Invasive aquatic weeds influence abundances of larval mosquitoes and other invertebrates

Authors: PORTILLA, MARIBEL - University Of California, Davis; Moran, Patrick; LAWLER, SHARON - University Of California, Davis

Submitted to: Journal of Aquatic Plant Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/3/2020
Publication Date: 8/31/2021
Citation: Portilla, M.A., Moran, P.J., Lawler, S.P. 2021. Invasive aquatic weeds influence abundances of larval mosquitoes and other invertebrates. Journal of Aquatic Plant Management. 59s:33-40.

Interpretive Summary: The 68,000-acre Sacramento-San Joaquin River Delta of northern California consists of the freshwater floodplain where these two large rivers meet. The original marshlands and tidal channels have been drained and modified into a complex system of canals, sloughs, small lakes, farmland, small towns and cities, and remnant wetlands. The freshwater Delta provides irrigation water for over $30 billion in crops in the Delta and Central Valley, provides drinking water for 27 million people, supports $300 million in recreational boating, and contains two major deepwater shipping channels. Non-native aquatic plants have invaded the Delta, blocking access to water for boating and fishing, and reducing business for marinas and supporting industries. These weeds also block water flow to pumping facilities, impede flood control systems, and displace native plants and animals. Water hyacinth, a floating aquatic weed, forms dense mats on the surface, while Brazilian waterweed or egeria roots on the bottom of the water column and grows up to cover the water surface. The Contra Costa and San Joaquin County Mosquito and Vector Control Districts, among others, are concerned that dense aquatic weeds provide habitat for mosquitos, including those in the genus Culex that can transmit West Nile Virus to animals and humans. In this study, hyacinth and egeria were planted in large (350-gallon) tanks at a research farm in Davis, CA located near natural mosquito habitat and were allowed to grow to cover the water surfaces of the tanks, while other tanks were left with no plants and open water. Larval mosquitos and their competitors and predators were then sampled once per week for 11 weeks. In the early part of the study (3rd to 6th weeks), contrary to expectation, mosquitos were 10-fold less abundant in hyacinth tanks and 20 to 30-fold less abundant in egeria tanks compared to open water tanks. Competitors (other aquatic soft-bodied immature insect larvae that feed on the same things mosquito larvae do, and larvae of other invertebrates) were three- to six-fold less abundant in hyacinth compared to open water and egeria treatments and aquatic larval flies of a representative competitor family-Ceratopogonidae-were up to 14-fold less abundant in the 3-6 week period. Predators, such as dragonfly larvae, did not vary among treatments in the early weeks, and there were no differences among tanks in mosquito, competitor, or predator abundance in the last four weeks of the study (weeks 8-11). Contrary to expectation, dense healthy aquatic weeds did not favor adult egg-laying by mosquitos or the development of dense mosquito larval populations. The results suggest that mosquito vector control districts should focus first on open still water when targeting aquatic habitats for mosquito control with natural or synthetic 'larvicides'. Additional ongoing studies are examining the effect of herbicide application and subsequent slow death and decay of hyacinth and egeria on mosquito populations.

Technical Abstract: Many aquatic plants provide landscape and habitat structure that affects aquatic invertebrates, including mosquitoes. As invasive aquatic weeds, water hyacinth or hyacinth (Eichhnornia crassipes) and Brazilian waterweed or egeria (Egeria densa) can modify and dominate aquatic habitats, altering natural ecosystems. Invasive aquatic vegetation could play a role in determining larval habitat quality for mosquitoes, but this topic is largely unexplored. Some invasive aquatic weeds harbor mosquito species, while others may repress them. In this mesocosm study, we measured wild larval mosquito abundances, along with numbers of naturally recruited predators and competitors, in three different aquatic habitat types: open water, water covered with hyacinth, or water with egeria. Both early (3 to 6 weeks) and late (8 to 11 weeks) in the development of aquatic weed populations in mesocosms, we found abundant larval mosquitos in open water. Abundance was 10-fold lower in hyacinth and 20- to 30-fold lower in egeria. Competitors were three- to six-fold less abundant in hyacinth compared to the other treatments, and larval flies of a representative competitor family-Ceratopogonidae-were up to 14-fold less abundant in the 3-6 week period. Predatory insects did not vary substantially among treatments in the early weeks, but were over three-fold more abundant in tanks with Egeria than in tanks with hyacinth in weeks 8-11. Healthy water hyacinth decreased habitat quality for larval mosquitos and their competitors. The results have implications for targeting of sites for mosquito control in relation to aquatic weed invasions.