ABSTRACT: An increase in number of Culex spp. found in CDC light traps in 1995 over the previous two years in spite of drought conditions led to a search for possible reasons for this change. One factor that stood out was that the increased number of Culex spp. seemed to be associated with bodies of water where water chestnut was abundant. The Lakes District of the Charles River in Newton and Waltham, Massachusetts was chosen as a survey site, and mosquito larvae were sampled from areas of either high water chestnut and high water lily densities in 1995 and 1996. It was found that mosquito larvae were abundant in areas of high water chestnut density and infrequent or missing in all other areas. Larvae were collected in 1996 and identified to species. All were found to be Uranotaenia sapphirina.

INTRODUCTION

Trapa natans is an old world water chestnut species that was brought to New York State in the late 1800s as an ornamental plant. It grows well in shallow nutrient-rich lakes and rivers (Methe et al. 1993). In 1983 a survey of aquatic vegetation in the Lakes District of the Charles River did not list water chestnut among the plants found in the area (MDC 1994). By 1994, a second survey found water chestnut to be one of the most common plant species in this area, along with fanwort, Eurasian milfoil, and white water lily. All but the white water lily are introduced species (MDC 1994).

The water chestnut is an annual plant that germinates from seeds in the early spring. The stem is cord-like and can grow as long as 16 feet. The plant has two types of leaves: submerged and floating. The submerged leaves are paired and feathery. The stalks of the floating leaves contain air bladders; the leaves form rosettes on the water surface. Each seed gives rise to 10-15 rosettes, and each rosette can produce up to 20 seeds. This gives the water chestnut an amazing ability to rapidly spread throughout a body of water. Water chestnuts begin to flower in July and the chestnuts ripen in about a month. The plants continue to flower until killed by frost. Mature nuts sink to the bottom of the lake or river where they overwinter.

In 1995 we noticed that over the course of a few years the number of Culex spp. in our CDC light traps had been rising although several years of drought conditions had resulted in a negative impact on catch basins and permanent bodies of water in our district (Fig. 1). In fact, in 1995 the number of Culex spp. in traps adjacent to certain sites had nearly doubled over the previous two years (Fig. 2). Two things stood out while searching for a reason for this increase: a warmer than normal winter temperature for the '94-'95 winter season, and an increase in the amount of water chestnut in some of the sites that produced Culex spp. Mild winter temperatures could conceivably decrease the amount of mortality in overwintering adult mosquitoes, leading to an increase in the breeding population the following spring; but this was not easily testable. Instead, what we chose to study was the idea that increased oviposition sites created by an increase in vegetation at these sites could also lead to an increase in the population of Culex spp. It was felt that water chestnut could provide additional oviposition sites for Culex spp. because, although the plant grows in relatively deep water, shallow water pockets are formed between the surface leaves and the submerged leaves. The dense mats of water chestnut trap organic matter, providing not only shelter but also a food source for developing mosquito larvae. This would not be the case for plants such as the water lily, that only has surface leaves, and water milfoil, that is mostly subsurface. Duckweed, also very common at these sites, has been found to decrease oviposition by Culex spp. The reasons for this decrease may be mechanical, chemical, or both (Rajendran & Reuben 1991; Eid et al. 1992).

MATERIALS AND METHODS

We used a canoe to access water chestnut sites on the Charles River. We sampled in several different types of vegetation: low density of water chestnut, high density of water chestnut, water lily, and mixed other vegetation. We measured water depth at each of these sites. Larvae were collected at several different sites and taken back to the lab for identification.

RESULTS AND DISCUSSION

We began our sampling on 30 Aug. 1995. In areas with other vegetation, mostly water milfoil and duckweed, we found no larvae in 55 dips. In 50 dips in dense water lily mats, we found no larvae and 1 pupa. In areas of low water chestnut density (a few plants), we found one 2nd stage larva in 25 dips. In areas of high water chestnut density, with many overlapping layers of plants, we found 52-1st stage larvae, 38-2nds, 17-3rds, 2-4ths, and 12 pupae for a total of 109 larvae and 12 pupae in 45 dips (Fig. 3). Water depth at all of these sites was approximately 4 feet. We wanted to bring some of these larvae back to the lab to identify, but found that the collection jars had not been placed in the canoe. As this was the end of our adult trapping season, and mechanical harvesting of the water chestnut had started, we decided to continue the study in 1996.

We first went out on the Charles on 3 July 96, but found no mosquito larvae. We went out again on the 23rd of July, but still found no larvae. We finally started finding mosquito larvae in early August, but in the meantime, the MDC had started harvesting the water chestnut in an attempt to eliminate or contain the spread of this species. Mechanical harvesting is one of the most effective ways of controlling water chestnut (MDC 1994) so that they do not block waterways. We sampled every week for the next 5 weeks, often running just ahead of the mechanical harvester, and again found mosquito larvae, although in smaller numbers, only in areas with dense mats of water chestnut. These areas ranged in depth from 1-10 feet. In all, we took 250 samples in water chestnut and found 37 mosquito larvae. We took 100 samples in water lily mats and found no mosquito larvae (Fig. 4). The number of Culex spp. found in the light traps in 1996 was higher than the year before (Fig 1), but the number of Culex spp. found in traps adjacent to known water chestnut sites had decreased (Fig 2). If Culex spp. are using water chestnut as oviposition sites, the possible reasons for this decrease are: (1) increased and earlier harvesting of the water chestnut, (2) increase in other oviposition sites due to increased rainfall throughout the season, and (3) decrease in local overwintering populations due to increased snow and cold in the '95-'96 winter season. This year we did remember to take collection jars out in the canoe and found that all the mosquito larvae we collected were Uranotaenia sapphirina, a bird feeding species of little economic importance. So, do Culex spp. use water chestnut as oviposition sites? We can prove that water chestnut provides habitat for any number of aquatic species including Uranotaenia sapphirina. But, although we can't rule out the possibility of Culex spp. using these sites, we also haven't proven that they do.

ACKNOWLEDGMENTS

I'd like to thank my coworker, Eric Rastauskas, who not only helped with the sampling, but also provided the canoe used in the study, and the motor so we didn't have to paddle up and down the Charles.

LITERATURE CITED

Eid, M.A.A., M.A.E. Kandil, E.B. Moursy and G.E.M. Sayed. 1992. Studies on isolating insecticidal components from the duck-weed, Lemna minor, effective against the mosquito, Culex pipiens pipiens. Bull. Ent. Soc. Egypt. Econ. 19:107-114.

Methe, B.A., R.J. Soracco, J.D. Madsen and C.W. Boylen. 1993. Seed production and growth of water chestnut as influenced by cutting. J. Aquat. Plant Manage. 31:154-157.

Rajendran, R. and R. Reuben. 1991. Evaluation of the water fern Azolla microphylla for mosquito population management in the rice-land agro-ecosystem of south India. Med. Vet. Entomol. 5:299-310.

The Metropolitan District Commission (MDC). 1994. A Survey of the Aquatic Plant Community of the Charles River Lakes District, Waltham and Newton, Massachusetts. Aquatic Control Technology, Inc. and Fugro East, Inc., Northborough, MA.

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