Utricularia gibba

U. gibba is an annual or perennial submerged or free-floating carnivorous aquatic plant. It has been identified as such a specialist invasive species and may outcompete native bladderworts in lowland wetland ecosystems in countries where it is introduced. It was intentionally introduced, as an aquarium plant, to New Zealand in 1980, where it is now fully naturalized.

U. gibba is predominantly dispersed by water fowl. It can also rapidly colonise new water bodies by stem fragmentation and via its seeds. It forms a mat over the water surface which reduces light to plants growing beneath and could be a problem for irrigation and drainage. U. gibba is on the National Pest Plant Accord list of New Zealand and is designated as an Unwanted Organism, and banned from sale, propagation and distribution.

U. gibba has a global distribution, with pan-tropical distribution in particular (Taylor, 1994). It occurs naturally in North America and is well distributed in the following areas: along the east coast throughout Florida and Louisiana, to Quebec and eastern North America; from southern British Columbia to California; and the West Indies (USDA-NRCS, 2012). It is also well distributed in Central and South America (GBIF 2012; Tropicos, 2012); the western Mediterranean (Greuter et al., 1989); southern Africa (African Plants Database, 2012); China, India, South West Asia, the Indian Ocean islands, the Pacific islands (Zhenyu and Cheek, 2011); and Australia (CHAH, 2009; Rowe and Brown, 1992).

In its non-native distribution, it is introduced in the Hawaiian Islands (USDA-NRCS, 2012) and Slovakia and Hungary (DAISIE, 2012). In New Zealand (North Island) it is extensively naturalised (Webb and Sykes, 1997; Salmon, 2001). Further information about the distribution of this species can be found in FBIS (2005).

U. gibba is commonly cultivated as an ornamental plant (Biosecurity New Zealand, 2008); it may have been introduced originally as an aquarium plant (Webb et al., 1988). It is predominantly dispersed in New Zealand by water fowl and can rapidly colonise new water bodies by stem fragmentation and seed (Compton et al., 2012), and natural spread between catchments is likely. Currently, the areas in which it appears to be spreading by natural means are limited, and the risk of long-distance spread thus reduced (Biosecurity New Zealand, 2008).

It has also failed weed risk assessments for New Zealand, so further introductions, at least in this region, are unlikely (Champion and Clayton, 2000; 2001). In addition, Plantlife (2010) has identified an urgent need for a detailed risk assessment of U. gibba in the UK; the introduction of this plant into the UK is therefore unlikely. U. gibba is included in the list of environmental weeds in New Zealand (Howell, 2008) and in the First Schedule of the National Pest Plant Accord (NPPA). All plants on the list are designated as Unwanted Organisms, and are banned from sale, propagation and distribution throughout New Zealand (Biosecurity New Zealand, 2008).

U. gibba is frequent in canals, reservoirs, pools and is well extended in wetlands. It also occurs along lake edges, in lowland marshes and fens (Taylor, 1989; GISD 2012) and in shallow still or slow-flowing water (Rowe and Brown, 1992). In China it is found in bogs and rice fields (Zhenyu and Cheek, 2011). The invaded habitats in New Zealand include lowland freshwater wetlands (Landcare Research, 2012). It is found in pools and backwaters associated with Scirpus spp. in New Zealand (Landcare Research, 2012).

Genetics

In the highly specialized carnivorous family Lentibulariaceae, U. gibba has been found to exhibit significantly lower values of nuclear holoploid genome sizes, with 88 Mbp (megabase pairs of DNA) (Greilhuber et al., 2006). The basic chromosome number of U. gibba is 12n= 28 (Zhenyu and Cheek, 2011). The species closest to U. gibba genetically is U. bremii (Rahman, 2007).

Reproductive Biology

The flowers are hermaphrodite and pollinated either by insects or by self-pollination (Salmon, 2001). U. gibba regenerates naturally from seeds (Champion and Clayton, 2000; Compton, et al., 2012). The vegetative reproduction is by re-sprouts from stem fragments or from rhizomes (NZPCN, 2010). U. gibba differs from all these species in that it has no winter buds in its leaf axils, which are the most common propagation method for other species of Utricularia (Zhenyu and Cheek, 2011).

Physiology and Phenology

In more suitable habitats, U. gibba flowers during the warm time of the year, or year-round in tropical regions (Rice, 1994). In the northern hemisphere, U. gibba is a perennial plant, flowering June-August, when yellow flowers are produced that protrude above the water surface (Rook, 2004); in China, flowering takes place from April to November and fructification from May to December (Zhenyu and Cheek, 2011). In New Zealand and Australia the flowering season is in autumn and can be extended until spring (Rowe and Brown, 1992; Landcare Research, 2012). U. gibba overwinters as a complete plant, not as winter buds, and drops to the bottom of the water where it remains dormant through the winter.

Bladderworts (Polypompholyx and Utricularia spp.) are aquatic carnivorous plants with bladder traps interspersed among photosynthetic leaves, and can only use free CO2 (not HCO3-) for photosynthesis (Adamec, 1997). Bladders contain some chlorophyll, but their rate of photosynthesis is lower than that of leaves and thus entails considerable maintenance costs (Adamec, 2006). Bladder production in Utricularia is a strategy that offsets the ecological disadvantages associated with nutrient-poor environments (Guisande et al., 2007).

