The term “invasive species” is used often in the pest control industry. But what does it really mean, and why should we be so concerned? This article will examine the concept of invasive species, with a specific focus on mosquitoes and some fairly recent introductions into the United States, and the threats they may pose.
An invasive species may be defined as a living organism, including but not limited to, plants, parasites, pathogens, fungi and animals (including insects) that is non-native to an ecosystem and begins to spread out or expand its range from the original site of introduction. Even an organism’s seeds or eggs may be invasive. Additionally, the species must have the potential to cause harm to the environment, the economy or human health, which brings the pest control industry into the arena.
Invasive species are often spread by humans and the goods they use as they travel about the world. Most invasive species, such as the West Nile virus, Zika virus, zebra mussels, kudzu vine and starlings are considered accidental introductions. Some invasive species, however, are originally brought to a new area or region for a specific purpose. A good example is the Asian carp, which was imported into the United States for use in aquaculture ponds. Unfortunately, through flooding and accidental releases, these destructive fish have ended up in the Mississippi River system (and hence access to many of the country’s rivers and streams), resulting in all types of problems.
Some invasive species are the result of pets that escaped or were released into the wild. Consider the example of the Burmese pythons that are now plentiful in the Florida Everglades where they have no natural predators. These huge snakes, which may grow up to 20 feet long (!), are feeding on many local species in the Everglades, especially the wading birds.
When a new and aggressive species arrives in an area, there may be no natural predators, diseases or other controls to check its population growth. The result is often a dramatic, negative impact on the local wildlife. The brown tree snake (BTS) was accidentally introduced to Guam, an island in the South Pacific, in the late 1940s or 1950s. With no predators and lots of food (wildlife) present, the snake quickly multiplied with estimates as high as 10,000 snakes per square mile at one time! Over the years, the BTS has been responsible for the extinction of some of the island’s forest-dwelling birds, has repeatedly knocked out power to parts of the island, and has even gnawed on the fingers and toes of sleeping children.
According to the U.S. Geological Survey, there are about 6,500 non-native invasive species in the United States. Collectively, they cause more than $100 billion (that’s 100 with 9 more zeros!) worth of damage every year to the U.S. economy. Examples of these costly effects include crop damage; threats to fisheries; clogging of water facilities and waterways (which can result in heavy mosquito production); increased risk of fires; negative impacts on ranchers and farmers; pathogen transmission and resulting disease to animals and humans; and in the case of mosquitoes, just plain old annoyance.
INVASIVE MOSQUITO SPECIES. Since invasive insects are of most interest to PCT readers, what follows is a review of four mosquito species and what to know when you encounter them.
Aedes albopictus, Asian tiger mosquito (ATM) The Asian tiger mosquito is black and white, with a distinct white line on the thorax (the body segment behind the head to which the wings and legs are attached). This species is native to tropical and subtropical areas but in the past few decades has spread to many countries. In the United States, it was first discovered in Texas in the mid-1980s and has since spread to about 1,350 counties in all or parts of 40 states.
The eggs of the ATM are highly resistant to drought and are regularly moved around the world in things such as used tires and lucky bamboo plants. One of those methods is undoubtedly how they came to the United States. ATMs are aggressive biters, primarily during the daytime, and they utilize a wide host range, including humans, to acquire their blood meals for egg production. This species has been found naturally infected with several viruses and is a potential vector for several human pathogens that cause diseases such as dengue fever, Zika, yellow fever, chikungunya and others.
It prefers to breed in artificial containers, e.g., used tires, bird baths, buckets, clogged gutters and bottle caps, as well as natural containers such as treeholes. Children seem to be particularly sensitive to bites of this species and can develop secondary infections from repeated scratching. In the author’s opinion, this mosquito is the leading reason why backyard mosquito control services have dramatically increased, along with associated revenue, in the past decade or so.
Aedes japonicus, Asian bush mosquito (ABM) As its common name implies, this mosquito species is native to East Asia. In the United States, it was first discovered in Connecticut, New York and New Jersey in 1998. It has since spread to most of the Eastern U.S. as well as the Midwest, all the way to the Canadian border. It also has been found in the Pacific Northwest as well as Hawaii. The ABM has a distinctive, lyre-shaped pattern on its thorax that appears golden.
