MSU Extension

The excessive heat may be temporarily gone from what I consider to have been a horrible summer, but other unpleasantness remains in the form of large populations of mosquitoes. It’s been a “good” year for many species because they thrive in hot weather and in habitats created by sporadic, heavy rainfall. Lots of rain in May led to large June populations. Those June populations laid eggs that made it through the July drought and hatched with heavy rains toward the end of the month. This resulted in more broods of summer floodwater mosquitoes (e.g., Aedes vexans and A. trivittatus).

One of the indicators of summer floodwater populations is the appearance of a very large species (largest in Michigan) commonly called “gallinippers.” This is Psorophora ciliata and it can be alarming when it lands on your arm to attempt to feed. Adult females can be over half an inch long in body length and they have very hairy legs with yellowish bands. They are never very abundant, but you won’t forget them if one tries to bite you. On the positive side, their larvae feed upon other mosquito larvae, so they have a beneficial aspect.

If warm conditions persist and heavy rainstorms occur every few weeks, large numbers of mosquitoes could be with us well into September. (Not to alarm anyone, but some areas in Michigan have seen substantial emergences of these mosquitoes in October!) The question is, what can you do about it?

Unfortunately, when mosquitoes have reached the adult stage in large numbers, options are few. There are many marginally effective backyard spraying and fogging treatments (available at local hardware and home stores) that typically use a synthetic pyrethroid (e.g., permethrin) as the active ingredient. Some can be sprayed at yard borders and have residual (several weeks) effect. Obviously, you will want to strictly adhere to application instructions and restrictions. Pyrethroids have low mammal and bird toxicity, but overspray into ponds, for example, might harm fish. Note also that these compounds are not mosquito specific, so most other insects will be affected.

A commercial option that should be avoided is the timer-based automatic sprayers. These units, called “Mosquito Misters,” are analogous to automatic sprinkler systems and simply spray or mist insecticide from a reservoir at the determined time intervals from a series of nozzles placed in and around the property. This system is costly, inefficient and violates the sound principles of integrated pest management. If mosquito problems on your property are severe enough for you to consider such a system, I suggest that you contact a mosquito control company instead. Consider hiring a service as part of a neighborhood group – mosquitoes can and will move around frequently from yard to yard. Some of the floodwater species are known to travel miles in search of hosts.

Other potential alternatives for adult control include garlic-based oils applied as sprays. I’ve had no experience with these products and, to my knowledge, there has been little peer-reviewed reporting on their efficacy. The idea that consuming garlic makes one less attractive to mosquitoes is a myth, and there is little evidence to indicate it is useful as a repellent (for mosquitoes, not irritating people). If someone tries a garlic product, please let me know how it works, but I remain skeptical.

If you’re like me and prefer not to deal with broad-spectrum adulticides or unproven products, then your options lie in avoidance (stay inside!) and the judicial use of repellents. Spending the evening on your deck or patio can also be made more tolerable with some well-placed fans. There is an increasing variety of mosquito repellants available that can be applied to exposed skin and many common fabrics; cotton and nylon are OK, but certain synthetics such as rayon may not hold up to higher concentrations of DEET. DEET-based products remain the standard for effectiveness and safety, but relatively new products with picaridin (supposed to be less irritating than DEET, sold as Cutter Advanced), IR3535 (in some Avon products) or lemon-eucalyptus oil derivatives (a Repel product) are quite effective. There are several new DEET products such as the OFF dry spray line that, in my experience, are very effective and don’t have the irritating oily feel of other formulations.

Additional products based on botanical derivatives (e.g., Bite Blocker with soybean oil) can be effective for short periods of time, but if you want something to last for more than two hours after application and to work for ticks as well as mosquitoes, use one of the products mentioned above. There are also several lines of clothing impregnated with permethrin (e.g., Buzz Off) that keep mosquitoes from landing on materials. You can also apply this to several types of fabrics yourself and it’s supposed to last through several washings. Note that this is the same principle used in bednets to fight malaria in Africa, but also note that the clothing doesn’t provide a whole body shield. Mosquitoes will readily land and bite on exposed skin adjacent to the material.

