Minimising pesticide residues in stored grain by use of mixtures

Insect pests can cause serious losses in grain stored in large quantities for long periods. In the Philippines, for example, scientists have estimated that weight loss in unprotected milled rice stored for 3 months can reach 25%.

Pesticides are used widely for pest control in grain storages both in Australia and Southeast Asia and it is likely they will be required for the foreseeable future. However, although residue levels are low and are generally considered safe by health authorities, consumers are becoming increasingly concerned about possible long-term effects on health and the environment. Clearly residue levels should be reduced wherever practicable. One method of reducing application rates and, in turn, residue levels is to use mixtures, which can have greater potency than the individual components used separately.

Scientists of the Entomology Branch of the Queensland Department of Primary Industries, in conjunction with counterparts in Malaysia and the Philippines, are to develop this concept for ACIAR. The collaborators will study the enhancement of insecticide potency using mixtures with a view to lowering the amounts of insecticides that have to be applied to grain to kill pests.

Initially the team will culture insects and do bioassays on treated container surfaces and in treated grain in the laboratory. They will employ data analysis to optimise mixtures, and determine relationships that have toxicological application - in particular, they will identify synergists that increase the potency of synthetic pyrethroids. In the later phase of the project, the researchers will conduct field trials in Australia, Malaysia and the Philippines to ascertain the best insecticide combinations to use in commercial grain storages.

The Queensland group will be responsible for assessing the performance of grain protectants and building-fabric treatments, and for the development of optimal mixtures to protect against major pest species. The group will also study the effect of synergists that have a crucial role in the activity of synthetic pyrethroid insecticides. From these studies it should be possible to estimate minimum effective doses for combinations of candidate grain protectants. An Australian collaborating institution, the Australian Wheat Board, will cover analytical aspects. The Board will develop appropriate methodology, analyse insecticide concentrates, and estimate residue levels on treated commodities. It will also study the kinetics of decay of residues under different environmental conditions.

The overseas research teams will be responsible for all activities, including field trials, carried out in their own countries. They will also be solely responsible for work on Trogoderma granarium, a major pest species that does not occur in Australia. Field trials on rice, maize and soybean will begin during the second year of the project. They will involve treatment of building fabrics, bag stacks, and grain bulks in commercial storages using treatments and application rates suggested by the basic studies. The field trials will finish in the third year.

The Malaysian and Philippine project leaders will visit Australia to discuss experiment design and analysis, and data interpretation and reporting with the Queensland scientists who have specialised in this area. They will also have the opportunity to observe recent developments in laboratory techniques. Two Australian scientists will visit Malaysia and the Philippines at times set to coincide with coordination meetings.

The project should result in reduced losses of grain due to pest infestation during storage in the humid tropics. Pesticides will be applied in a manner designed to maximise the potency to pests and to minimise residues. The technology will be applicable to central handling systems, cooperatives, village stores and farms in Malaysia, the Philippines, and other countries in the region.

The results will be equally relevant to Australia, and should provide considerable benefits to the grain industries. Further, as quarantine restrictions preclude work on Trogoderma granarium in Australia, data gathered on the susceptibility/resistance of this species to insecticides will be valuable if the pest ever gains entry and becomes established here.