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
517-353-3379
FAX 353-4354
kaufma15@msu.edu