Hydrilla verticillata threatens South African waters J.A. Coetzee1 and P.T. Madeira2 Summary South Africa’s inland water systems are currently under threat from hydrilla, Hydrilla verticillata L. Royle (Hydrocharitaceae), the worst submerged aquatic weed in the USA. The presence of the weed was confirmed for the first time in South Africa in February 2006, on Pongolapoort Dam in KwaZulu-Natal. An aerial survey revealed that the infestation on this dam covers approximately 600 ha, which is far greater than initially thought. Despite reports that it may be present in other water bodies, surveys have shown that it is restricted to Pongolapoort Dam. We conducted a boater survey which showed that there is significant potential for this devastating weed to spread beyond Pongo- lapoort Dam, and containment of hydrilla is of utmost priority. Research into the suitability of the already established biological control agents, Hydrellia pakistanae Deonier and H. balciunasi Bock (Diptera: Ephydridae), from the USA, as potential agents in South Africa, is also being conducted. However, the South African hydrilla biotype is different from the biotypes in the USA, and this needs to be borne in mind when considering which agents to release. Keywords: potential spread, management, genetic analysis. Introduction Current distribution of The confirmation of Hydrilla verticillata L. Royle hydrilla in South Africa (Hydrocharitaceae) (hydrilla) in South Africa from and potential for spread Pongolapoort Dam, KwaZulu-Natal province (KZN), in early 2006 (L. Henderson, personal communication, Hydrilla is one of the most problematic submerged 2006) prompted immediate action to contain and con- plants worldwide, invading both tropical and temperate trol this weed, and prevent further spread to other wa- regions because of its tolerance to a wide range of envi- ter bodies around South Africa. At present, it appears ronmental conditions (Cook and Lüönd, 1982). It is not that hydrilla is restricted to Pongolapoort Dam, which clear how or when hydrilla entered South Africa, and is the centre of a multimillion rand tourist industry. It so the first step in the hydrilla biocontrol programme in is imperative that we gain an understanding of the dy- South Africa was to determine the extent of its distri- namics of the hydrilla invasion in South Africa, and po- bution. Following reports that hydrilla’s presence was tential for its control, because there is a knowledge gap suspected in a number of water bodies in KZN, both in South Africa surrounding submerged aquatic plants, aerial and boat surveys were undertaken, which con- particularly from a biological control aspect. firmed that hydrilla is currently restricted to an area of about 600 ha in Pongolapoort Dam. However, heavy rains in early January 2007 resulted in the flooding of this area, and the dam increased in capacity from 73% to 92% full in 1 week. The possibility that hydrilla 1 ARC–Plant Protection Research Institute, P/Bag X134, Queenswood has spread throughout the dam, and into the Pongola 0121, South Africa. Current address: Department of Zoology and En- tomology, Rhodes University, PO Box 94, Grahamstown 6140, South River below should not be ruled out, and warrants fur- Africa. ther investigation. 2 USDA-ARS, Invasive Plant Research Laboratory, 3205 College Av- While containment of hydrilla in Pongolapoort Dam enue, Fort Lauderdale, FL 33314, USA. is currently the main control strategy in South Africa, Corresponding author: J.A. Coetzee <[email protected], paul. [email protected]>. there is potential for this plant to spread. In the USA, © CAB International 2008 the main mode of spread of the weed is via recreational 665 XII International Symposium on Biological Control of Weeds boaters and fishermen (Balciunas et al., 2002) as frag- Fluridone does have non-target side effects on other ments of the plant get caught in anchors and propellers aquatic vegetation and fish, which has led to sublethal and are then transported between water bodies. It is doses being applied as this minimizes these effects, and therefore very likely that hydrilla may spread through- is more cost-effective. However, hydrilla has become out South Africa in this manner, particularly because resistant to these doses in the USA, which has compli- Pongolapoort Dam attracts thousands of tourists annu- cated control programmes (Michel et al., 2004; Dayan ally, and because it is home to the annual Tiger Fish- and Netherland, 2005). So it becomes a risk–benefit is- ing Festival, the biggest tiger fishing competition in sue in South Africa – is it worth using a lethal dose the southern hemisphere that attracts fishermen from that will remove large amounts of the plant, but that all over South Africa, and neighbouring Swaziland and will impact the fauna and flora in the dam, against the Mozambique. potential of resistance developing if sublethal doses are Fishermen are primarily responsible for hydrilla’s used, thereby ruling out the most effective control strat- spread