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March 2007

RIRDC Publication No 07/043  RIRDC Project No DAN-203A

 

Executive Summary
Importance of this research
This project has focussed on the three main invertebrate pests affecting the establishment of aeriallysown rice crops: bloodworms, snails, and aquatic earthworms. Research has concentrated on expanding the options available to rice growers for controlling these pests, and on developing strategies that minimise the use of synthetic pesticides. Reduced pesticide use helps to conserve biodiversity and also reduces the cost of crop production.

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

• Evaluate alternatives to broad-spectrum insecticides for bloodworm control.
• Complete a study on the population dynamics of bloodworms in rice crops to provide a better understanding of which species (other than Chironomus tepperi) may require management.
• Conduct commercial-scale trials of chlorothalonil for snail control as a precursor to a registration claim for this material. Conduct preliminary trials of niclosamide, another compound with potential for use against rice snails.
• Complete a pesticide screening program against the aquatic earthworm Eukerria saltensis in order to identify chemicals that may provide sustainable control of this pest in flooded crops.


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

• Bacillus thuringiensis var. israelensis (BTI) is a bacterial insecticide used extensively against mosquito larvae. Laboratory and field trials demonstrate that BTI effectively controls bloodworms in rice at acceptable application rates, and does so selectively, having little impact on predatory species.
• Several new compounds have been identified that have the potential to provide selective bloodworm control at very low application rates. The neonicotinoid chemical group in particular has significant potential.
• Ecological studies on the colonisation of rice fields by bloodworms show the bloodworm community during crop establishment to be highly diverse, with 17 species recorded in the first 35 days after flooding. Procladius paludicola, a predatory species, was the most common, followed by Chironomus tepperi, which is known to be a major crop pest. Together, these species accounted for 89% of bloodworms collected.
• Previous studies have shown that ovipositing C.tepperi midges strongly avoid laying their eggs into water that already contains bloodworm larvae, and identifying the compounds released into the water by bloodworms may lead to a novel control strategy for this species. Physical treatments and trials with fractionated samples conducted in this study indicate that the repellent compound is a large hydrophobic non-proteinaceous molecule, and preliminary studies using GC/MS support the theory that it is a fatty acid.
• Chlorothalonil trial results from the Griffith and Coleambally areas were highly variable, and subsequent investigations showed this compound is strongly affected by soil type. Higher soil adsorption correlated to lower efficacy, and it is unlikely that chlorothalonil will be a robust alternative to copper sulfate for snail control unless changes to formulations can overcome its rapid adsorption to some soils.
• Preliminary trials with niclosamide have shown this material to be effective at low application rates and its chemical characteristics suggest it is unlikely to be affected by variations in soil type.
• Eighteen compounds were screened for activity against aquatic earthworms, concluding the laboratory testing program. Of the 38 compounds evaluated, only 8 have sufficient activity to justify field evaluation.


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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