The Asian tiger mosquito (Aedes albopictus) is among the few species of mosquito for which a correlation between critical photoperiod—the day length that triggers half or more of a population to enter diapause at a certain point in autumn—and latitude and altitude has been established. A look at the body of research critical photoperiod in mosquitoes suggests more data on additional species could better align surveillance and management efforts with mosquitoes’ seasonally active periods. (Photo by Jim Occi, BugPics, Bugwood.org)

By Ed Ricciuti

Ed Ricciuti

As summery warmth progressively extends longer into the fall, so too does the time at which mosquitoes naturally respond to decreasing day length by switching off their activity heading into life-extending dormancy. Consequently, more mosquitoes hang around longer, including those that can transmit pathogens that cause diseases such as dengue fever, Zika, West Nile, yellow fever, and chikungunya.

The news isn’t all bad, however, as researchers at Ohio State University suggest that strategies to monitor and control mosquitoes can be enhanced by factoring in the timing of this shutdown, called diapause. The general idea is to zap harmful mosquito populations while they are still buzzing about in larger numbers, before they cease activity, according to a new research review published last week in the Journal of Medical Entomology.

Once mosquitoes lapse into diapause, they are, in effect, frozen in time. The timing of diapause is a genetically based survival mechanism, as evidenced by the fact that different populations initiate diapause in response to day lengths, or photoperiods, that best suit the seasonal patterns of their location. “Entering diapause halts the mosquito’s development no matter what stage at which the species enters it,” says lead author Caitlin S. Peffers, a master’s degree student in the department of entomology at Ohio State. “Mosquitoes that enter diapause live months longer than their nondiapausing counterparts. Without diapause, mosquitoes would not be able to survive harsh winters in temperate regions and could not start the next generation for the coming spring in these areas.”

Caitlin S. Peffers

The photoperiod that triggers half or more of a population to enter diapause at a certain point in autumn is called the critical photoperiod (CPP). It is specific to species. Other environmental cues such as falling temperatures play a part, too, but diurnal light is the key. In the northern hemisphere, the higher the altitude and latitude of a locality, the sooner CPP occurs. This correlation makes sense in an evolutionary context, since killer cold starts sooner as altitude and latitude increase, so diapause must start earlier than in warmer climes.

“Given temperature increase over recent decades,” the researchers write, “it is not surprising the CPP response of some mosquitoes has edged toward shorter, more southern day lengths as growing seasons have become longer.”

It is happening fast for some species, sometimes with only a few years. One study showed that, from 1972 to 1996, the CPP of the pitcherplant mosquito (Wyeomyia smithii) collected between 30 and 50 degrees north latitude, had decreased by several minutes, allowing mosquitoes to enter diapause later in the year.

“It is amazing to see such rapid change … in response global warming,” says Peffers. “It benefits mosquitoes to be able to adapt quickly to new climate conditions.”

The CPP of a species is not written in stone, however, but rather it adapts to changing conditions such as climate warming. Coincidentally, with the earlier CPP, some species of mosquitoes have been expanding their range in North America, including the yellow fever mosquito (Aedes aegypti) and the Asian tiger mosquito (Ae. albopictus), both important disease vectors. The Asian tiger mosquito is among the few species for which a correlation between CPP and latitude-altitude has been established. Predicted temperature rise linked to global warming, say the researchers, could open up virgin territory to new species of mosquitoes by making higher latitudes habitable for them.

The timing of CPP for most mosquito species, Peffers and colleagues write in their review of research on CPP and mosquitoes, is not well studied. “It’s a lot of work to determine the CPP of wild populations across a latitudinal gradient. Traveling to enough locations, maintaining wild mosquitos in a lab, and determining diapause status are very time consuming and laborious tasks,” Peffers says.

Characterizing vector mosquitoes’ critical photoperiods could inform management efforts by making sure they match them up with the mosquitoes’ active periods—and avoiding unnecessary chemical applications when target species are likely to have already entered diapause, the researchers note.

This could also reduce pesticide waste and damage to nontarget organisms. To do so, say the authors, the time of diapause for different species must be predictable and “latitude and altitude must be known for more species.”

Their paper, Peffers says, is the first to compile information on CPP in several mosquito species. She believes it is also the first to suggest use of the CPP in timing mosquito monitoring and control.

“Long-term studies on how CPP changes over time and in response to temperature such as that done with Wy. smithii  and Ae. albopictus are necessary to determine whether characterizing the CPP of vector populations can inform or improve mosquito control,” the report states. “Further, with the increase in global trade, the risk for introducing invasive disease vectors is ever-increasing.”

The researchers suggest that more study is needed on how variables other than day length impact the CPP response. “While the relationship between latitude and CPP is a very robust trend in several insect species,” Peffers says, “we still need more information on how other factors such as nutrition and temperature impact CPP in the field—as well as how climate change will change the seasonality of mosquitoes and if this will impact the predictability of CPP—before we can use it to make monitoring and control practices more efficient.”

Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.