As tiny plastic particles infiltrate seemingly every environment on Earth, researchers are examining their effects on creatures big and small. A new study finds mosquito larvae may be resilient to microplastic consumption, but further research is needed. Shown here are larvae of mosquitoes in the genus Culex. (Photo by James Gathany, CDC Public Health Image Library)

By Timothy W. Schwanitz

Timothy W. Schwanitz

We live in the era of microplastics: These small plastic particles can be found in the most far-flung parts of the globe, like remote regions in Antarctica. The omnipresence of microplastics raises many questions about how organisms will react.

Mosquitoes are an interesting group for investigating these questions, since some are particularly prone to encountering humanity’s petroleum products—their aquatic larvae often breed in water in plastic containers, which may have higher-than-background concentrations of microplastics. A recent study shows that it is even likely that mosquitoes transmit small quantities of microplastics to humans when they bite humans, underscoring the extent to which these piecemeal plastics dominate our environment.

Does the consumption of all these microplastics impact the health of the mosquito larvae? Two researchers at the University of British Columbia, Markus Thormeyer and Michelle Tseng, Ph.D., set out to address this question in their short communication published in February in the Journal of Medical Entomology. They used common fitness measures in the mosquito species Culex pipiens and Culex tarsalis to assess how different concentrations of plastics influence development.

Investigating the Effects of Microplastics

Markus Thormeyer

The authors tested the effects of microplastic exposure by tracking common mosquito fitness metrics: development time, growth rate, and wing length (a proxy for body size). To isolate the effects of microplastics without any confounding effects from group rearing or plastic containers, the researchers took on the task of rearing 120 larvae individually in glass beakers. “Caring for them was a bit monotonous at times, but not too bad if I threw a podcast on,” says Markus Thormeyer, a Ph.D. student and first author on the study.

The authors chose zero microplastic particles per milliliter (MP/mL) as a negative control, and they selected 20,000 MP/mL as their high concentration because it is commonly used in studies investigating the effects of microplastic consumption in mosquitoes. However, what relation does 20,000 MP/mL have to real-world concentrations of microplastics?

Thormeyer and Tseng did some digging in the literature and decided that 200 MP/mL more accurately reflects the microplastic particle concentrations that mosquito larvae are likely to encounter in nature, and so they chose 200 MP/mL as their low concentration treatment.

The authors reared 120 mosquito larvae individually in glass beakers to isolate the effects of different microplastic concentrations on their development. (Photo courtesy of Markus Thormyer)

So, why have previous studies used concentrations orders of magnitude higher? Thormeyer explains that very high experimental concentrations make sense given how new the field of microplastics research is.

“I think using environmentally unrealistic doses of microplastics is called for in experiments where you want a proof of concept or you’re looking to limit-test at which dose we might see an effect,” he says.

In this study, however, the authors wanted to instead get a handle on the effects of more realistic microplastic concentrations.

Some prior experiments had exposed mosquitoes to microplastics only as older third- or fourth-instar larvae. To more accurately reflect natural conditions, Thormeyer and Tseng exposed their mosquito larvae as first instars, one day after hatching. Additionally, the investigators hatched mosquito eggs collected from the wild rather than using eggs from lab-reared colonies.

The researchers found that microplastic consumption did not impact the three measured mosquito fitness metrics. Neither the high nor the more realistic low microplastic particle concentrations affected development time, growth rate, or wing length. The authors also did not see any sex-specific differences resulting from the effects of different microplastic concentrations.

Future Directions for Microplastics and Mosquitoes

This study’s findings conflict with some others. A different study by researchers in Brazil found that fourth instar Culex quinquefasciatus mosquitoes exposed to 20,000 MP/mL exhibited delayed growth.

One potential reason for the discrepancy lies in the different size and shape of microplastic particles used in the two studies. Thormeyer and Tseng employed one size of spherical beads, but, as Thormeyer points out, “In the environment, we can see all kinds of shapes like fibers, fragments, and films. I think incorporating different-shaped microplastics into microplastic-exposure experiments could produce some pretty cool results.”

In addition to particle size and shape, temperature could also affect mosquito larval development. “Other studies have shown that warming increases resource consumption rates. We predict that organisms will have elevated chances of consuming microplastics at warmer temperatures,” the authors note.

There are also a lot of interesting potential research avenues in physiological metrics to assess the effects of microplastics—as Thormeyer and Tseng explain in their paper, small effects not seen in developmental metrics can be measured with physiological metrics such as immune responses. Changes in immune responses are especially relevant in mosquitoes, as these changes could potentially alter how mosquitoes transmit pathogens.

Timothy W. Schwanitz is an insect neuroethologist currently pursuing a Ph.D. in the McBride Lab at Princeton University. He majored in English and entomology during his undergraduate studies because of his broad interest in both books and bugs. Email: ts5103@princeton.edu.