Bad season for Michigan mosquitoes may mean good Memorial Day for humans

• Dry weather should mean fewer mosquitoes over the holiday
• That could change later in the season, depending on rainfall
• Climate change is adding to Michigan’s blood-sucking problems

A dry May should mean fewer mosquitoes over Memorial Day weekend in Michigan.

If you just want the good news, you can stop reading now.

Michigan State University entomology professor Edward Walker cautions that the below-average number of biting bugs flitting around picnic baskets this holiday weekend doesn’t mean they won’t be bad later in the summer or fall — that depends on rainfall.

Oh, and climate change is likely adding to the squadrons of blood-sucking demons.

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The prevalence of mosquitoes relies heavily on increased moisture in their habitats. Michigan is home to more than 65 mosquito species, but only the females of a select few species, seeking protein for reproduction, bite and suck blood from hosts such as humans.

Mosquito eggs require exposure to water to hatch, and members of a group called “floodwater mosquitoes” rely on heavy rainfall to spur on hundreds of egg hatches at once. With a lifespan of at most 3 months for females, a dry spell of just a few weeks can prevent hundreds of baby bloodsuckers from hatching.

The lower peninsula has seen lower than normal precipitation throughout the month of May, which should lead to a relatively bite-free Memorial Day weekend. However, mosquito eggs can survive up to 8 months without moisture, so a downpour in the summer months might unleash an onslaught of baby mosquitoes who have been biding their time. 

Some species are more dependent on permanent water conditions such as lakes or sewers and, paradoxically, fare better with less precipitation, according to Walker. This variety, along with hard-to-predict weather patterns, makes it difficult to foretell the severity of mosquitoes in the coming months.

“Being unable to predict how much rain we're going to get or how intense of rainfall we're gonna get, it's hard to say, hard to predict that now we have other mosquitoes that are less dependent on rainfall,” Walker said.

Walker says that climate change has likely impacted the frequency and severity of tiny nuisances. Besides seasonal variances in precipitation, rising temperatures have extended the presence of mosquitoes throughout the year. The creatures can both hatch sooner and persist longer into the autumn months, with Walker finding that species that previously disappeared mid-September are now sticking around well into October. 

Annually rising temperatures have also started to push species thriving in warmer environments northward. One such species is the Asian tiger mosquito, which first made its way to Michigan in 2017 despite originating in the more temperate climates of Southeast Asia. 

“The brown rice field mosquito, a mosquito associated with rice agriculture in places like Arkansas, Texas, Louisiana, we now have established populations of the brown rice field mosquito in certain parts of Michigan,” Walker said. “The only explanation I can think of is that our more moderate winters and our favorable climate conditions are allowing the southern species to expand their ranges north and establish here.”

Along with causing itching and swelling, Michigan mosquito bites can infect some unlucky humans with the West Nile virus, which sickened 12 Michiganders in 2022, and the Jamestown Canyon virus, which infected one. Another rare but often deadly virus carried by some mosquitoes is Eastern Equine Encephalitis (EEE), which, with no known vaccine or treatment, kills approximately one-third of those infected. EEE outbreaks have occurred in Michigan in horses since the 1940s, and have sporadically been reported in humans since the first reported case in 1980. The largest human outbreak occurred in 2019, infecting 10 and killing 6 individuals in southwestern Michigan counties.

Species new to an area can affect the transmission of pest-borne viruses for residents. Walker pointed out the EEE-carrying Culex Erraticus as one southern species that has made its way to the Michigan peninsula. 

“The implications are that we could see more regular transmission of [EEE] virus,” Walker said. “And indeed we have been, over the past, of the years since the 1940s, our triple-E episodes have increased in frequency and regularity. So we see activity almost every year, and formerly we only saw it sort of sporadically.”

Staying inside is the best way to prevent mosquito bites, and the state health department has encouraged Michiganders to apply insect repellents containing Environmental Protection Agency-approved products, maintaining window and door screening and watching out for standing water sites that might become mosquito breeding grounds.

But calls to remain indoors and don long sleeves might not be popular among those looking to enjoy a Michigan summer. Walker says organized pest control programs employed by state and county governments can make up for the limitations of personal protective measures. A number of county-wide initiatives are located in the Saginaw Bay area, while smaller, township-based programs contract private companies to keep a lid on mosquito and tick populations.

Walker says Michiganders should take precautions not only for their own safety, but also for that of their furry friends. Dogs can become infected with canine heartworm, which is becoming increasingly prevalent with the growth of coyote populations, Walker said. If a mosquito bites an infected coyote, it might then transmit the disease to a dog it bites.

“If that dog is not protected by medicines from their veterinarians, that dog can become infected, and it can be quite a severe illness, [it] infects the heart and the pulmonary circulation of the dog.” Walker said. “People in Michigan really should not trivialize protecting their dogs from dog heartworm with the preventative medicines that the veterinarians prescribe.”