The summer of our discontent

Editor’s note: This article was revised by Dave Smitely, MSU Entomology and was first published in the August 8, 2008 Landscape Alert.

Although substantial rains have subsided in the past few weeks, mosquito populations are still reaping the benefits from wet spring and early summer periods. I’d considered last year one of the worst in my locale in the past decade, but this year has probably equaled that. Why is this so, what can you do about it, and what might it mean for mosquito-borne disease risks?

The simple explanation is that regular rain events earlier in the year both induced hatching and sustained larval habitats. In my area, there were several storms that produced over an inch of rain. This has helped launch broods of summer floodwater mosquitoes (e.g., Aedes vexans and A. trivittatus). In June and to some extent even now, these species overlapped with remaining spring emerging species (recall our wet spring) to create a large crop of nuisance biters. Additionally, a cattail marsh species, Coquilletidia perturbans that normally starts to peak around July 4 has added to the problem in some areas.

Large floodwater mosquito populations often result from the hatching of eggs laid several years ago. Think of them as mosquito “seeds.” The eggs are laid in ground depressions that may not hold standing water every year. When we get large rain events in the summer, these habitats produce mosquitoes from the “seed bank” to add to the normal background populations. I’m pretty sure this occurred in many parts of Lower Michigan last year and this robust population laid eggs that hatched this year. The main culprit in my area this year, A. trivittatus, is a small, aggressive mosquito that will bite during the day and often attacks in swarms. It commonly breeds in flooded areas next to streams and rivers, but can also exploit other wetland habitats. Many flood zones contained standing water long after the normal spring peak this year. Coupled with hot weather a few weeks ago, these environments were ideal production centers for A. trivittatus. Had summer temperatures not been a bit cooler than normal, we would have experienced this mosquito plague even earlier. If continual cycles of flooding and drying occur during a single summer, the floodwater mosquitoes such as A. trivvitatus can continually breed (multiple generations each year) and maintain these intolerable levels. The good news is that large, flooding rainfall events appear to be diminishing, so many larval habitats will disappear and adult populations will wane. Additionally, temperatures appear to be moderate thus far, and this will further delay the next onslaught. However, another series of storms with heavy rainfall and more typical summer temperatures could very well continue the outbreak. Since we’re heading into the part of summer when species from permanent water bodies are also beginning to peak, our misery index could easily rise again.

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 ½ 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.

How does one deal with the onslaught of summer species? Unfortunately, when they’ve reached the adult stage in large numbers, options are few. There are many effective backyard spraying/fogging treatments 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.

If you’re like me and prefer not to deal with broad-spectrum adulticides, 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) or lemon-eucalyptus oil derivatives (a Repel product) are quite effective. Other 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 2 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.

A recently available personal barrier repellent, OFF clip-ons, also uses 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 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/biting attempts 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 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. You should also be aware that there’s been a spike in the numbers of rabies-positive bats in recent years, so it’s hard to recommend any practice that would increase the potential for contact with these creatures.

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. WNV and similar diseases tend to be more pronounced in hot, dry years and I wouldn’t characterize the first half of our summer as such. 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.

In the area of “what’s new” for mosquitoes in Michigan, you’ll be happy to know that we have a recently established invasive species. Ochlerotatus (Aedes) japonicus actually arrived here in about 2003 (1998 in the eastern United States) and we’ve been tracking its progress in certain areas. It’s a medium-large mosquito with distinctive markings but isn’t necessarily an aggressive biter. Its original range is Japan/northern Asia, and can easily overwinter in our climate in contrast to other invasive mosquito species such as the Asian tiger mosquito. It appears to be a highly competent vector of WNV and similar diseases, and we know it feeds on birds and mammals, including humans. The reason I point it out here, is that it seems very well-adapted to breeding in artificial containers around human dwellings, with a particular fondness for plastic. I’ve found it in buckets, dog dishes, kiddie pools, artificial ponds (with plastic or concrete liners) and a number of miscellaneous containers. In some cases, it appears to be displacing native mosquitoes including Culex. We don’t know its role in any disease cycle yet, but it’s clearly one to watch in the event of any new disease introduction into North America. If anyone notices large larvae in plastic containers around the house or yard, please place a few in alcohol (rubbing alcohol is fine) and send them to me:

Michael G. Kaufman, Ph. D.
Research and Extension Specialist
Department of Entomology
Michigan State University
E. Lansing, MI 48824-1115
FAX 353-4354  

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