I cannot recommend OFF clip-ons, which use a pyrethroid type of insecticide (metofluthrin) dispersed with a small fan as a repellant. I recently tried using one of these units, but it was almost totally ineffective against the swarms of A. trivittatus that attacked my dog and me when we walked near the edges of the lawn or along country roads. It did appear to inhibit landing and biting attempts on parts of my torso when I used it while sitting on the patio, but it did not eliminate repeated mosquito attacks to my head, face and lower legs. I doubt most people will want to wear three to five of these units for full “coverage” and I suspect no one will want to wear one as a necklace to keep A. trivittatus away from the head and neck – the packaging label warns against inhalation of the vapors (something that’s probably hard to avoid, in my estimation).

Unfortunately, there are no great options for barrier repellants yet. Landscaping plants and citronella candles have not been shown to be more effective than smoke-producing candles in keeping mosquitoes at bay. However, research of area-wide repellants is a hot area, so expect to see more products of this type in the next few years.

I’d love to be able to recommend attracting bats as a means to reduce mosquito populations, but the idea that they are mosquito-eating machines is simply a myth. Of course they can and do eat mosquitoes, but they almost certainly could not survive by doing so. The myth arose from a study that reported bats would need to eat several thousand mosquitoes (or mosquito-sized insects) a night to meet energy demands, and from counts of mosquitoes eaten by bats in cages where they were the only prey item. Any accounts of bats controlling mosquito populations are anecdotal, as are those indicating birds such as purple martins are effective. I have lots of bats and insectivorous birds on my property and the mosquitoes are clearly uncontrolled. You should also be aware that bats are known to carry rabies, so it’s hard to recommend increasing their populations near human dwellings.

Likewise, it would be nice to be able to recommend propane-powered devices such as Mosquito Magnets that attract and kill mosquitoes via carbon dioxide (sometimes with an octanol supplement) plumes and a fan. However, there is no evidence that they reduce biting rates in a realistic setting and in fact may be drawing in mosquitoes from other areas. The running joke is that if you want these traps to work for your yard, buy one for your neighbor. They are also not equally effective in trapping all species – we know this from our own mosquito traps that are based on the same attractants. The only study showing substantial reduction in biting rates after use of these devices took place on a very small island with a well-defined mosquito population.

The standard dogma about eliminating breeding sites on your property – eliminating or frequently changing any standing water such as that in birdbaths –  still holds true, but it will have little effect on the crops of floodwater mosquitoes that have been the bane of this summer thus far. As mentioned, most of the mosquitoes biting you during the day while you’re trying to weed your garden, or in the evening when you’re relaxing on the patio, have developed elsewhere and have potentially flown into your backyard – unless you live along a floodplain – from miles away. This is not to say that your efforts to eliminate breeding sites are useless. The artificial containers around your home can be excellent larval habitats for many of the species that transmit human diseases.

The large populations of nuisance mosquitoes seen this summer do not necessarily portend an increase in risk of mosquito-borne disease. Some floodwater species appear to be competent vectors of West Nile virus (WNV), for example, but it’s thought their role is minor. They have, however, been implicated in transmission of dog heartworm, so make sure your pets are current with their medications.

The primary vectors of West Nile, species of Culex mosquitoes, do not appear to be in unusual abundance this year and our testing of mosquito pools for WNV and other mosquito-borne viruses have not yielded a single positive from Michigan samples. It may simply be that the disease is slower to emerge this year because of climate conditions or bird population factors. The same conditions (heavy rains) that encourage high populations of floodwater mosquitoes may be washing out some larval habitats (e.g., storm water catchbasins) of the Culex species. However, be aware that late summer is usually when Culex populations peak, and they also appear to be more likely to feed on humans during this period. Although the risk may be relatively low this year, I’d still urge precautions and the use of repellants, particularly during the evening and nighttime hours.

Related article:

• Mosquito super emergence, Howard Russell, MSU Diagnostic Services