Mosquito research by Maribel Portilla and Sharon Lawler recently included an experiment on how Egeria, or Brazilian water weed, affects mosquito populations. Egeria is a submersed aquatic weed that can cause problems in channels and irrigation waterways. Mosquitoes also breed in stagnant waters, but, we asked, is this because of the weeds?

We used large cattle-watering tanks to simulate stagnant, back-water areas with and without infestations of Egeria (Figure 1). Five tanks had no weeds, five had healthy Egeria, and five had Egeria that was sprayed with the herbicide fluridone. We added zooplankton and insects colonized naturally.

Figure 1. An Egeria flower in an experimental tank. Photo: Maribel Portilla.

Interestingly, there were fewer mosquito larvae in tanks with dense growths of Egeria compared to weed-free tanks. Because numbers of beneficial predators were similar among treatments, we hypothesized that the plants might be interfering with mosquito survival. We tested this idea by putting sets of 15 early-stage mosquito larvae into all tanks, in cages that excluded predatory insects.   The San Joaquin Mosquito and Vector Control District supplied the test mosquitoes.

Sure enough, on average half the number of mosquitoes survived in the tanks with Egeria as in the weed-free tanks (Figure 2). Egeria that was dying after fluridone treatment had a more moderate effect on mosquito survival. Egeria might discourage mosquito growth by competing for nutrients with the algae that mosquitoes eat. Alternatively, it might exude plant chemicals that harm mosquitoes. We are planning future research on these possibilities.

Figure 2. Mosquito larvae survival in tanks with and without Egeria.

Dr. Sharon Lawler and Maribel Portilla are professor and graduate student, respectively, in the Department of Entomology and Nematology, UC Davis. This study was conducted in cooperation with the Delta Region Areawide Aquatic Weed Project (DRAAWP).