Hydrilla verticillata (L.f.) Royle (Hydrocharitaceae), a submersed aquatic macrophyte, is native to Asia, Australia, Europe, and Africa (Buckingham & Bennett 1996). This invasive species was first introduced to North America in the early 1950s in the Tampa Bay area (Schmitz et al. 1991) and dispersed rapidly to become one of the worst aquatic weeds in the US (Balciunas et al. 2002). Hydrilla is controlled mainly with chemical herbicides (Fox et al. 1996). However, both mechanical removal (Serafy et al. 1994) and biological control agents (Wheeler & Center 2001) play some role. Since the discovery of herbicide resistance in hydrilla populations in Florida (Michel et al. 2004), biological control has become a high priority. Foreign exploration for classical biological control agents of hydrilla was conducted in China between 1989 and 1995 and resulted in the discovery of many herbivores (Buckingham 1998). Among the most promising were 2 unidentified Macroplea species (Coleoptera: Chrysomelidae) whose specificity was unknown (Buckingham 1998). This note documents the rediscovery of this insect at several sites in China, a brief description of its biology, and its field host range.
From Jun 2006 to Nov 2008, we conducted intensive field surveys in China for biological control agents of hydrilla and found the beetle Macroplea japana (Jacoby) at 6 of 91 survey sites. These survey sites were located in Hubei, Hunan, Jiangxi, Jiangsu, Guangdong, Guangxi, Guizhou, Yunnan, Sichuan, and Chongqing provinces (Fig. 1). We collected larvae or pupae by hand-pulling the host plants from water. The plant samples with larvae and pupae were put in plastic bags and then in a cooler, prior to transport to the lab for observing immature development of the beetle in the lab. These field-collected samples were reared in 3-L plastic containers underwater. Mating adults were placed in plastic pots (10 pairs per pot), each with a host plant for oviposition. The leaves with eggs were excised from the plant and placed in Petri dishes (9 cm; 1 leaf/dish) containing tap water.
We found M. japana in Hubei (Xiaochang and Jiayu County), Hunan (Fenghuang County, Changde City), and Guizhou (Guiding County, Guiyang City) provinces. This species has also been reported from the Chinese Provinces Jiasu, Gansu (possibly), Heilongjiang, Liaoning, and from Japan (Tan et al. 1985; Hayashi & Shiyake 2001; Kölsch et al. 2006).
In the field and laboratory, M. japana adults were observed on submerged host plants; however, when disturbed, they occasionally would emerge from the water and fly. Adults fed on submerged leaves of host plants. Adults were occasionally seen underwater with air bubbles that might serve for respiration attached to their hind legs and abdomen. After emergence from the pupal case, males immediately located a female and mated (Fig. 2A). Seven to 12 d after mating, females lay a row of 3 to 7 eggs among 2 or 3 leaves bound together in the hydrilla tips (Fig. 2B). Eggs are oblong with a yellow core. In total, 11 to 15 eggs are laid on each tip. Seven to 15 d after oviposition, the eggs begin to hatch. Larvae developed through 3 instars. First instars are yellow, 1 mm long, and attach to the stems and roots of hosts by spiracle hooks on their terminal abdominal segment. After ecdyses, the exuviae of first and second instars remain attached to the roots. If the host quality deteriorates, larvae will move to undamaged parts of the same plant or to a new plant and continue feeding. Mature larvae are white or pale green (Fig. 2C). The exarate pupae are white or yellow initially but turn dark brown with age and attach to stems near or on the roots (Fig. 2D). Field observations indicate that M. japana has 1 generation per year and lives through the winter underwater. Most often, overwintering individuals were attached to the base of plants as pupae or as adults, but occasionally as mature larvae or adults inside cocoons.
Field host range of M. japana was determined on site by searching the different plant species adjacent to hydrilla. We found the larvae and/or adults of M. japana feeding on 9 plant species including H. verticillata, Vallisneria spiralis L. (Hydrocharitaceae), Ottelia acuminata (Gagnep.) Dandy (Hydrocharitaceae), Nymphoides peltatum (S.G.Gmel.) O.Ktze. (Menyanthaceae), Potamogeton malaianus Miq. (Potamogetonaceae), P. maackianus A. Benn., P. perfoliatus L., Myriophyllum verticillatum L. (Haloragaceae) and Alopecurus aequalis Sobol. (Poaceae). These results indicate that M. japana utilizes many host species in the field and thus will not be considered further as a biological control agent.
We thank Kai Wu, Wei Huang, Yong Fan for field assistance, M. Hayashi for identifying the beetle specimens, advice and providing literature references, G. Köelsch and T. Sota for also providing literature. Funding was provided in part by Florida Department of Environmental Protection (grant # SL849) and USDA/ARS Ft. Lauderdale, FL.
The beetle Macroplea japana was evaluated for its potential as an agent for biological control of hydrilla. Our field surveys and lab observations indicate that all stages of this beetle live underwater feeding upon the base of their host plant near the hydrosoil. Surveys for the beetles were conducted in 10 provinces but they were found only in Hubei, Hunan, and Guizhou. In the field, we found the beetle fed and completed its development on 9 plant species in 7 different genera. Considering this wide host range, it is unlikely that M. japana will be developed as a biological control agent for hydrilla.