"removing the rats immediately improved bird, tree and crab survivals rates."
TNC Palmyra Program Director
By Jan TenBruggencate
Wildlife managers who removed black rats from Palmyra Atoll have discovered their efforts also rid the island of a disease-carrying mosquito. Researchers say the unexpected result marks the first time a mosquito has been eradicated by removing its blood source host—an outcome that could open new avenues for controlling mosquitoes and mosquito-borne diseases elsewhere.
“The unplanned eradication of Aedes albopictus greatly reduces the potential for transmission of diseases harmful to both humans and wildlife,” said Alex Wegmann, Palmyra program director for The Nature Conservancy.
The mosquito, known as the Asian tiger mosquito for its stripes, can transmit zika, chikungunya and dengue to humans, as well as West Nile virus to both humans and birds.
Black rats and the Asian tiger mosquito were both unwanted immigrants to Palmyra, a pale blue atoll located 1,000 miles south of Hawai`i that is jointly managed by The Nature Conservancy and the U.S. Fish and Wildlife Service. The atoll is a major environmental resource, a rare patch of mid-ocean dry land that is home to one of the largest remaining Pisonia forests, large land crabs like the coconut crab, and 11 seabird species.
Restoring Ecological Balance
But with rats preying on these and other wildlife, Island Conservation, a global invasive species control organization, was brought in to lead an eradication effort in 2011 to help restore the atoll’s ecological balance.
“Rodents like rats and mice are destructive ecosystem engineers that impact every link of the food chain on islands and disrupt processes like seed dispersal, pollination and native predator-prey relationships,” said Wegmann, who led the rat eradication effort for Island Conservation before joining The Nature Conservancy. “At Palmyra, rats did all of this and more, including eating eggs and chicks and preventing native trees from reproducing by eating their seeds and seedlings.”
According to Wegmann, removing the rats immediately improved bird, tree and crab survivals rates. It allowed seabird populations to safely thrive, re-balanced the important sea-to-land-to-sea nutrient cycle seabirds provided to the atoll, and restored land crabs to their place as the native engineers of Palmyra’s rainforest.
No Blood Source Host
Not long afterwards, U.S. Geological Service researchers at Palmyra began noticing fewer attacks from the daytime Asian tiger mosquito. Upon closer study, they discovered the rats provided the dominant blood meals required by the mosquitoes. With the rats gone, there were no other mammals on the island to support the mosquito population. A scientific paper on the disappearance was published in February 2018 in the journal Biology Letters.
“Anecdotal biting reports, collection records, and regular captures in black-light traps showed the species was present before rat eradication. Since then, there have been no biting reports and no captures over two years of extensive trapping,” say the paper’s authors.
Researchers believe that this is the first time an introduced mosquito species has been removed by killing off its host, a finding that provides useful clues for future rodent eradication efforts.
“It suggests that preferred host abundance can limit mosquito populations, opening new directions for controlling important disease vectors that depend on introduced species like rats,” the authors say.
Targeting Another Pest
While the Asian tiger mosquito is gone, Palmyra still has another unwanted buzzer, the night-biting southern house mosquito, Culex quinquefasciatus. It can carry avian malaria, as well as human diseases like Zika virus, encephalitis and West Nile virus.
The Nature Conservancy is working with the USFWS, Verily Life Sciences and researchers from the University of California at Santa Barbara and the University of Maryland on its eradication.
Their plan is to raise millions of mosquitoes and to infect the males with a strain of Wolbachia bacteria that acts as a kind of birth control. By flooding the population with mosquitos that can’t reproduce, the team hopes to wipe them out entirely.
“The eradication of mosquitos from Palmyra will protect the atoll’s wildlife and the people who travel there to study it,” Wegmann said. If successful, a similar technique could be used in Hawai’i to protect endangered forest birds from carriers of avian malaria.