common name: hydrilla tip mining midge
scientific name: Cricotopus lebetis Sublette (Insecta: Diptera: Chironomidae)
Insects of the family Chironomidae, commonly known as midges, are often the most abundant group of insects inhabiting freshwater environments (Pinder 1986). Midges are fragile and mosquito-like in appearance but they do not bite. The larvae of most midges are aquatic and feed primarily on algae and decaying organic matter. A few species, however, are capable of mining the soft tissues of submersed plants and utilizing the living plant material as a food source (Pinder 1986). Recently, this feeding strategy has been studied in some detail in the genus Cricotopus because of the realization that it could be exploited for the biological control of the alien aquatic weed Eurasian watermilfoil, Myriophyllum spicatum L. (MacRae et al. 1990) and possibly hydrilla, Hydrilla verticillata (L.f.) Royle (Cuda et al. 2002).
Hydrilla is a submersed aquatic plant endemic to the Old World tropics that was introduced into Florida by the aquarium industry in the late 1950s from Sri Lanka (Langeland 1990). After its discovery in the Crystal River watershed in 1960, hydrilla continued to expand its range statewide and to increase in severity in water bodies already infested. The dense surface mats associated with severe hydrilla infestations cause problems because they hinder navigation and flood control, interfere with recreational activities, and reduce the biodiversity in aquatic ecosystems (Haller 1978). Between 1980 and 2004, approximately $158 million in state and federal funds were spent managing hydrilla in Florida public waters with non-biological control methods (FLDEP 2004). With the recent discovery of herbicide resistance in hydrilla (Michel et al. 2004), there is renewed interest in biological control.
In 1992, USDA researchers discovered midge larvae attacking the apical meristems of hydrilla in the Crystal River watershed in Citrus County, Florida (G.R. Buckingham, personal communication), and that the damaged hydrilla at one site was stunted and unable to grow to the surface. The hydrilla-attacking midge was subsequently identified as Cricotopus lebetis Sublette, a species possibly new to Florida (Epler et al. 2000). Because previous research implicated midge larvae as causal agents of damaged stem tips on stunted hydrilla plants in Africa (Markham 1986), this tip mining midge may have some potential as a biological control agent.
The midge genus Cricotopus is represented in North America by four subgenera (Epler 1995). Two of these subgenera occur in Florida and contain at least eight species (Epler 1995). The actual distribution of C. lebetis will not be known with any certainty until it can be determined if it is an immigrant that was accidentally introduced along with hydrilla, or an indigenous species that has developed a new association with hydrilla.
Adult: The adult midge is small, only 3 to 4 mm in length, and fragile. Both sexes are pale green in color with black markings on the thorax and a pair of adjacent dark bands on abdominal segments 2 and 3, and 5 and 6. The black markings on the thorax and the coarse banding pattern on the abdomen give the midge a darker appearance. The sexes can be readily distinguished by the condition of the antennae and the shape of the abdomen. In females, the antennae are short and the abdomen is as wide as the thorax. In contrast, the males possess long antennae with distinct whorls of hair and have a narrow, tapering abdomen.
Figure 1. Dorsal views of adult female (left) and male (right) "hydrilla tip mining midge", Cricotopus lebetis Sublette. Photograph by Jerry F. Butler, University of Florida.
Figure 2. Lateral views of adult female (left) and male (right) "hydrilla tip mining midge", Cricotopus lebetis Sublette. Photograph by Jerry F. Butler, University of Florida.
Egg: The egg mass is linear shaped, and contains from 50 to 250 eggs diagonally-arranged in one or two rows encased in a sticky gelatinous tube. The eggs are white in color when first laid, and resemble a string of pearls. Within 24 hours the eggs that have been fertilized turn grayish-brown, and red eyespots of the fully formed embryo appear just prior to hatching.
Figure 3. Two views of egg masses of "hydrilla tip mining midge", Cricotopus lebetis Sublette. Photograph by Jerry F. Butler, University of Florida.
Larva: The larvae of Cricotopus lebetis can be identified by the color and general appearance of the body. Live or freshly-preserved specimens have a characteristic green body color with a broad dark blue band around the thorax. After the body color fades in preserved specimens, the larvae can be separated from other midge larvae by the presence of a pair of lateral setae on each abdominal segment.
Figure 4. Larva of "hydrilla tip mining midge", Cricotopus lebetis Sublette. Photograph by Jerry F. Butler, University of Florida.
Figure 5. Hydrilla tip damage and larva of "hydrilla tip mining midge", Cricotopus lebetis Sublette. Photograph by Jerry F. Butler, University of Florida.
Pupa: The pupa is a nonfeeding stage. The wings and other adult features that have been developing internally are visible. Breathing horns or "trumpets" that are usually present on the prothorax in species that have free-swimming pupae are lacking. A pupa destined to become an adult female of Cricotopus lebetis will have a full complement of eggs apparent in the abdomen.
Figure 6. Female pupa of "hydrilla tip mining midge", Cricotopus lebetis Sublette. Photograph by Jerry F. Butler, University of Florida.
