Weeds - Phase 1 - Research Highlights

Cabomba and Alligator Weed

What the report is about?

Alligator weed (Alternanthera philoxeroides) and Cabomba (or Caroline fanwort, Cabomba caroliniana) are two aquatic weeds of national significance. Due to the weeds' economic, social and environmental impacts and the difficulty of controlling them by using conventional methods, biological control is recognised as an important research component in the national strategies for dealing with the weeds.

This project represents the culmination of two previous projects, begun in 2003, which identified, prioritised and started the host specificity testing of several potential invertebrate agents. The goal of this project was to provide safe and sustainable control of the two introduced aquatic weeds, focusing on two priorities: biological control and improvements in best-practice methodology.

Click here to see the full report.

What are the relevant regions in Australia?

Cabomba (Cabomba caroliniana) is a serious aquatic weed, introduced to Australia as an aquarium plant, that has become widely naturalised in Australia where it invades lakes, potable water supply dams, farm dams, creeks, rivers and drainage canals, often forming dense submerged mats. Alligator Weed was introduced to Newcastle (NSW) as a culinary herb, but is now a serious aquatic weed throughout Australia.

Aims/objectives

This project had four primary components:

• testing the host specificity of the beetle Systena nitentula, a potential biological control agent for alligator weed
• testing the host specificity of the weevil Hydrotimetes natans, a potential biological control agent for cabomba
• examining the effect of shading as a management tool for cabomba
• examining the interaction between inundation, selective herbicide use and mowing in the management of alligator weed.

Methods used

In testing S. nitentula, 10 newly hatched larvae were placed on the washed roots of test plants in a 20-centimetre Petri dish lined with filter paper. Each Petri dish was sealed with paraffin film to retain moisture. More roots were added to each dish as required, to ensure that plenty of root material was available for the developing larvae.

Larval development to each instar and the number of adults that emerged were recorded. The adults were allowed to continue to develop in the boxes for observation, mating and oviposition. Eggs were collected and placed in a Petri dish on moist filter paper. The number of eggs hatched was noted in order to determine fecundity. Newly hatched larvae and the adults from each test plant were then placed in a cage with the same species of test plant for observation of continuing development.

Eight plant species were tested:

• Alternanthera angustifolia-native (narrow leaved joyweed)
• denticulata (green)-native (Lesser joyweed)
• denticulata (New Zealand, large-leafed)-exotic
• denticulata (red-leafed)-native
• nana-native (Hairy joyweed)
• nodiflora-native (Common joyweed)
• sessilis (Australia)-native (Sessile joyweed or dwarf copperleaf)
• sessilis (New Zealand)-exotic.

It should be noted that multiple plant species have been included that have the same epithet. These "species" have differing morphology or colour, which may indicate that they may actually be different species or different varieties. Therefore, they have been included as separate entities in the host testing for added safety. The authors are currently collaborating with New Zealand Landcare on a project examining the genetic differences among these plants.

Six H. natans adults and one stem of a test plant were placed in 2-litre jars. The jars were sealed with a piece of gauze and rubber bands. The adults were removed from the test plants after four days and were placed in a holding container with fresh cabomba plants. The test plants were checked for feeding damage and any larval development.

Three replications of the following plants were used:

• Cabomba caroliniana (Cabomba)
• C. aquatica (Fanwort or Giant cabomba)
• Brasenia schreberi (Watershield)
• Hydrilla verticillata (Hydrilla)
• Nymphoides indica (Water snowflake)
• Ludwigia peploides (Water primrose)
• Myriophyllum papillosum (Water milfoil)
• Monochoria virginalis (Monochoria)
• Ottelia ovalifolia (Swamp lily)
• Utricularia gibba (Humped or floating bladderwort)
• Ceratophyllum demersum.(Rigid hornwort or coontail)

The test was terminated after eight weeks.

A choice test was conducted to determine whether H. natans adults would choose C. caroliniana over the most closely related Australian species, B. schreberi. Twenty Hydrotimetes adults were placed in the centre of an aquarium tank with the C. caroliniana and B. schreberi plants at each end separated by a mesh screen with openings large enough for the weevils to move through it. The location of each adult was recorded after 24 hours.

Researchers also conducted a no-choice test to determine how long adults could survive on C. caroliniana compared with B. schreberi. Four adult weevils were placed in an aquarium with an abundant amount of either C. caroliniana or B. schreberi. Adults were left on the material for two months, fresh plant material being added as needed.

For the shade trial for cabomba, researchers covered a small farm dam with black builders' plastic. Before covering the dam, researchers examined biomass by collecting plant material in underwater quadrats (0.25 x 0.25 metres). The material was taken to the CSIRO laboratories in Brisbane, sorted into species and dried to constant weight. Researchers also took water quality data four times during the project in order to determine if the plastic was affecting dissolved oxygen, turbidity, conductivity, nitrogen and phosphorus.

The plastic stayed in position for more than four months, after which researchers surveyed the dam internally by snorkelling in a grid pattern and externally by examining the perimeter. Any material found was collected and dried at CSIRO, and its dry weight was recorded.

Researchers revegetated the dam with native aquatic and edge species two weeks after removal of the plastic. The species used were Philydrum lanuginosum (frog's mouth), Nymphoides indica (water snowflake), Lepironia articulata (lepironia) and Baumea rubiginosa (soft twigrush). The vegetation was then monitored at monthly intervals.

For the inundation trial, researchers investigated the effect of anthropogenic disturbance (herbicidal and mechanical) along a natural inundation gradient (20 to 282 days) on the biomass and resource allocation of the invasive wetland plant alligator weed (Alternanthera philoxeroides) and two co-occurring competitor plants, the introduced grass kikuyu (Pennisetum clandestinum) and the native grass couch (Cynodon dactylon), over two years.

Results/key findings

S. nitentula was imported from Argentina for testing on native and economically important non-target plants for the host specificity testing process. The beetle was tested and it was found it could complete its life cycle on several native Alternanthera species. It is therefore not suitable for release in Australia and no application has been submitted.

H. natans was imported from Argentina for testing on native and economically important non-target plants for the host specificity testing process. Preliminary testing was completed on the weevil and it was found it does not complete its life cycle on the most closely related species in Australia, Brasenia schreberi. However, testing was unable to be completed on the other species because sufficient numbers of the weevil could not be reared in our quarantine.

Several additional potential control agents were identified for subsequent testing.

Data was collected on the shade experiment with cabomba. It was found that, although shade will reduce the biomass of cabomba, it is not an effective eradication technique because vegetative propagules remain in the sediment.

Researchers also found that inundation duration is an important mechanism that allows alligator weed to outcompete desirable forage grasses: about 30 days' inundation allows the weed to outcompete kikuyu.