This species is highly tolerant of cooler weather, being able to overwinter in the egg stage in sub-zero weather. This fact helps explain its northerly expansion in the U.S. It is well-adapted to living around humans and will feed on them as well as other mammals and birds.
In the laboratory, it has been shown that the ABM can be infected with several viruses. Also, in the U.S., wild-caught specimens were found to be infected with West Nile virus as well as La Crosse virus. Much like the Asian tiger mosquito, this species breeds in natural containers (e.g., rock pools) as well as artificial containers. More field and laboratory work is required in order to elucidate the specific role that this species may play in the transmission cycles of these two human pathogens, as well as others in the U.S.
Aedes scapularis (no common name) This species has a very broad distribution, occurring along the Rio Grande border in Texas and then south through Latin America and most of South America. It is also found throughout the Caribbean except in Puerto Rico. In 2020, surveillance efforts revealed that it has become established in two counties in southern Florida. As this is a relatively recent invasion, it is still unclear how far this species will spread but it is a good bet it will at least establish in the Southeast United States.
In its native habitat, Ae. scapularis can be found in forested as well as urban settings. It is a crepuscular feeder (feeds during changes of light intensity) but will readily feed during the daytime. Importantly, it has a very wide host range, feeding on humans, other mammals, birds, reptiles and amphibians. When a mosquito species displays such a wide host range as this, it is usually much more likely to become involved in transmission of pathogens.
This species has been found naturally infected with many different pathogens and in its native habitat is a known vector of dog heartworm, yellow fever and Venezuelan equine encephalomyelitis. Larvae can be found in temporary ground pools, rock holes and crab holes so it is likely that Ae. scapularis will eventually take up residence in artificial containers.
Aedes notoscriptus, Australian backyard mosquito (ABM) The ABM is native to Australia, where it is widespread, and also occurs in New Zealand, the Western Pacific and Indonesia. It is closely associated with urban areas and much like the species already discussed, lays eggs in a wide range of natural and artificial containers, especially in people’s backyards! Most of the biting occurs in the afternoon and early evening. This species will readily bite humans but also will feed on other animals opportunistically.
Fairly recently, the ABM was discovered in Southern California, presumably having hitchhiked from Australia. In Australia, this species has been implicated in the transmission of some nasty viruses and it is also an effective vector of dog heartworm.
REASONS FOR CONCERN. From a public health and quality-of-life perspective, there are several reasons to be concerned about these invasive mosquito species, as well as others that eventually will make their way to the United States and become established here.
Annoyance. As we have seen, these invasive species acquire their blood meals from a wide range of hosts, including humans. The Asian tiger mosquito is a very aggressive daytime feeder and can quickly ruin any type of social event or family activity in the backyard. And as previously mentioned, the bites, especially on children, can become infected due to repeated scratching.
Introduction of New Pathogens. Although not highly likely, there is the possibility that a mosquito carrying a virus or other pathogen from its native habitat could make its way to the United States via airplane, ship or other mode of transportation. This scenario has played out several times with what is known as “airport malaria,” where an infected mosquito hitches a ride on an airplane and just like the passengers, disembarks at the destination, bites a local person and an outbreak of malaria ensues.
Endemic Disease Cycles. As invasive mosquito species establish in a new area and their population numbers increase, it is possible, and in some cases probable, that they will become involved in the endemic (regularly found) disease cycles. This is especially true of species that feed on humans as well as other hosts, as they may become “bridge vectors;” that is, spreading pathogens from animals to humans. This could result in the geographic spread of a disease, localized outbreaks where the disease has not occurred before or perhaps even in an epidemic.
Geographic Expansion. Many of these invasive species are easily transported during human activity. Most of the time, this probably occurs via the eggs, which as we have noted, are highly resistant to drought and can survive in things such as used tires for months or perhaps even years. Accordingly, the pest management industry will undoubtedly see these invasive species continue to expand their range.