Figure 7. Damaged hydrilla tip with pupa of "hydrilla tip mining midge", Cricotopus lebetis Sublette. Photograph by Jerry F. Butler, University of Florida.
Both male and female midges live from one to three days and do not feed. The adults mate on a suitable substrate in daylight. Male swarming behavior that is a prerequisite for mating in many species of the Chironomidae was not observed in this species. Shortly after mating, the female oviposits on the surface of the water. The female inserts the tip of her abdomen beneath the water surface where she deposits a single ribbon-like egg mass surrounded by a gelatinous matrix and dies soon afterwards. The egg stage lasts 36 to 48 hours.
Figure 8. Mating adults, "hydrilla tip mining midge", Cricotopus lebetis Sublette. Photograph by Jerry F. Butler, University of Florida.
Larval hatching is synchronous. The neonates are very active but remain inside the tubular gelatinous matrix for several hours, crawling from one end to the other. Eventually, they exit the gelatinous matrix from one of its ends, or occasionally from the middle. The larvae at this stage of their development are free-swimming and vulnerable to predation. However, their translucent color and small size may afford them some protection until they can enter a shoot tip. Once inside the plant, the larvae mine and feed on the vascular tissues of the apical meristems of the hydrilla shoots (one larva per shoot tip). As they develop to maturity, their feeding activity creates a 1 to 2 cm tunnel inside the stems which eventually kills the shoot tips and induces their abscission. The tunnels created by the developing larvae inside the shoot tips probably protect them from predators but also function as pupal cases. The larvae complete their development in nine to 22 days.
Pupation occurs inside the hydrilla stem. Before pupating, the mature larva completely severs the tip of the shoot to create an escape route for the fully-developed pupa, and caps the opening of the tunnel with plant fibers excavated from the stem wall. The preparation of pupal case by the the last instar larva is what actually induces abscission of the shoot tip. The pupal stage lasts 24 to 48 hours. Adult emergence occurs after the sedentary pupa exits the stem by repeatedly undulating its abdomen to break through the fibrous cap, and slowly swims to the surface aided by an air bubble released inside the pupal skin.
Cricotopus lebetis may have some potential as a biological control agent of hydrilla. The larvae of this herbivorous midge mine the meristematic tissues of the plant and in the process disrupt shoot growth. By severely damaging or killing the apical meristems, the developing larvae may prevent new stems from reaching the surface. This type of damage is desirable for managing hydrilla because it would eliminate most of the adverse effects caused by the formation of the dense surface mats, such as changes in biodiversity, water chemistry, circulation and temperature.
- Cuda JP, Coon BR, Dao YM, Center TD. 2002. Biology and laboratory rearing of Cricotopus lebetis (Diptera: Chironomidae), a natural enemy of the aquatic weed hydrilla (Hydrocharitaceae). Annals of the Entomological Society of America. 95: 587-596.
- Epler JH 1995. Identification manual for the larval Chironomidae (Diptera) of Florida, revised edition. Bureau of Surface Water Management, Florida Department of Environmental Protection, Tallahassee.
- Epler JH, Cuda JP, Center TD. 2000. Redescription of Cricotopus lebetis (Diptera: Chironomidae), a potential biocontrol agent of the aquatic weed hydrilla (Hydrocharitaceae). Florida Entomologist 83: 172-180.
- Florida Department of Environmental Protection, Bureau of Invasive Plant Management. 2004. Status of the aquatic plant maintenance program in Florida public waters: Annual report fiscal year 2003-2004. http://www.dep.state.fl.us/lands/invaspec/2ndlevpgs/pdfs/Aquatic%202003-04.pdf (7 June 2005).
- Haller WT. 1978. Hydrilla: a new and rapidly spreading aquatic plant problem. Circular S-245. Gainesville: Institute of Food and Agricultural Sciences, University of Florida.
- Langeland KA. 1990. Hydrilla (Hydrilla verticillata (L.F.) Royle), a continuing problem in Florida waters. Circular No. 884. Cooperative Extension Service, IFAS, University of Florida, Gainesville.
- MacRae IV, Winchester NN, Ring RA. 1990. Feeding activity and host preference of the milfoil midge, Cricotopus myriophylli Oliver (Diptera: Chironomidae). Journal of Aquatic Plant Management 28: 89-92.
- Markham RH. 1986. Biological control agents of Hydrilla verticillata, final report on surveys in East Africa. Miscellaneous Paper A-86-4, U.S. Army Corps of Engineers Waterways Experiment Station, Vicksburg, MS.
- Michel A, Arias RS, Scheffler BE, Duke SO, Netherland MD, Dayan FE. 2004. Somatic mutation-mediated evolution of herbicide resistance in the nonindigenous invasive plant hydrilla (Hydrilla verticillata). Molecular Ecology. 13: 3229-3237.
- Pinder LCV. 1986. Biology of freshwater Chironomidae, p.1-23. In Mittler TE, Radovsky FJ, Resh VH. (eds.), Annual Review of Entomology 31. Annual Reviews, Inc., Palo Alto, CA.