Phosphine resistance in insect pests of stored grain

• State Administration of Grain Reserves, China
• State Internal Trade Bureau, China
• Central Food Technological Research Institute, India
• Central Food Technological Research Institute, India

Storing and handling of grain surpluses and imports may create problems that lead to losses in grain quantity and quality. In Australia, India and China, management of stored grain pests remains a vital component of reducing losses and maintaining grain quality in domestic and export markets. Phosphine is the most widely used fumigant for controlling stored product pests - thus any loss of efficacy or failure of control recommendations will be a major concern, as there are presently no alternative fumigants available.
Insect resistance to phosphine has become a problem for Australia, China and India. Initially levels of resistance were low, but in the mid 1990s some insect strains in several countries developed high levels of resistance. The importance of these strains to commercial fumigations that use gastight storages is yet to be determined, but they pose a serious problem to storage systems common in China and India, where short fumigation times are used in inadequate structures.

Insecticides and fumigants are the most effective and flexible means of controlling pest activity in grain storage and handling, with phosphine being used as the main fumigant of grain in Australia, China and India. However, resistance to phosphine fumigation is emerging among many major pest species in Australia, China, India and many other countries. The aim of this project is to develop and implement management practices which limit the current problem of phosphine-resistant pest species and delay the further development of phosphine resistance.

The research program for the project included testing the responses of major insect pests to different dosages and lengths of exposure to phosphine and relating laboratory levels of resistance to the current methods of phosphine fumigation used in the field. This work was designed to formulate recommendations for modifying dosages and methods of application for phosphine fumigation. It was also the basis for a fumigation strategy that gives zero survivors of the most resistant strains.
The scientists also tested phosphine-resistant strains with other protectants and fumigants for cross resistance and determined the most appropriate treatment for use in rotation with phosphine. They also defined the genetics and fitness of the phosphine-resistant strains to determine the likely future selection for resistance and the maximum resistance levels likely to arise from these strains.

The project established that resistance within several major pest species posed a serious threat to the safe storage of grain in some situations. In China and India laboratory and field experiments led to formulations for dosage requirements for successful fumigation of resistant insects and provided a better understanding of resistance development and its management.
Very high levels of phosphine resistance were detected in several Chinese strains of stored grain pests (especially Rhizopertha dominicina and Cryptolestes). Field evaluations of phosphine use in China indicated that fumigation failures (i.e. incomplete control of pests leading to selection of phosphine-resistant strains) were inevitable while the prevailing practices persisted. A similar pattern emerged in India, while low-level resistance was detected amongst Australian isolates.
In Australia, scientists completed genetic studies on Chinese phosphine-resistant strains of three species. Results showed that two or more genes were apparently responsible for controlling phosphine resistance. Meanwhile studies in China provided information on the genetics and ecological fitness of resistance, and on cross-resistance to other chemicals, all of which help in understanding the evolution of resistance and in designing measures to counter it.
Studies in Australia showed that fumigations according to the protocols recommended for the CSIRO-patented Siroflo fumigation process (and similar procedures in Australia) did not control two resistant strains from China, therefore these protocols required urgent revision.
In China, field experiments in grain warehouses identified deficiencies in current practices, largely associated with inadequate storage sealing and gas-tightness. Modifications of fumigation techniques have led to improved maintenance of lethal dose (a higher concentration for longer) and compliance with agreed standards for fumigation success. The information on fumigation practices and modifications have helped the project team to formulate new recommendations for phosphine use in China to minimise the impact of phosphine-resistant strains.
In India the scientists formulated recommendations to improve fumigation performance and reduce development of resistance. Further research was recommended to show that a slow-release phosphine formulation could be used as a significant means of improving fumigation performance.
As a result of the project findings concerning phosphine resistance, Queensland and New South Wales authorities doubled the recommended dose for phosphine fumigation of grain.