A vernal pool photographed in mid-May, when the majority of early season mosquitoes will have finished development and emerged as adults. Photo by Andrew Mackay.
Early Spring Univoltine Mosquitoes of Illinois
Mosquito Life Cycle Strategies
When we think about mosquitoes, it is often of the species that are common around our homes during mid- to late summer and which annoy us, and possibly transmit disease agents to humans or other animals.
Common species that people are likely to be familiar with, or at least have been exposed to, include the invasive Asian tiger mosquito (Aedes albopictus Skuse), now present in many parts of the state, the northern house mosquito (Culex pipiens Linnaeus), an important vector of West Nile virus, or Aedes vexans Meigen, often described as a “nuisance mosquito.” One life history trait that these species have in common is that they are multivoltine, which is to say that these species have multiple broods of offspring and multiple, overlapping generations each year, which allows them to increase in abundance over the course of the summer and, if they are involved in maintaining transmission of a virus such as West Nile, allow for the ramping up in prevalence of that virus in local host communities.
About a quarter of the nearly 70 mosquito species present in Illinois have a different life cycle strategy of univoltinism, species which produce only a single generation per year. Much less attention has traditionally been directed towards these species as they were not thought to be involved in pathogen transmission to the same extent, yet they contribute to a considerable part of mosquito species richness and fill an important ecological niche.
Mosquitoes Associated with Vernal Pools
A common life cycle type that we see in certain mosquitoes of the Aedes genus, such as Ae. stimulans Walker, is closely associated with the seasonal presence of woodland or vernal pool habitats, with wooded areas that are flooded by spring rains or snow melt providing the habitat for the larvae of these species to develop. These habitats typically have a thick layer of leaf litter, and consequenty the water will contain a high level of tannins. Because of the ephemeral nature of these vernal pools, there is a punctuated end date by which larval development of that year’s brood has to be completed. This can result in incredible densities of ravenous female mosquitoes in these woodland habitats during spring, all seeking a blood meal source (typically a mammal host) so that they can complete reproduction. After finding a blood meal, for instance from white-tailed deer or another mammal, the females will lay their eggs in the leaf litter in depressions likely to flood next year, when the first instar larvae within them will hatch and give rise to the next year’s generation.
Within the vernal pools, these mosquito species can reach high levels of abundance as larvae. They are thought to serve as an important food source for a variety of vertebrate and invertebrate species that co-occur in these habitats, including salamanders and invertebrate predators such as diving beetles or dragonfly nymphs, and consequently only a small percentage of mosquito larvae will survive to adulthood. A wide range of invertebrates is associated with vernal pools, many of which are temporary water specialists (for example, fairy shrimp). Although the ecological role of mosquitoes in these habitats remains understudied, it is likely that they contribute to the environment and food webs in important ways.
Primarily Univoltine Species
Another set of Aedes spp. follows a slightly different life cycle strategy. They are primarily univoltine, with larvae that hatch in the spring forming the largest proportion of that year’s population, but in some years hatching may occur a few more times throughout the summer if rains flood their larval habitats again. These species include Ae. canadensis Theobald, a species that primarily uses vernal woodland pools, and Ae. sticticus Meigen, a species most strongly associated with shaded woodland pools in or near floodplains. It has been suggested that the eggs of these species may need to be inundated multiple times before hatching, leading to the somewhat more staggered emergence over the course of the year.
Early Spring, Univoltine Mosquitoes and Pathogen Transmission
Many univoltine species likely do not contribute to transmission of viruses and other vector-borne pathogens to a meaningful extent. In some cases, however, it appears that they can play an important role. One of these species, Ae. canadensis, feeds on a wide range of animals including box turtles, but also mammals such as humans. It has been described as a vector of several viruses of human concern, including eastern equine encephalitus virus, La Crosse virus and Jamestown Canyon virus.
To date, there have been no documented cases of eastern equine encephalitus virus in Illinois in humans, but cases have occurred in neighboring states such as Wisconsin and Indiana, and our research group has detected the primary mosquito species responsible for transmission (Culiseta melanura) both in southern Illinois and northeastern Illinois habitats. La Crosse virus is a leading cause of encephalitis in children in the United States. The primary mosquito involved both in maintaining a transmission cycle of this virus among chipmunks, squirrels and other small mammals, and with transmitting this virus to humans, however, is the eastern treehole mosquito, Ae. triseriatus.
Jamestown Canyon virus belongs, like La Crosse virus, to the California serogroup viruses. Exposures to this virus in humans are often asymptomatic, but can include fever, fatigue, and headache and in some cases severe disease, including encephalitis. White-tailed deer are the primary vertebrate host of this virus responsible for maintaining transmission. A study performed on deer in forest preserves in the Chicago area found that 34 percent of deer had specific antibodies to Jamestown Canyon virus (i.e., indicating prior exposure), highlighting that this virus is present in the state. The seasonal ecology of this virus is very complex and regionally variable, potentially involving several mosquito species as vectors, including Ae. canadensis.
An important question with transmission of mosquito-borne viruses is how, in temperate zones, the virus overwinters, as the seasonality of transmission is limited to the activity of biting adult mosquitoes, and infectivity in hosts limited as well. It has been shown that a few early season mosquito species, including Ae. stimulans, Ae. provocans and Culiseta inornata, can transmit this virus transovarially, in other words a female will oviposit and the first instar larvae developing in the eggs they lay will already be infected and ready to infect white-tailed deer when they emerge the following spring. Adult Ae. stimulans females, when they emerge from their woodland pools in the spring are aggressive and robust biters and readily feed on white-tailed deer. This species is common in northern Illinois and is thought to play an important role in the overwintering of the virus in other areas, including Indiana. Other mosquitoes, including possibly Anopheles species, have been suggested to be involved in transmitting the virus to humans later in the summer. To what extent Ae. stimulans or other mosquito species are overwintering hosts of Jamestown Canyon virus in Illinois, and the contribution of other mosquito species to transmission of the virus later in the season are questions that our lab is currently investigating through a project supported by the Illinois Department of Public Health. This is a pattern of transmission that is somewhat different from many other mosquito-borne diseases, with some unique challenges, which suggests that the role of early season mosquitoes is worthy of further study.
Dr. Andrew Mackay is a vector ecologist at the Illinois Natural History Survey Medical Entomology Laboratory. His research focuses on understanding the distribution and ecology of mosquitoes, the role they play in transmission of vector-borne diseases, and development of novel surveillance and control methods.
Dr. Chris Stone is a medical entomologist and director of the Illinois Natural History Survey Medical Entomology Laboratory, and Affiliate of the Department of Entomology and the National Center for Supercomputing Applications at the University of Illinois. His research interests are focused on how the environment shapes vector-borne disease transmission, as well as surveillance and control of mosquitoes, ticks, and the pathogens transmitted by them.
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