The Diversity of Chironomidae Associated with Hydrilla in Florida, with Special Reference toCricotopus lebetis(Diptera: Chironomidae)

Abstract

Hydrilla verticillata (L.f. Royle) (Hydrochari tales: Hydrocharitaceae) hereafter hydrilla is an aquatic plant that is widely distributed in the Old World. A dioecious form of hydrilla was in troduced into Florida in the 1950s, and has since spread across the southern USA (Schmitz et al. 1991; Langeland 1996). Hydrilla is a highly ag gressive weed which is known to displace native vegetation (Haller & Sutton 1975; Hofstra et al. 1999; Van et al. 1999), impede boat traffic and dis rupt water movement (Schmitz et al. 1991). In 1992, the chironomid Cricotopus lebetis Sublette (Diptera: Chironomidae) was discovered in Crystal River, Florida attacking the apical meristems of hydrilla, and may have potential as an augmentative biological control agent (Cuda et al. 2002, 2011). The origin of the midge is un known (Epier et al. 2000), and its distribution in Florida has not been determined. The midge dam ages hydrilla by mining in the apical meristem, which causes tip abscission (Cuda et al., 2002). The objectives of the current study were to exam ine the distribution and abundance of C. lebetis, and more generally of chironomid midges, associ ated with hydrilla in Florida. Six water bodies in Florida were surveyed to examine the distribution and abundance of chi ronomids associated with hydrilla; Lake Rowell (Bradford Co.) and Wacissa Springs (Jefferson Co.) in North Florida, Lake Tohopekaliga (Osecola Co.) and Bulldozer Canal (Brevard Co.) in central Florida and Lake Istokpoga (Highlands Co.) and Lake Okeechobee (Okeechobee Co.) in south-cen tral Florida. Each location was sampled quarterly from Jan, 2011 to Jun, 2012 for a total of 6 sam ples from all locations except Wacissa Springs, which was sampled 5 times. On each sampling oc casion, a 4-pronged steel hook attached to a rope was thrown into the water and dragged along the hydrosoil to collect hydrilla. Hydrilla plants were removed from the hook and placed in plastic bags (46 x 61 cm, -16 liter capacity) partially filled with water from the corresponding sample site. Sufficient hydrilla was collected on each sampling occasion to fill two bags approximately two-thirds full. A separate plastic container was filled with water from the corresponding site. Bags were placed in a cooler and transported to the labo ratory. Apical portions (5-8 cm) of 300 intact hy drilla tips were haphazardly selected from each sample and placed in open plastic containers (34 x 28 x 15 cm, L x W x H) in water collected from the corresponding site. Containers were placed individually in fine mesh emergence cages (50 x 50 x 50 cm) and aerated with an aquarium pump. Cages were monitored daily for 14 days for midge emergence. Adults were collected daily by aspira tor and placed in vials containing 95% ethanol. Specimens were initially sent to J. H. Epler for authoritative identification, but once a reference collection was assembled, most midges were lo cally identified. Species richness at each location was deter mined as the total number of midge species recov ered. Diversity was calculated using the Shannon Index, which combines aspects of species rich ness and evenness (how equally individuals are distributed among species) (Shannon 1948). The Shannon index measures the predictability that an individual selected in a sample will be a given species. Higher values of the Shannon index indi cate that individuals are more evenly distributed among species, whereas low values indicate that some species are much more common than others. The Shannon Index was calculated for each sampling location as follows: