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Last Updated: June 2005
Order:
Diptera
Family:
Epydridae
General/Historical:
Hydrellia pakistanae is a small leaf-mining fly in the family Ephydridae. It originates in Pakistan and India and was first released in the United States on Lake Patrick, FL, in 1987. It is very similar in habit and appearance to another introduced ephydrid, H. balciunasi, and two native Hydrellia frequently found in association with hydrilla in the southeastern United States.
Plant Host(s):
Hydrilla verticillata
Description:
Adult H. pakistanae are small flies about 2 mm in length, which reside almost exclusively on or near hydrilla infestations. Adults can be observed resting on floating hydrilla as well as on other emergent aquatic vegetation in the immediate area of the hydrilla infestation. They resemble, superficially, the small gnats that are often seen near small ponds and other aquatic systems. They appear to hop along the water surface from one resting place to another instead of actually flying.Adult Hydrellia are relatively difficult to identify in comparison to other species of aquatic plant herbivores included in this system. The difficulty arises because of its small size, lack of any obvious distinguishing characters, its similarity to introduced and native Hydrellia, and the required use of external reproductive characters for identification. However, with some practice and use of a good grade of dissecting microscope identification is relatively easy.Adult male Hydrellia pakistanae can be distinguished from other commonly collected native and introduced species by several characters, including 1) the length of the thorax in comparison to the abdomen length, 2) presence of crossed or cruciate macrochaetae, and 3) shape of the macrochaetae.The abdomen in male H. pakistanae is relatively short and is roughly the same size as the thorax.In contrast, all commonly encountered native species have abdomens that are 1.5 to 2 times longer than the thorax.However, be aware that the other introduced Hydrellia, H. balciunasi, has similar abdomen/thorax configurations as H. pakistanae. That is, the abdomen is roughly the same size as the thorax.The macrochaetae are small hair-like structures associated with the external reproductive structures that are thought to be responsible for holding the female in place during copulation. In both species of introduced Hydrellia, the macrochaetae are crossed or cruciate.The most important character for distinguishing H. pakistanae from H. balciunasi is the shape of the macrochaetae. The macrochaetae in H. pakistanae are small and more distinctly needle-shaped than those found in H. balciunasi.Female Hydrellia are also distinguished from native and other introduced Hydrellia by the morphology of the genitalia. In females the shape of the cerci is most important.In H. pakistanae the cerci are hooked or L-shaped as compared to long and narrow, short and round, or roughly triangular (arrow- or diamond-shaped) as in H. bilobifera, H. discursa, and H. balciunasi, respectively. Eggs are laid on just about any emergent aquatic vegetation including hydrilla and areas near hydrilla infestations. Females lay eggs singly, and each female can lay upwards of several hundred eggs for the length of her reproductive period. Eggs hatch in 3 to 4 days depending on temperature.Larvae emerge from the eggs and enter the water in search of hydrilla. Larvae tunnel or mine hydrilla leaves, feeding and destroying about 9 to 12 leaves during the 3 larval stages.Third instar larvae pierce the stem tissues with two needle-like projections and subsequently pupate. It is believed that piercing the stem allows the pupae to obtain oxygen. The pupae are housed within a protective case known as the puparium; which is formed from the last larval cuticle. The pupae are roughly cigar-shaped and can be easily confused with axillary buds. The pupal stage lasts from 6 to 15 days, after which the emerging adult floats to the surface in an air bubble.
Collection Techniques:
The best collection technique involves collecting adults by vacuuming them from the water surface using specially designed hand-held vacuums. The adults can either be released directly into new sites or placed into large containers to allow them to lay eggs on partially submersed hydrilla. Larvae can be collected from infested plant material using a Berlese funnel. Infested plant material can be harvested and moved to new locations.
Feeding Damage:
H. pakistanae larvae feed on the internal leaf tissues, leaving distinct tunnels between the leaf surfaces. Each larva is capable of destroying from 9 to 12 leaves during its larval feeding period. After larvae feed on a leaf, it appears almost entirely clear, with only limited amounts of green tissue remaining near the leaf margins. Do not confuse the tunneling of hydrilla larvae with typical hydrilla leaf clearing, known as bleaching, which occurs commonly with hydrilla mostly during the summer months. In the case of bleaching, the clearing begins at the distal leaf tips and proceeds to where the leaf attaches to the stem. In extreme cases of bleaching, entire sections of the stem will contain leaves that are entirely clear.
Hydrellia pakistanae feeding on Hydrilla.
Plant Effects:
From a distance, the hydrilla mat containing large amounts of H. pakistanae feeding appears browned; upon closer examination, one can observe distinct areas along the stem where feeding has occurred. Overall damage to hydrilla is by a reduction in total photosynthetic area caused by the leaf damage, thereby reducing growth and vigor and leading to a decrease in the competitiveness of the hydrilla. In addition, some evidence suggests that buoyancy of the plant is reduced by such feeding. Several Hydrilla declines have been attributed to the feeding action of this species. These include Sheldon Reservoir, TX and Lake Seminole, FL. Small scale experimentation in both tanks and small ponds have shown significant impacts to biomass and tuber production by the sustained feeding action of this species.
Operational Status:
Hydrellia pakistanae has proven to be very successful in damaging and impacting infestations of hydrilla. Establishment is high at most sites with established populations occurring throughout Florida, north to Muscle Shoals, AL, and West to Austin, TX. Populations of this species at several sites have reached >60% of the leaves damaged. When the percentage of leaves damaged exceeds 40%, large holes develop in the mat and large portions of the mat sink. At sites in Muscle Shoals, AL, continual high amounts of damage over several growing seasons have apparently limited the regrowth of the hydrilla in subsequent years.
Contact Personel:
Dr. Alfred Cofrancesco U.S. Army Engineer Research and Development Center - CEERD-EM-W 3909 Halls Ferry Road Vicksburg, MS 39180 Phone: (601) 634-3182
Dr. Michael Grodowitz U.S. Army Engineer Research and Development Center -CEERD-EE-A 3909 Halls Ferry Road Vicksburg, MS 39180 Phone: (601) 634-2972
Dr. Ted Center USDA, ARS Invasive Plant Research Laboratory 3205 College Ave. Ft. Lauderdale, FL 33314 Phone: (954) 475-0541 Ext. 103
Dr. Joe Balciunas U.S. Department of Agriculture Western Regional Research Center 800 Buchanan Street Albany, CA 94710 Phone: 510-559-5975
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