Journal of the American Mosquito Control Association, 19(4):361-366'2003 Copyrighi @ 2003 by the American Mosquito Control Association' Inc'

THE IMPACT OF WETLAND VEGETATION DRYING TIME ON ABUNDANCE OF MOSQUITOES AND OTHER INVERTEBRATES MICHELLE

R. SANFORD,T JOE B. KEIPER'

n'ro WILLIAM

E' WALTONT

down emerABSTRACT. Vegetation management for constructed treatment wetlands often involves knocking practices gent vegetation witf,heavy equiptient and inundating the dead vegetation after a period ofdrying. Such drying of. the length between relationship the We studied mosquitoes. create favorable conditions foi la.uul period for an emergent macrophyte, Typha-sp., and the abundance of aquatic inveftebrates in replicated 0.18was significantly more abundant in pools containing vegetation m, wading pools. The mosquiio,'Cutei'tarsafis, pools containing vegetation aged 5 wk, freshly cut vegetation, or to compared inundation for i wk before aged riithout vegetation. Potential larval mosquito food resources (particles between 2 and 6l pm in equivalent the spherical di-ameter) in the 2-wk aging treatment did not differ significantly from the other treatments during 5-wk experiment. The abundance of other larval culicids, nonculicine Diptera, and potential mosquito predators (i.e., Dyiiscidae and Aeshnidae) did not differ significantly among the vegetation aging treatments. KEY

WORDS

Culex, ce:nstntcted wetlands, vegetation management, Diptera, colonization

INTRODUCTION Constructed wetlands located near housing developments and densely populated areas can produce large mosquito populations, which cause a nuisance and may pose a serious health hazard as vectors of pathogens causing diseases in humans (Walton et al. 1998, Russell 1999, Walton 2002). In addition to providing water treatment, man-made wetlands may be used to enhance wildlife habitat and therefore maintaining the diversity of inveftebrate populations, a major food source for migratory waterfowl, is a goal of wetland managers. Large areas of senescing emergent vegetation can reduce the efficacy of constructed wetlands to purify water (Sartoris et al. 2000) and may lower habitat quality. Vegetation management in wetlands may be needed to maintain a properly functioning wetland as well as to control mosquito populations (Scholssberg and Resh 1997, Thullen et al.2OO2). Vegetation management practices such as mowing and discing may leave large mats of senescent vegetation in wetlands that provide a refuge for mosquitoes after inundation (Schaefer and Miura 1985). Berkelhamer and Bradley (1989) suggested that dense mats of rotting vegetation provide nutritional enhancement to developing larvae. Plant decay also may attract gravid mosquitoes. Infusions of decaying vegetation, such as Bermuda grass (Isoe et al. 1995a), hay (Hazard et a|. 1967), and bulrush (Walton and Workman 1998) attract gravid Culex. Organic enrichment sustained oviposition and larval production in field microcosms (Rodcharoen et al. 1997). Hazard et al. (1967) demonstrated that bacteria are responsible for the ovipositional attractancy of these infusions. Vegetation management at the Prado Constructed Wetlands in western Riverside Countv. California. I Department of Entomology, University of California, Riverside, Riverside, CA 92521. 2 Department of Invertebrate Zoology, Cleveland Museum of Natural History, Cleveland, OH 44106.

usually involves draining marshes, knocking over the vegetation with heavy equipment, drying the vegetation, and then inundating the dried plant material. Although these practices are thought to improve water-quality performance of treatment wetlands by enhancing denitrifying bacteria populations, larval mosquito abundance in 3- and 5-ha marshes at the Prado Constructed Wetlands increased significantly after vegetation management (Keiper et al. 2003). In this study, we examined the relationship between the length of time that vegetation is dried before inundation and the abundance of mosquitoes and other invertebrates in small plastic wading pools.

MATERIALS

AND METHODS

Study site: This study was conducted at the Prado Constructed Wetlands during August and September 1999. The Prado Wetlands consist of approximately 50 marshes and ponds encompassing 186 ha. The wetlands were designed to remove nitrate from the Santa Ana River (Mills et aI. 1998). Emergent vegetation is dominated by bulrush (Schoenoplectus califurnicus (Meyer) Sojdk) and cattails (Typha sp.). Vegetation aging experiment: We used a randomized complete block design consisting of 4 treatments within each of 5 blocks of small plastic wading pools (diameter : 107.3 cm, depth : 2O.3 cm) situated on berms adjacent to the marshes at 5 sites. Three vegetation treatments were created by using l-m-long stems of cattails that had been aged in the field for 5 wk, 2 wk, and 0 wk (fresh). Plant material was aged in full sun on the banks of the marshes. To add equivalent plant biomass to each pool, 5 stems (1 m long) of Typha sp. were added to the most aged treatment (5 wk), 4 stems were added to the intermediate treatment (2 wk), and 3 stems were added to the least aged treatment (0 wk). The control treatment assessed mosquito abundance in the water used to fill the pools and contained no vegetation.

361

362 Table l'

JounNel oF THE AMERTcaN Mosquno

CoNrnol

AssocnrroN

Vor-. 19,No.4

Proportion of total collection, functional feeding group association, and days until lst collection of invertebrate taxa through the course of the study. Functional Taxa

sDays aJJ until urrlrr

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