Assessing target and non-target effects of aquatic weeds and weed control operations on mosquitoes and other organisms
Dr. Sharon P. Lawler, Professor of Entomology and Nematology, University of California, Davis
To improve mosquito and vector control activities by quantifying how abundances of mosquitoes and beneficial aquatic invertebrates respond to living versus decaying, herbicide-treated water hyacinth (Eichornia crassipes) and Brazilian water weed (Egeria densa).
- Use replicated artificial wetlands to compare abundances of mosquitoes and beneficial invertebrates (insects, amphipods and etc.) in beds of water hyacinth (Eichornia crassipes) vs Brazilian water weed (Egeria densa) vs areas of open water.
- Test whether mosquitoes change in abundance in aquatic weeds that have been treated with herbicide, and quantify any impacts of herbicide treatments on beneficial macro-invertebrates.
- Survey mosquitoes and other invertebrates in replicated areas of the Delta that have either dense growths of water hyacinth and Brazilian water weed or decaying, herbicide-treated weeds.
This project is aimed at advancing ecological understanding of the role of invasive aquatic weeds in influencing the population sizes of potentially disease-vectoring mosquitoes and beneficial aquatic invertebrates (insects and others). Results will be of interest to mosquito and vector control districts, boaters and other private citizens, public health agencies and aquatic ecologists. The Culex mosquitoes found breeding in Delta waters can vector West Nile virus, St. Louis Encephalitis virus, and other pathogens to humans, livestock, and wildlife.
Aquatic plants provide food to animals and they shelter a variety of organisms, including larval fishes and aquatic insects, other arthropods, worms, and snails. However, when they become dense and weedy they block waterways and potentially promote larger populations of mosquito larvae by giving them places to hide from beneficial predators.
There have been few studies that compare different aquatic weeds with regard to mosquito harborage or with regard to providing habitat for the insects that fishes rely upon. Virtually no studies address how herbicides and aquatic weed decomposition affect macroinvertebrates. We hypothesize that water hyacinth will harbor more mosquitoes than Brazilian water weed or open water because it creates ‘pockets’ of water at the surface where mosquitoes can escape predators. We also hypothesize that herbicide applications may temporarily benefit mosquito larvae as weeds decompose. Decomposing weeds may deplete dissolved oxygen in the water, killing beneficial species or driving them away. However, larval mosquitoes breathe air through ‘siphons’ and so are not vulnerable to low dissolved oxygen in the water.
Mosquitoes and beneficial invertebrates are often patchy in natural habitats, which can make it difficult to understand which factors drive their abundances. To produce quantitative studies of aquatic systems, many scientists use replicated artificial ponds, or `mesocosms'. We are performing mesocosm experiments that will allow us to measure differences in abundances of mosquitoes and other aquatic invertebrates in beds of two kinds of invasive weeds (water hyacinth and Brazilian water weed) and in open water, and to estimate the effects of herbicide treatments on the survival and growth of mosquito larvae (Figure 1). In 2015, with assistance from U.S.D.A., The U.C. Davis Putah Creek Reserve, the San Joaquin Mosquito and Vector Control District, and CA Boating and Waterways, we prepared 35 water tanks as mesocosms (each held 1,325 liters). Ten each had heavy growths of water hyacinth or Brazilian water weed, and 15 were weed-free. After insects colonized, we began weekly sampling for mosquitoes and other invertebrates, and we used caged mosquito larvae to measure mosquito survival and growth. This allowed us to see if mosquito abundances differed between weed species or open water.
Soon after mesocosm establishment and before weeds were sprayed, we found more mosquito larvae in mesocosms with weeds, as expected. After several initial samples were completed, we applied glyphosate to five of the water hyacinth tanks and five weed-free tanks, and applied fluridone to five of the Brazilian water weed tanks and five weed-free tanks; unsprayed tanks of each type served as controls. We continued to measure invertebrates and the growth and survival of mosquitoes in all tanks over the next few weeks. Afterward, we removed wild mosquitoes with a short-acting larvicide and put nets over the tanks, so that we could measure the growth and survival of a standard-sized group of mosquito larvae in each tank when weeds were actively decomposing. Preliminary results for this part of the study showed that about two weeks into weed decomposition, there were many more mosquito larvae in decaying water hyacinth than in living water hyacinth (Figure 2; difference was statistically significant). Results were inconclusive for Brazilian water weed and so part of the experiment will be repeated in 2016.
To determine broad-scale associations between vector mosquitoes, other taxa, and weeds, in 2016-2017 we will conduct surveys of mosquito larvae and other invertebrate within weed beds in approximately 20 backwater channel areas in the Southern Delta, again focusing on water hyacinth and Brazilian water weed. We will also run light traps to collect adult mosquitoes in these areas. We will survey weed beds of each type at least three times before and after scheduled herbicide treatments applied by California Boating and Waterways, as well as surveying untreated sites for each weed. Post-treatment samples will allow us to document changes in mosquitoes and other invertebrates.
In combination, mesocosm experiments and surveys will advance knowledge about the kinds and abundances of larval mosquitoes that exploit various aquatic macrophytes, as well as the effects of weed treatments on mosquito abundances. Additionally, data on other macroinvertebrates will enhance understanding of the ecology of the Delta. We anticipate that our results will be of great interest to Mosquito and Vector Control Districts, other public health agencies, environmental agencies, and many private citizens.