A survey of mosquitoes in Pennsylvania discovered 12 mosquitoes parasitized by larval mites, such as this Aedes japonicus with a larval mite (red) attached to its head Water mites can reduce the survival and reproductive success of mosquitoes, and entomologists are investigating such parasitism to evaluate its potential as a method for biological control efforts. (Photo credit: Michael Hutchinson)

By John P. Roche, Ph.D.

The Asian tiger mosquito (Aedes albopictus) and the mosquito species Aedes japonicus (sometimes also known as the Asian bush mosquito) can spread viruses dangerous to humans. Both mosquitoes are invasive species in the United States. Some mosquito species have been found to be parasitized by larval water mites—tiny, brightly covered arachnids that live in fresh water habitats. Parasitism by water mites is significant because it can reduce the survival and reproductive success of mosquitoes.

Thomas Simmons, Ph.D., and Anna Manges of Indiana University of Pennsylvania and Michael Hutchinson of the Pennsylvania Department of Environmental Protection, were interested in whether or not Asian tiger mosquitoes and Asian bush mosquitoes are parasitized by water mites. To answer this question, they looked at thousands of mosquitoes in Pennsylvania and checked for the presence of water mites. The results of their study were published this month in the Journal of Medical Entomology.

Since 2000, the Pennsylvania Department of Environmental Protection West Nile Virus Control Program (PADEP-WNV) has collected mosquitoes to test for the presence of West Nile virus in mosquito populations. In 2016, Simmons and his team checked for the presence of water mites in a subset of the mosquito species collected by PADEP-WNV, focusing on species for which water mite parasites had not been seen before or had only rarely been seen. Simmons and his colleagues found that, out of 146,607 mosquitoes from these species examined, 12 were parasitized by mites. Of these 12, 10 were water mites and two were terrestrial mites.

This was the first time that a water mite was observed on an Asian tiger mosquito and only the second time a water mite was observed on an Asian bush mosquito. “Parasitism of invasive mosquito species of public health significance is the most important aspect of our discovery,” says Simmons. “This is particularly interesting since the water mites are likely native species not imported with their invasive hosts.”

Most surprising, Simmons says, “is that parasitism of Aedes albopictus and Aedes japonicus by water mites has not been previously documented, at least not in the published literature, given the large number of these mosquito species that are collected for disease surveillance. Parasitism of other Aedes species by water mites has been previously and not uncommonly documented in the literature.”

One factor that makes this research important is that water mites can affect the survival and reproductive success of mosquitoes. How water mites do this, Simmons says, “is by imbibing and engorging on body fluids from the host. This may interfere with water balance, and, due to the size or density of water mites, they may physically interfere with flight.” Thus, Aedes populations that are parasitized by water mites can have reduced success, which in turn might reduce disease transmission. As a result, water mites offer an avenue to explore for potential biological control.

“But, in order for water mites to adversely impact their mosquito hosts,” Simmons says, “the intensity of parasitism has to be fairly high.” The intensity of water mite parasitism of Aedes mosquitoes is typically low, as seen in this study, so to serve as a useful biological control agent, parasitism would have to be augmented to be considered. “But water mites might be helpful allies in the field on their own,” Simmons says, “especially against invasives that might not have natural defenses against our native mites.”

When asked about next steps in research building on these findings, Simmons says, “The first step is to determine if these associations are more common than they appear to be based on the paucity of reports in the past. This year we have intensified our surveillance and are screening for water mites on all of the Aedes albopictus and Aedes japonicus mosquitoes that are collected by PADEP-WNV, not just a subsample. Simmons and colleagues have already found another Asian tiger mosquito and several more Asian bushmosquitoes parasitized by mites.

The results of this study are an incremental advance in knowledge of great interest. But the field of study as a whole is of immense significance because, with the increase in international travel and trade causing introduction of invasive mosquito species and the increase in global temperatures expanding the range of many insects that transmit human disease, an in-depth understanding of mosquito community ecology is essential for monitoring and management plans.

John P. Roche, Ph.D., is an author, biologist, and science writer dedicated to making rigorous science clear and accessible. He has published 190 articles, authored books, participated in numerous research and curriculum-development projects, and written and taught extensively about science. For more information, visit https://authorjohnproche.com.