How an Invasive Species Spreads: The Case of Aedes notoscriptus in Southern California
By Andrew Porterfield
The mosquito Aedes notoscriptus is no longer a stranger to California. The Australian native was first reported in Los Angeles County in 2014, its first appearance outside of Australia, New Zealand, and the southwest Pacific. Of course, invasive insect species aren’t unusual (in California and elsewhere), but this species was noted for its rapid spread and easy adaptation to urban areas.
By 2019, immature and adult Ae. notoscriptus mosquitoes had been collected from 44 cities in three California counties. Yet, the mosquito—sometimes known as the striped mosquito or the Australian backyard mosquito—remains relatively understudied. Knowing how the species adapts (and how quickly) will help provide clues to managing it. This knowledge is also important because Ae. notoscriptus seems well poised to rapidly spread globally. In a paper published in October in the Journal of Medical Entomology, entomologists from vector control districts in Los Angeles and San Diego counties and the California Department of Public Health present findings on the spread and adaptations of the mosquito.
Aedes notoscriptus will take its blood meals from nearly any animal (mammal—including humans—or bird), at night or during daytime. It prefers temperate and tropical forests and uses water-holding containers such as tree holes, bamboo stumps, leaves, and rock pools to develop larvae. It also has adapted well to urban areas, using artificial water containers. It has entered previously inaccessible areas of Australia and traveled to New Zealand, in addition to becoming an urban nuisance in California. And Ae. notoscriptus has not been as well studied as its cousins Ae. aegypti and Ae. albopictus, because it was not associated with diseases like West Nile virus, but it has been associated with Ross River and Barmah Forest viruses in Australia and with canine heartworm in California.
The California study, led by Marco Metzger, Ph.D., at the California Department of Public Health, looked at data on larval habitat, seasonality of adults, and adult trap preferences to track the progress of this mosquito’s spread, and the researchers attempted to determine the origin of the mosquito in the United States.
The first detection of Ae. notoscriptus in the U.S. was in Monterey Park, a city in Los Angeles County, during a surveillance trapping study for West Nile virus. The female was caught in a carbon dioxide-baited trap. Two months later, in nearby Montebello, a request from a resident to control day-biting mosquitos resulted in trapping larvae and adults of what turned out to be Ae. notoscriptus. Its spread was tracked by the county’s West Vector & Vector-Borne Disease Control District, starting with seven detection sites in 2015, rising to 57, 153, 164, and 155 each year through 2019. The mosquito was discovered in adjacent Orange County in 2017 and in San Diego County in 2018.
The establishment of Ae. notoscriptus triggered a necessary shift in control strategies. The vector control agencies had to pivot from individual property inspection and trapping to neighborhood (public) approaches, persuading the public to help with mosquito control (e.g., emptying water-holding containers, wearing protective clothing, using insecticides).
In total, 744 detections of Ae. notoscriptus were made in the period between 2014 and 2019. Traps that collected mosquitoes were carbon dioxide-baited (some with an added human-scent lure), which were the most successful, and gravid traps. Seasonality was recorded, as larvae and adults were collected every month. The largest numbers were collected in August, and the lowest numbers in February.
“The discovery and spread of Ae. notoscriptus in southern California was nearly simultaneous with the documented invasions of Ae. albopictus and Ae. aegypti and created an unprecedented burden on local vector control agencies seeking to protect the public from mosquito-borne pathogens and biting nuisance,” the authors write. “Aedes notoscriptus was recognized to pose a lower public health threat in California relative to the invasive vectors Ae. aegypti and Ae. albopictus; however, there were still public and veterinary health concerns associated with this species.”
The adult Ae. notoscriptus mosquito can survive domestic and international air travel, leading vector control officials to look at airports like Los Angeles International as a possible entry point into California. However, trapping in the area was not successful. In 2018, evidence turned up pointing to the first introduction of the mosquito in San Diego County, where a bromeliad collector had more than 200 plants on the property and traded plants with other collectors in Los Angeles and Orange counties. While this may have been the first source, the mosquito adapts well to urban southern California, and increased detections indicated multiple introductions. The mosquito has a short flight range, and spreading was probably aided by hitching rides on cars and trucks.
“The invasion and establishment of Ae. notoscriptus in southern California underscores the potential of a new exotic species to spread globally,” the study authors write. “This species is becoming well established in the urban environment and may be expanding beyond the urban matrix. With year-round reproduction, seasonal control efforts may need to be replaced with on-going routine treatments if population reduction is needed.”
Journal of Medical Entomology
Andrew Porterfield is a writer, editor, and communications consultant for academic institutions, companies, and nonprofits in the life sciences. He is based in Camarillo, California. Follow him on Twitter at @AMPorterfield or visit his Facebook page.