The biomass of U. gibba is about 16.8±1.7 g and the largest portion of the biomass is allocated to leaves, stolons and traps (86.66%), whereas the amount of biomass allocated to reproductive structures is 13.44%, a relatively small amount. Porembski et al. (2006) suggest that aquatic Utricularia species rely on vegetative fragments for reproduction and dispersal rather than on seeds.

Associations

In the western USA, U. gibba is associated with species such as algae, Canadian waterweed (Elodea canadensis), watershield (Brasenia sp.), pond lily (Nuphar sp.), purple marshlocks (Potentilla palustris), hoary sedge (Carex canescens) and common rush (Juncus effusus ssp. pacificus) (MDNR, 2005).

Environmental Requirements

The majority of aquatic carnivorous plants usually grow in soft or medium-hard, acid or neutral oligotrophy water with humic acid, which is favourable for the development of carnivorous aquatic plants in general (Adamec, 1997), including the rootless and free-floating bladderwort U. gibba, which is an obligate wetland species (US Fish and Wildlife Service, 1988).

U. gibba is specially adapted to low-nutrient environments such as bogs and swamps (Biosecurity New Zeland, 2008), and increases in abundance when the conditions change from oligotrophic to mesotropic; however, with further change in that direction it decreases in abundance (Preston and Croft, 1997). U. gibba prefers slow-moving, warm water and has moderate shade tolerance (NZPCN, 2010). These factors best explain the occurrence of U. gibba in New Zealand (Compton et al., 2012).

U. gibba appears to be restricted to shallow water, where it forms floating rafts amongst and on the immediate open edge of the reeds (HEAR, 2005). In New Zealand's lakes, where it is invasive, it appears to grow deeper than 3.0 m (Wells and Champion, 2010).

In its western area of distribution in the USA, it is found at altitudes of 50 to 150 m (MDNR, 2005). In China it is found from near sea level to 900 m (Zhenyu and Cheek, 2011), but can also be found as high as 2500 m (Taylor, 1989).

Rook (2004) reports that in America, U. gibba is occasionally eaten by muskrats, ducks and other waterfowl.

General

• Pet/aquarium trade

An interactive key for invasive plants in New Zealand has been developed by Dawson et al. (2010).

Utricularia species can be distinguished from all other plants by the bladders on the leaves when these are present. Otherwise, they can be distinguished by the finely dissected compound leaves which are divided in two; this is as opposed to Batrachian Ranunculus species, which are divided in three; or Cabomba caroliniana, which are divided more than three times. Plants are also not rooted in the substrate. U. gibba is difficult to distinguish from C. caroliniana in its vegetative condition, but it could be distinguished from other bladderworts by its bottom-creeping habit and by leaves which radiate from the base of the flower stalk.

In New Zealand, as regards non-native distribution, the only bladderwort resembling U. gibba is the rare U. protrusa, which has many-branched, filamentous leaves, 2-3 mm long bladders, 9 mm diameter flowers (rarely seen). It is found only in still, nutrient-poor water (NZPCN, 2010).

The internal glands of the bladders could be used in the identication of Utricularia species (Yuen-Po et al., 2009).

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

U. gibba spreads naturally in its non-native range and is too widespread and to enable it to be managed effectively (Wells and Champion, 2010). It is difficult to control due to a lack of effective control methods and its aquatic habitat (Biosecurity New Zealand, 2008), as well as its resistance to sprays (HEAR, 2005).

Prevention

U. gibba is on the National Pest Plant Accord list; all the pest plants included on the list are designated as Unwanted Organisms and are banned from sale, propagation and distribution throughout New Zealand (Biosecurity New Zealand, 2008).

Early Warning Systems

U. gibba is included on the weed list of New Zealand and is included in early-warning biosecurity systems.

Public Awareness

In New Zealand, the regional council determines the status of U. gibba and is responsible for control and/or advice on control (Biosecurity New Zealand, 2008).

Physical/Mechanical Control

At small sites, such as in pools, it can be controlled by mechanical removal or by using weed matting.

Biological Control

Sclerotinia sclerotiorum (Lib.) de Bary, a naturally occurring pathogen of many weeds, has been tested on U. gibba but did not show any potential as a control agent (Waipara et al., 2006).

Chemical Control

No information is available on any chemical control methods attempted on this species.

Monitoring and Surveillance (incl. Remote Sensing)

U. gibba is included in Surveillance Pest Plants as a species that have been identified as having significant impacts on the biosecurity values of the Auckland region (New Zealand). This measure will reduce the further spread and establishment of the species throughout the region by prohibiting its sale, propagation, distribution and exhibition (Auckland Council, 2012).

Further work is needed on reproduction strategies in native and introduced ranges. More research needs to be carried out on control methods, including biocontrol studies, particularly in New Zealand, where U. gibba is already invasive.

16/07/12 Original text by:

MA Duenas, consultant, Spain