Invasive Crayfish Increase Number of Mosquitoes in Southern California Mountains

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A new study published in Conservation Biology ties the presence of invasive crayfish to higher numbers of mosquito larvae within the Santa Monica Mountains, CA. The area is only about an hour away from the heart of Los Angeles, and is home to diverse wildlife, which already face threats from urbanization and a shifting climate.  

Non-native species can throw ecosystems into disarray by replacing native species without filling their roles in the environment. This can have serious consequences for the various services that healthy ecosystems provide for people — including disease control.

To determine whether a similar scene was playing out in southern California, researchers from the University of California, Los Angeles, U.S. Geological Survey, and Pepperdine University, along with collaborators at the Las Virgenes – Triunfo Joint Powers Authority and Mountains Restoration Trust investigated the effect that invasive red swamp crayfish (Procambarus clarkii) have on native, juvenile dragonflies and mosquito larvae within the Santa Monica Mountains National Recreation Area. Within local streams and pools, juvenile dragonflies, or dragonfly “nymphs,” typically play a key role in consuming mosquito larvae and reducing their numbers.

Dragonfly nymph

Photo of a dragonfly nymph in a pond. Dragonfly nymphs are known predators of mosquito larvae, and play a critical role in controlling mosquito populations in California's Santa Monica Mountains (Credit: Chris Brown, USGS Western Ecological Research Center. Public domain.)

Had this changed with the introduction of the red swamp crayfish? Which is the deadlier predator — crayfish or dragonfly nymph?

“We had an idea of what the answer might be, because natural vernal pools in southern California tend to have a lot of dragonfly nymphs, no crayfish, and no mosquito larvae,” says Robert Fisher, USGS research biologist and co-author on the paper.

The scientists led field surveys and collected crayfish and dragonfly nymphs, bringing them into the laboratory to test whether crayfish affected nymph and mosquito larvae abundance, and dragonfly nymphs’ efficiency as predators. In addition, field crews spent months in the Santa Monica Mountains, carefully recording abundances of dragonfly nymphs and mosquito larvae in streams with and without the red swamp crayfish. Back in the laboratory, other researchers watched as crayfish and dragonfly nymphs hunted mosquito larvae separately and together.

The results were striking. On average, dragonfly nymphs in the lab consumed 70 percent of all mosquito larvae within the first hour. When crayfish were the primary predator, or when dragonfly nymphs and crayfish were placed in the same tank, they consumed only 12 and 7 percent of mosquito larvae in the same time period. Within two days, the crayfish had consumed the dragonfly nymphs themselves.

Out in the streams and pools of the Santa Monica Mountains, the team noted that freshwater bodies with red swamp crayfish tended to have far fewer dragonfly nymphs and many more mosquito larvae.

The red swamp crayfish is native to the southern and southeastern United States. Since its introduction to California in the early 1900s, this vibrant crustacean has invaded streams and displaced top predators in aquatic ecosystems. Its presence could be particularly problematic in urbanized streams with slow-moving or still water, where mosquitoes thrive.

Anopheles larva

Photo of an Anopheles mosquito larva, a genus of mosquitoes found in the Santa Monica Mountains. (Credit: Harry Weinburgh, CDC. Public domain.)

“Sometimes people are not overly concerned about invasive species, but this study nicely illustrates that disruptions produced by invasive crayfish have the potential to come back and negatively impact human health,” says Lee Kats, Vice Provost for Research and Strategic Initiatives, Pepperdine University, whose decade-long study of Santa Monica Mountain streams led to the lab and field experiments.

“This study is important because it informs us of the unique way native predators can naturally keep mosquitoes from causing huge public health risks. It’s a testament of how strong our ecosystems are when they function normally, without introduced crayfish and why protecting natural resources is so important,” says Gary Bucciarelli, lead author and research director of the Stunt Ranch Reserve at UCLA.

In environments where invasive crayfish can directly and indirectly lead to increased mosquito populations, these results can inform the management of freshwater habitats for biodiversity and a natural source of disease control.