Hydrellia pakistanae and H. balciunasi, insect biological control agents of hydrilla: boon or bust? Michael J. Grodowitz,1 Michael Smart,2 Robert D. Doyle,3 Chetta S. Owens,2 Robin Bare,2 Christie Snell,2 Jan Freedman1 and Harvey Jones1 Summary Of four insect species released in North America for the management of hydrilla ( Hydrilla verticillata, Hydrocharitaceae), only the two leaf-mining flies Hydrellia pakistanae and H. balciunasi have become established. While the flies have exhibited impressive range extensions since their first release in 1987, populations at most sites have remained below what was considered damaging. Recently, modest to large increases in fly populations followed by hydrilla declines have been observed at several sites including Lake Seminole, Florida, Coleto Creek Reservoir, Texas, and Sheldon Reservoir, Texas, United States of America (USA). Long-term, large tank experimentation has shown that even modest levels of fly damage can significantly reduce hydrilla biomass (50%) and tuber numbers (25%), appar- ently by reducing photosynthesis and thereby decreasing plant vigour and production. Field studies have also substantiated these findings where lower numbers of tubers (60%) were observed at sites on Lake Seminole impacted by fly feeding. While more detailed field evaluations are needed, it appears that these agents have the potential to suppress hydrilla populations over the long term. However, a complex of factors can influence their effectiveness, including temperature, plant nutrition, especially protein levels, crowding and the presence of a capable pupal parasite. Further research is needed, including overseas work to identify additional agents and the implementation of new release programs. Based on field surveys, fly releases may increase the likelihood of impact since US release sites now have as much as seven-fold higher fly numbers and associated damage than non-release sites. Keywords: biological control, Hydrellia, Hydrilla verticillata. Introduction subsequently in Louisiana, Alabama, Georgia, Texas and California, United States of America (USA). Its Beginning in 1987, two species of leaf-mining flies in distribution has expanded considerably, now extending the family Ephydridae were introduced to North throughout the Florida peninsula, upwards into the America for the management of hydrilla (Hydrilla vert- Florida panhandle and Georgia, mainly on Lake Semi- icillata, Hydrocharitaceae) (Center et al. 1997, nole, north and west into Alabama, and throughout Grodowitz, et al. 1997). The first species, Hydrellia many locations in eastern and south-eastern Texas. pakistanae Deonier, was introduced into Florida and Direct impact to hydrilla by H. pakistanae has been observed at several locations mainly in northern Alabama, Texas, and Florida (Grodowitz et al. 1995, 1 US Army Engineers Research and Development Center (ERDC), Grodowitz et al. 1999, 2000b), but long-term moni- CEERD-EE-A, 3909 Halls Ferry Road, Vicksburg, MS 39180 USA. toring for impact has been limited. In many areas, intro- 2 US Army Engineers Research and Development Center (ERDC), duced Hydrellia spp. population levels and associated CEERD-EE-A, Lewisville Aquatic Ecology Research Facility, Lewis- damage have been low (Grodowitz 1999, Wheeler & ville, TX 75056 USA. 3 Baylor University, Department of Biology, Waco, TX 76798 USA. Center 2001). Unfortunately, factors accounting for Corresponding author: Michael J. Grodowitz such low populations have not been quantified, but may <[email protected]>. include high levels of parasitism, plant nutritional 529 Proceedings of the XI International Symposium on Biological Control of Weeds quality and other forms of predation (Wheeler & Center than H. pakistanae and this may account for the differ- 1996, Dr Jim Cuda, pers. comm., unpublished data). ences in the release effort. The three other introduced insect agents have had limited, if any measurable success. These include the Expansion closely related leaf-mining fly H. balciunasi Bock (Grodowitz et al. 1997), the tuber-feeding weevil Another important measure of success for an agent Bagous affinis Hustache and the stem-feeding weevil B. is its ability to disperse extended distances after initial hydrillae O’Brien (Grodowitz et al. 1995). Of these releases have been discontinued. Hydrellia pakistanae three species, the only agent to become established has has exhibited impressive range expansion since 1987. been H. balciunasi, but expansion in distribution and Considering that this species was released at only about population size has been severely limited. Recent 30 locations in 5 states, it is impressive that it is found surveys have shown the presence of this species at sites in almost every location examined. During