06
Sep
Invasive Crayfish Found to Increase Mosquito Populations
(Beyond Pesticides, September 6, 2018) Red swamp crayfish have long been associated with Louisiana and Gulf Coast bayous, but the species has found new homes throughout the world over the last 70 years, and evidence is showing that their introduction may alter ecosystems in ways that increase mosquito populations and human disease risk. Â In late August, researchers at the University of California, Los Angeles and Pepperdine University published new evidence in the journal Conservation Biology associating non-native crayfish in California streams with declines in dragonflies that would otherwise eat local mosquito populations. As mosquito species and disease vectors shift in response to a changing climate, influxes of non-native species and their impacts on ecosystems add another layer of complexity to communities aiming to manage mosquitoes and other vector-borne diseases.
Red swamp crayfish (Procambarus clarkii) are native to the U.S. Gulf Coast, but as a result of the aquarium pet trade, and their use as fish bait, have established opportunistic populations in wild ecosystems throughout the world – in every continent except Australia and Antarctica. In Southern California, researchers observed higher levels of mosquitoes in streams where the non-native crayfish have been found. To further understand the impact of this species on the native ecosystem, both laboratory and field studies were initiated.
Streams in Southern California were sampled, finding a strong correlation between the presence of crayfish and the number of mosquito and dragonfly nymphs, which are major predators of mosquito larvae. In streams where only dragonfly nymphs were found, no mosquito larvae were discovered. However, in steams that had crayfish present, both mosquito and dragonfly larvae were found.
Researchers then looked into whether the presence of crayfish was depressing the ability of dragonflies to hunt mosquito larvae. Laboratory tests found while dragonfly nymphs consumed roughly 70% of mosquito larvae provided when alone for a 60 minute period, when in the presence of crayfish, this number dropped to seven percent. Crayfish alone only consumed 12% of mosquito larvae provided, a fraction of what the dragonflies controlled.
In the lab, dragonflies were closely observed when they were put in tanks alongside crayfish. Researchers found that, rather than adjust their behavior based on the novel species, dragonfly nymphs exhibited what could be considered fear and confusion. Dragonflies were generally inactive when in the presence of red swamp crayfish, either hiding in the corner, or resting on crayfish claws or body.
The sum of this study points to significant concerns for disease management and ecosystem stability in Southern California streams and waterways. In the study, researchers either observed crayfish in a stream, or did not – there were no streams where crayfish were found at one location in the stream but not another. However, at each stream where crayfish had opportunistically colonized the area, there were higher levels of mosquito larvae, and lower levels of their main predator, the dragonfly nymph.
By depressing dragonfly predation, thereby increasing mosquito populations, non-native red swamp crayfish precipitated a trophic cascade that could lead to increased rates of human disease transmission. Beyond Pesticides recently covered pesticide-mediated trophic cascades in the Pesticides and You article Pesticide Use Harming Key Species Ripples through Ecosystem. In the case of California ecosystems, non-native crayfish are degrading the ecosystem, and causing stress much like the influx of a toxic pesticide.
However, while there are answers available that can readily replace toxic pesticide use and its ability to contaminate ecosystems through alternative practices and products, species removal is a much more difficult undertaking. But there are novel options being considered for this particular species. In Germany, for instance, the city of Berlin has recently licensed the wild catch of crayfish for restaurants.
Ultimately, this issue will need to be another consideration for mosquito managers as part of an integrated mosquito control framework. For instance, without natural dragonfly predators, streams that are found to contain crayfish should be made a priority target for use of a least-toxic larvacide like bacillus thuringiensis.
For more information on how to address mosquitoes and their diseases safely in your community and backyard, see Beyond Pesticides’ Mosquito Management homepage.
All unattributed positions and opinions in this piece are those of Beyond Pesticides.
Source: National Geographic, Â Conservation Biology
