to increase, particularly later in a season.
- Occasional invaders and other pests will enter homes more often. Large pest populations alone can be responsible, but drought conditions and extreme temperatures can drive pests, seeking moisture, food and cooler temperatures, into homes.
- Range expansion (generally to the north) of existing native and exotic nuisance pests.
- The number and frequency of invasive species will increase.
- Medically important arthropods will become more important as their ranges and populations increase.
Two recent examples, from the St. Louis area, illustrate weather-influenced effects on pests. In 2012, termites swarmed unusually early (February) following the extremely mild winter of 2011-12. The mild winter was followed by a very warm and dry summer and this was associated with a significant increase in houses invaded by brown recluse spiders that normally reside outdoors.
The fossil record supports rapid insect movement northward in response to warming climate. Fossils from the end of the last ice age demonstrate that rapid, poleward shifts of insects accompanied overall warming, including warmer winters. In addition to the direct impacts of warming on insects, volatile weather and warming can disrupt the relationships among species that help to prevent the spread of ‘‘nuisance’’ species. This is a topic that is difficult to study and remains poorly documented.
Range expansions and population increases of medically important species will likely be the most significant consequence of climate change. For example, within the next 75 years it is estimated that the range of the red imported fire ant in the United States could expand northward by more than 80 miles and expand in total area by 21 percent as warmer winters make new areas suitable for their survival.
Similarly, the Africanized bee, which entered California and the desert southwest in the early 1990s, has continued to spread northward and its northern limits will be influenced by warmer winter temperatures.
What About Mosquitoes? The more serious issue is, perhaps, that climate warming, urbanization and vegetation changes will affect global patterns of vector-borne (primarily mosquitoes and ticks) diseases. This will occur as the geographical distribution of vectors expands. Populations of vectors will increase in response to extended seasons suitable for development and less severe winters. Epidemics of insect-borne diseases such as West Nile virus and dengue also could become more common in response to an increase in the frequency of extreme climatic events. Infections involving two or more species — mosquitoes, ticks, deer, birds, rodents and humans — re?ect changing ecological and climatic conditions as well as social changes (e.g., suburban sprawl).
Diseases carried by mosquito vectors are particularly sensitive to meteorological conditions. Excessive heat kills mosquitoes. However, within their survivable range, warmer temperatures increase their reproduction and biting activity, and the rate at which pathogens mature within them.
Temperature thresholds limit the geographic range of mosquitoes. Yellow fever and dengue fever are both carried by Aedes aegypti, which is restricted by the 10°C (50°F) winter isotherm. Freezing kills Aedes eggs, larvae and adults. Thus, expanding tropical conditions can increase the ranges and extend the season with conditions allowing disease transmission. Warm nights and warm winters favor insect survival.
In the United States, warm winters have been demonstrated to facilitate overwintering and thus northern migration of the ticks that carry tick-borne encephalitis and Lyme disease. Mild winters are expected to allow a greater than 60 percent range increase of deer ticks in the United States by the end of this century.
Malaria is the bane of mankind. Today, half of the world’s population is exposed to malaria on a daily basis. Deforestation, drug resistance and inadequate public health measures all have contributed to the recent resurgence. Could malaria once again become a problem in the United States?
Since 1976, several vector-borne diseases have reappeared in temperate regions. Anopheline mosquitoes long have been present in North America and malaria circulated in the United States in the early 20th century. However, by the 1980s, transmission was limited to California after mosquito control programs were instituted. Since 1990, small outbreaks of locally transmitted malaria have occurred during hot spells in Texas, Georgia, Florida, Michigan, New Jersey, New York and Toronto. Malaria has returned to South Korea, areas of southern Europe and the former Soviet Union. Malaria has recolonized the Indian Ocean coastal province of South Africa, and dengue fever has spread southward into northern Australia and Argentina.
These changes are consistent with climate projections, although land clearing, population movements, and drug and pesticide resistance for malaria control all have played parts. Warm weather facilitates malarial transmission. At 20°C (68°F), falciparum malarial protozoa take 26 days to incubate, but at 25°C (77°F) they develop in 13 days. Anopheline mosquitoes, which are carriers of malaria, live only several weeks. Thus, warmer temperatures permit parasites to mature in time for the mosquito to transfer the infection.
Final Thoughts. Warming encourages the spread of infectious diseases, but extreme weather events also can have major impacts on public health. Large-scale weather patterns have shifted. The warming of Eurasia, for example, has intensi?ed the monsoons that are strongly associated with mosquito and water-borne diseases in India and Bangladesh. The U.S. southwest monsoons also may have shifted, with implications for disease patterns there as well. Extremes can be hazardous for health. Prolonged droughts fuel ?res, releasing respiratory pollutants. Floods foster fungi, such as the house mold Stachybotrys atra, which may be associated with a hemorrhagic lung disease. Floods greatly increase mosquito-breeding sites and also pollute waterways.
Continued global warming and icecap melting will result in significant rises in sea level. This, in turn, will lead to coastal flooding and increased breeding habitat for brackish water mosquitoes such as Ochlerotatus taeniorhynchus, Aedes squamiger and Aedes sollicitans. In the future, nuisance biting mosquitoes are likely to become a much greater problem in urban coastal areas.
Whatever its cause, global climate change is occurring and bringing with it new pests, diseases and opportunities for PMPs. Read your professional literature and stay informed!
Steven R. Sims, Ph.D., is with Blue Imago Consulting, St. Louis, Mo. Arthur G. Appel is professor and chair of the Department of Entomology and Plant Pathology at Auburn University, Auburn, Ala.