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Mark M. Stevens
March 2007 RIRDC Publication No 07/043 RIRDC Project No DAN-203A
Background
Bloodworms are the larvae
of chironomid midges. Female midges lay their eggs into rice bays soon
after flooding, and the larvae of some species attack the roots of aerially-sown
rice plants. Seedling losses in aerially-sown crops can exceed 80% if bloodworms
are not controlled. Rice snails are currently controlled with copper sulfate,
and the long-term use of this material is not considered to be environmentally
sustainable, since copper can accumulate in soils. Finding a suitable alternative
to copper sulfate has been one of the most difficult challenges in the
NSW rice pest management program. As snails are molluscs, rather than insects,
they generally do not respond to conventional insecticides.
Aquatic earthworms are the only introduced invertebrate pests in NSW rice crops. Although crop rotations can be used to minimise their impact, there are currently no effective control procedures for aquatic earthworms once the crop has been sown.
Objectives/aims of the research
Methods
Laboratory bioassays and
replicated small-plot field trials were used to evaluate bloodworm control
materials and niclosamide for snail control. Bloodworm recovered from the
untreated (control) plots in previous field trials were used to complete
the study on bloodworm population dynamics.
Trials of chlorothalonil were conducted under APVMA permit on commercial rice farms in the Griffith and Coleambally areas, and results were evaluated in conjunction with data from chemical adsorption studies conducted by Dr Stuart Helliwell of Charles Sturt University. The response of aquatic earthworms to pesticides was examined using standardised laboratory bioassay methodology.
Key findings
Implications for relevant
stakeholders
Commercialisation of BTI
for bloodworm control will allow many rice crops to be grown without any
synthetic insecticides, providing substantial benefits to the environment,
however BTI will be more expensive than generic compounds, and this may
limit uptake. Further development of neonicotinoids for bloodworm control
may allow improved biodiversity conservation relative to existing treatments
at a more acceptable cost. Studies on bloodworm population dynamics have
identified several additional species that need to be evaluated for their
potential role as crop pests.
The failure of chlorothalonil to provide robust snail control means that rice growers will continue to rely on either copper sulfate or crop rotations for snail control until niclosamide can be further developed and commercialised. There are several compounds that show potential for salvage control of crops infested with aquatic earthworms, however these materials require field evaluation before any decisions on registration can be made.
Recommendations
Future bloodworm control
trials should be focussed on neonicotinoid compounds such as imidacloprid
and acetamiprid. Residue studies with niclosamide should be conducted as
a precursor to commercial-scale evaluation of this compound for snail control.
DNA-based techniques for confirming rice consumption by aquatic insects
need to be developed to help determine the pest status of bloodworm species
that are not easily cultured in the laboratory.
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