in the USA. Therefore we conducted a survey at egy against it? the annual Tiger Fishing Festival in September 2006 to The most sustainable long-term strategy to control determine the potential for hydrilla to spread through- hydrilla should be biological control. The option of us- out South Africa by assessing boating behaviour of the ing the two species of ephydrid flies, Hydrellia paki- fishermen, and whether they were aware of the pres- stanae Deonier and H. balciunasi Bock (both Diptera: ence of hydrilla on the dam. One hundred sixty-three Ephydridae) as potential control agents against hydrilla fishermen were asked questions from a structured ques- in South Africa is being investigated, because these are tionnaire. The results showed that 51% of the fishermen the only two agents that have established in the USA interviewed only used their boats once on Pongolapoort (Center et al., 1997; Bennett and Buckingham, 2000; Dam, and that was at the September competition. The Grodowitz et al., 2000; Wheeler and Center, 2001). In results also showed that 14.1% of the fishermen used addition, the weevil, Bagous hydrillae O’Brien (Cole- their boats only on Pongolapoort Dam between Janu- optera: Curculionidae), which was tested and released ary 2005 and September 2006, and 20 respondents in the USA to control hydrilla but never established (12.2%) never used their boats anywhere except once because it requires periods of drought for pupation at the 2006 fishing competition, between January 2005 (Grodowitz et al., 2000), is being considered as a and September 2006. control agent in South Africa because drawdowns are However, analysis of the number of times fisher- implemented on Pongolapoort dam. Permits have been men used their boats in South Africa highlighted that granted to import the flies and the weevil into South dams outside of KZN were visited more frequently Africa from the USA, so that host specificity testing than those in KZN, and the majority of fishermen trav- may commence. eled between 200 and 800 km to reach their fishing Expanded surveys are also being conducted by both destinations, emphasizing the potential for hydrilla to the USDA-ARS and CSIRO to find additional control spread around South Africa. Even though a contain- agents for hydrilla in the USA (Overholt and Wheel- ment strategy is in place on Pongolapoort Dam, this er, 2006), following the discovery that infestations of survey stressed that more water bodies in South Africa hydrilla are resistant to fluridone (Michel et al., 2004; need to be assessed for the presence of hydrilla as a Puri et al., 2007). Surveys in Burundi, Uganda and result of boating activities before the fishing competi- other central and east African countries, and in Suma- tion in September. tra and China have found promising agents (Overholt and Wheeler, 2006), which could be considered as ad- Management options ditional control agents in South Africa. Mechanical and chemical control has been the most widely used control methods in the USA, although Identification and origin of the their success is varied. Typically, mechanical control introduced biotype using is time-consuming and only offers temporary control, chloroplastic markers and its use has been dissuaded in South Africa, par- ticularly because new infestations can result from plant Hydrilla is a widely distributed species whose range fragments. Until a biocontrol programme can be imple- extends from New Zealand and Australia, through mented, chemical control is currently the most favour- Southeast Asia, north through China, into Siberia, and able option for hydrilla. It should be controlled using west into Pakistan, and it also has a local and disjointed herbicides as soon as possible because it is confined to range in Africa and northern Europe (Cook and Lüönd, only one system. The most effective herbicide to date 1982). Studies have identified more than 28 different against hydrilla in the USA is fluridone, which has been hydrilla biotypes, which could have important con- widely used for large-scale control (Dayan and Nether- sequences for biological control of the plant in South land, 2005). Trials with this herbicide will commence Africa. Four major biotype clusters and one minor out- as soon as it is imported into South Africa. lier cluster have been identified (Madeira et al., 1997, 666 Hydrilla verticillata threatens South African waters 1999). The USA has two hydrilla biotypes – a dioe- Alien Species Programme of KwaZulu-Natal Depart- cious strain that clusters closely with an Indian strain, ment of Agriculture and Environmental Affairs is ac- and a monoecious strain that clusters closely with an knowledged for financial assistance. accession from Korea (Madeira et al., 1997). Several biocontrol agents have been released in the USA, but only the leaf mining fly H. pakistanae References is causing significant damage (Wheeler and Center, 2001). Regional variation in both the host plant and the Balciunas J.K., Grodowitz, M.J., Cofrancesco, A.F.