As the temperatures start to slowly rise back up into more comfortable ranges, and Signs of Spring give way to Signs of Summer, Deborah’s and my thoughts inevitably turn to pulling out our folding chairs and getting outside in the evening to watch and feel the day come to a close. We usually get most of the month of April and then a good part of May (depending how fast summer really sets in) to enjoy long evenings outside, but by June something comes up that limits our time outside: mosquitoes.
There are always a few mosquitoes right away in the spring, but it takes some time and some continuous warm temperatures for the real swarms to arise. This staged invasion is due to the way that different mosquito species survive the winter. Our native mosquito species, by the way, don’t seem to be negatively affected by very cold winters or excessively stimulated by very mild winters. They are able to roll with our fluctuating climate and pull themselves back into their reproductive (and blood meal requiring) life stages with great ease and efficiency.
Some of our mosquitoes overwinter as cold resistant eggs while others hibernate as larvae. There are also a few mosquito species that overwinter as mated adult females. These adult females emerge early in the spring and are ready right away for a blood meal so that they can finish making their eggs. They are the first attacking wave in the early spring. Again, cold spring temperatures may delay their emergence, but the cold does not seem to significantly reduce their numbers. So there are a few mosquitoes active as soon as air temperatures get sufficiently warm, but the swarms don’t come until all of those overwintering eggs and larvae mature and begin to seek their blood meals.
There are so many things to say about mosquitoes, and so many different points of view to take about them!
Dr. Nora Besansky is a biologist at Notre Dame University who specializes in mosquitoes. She was interviewed on NPR back in February and spent most of her on-air time emphasizing how beneficial and important most mosquitoes are. Of the four thousand or so species of mosquitoes less than one hundred actually transmit human diseases. The rest occupy important places in many food chains (for mammals, birds, fish, amphibians, reptiles, and wide range of insects and other arthropods) and also function as pollinators for many species of plants (all adult mosquitoes, Dr. Besansky emphasized, drink flower nectar). Blood feeding by mosquitoes is only carried out by females and is needed to provide the protein and iron required to make viable eggs. The female mosquitoes can take a blood meal from a wide variety of potential hosts. The list of possible hosts, in fact, is nearly identical to the list of what can eat mosquitoes, and different mosquito species specialize on particular types of hosts.
Another perfectly reasonable point of view, though, concerning mosquitoes looks at the nearly one hundred mosquito species that can transmit diseases to and in between people. There are some serious diseases on the mosquito transmitted list! Malaria, yellow fever, Dengue fever, West Nile virus, and a whole slew of encephalitis syndromes are all carried by mosquitoes. Over the past winter, our awareness of a new mosquito borne virus spreading across South and Central America, the Zika virus, has triggered intense discussions about mosquito control procedure and the safety of travel to Zika affected areas. Each year, according to EB Medicine (an on-line medical reference site), seven hundred million people get a mosquito transmitted illness. Most of these cases occur in human populations residing in tropical and subtropical climate zones, and about half of all of these cases involve malaria.
So here in Western Pennsylvania (which for now anyway, is about as non-tropical or subtropical as you can get!), we have nothing to worry about, right? (I bet everyone reading this can feel the coming bad news that is about to extend this essay!).
Let’s talk about the mosquitoes that spread some of these diseases: Malaria is caused by a protozoan parasite called Plasmodium. Plasmodium is most commonly spread by mosquitoes of the genus Anopheles. The very good news is that Anopheles mosquitoes, unlike our more robust native mosquito species, cannot survive our long, cold winters and are, therefore, not found in Western Pennsylvania. Fortunately, malaria is not something we have to worry about locally!
When we talk about mosquito spread viral diseases, though, like yellow fever, Dengue fever, and Zika, we are especially (but not exclusively) talking about mosquito species that are in the genus Aedes. Aedes aegypti , in particular, is an active, wide-spread tropical and sub-tropical mosquito species and is often the species involved in the transmission of these illnesses to humans. Aedes aegypti has all sorts of specializations that make it very difficult to control. It can lay its eggs in very small pools and puddles of water. Its larvae can develop successfully in the tiniest volume of even the most stagnant water (think rain water collected in a discarded tin can or old tire!), and bacterial growth in these water sources actually stimulates egg development! The adult Aedes mosquitoes are most active at dusk and at dawn especially in shady areas, but they can bite (and spread their viruses) at almost anytime during the day and also at any time of the year. Adults can also live exclusively inside of houses and other buildings often taking their blood meals while the people in these buildings are asleep.
So where is the boundary between the temperate and subtropical climate zones? How about 250 miles from here? How about even closer than that? There are established populations of Aedes aegypti and other Aedes species in Washington, D. C.! There are also periodically occurring populations of Aedes albopictus (the “Asian tiger mosquito”) right here in Western Pennsylvania, and there is some indication that the Asian tiger mosquito is developing a tolerance for our cold winters and may be setting itself up as an established alien species! The Asian tiger mosquito can carry all of those diseases listed above and may represent a significant human health threat!
It is always “good news/bad news” when thinking about these ecological and medical problems. The really good news is that our cold winters limit the ability of these virus and parasite carrying mosquitoes to establish themselves in Western Pennsylvania. The bad news is that these mosquitoes are evolving to get around that limitation and, with the ongoing warming associated with climate change, the climate seems to be meeting the species changes half way! Mosquitoes and ticks, unfortunately, are some of our signs of spring and summer! I will talk about ticks next week in our first “Signs of Summer!”