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RAT ELIMINATION LEADS TO MOSQUITO EXTINCTION

A rare case of secondary extinction has seen not only rats but mosquitoes banished from the tropical paradise of Palmyra following a successful rodent eradication program.

Researchers from the University of Santa Barbara in the US have confirmed a rare case of secondary extinction on a remote atoll in the northern Pacific Ocean. The study, published in February in Biology Letters, confirms that the eradication of rats on Palmyra Atoll has led to the surprising secondary extinction of Aedes albopictus – the Asian tiger mosquito. This appears to be the first documented accidental secondary extinction of a mosquito species.

Palmyra Atoll, which lies around 1,600 kilometres south of Hawaii, was used as an operations base by the US military during World War II. It is believed that the arrival of the military brought two significant pests to the island: Rattus rattus, the common black rat, and the Asian tiger mosquito, known carrier of such diseases as dengue, yellow fever, Rift Valley fever, Chikungunya and Zika. The mosquitoes fed on the blood of the rats, while the rats’ habit of eating juvenile coconuts and leaving empty shells formed the ideal habitats for mosquito larvae.

In 2011, to help Palmyra recover from the ecological damage wreaked by the non-native rats, a US-led rodent eradication project was successfully implemented. Two helicopters and a team of 41 people distributed 38,000kg of rodent bait containing the anticoagulant brodifacoum (at a broadcast application rate of 75-85kg/hectare) [1].

The result was surprising: rather than being bitten more, people on the atoll reported that the aggressive day-biting mosquito had ceased biting altogether. In fact, in the two years after the aerial drop, extensive trapping (using blacklight and scent traps) showed no evidence of Aedes albopictus. (By contrast, the southern house mosquito, Culex quinquefasciatus, was abundant before and after rat eradication.)

Palmya Atoll (Credit: Erik Oberg / Island Conservation)

It seems that after rat eradication, humans and seabirds alone could not support positive population growth or maintenance of Aedes albopictus. With a post-war population of only 5-30 humans on the atoll, the mosquitoes were left with an insufficient number of hosts on which to feed. Had rat densities simply been reduced rather than eradicated, or had human densities been higher after rat eradication, human biting rates would have been expected to increase as mosquitoes switched host, from rats to humans.

“We believe that this is the first documented accidental secondary extinction of a mosquito,” said the paper’s co-author Hillary Young, an Associate Professor in UC Santa Barbara’s Department of Ecology, Evolution, and Marine Biology (EEMB). “We hypothesized that Aedes was eradicated from Palmyra primarily because its persistence depended on taking blood meals from rats.”

rat
Tree-dwelling black rat (Credit: Hillary Young / Dept of Ecology, Evolution and Marine Biology, UC Santa Barbara)

The research team’s inability to document an Aedes bite, or trap an Aedes mosquito, over two years of sensitive surveillance meets the World Health Organization’s standards for demonstrating mosquito eradication. Although mosquito eradications are usually fleeting due to mosquitoes’ ability to quickly recolonise, the lack of recovery over six years suggests that conditions on Palmyra have become unsuitable for Aedes.

“Normally we mourn species losses, but without introduced rats and mosquitoes, Palmyra Atoll is as close to paradise as you could imagine,” explained lead author Kevin Lafferty, an USGS ecologist and an adjunct EEMB faculty member at UCSB.

Associate Professor Young added, “Eradicating mosquitoes by eradicating a key host provides a relatively unexplored way to combat the diseases mosquitoes transmit. Our case study highlights this potential for synergies between public health and an increasingly common conservation intervention.”

The research suggests new directions for controlling these important disease vectors, especially in locations where introduced species are heavily targeted as hosts.

[1] Wegmann, A. et al. (2012). Pushing the Envelope in Paradise: A Novel Approach to Rat Eradication at Palmyra Atoll. Twenty Fifth Vertebrate Pest Conference. Published by University of California Davis.