surveys in Texas that are substantially removed from the orig- conducted in 2000 (Grodowitz et al. 2000b), new popu- inal release sites in Texas. lations of H. pakistanae were located at sites on the Rio The purpose of this paper is to review the existing Grande near McAllen and Rio Grande City, Texas, that evidence and present new information on the impact of are well over 300 km and 400 km, respectively, from the introduced Hydrellia spp. in the US. Information on the nearest deliberately established populations. establishment success, expansion in distribution, popula- Surveys conducted in 2001 showed that it was present tion increase and ultimate impact will mainly be directed in 50% of non-release sites examined in Louisiana even toward H. pakistanae, since much of the current work though it was released in only one isolated system has focused on this species. The information has been (Lake Boeuf) south-west of New Orleans, Louisiana. obtained from a variety of published and unpublished Sites examined in Louisiana in 2001 ranged throughout research from both controlled experimentation and the state and encompassed almost every considerable actual field studies. Limited information on abiotic or type of hydrilla habitat. Wheeler & Center (2001) noted biotic factors that could possibly be influential, including the occurrence of H. pakistanae in almost every site plant nutrition and parasitism, is also included. examined in Florida. In contrast, Hydrellia balciunasi has exhibited only minimal range expansion. As indicated previously, H. Results and discussion balciunasi was established in only two Texas sites (i.e. Establishment Lake Raven, Huntsville State Park, and Sheldon Reser- voir, near Houston) located in the eastern portion of the Since the first release of H. pakistanae in North state. Surveys conducted in the early- to mid-1990s America in 1987, over 3 million individuals have been failed to reveal its presence in any other location, even released at close to 30 different sites from Florida to with extensive sampling. However, in 1997, H. balciu- California. Establishment success has been high, with nasi was discovered in locations north and north-east of at least 70% of the release attempts having H. pakis- the original two release locations, often in combination tanae present six months or longer after terminating the with H. pakistanae. These sites include ponds at the introductions (Center et al. 1997). Surveys conducted Lewisville Aquatic Ecosystem Research Facility during 2001 and 2002 at release sites in Louisiana and (LAERF) and Cypress Springs Lake near the town of Texas have shown that establishment success may be Mount Pleasant, Texas. Reasons for its recent expan- higher, since H. pakistanae has subsequently been sion are unknown, but offer encouragement for its found at sites where it was thought not to have initially continued expansion success. established. However, observing the agent after such an Determining mechanisms for such large expansions is extended period subsequent to termination of releases difficult, at best, for these species. First of all, they may be due to natural expansion from nearby popula- appear to be relatively weak fliers and are often seen tions. hopping from one resting place to another instead of Compared to H. pakistanae, H. balciunasi has flying. Human or animal transportation of hydrilla sprigs shown substantially lower establishment success. For containing immatures seems plausible, but established example, establishment success for H. balciunasi was sites and associated population size of the introduced only 18% in 1997, nearly four-fold lower than what was Hydrellia spp. was minimal during this time, hence the observed for H. pakistanae (Grodowitz et al. 1997). In odds of man or animals carrying Hydrellia spp. laden fact, only two release sites have had verified establish- sprigs seem unlikely. Additional research is warranted. ment of this species, both of which are in Texas. Reasons for such low establishment success for H. balciunasi are unknown. However, much less effort Population increase went into its release in comparison to H. pakistanae. Another important criterion of success is the ability For example, H. balciunasi was introduced in only 11 of the released agents to substantially increase in sites in two states with less than 300,000 individuals. population size. While large expansion in distribution Hydrellia balciunasi was always more difficult to rear is desirable, it is often more important to have corre- 530 Insect biocontrol agents of hydrilla sponding increases in population levels to effect damage (Grodowitz et al. 1994). Collectively, these control. data demonstrate that substantial population increases In closed, controlled