Mosquitoes of Michigan -Their Biology and Control

Mosquitoes are by far the most dangerous animals on earth. It is hard to comprehend the amount of disease and the resulting sickness, death, and economic loss caused by the mosquito. Some scientists estimate between 500 and 700 million people get malaria worldwide each year. That’s more than twice the entire population of the United States each year. Malaria has since been virtually eliminated here in Michigan, but the threat of mosquito-borne disease is still very real. Of the 60 different species of mosquitoes found in Michigan many are known to be vectors (carriers or transporters) of important diseases such as West Nile virus, Eastern Equine Encephalitis, St. Louis Encephalitis, and the California Group of encephalitis. Mosquitoes are the most medically important insects in Michigan and the entire world, thus it is imperative that pest management professionals understand mosquito biology and control.

The all too familiar mosquito nuisance also threatens our health. Health can be thought of as a “state of well being.” A person who is pestered and bitten by mosquitoes has an impaired state of health. Individuals and their families are often kept from outdoor recreation and activities due to nuisance mosquito populations. The bite or multiple bites resulting from mosquitoes can cause varying levels of discomfort from a slight itch to a severe allergic reaction. Mosquito control has an important role in maintaining public health even in the absence of disease transmission.

MOSQUITO BIOLOGY

Mosquitoes, like other flies, undergo complete metamorphosis, having egg, larval, pupal and adult stages. Larvae are commonly referred to as "wigglers" and pupae as "tumblers." Larvae and pupae of mosquitoes are always found in water. The breeding source may be anything from water in discarded tires to water collected in plants, to pools, puddles, and swamps. Mosquito species differ in their breeding habits, biting behavior, flight range, etc. However, a generalized description of their life cycle is presented here and will serve as a useful basis for understanding mosquito biology and ecology. Most larvae in the subfamily Culicinae hang down just under the water surface by a breathing tube (siphon), whereas anopheline larvae lie horizontally just beneath the water surface supported by small notched organs of the thorax and clusters of float hairs along the abdomen. They have no prominent siphon. Mosquito larvae feed on suspended particles in the water as well as microorganisms. The mosquito larva has 4 instars (stages) and undergoes four molts (each successively larger), the last of which results in the pupal stage. Mosquito pupae do not feed and will "tumble" toward the bottom of their water source upon disturbance. They emerge as adult mosquitoes in two to four days. With optimum food and temperature conditions, the time required for larval development can be as short as seven days. When the adult is ready to emerge the pupal skin splits from the top, and the adult slowly emerges onto the water surface. The adult will remain on the water surface until its wings harden.

Adult mosquitoes of both sexes obtain nourishment for basic metabolism and flight by feeding on plant juices and nectar from flowers. The female mosquito is solely responsible for biting; the females of most species require a blood meal for egg development. They not only bite people, but also obtain blood from birds, mammals, reptiles or amphibians.

Michigan has three major classes of mosquitoes based on larval habitats (breeding sites) and life history: permanent water mosquitoes, floodwater mosquitoes, and artificial container/tree hole mosquitoes. These are general categories and some species may be found in more than one type of habitat, for example Culex pipiens larvae may be found in permanent water habitats (sewage lagoons), but may also be found in artificial containers (tires); the shared appeal is foul, polluted water.

Permanent water mosquitoes in Michigan, which include Anopheles and many Culex species, and the unique Coquillettidia perturbans, can be found in various permanent habitats, such as swamps, ponds, sewage ponds/lagoons, and ditches that do not usually dry up. These permanent water mosquitoes can be subdivided by the quality of water. Cq. perturbans and Anopheles, which include An. quadrimaculatus and An. perplexens, prefer clean or relatively unpolluted water, as found in swamps, ponds, and large ditches. Culex mosquitoes, primarily Cx. pipiens and Cx. restuans, prefer polluted/highly organic water, as is often found in sewage lagoons and catch basins. Anopheles and Culex females lay their eggs directly on the water surface, Anopheles individually and Culex in groups of attached eggs called rafts. These two groups are capable of many generations per year and typically overwinter as mated females. The Cq. perturbans is unique in that it is found in freshwater habitats that possess emergent vegetation. Both larvae and pupae possess a modified tube adapted to pierce and attach to the submerged roots and stems of aquatic plants for the purpose of obtaining oxygen. The adult female lays its eggs in rafts directly on the water surface. Cq. perturbans only has one generation a year (takes a year to go through its life cycle) and overwinters in the larval stage.

Floodwater mosquitoes are found in habitats that are temporarily flooded for a portion or for brief periods throughout the year. The mosquitoes’ ability to utilize these short-term breeding sites lies in their desiccation-resistant eggs. The eggs are laid in ground depressions which flood when conditions are right; this often occurs in the spring after snow melt and after summer storms. Eggs can remain viable up to 7 years after being laid. There are two groups of flood water mosquitoes, the spring floodwater mosquitoes and the summer floodwater mosquitoes.

Larvae of spring floodwater mosquitoes hatch from the eggs in late March, in pools of water formed by melted snow in the woods. The eggs occur in the leaf litter at the bottom of the pools. These larvae develop slowly because of low water temperatures, and emerge as adults in May, before the pools dry up. The female spring floodwater mosquitoes can be very long-lived, and may bite several times. They lay eggs in the woods where they will be flooded the following year. Spring floodwater mosquitoes have only one generation per year, so even if these eggs are flooded by summer rains, they will not hatch until the following spring. The species names of some spring floodwater mosquitoes are: Aedes stimulans, Aedes excrucians, Aedes provocans, and Aedes canadensis. There are several other species as well, but they all share the same type of life history pattern.

Summer floodwater mosquitoes include several of our most common pest mosquitoes in Michigan, such as Aedes vexans, Aedes trivittatus, and Aedes sticticus. The Psorophora mosquitoes are also in this group; they include the woodland nuisance Ps. ferox and Michigan’s largest mosquito Ps. ciliata. Both Aedes and Psorophora larvae hatch from eggs after rainfall in the summer (usually 1 inch or greater) in shallow flooded areas such as meadows, floodplains, woodlots, roadside ditches, highway right-of-ways, tire tracks, and cow hoof prints. The larvae develop very quickly (7-10 days) and several generations may occur each summer depending upon the frequency and intensity of rainfall. For any given summer, it is hard to predict in advance how bad the summer mosquitoes will be, because it’s difficult to predict rainfall.

Artificial container/tree hole mosquitoes not only make use of natural tree holes but also breed in discarded tires, rain buckets, or anything that may hold water. The most common species in Michigan are the native Aedes triseriatus and the exotic Aedes japonicus. The eggs are often laid on the habitat walls just above the water-line. When the next rain event occurs the eggs become submerged and hatch shortly thereafter. These species are capable of many generations per season. One may also find that several Anopheles and Culex species may occasionally lay their eggs in these habitats. The type of species found in the container is often dictated by the location and water quality of the container. Ae. triseriatus and Ae. japonicus will often be found in wooded habitats, whereas Cx. pipiens will often be present in those containers with rather organic water located in an open yard setting.