Transmission electron micrograph of Wolbachia within an insect cell. Wolbachia are a commonly found bacterial endosymbiont of insects, but a new study shows that, while Wolbachia may infect a wide variety of insect species, the rate of infection among individuals within those species is often low. (Image credit: Scott O’Neill, Public Library of Science, CC-BY-4.0)

Like all animals, insects carry within them a host of microbes that affect them in various ways. Entomologists and microbiologists study these microbes—known as endosymbionts—to better understand what role they play in insects’ lives and whether such microbes might be manipulated to affect insect populations in desired ways.

Three commonly studied bacteria, Wolbachia, Rickettsia, and Cardinium, are known to frequently affect reproduction in insects they infect. But the question of which insects they infect—and, importantly, which species and at what rate within those species—isn’t always clear.

This uncertainty became evident to Eric Sazama, Ph.D., as he completed his doctoral work at the University of South Dakota (USD). He and fellow researchers collected data from existing studies to build a dataset of Wolbachia infection among insect species, and they wanted to dig further into the data.

“Originally, we wanted to find out the overall impact that the presence of Wolbachia can have on an ecosystem,” Sazama says. “To do this, we needed to model the distribution of Wolbachia within the infected species. After modeling the distribution, we started to notice that Wolbachia, while infecting approximately 52 percent of insect species, may not be as common as that single estimate implies.”

Indeed, Sazama and fellow researchers Scot Ouellette, Ph.D., at the University of Nebraska Medical Center and Jeff Wesner, Ph.D., at USD, found that, in most insect species in their dataset (69 percent), less than half of the sampled individuals were infected with Wolbachia. A similar dynamic held for Rickettsia and Cardinium. Their findings were published this month in the journal Environmental Entomology.

The low rate of infection within insect species suggests that these bacterial endosymbionts aren’t likely to have strong effects on populations of most insect species, the researchers write. Sazama says the results also raise many questions for further study.

“These findings suggest that successful endosymbionts, such as Wolbachia, may be in a constant state of flux within many species of insects, since the vast majority of infected species were infected at a rate between 10 percent and 90 percent,” he says. “This leads to further questions on the evolution of resistance and persistence between the host and endosymbiont.”

Contributing to uncertainty about bacterial infection rates in insects, too, is the low sampling rate in much of the research Sazama and colleagues found. Many examples tested just one or a few individual insects within a species.

“We suggest more studies that investigate the presence of these endosymbionts in a greater variety of species with more individuals tested within those species,” Sazama says. “A problem with the current dataset is the lack of sample size in many species, especially in the lesser known endosymbionts like Rickettsia and Cardinium.”

Meanwhile, Sazama’s work examining Wolbachia on an ecosystem-level continues. “We also are still investigating the potential environmental impacts that Wolbachia has on ecosystems. Understanding this will allow us to see how vital of a role that endosymbionts have in ecosystems in general.”