A mosquito native to Missouri can thank the larvae of an invasive intruder for keeping parasites away, say researchers.
“The extra energy put into fighting an infection, or lost to consumption by a parasite, can lead to changes in behavior in the host. That can change its ability to escape predation or compete for space and resources,” says first author Katie M. Westby, postdoctoral research associate at the Tyson Research Center at Washington University in St. Louis.
“Thus, if an invasive species reduces parasitism in a species in a community, it may indirectly affect other members of the community.”
For the study, which appears in the Journal of Animal Ecology, researchers compared the parasite loads of members of the local eastern tree-hole mosquito species (Aedes triseriatus) with members of an invasive Asian rockpool mosquito species (Aedes japonicus), which scientists first spotted in Missouri in 2005.
Both are container-breeding mosquitoes, meaning you can find their eggs and larvae in water-holding containers such as tires and buckets.
For the field experiment, researchers set up a mosquito nursery with 40 buckets of rainwater with varying levels of leaf debris. Three times a week, they sieved through some of the buckets of muck to remove the larvae of the invasive species that hatched. They left the others alone.
In the water, mosquito larvae encounter a kind of parasite called a gregarine parasite when the larvae are filter-feeding. But this parasite is species specific, which means that if the wrong kind of mosquito larvae vacuums it up, it can’t complete its life cycle. And apparently that’s exactly what’s happening.
“We found the presence of the invasive rockpool mosquito significantly reduced parasite prevalence in the native tree-hole mosquito via a dilution effect,” says Westby, who dissected the mosquito larvae to look for the parasites in their midguts.
Invasion and infection
When the native mosquitoes were sharing space with invasives, parasites infected the natives 13 percent to 27 percent of the time; when the natives were alone, they had parasites 72 percent to 90 percent of the time.
Scientists call it “encounter reduction.” Quite simply, if the invasive eats the parasite first, the native never runs into it.
“Understanding the patterns and processes that manifest in infectious disease is fundamentally an ecological question—that is, it’s about species interacting with one another and with their environment,” says biologist Kim Medley, director of Tyson Research Center.
“In this case, invasion reduced infection by reducing encounters with parasites, but not by altering the abundance of the native species; rather, the mechanism was through removal of infectious particles from the habitat.
“Studies like ours can broadly inform how infectious disease manifests and how it changes with changes in biodiversity. This concept can be applied to numerous systems, including infectious disease in humans.”