Stored grain pests

Increasingly, grain growers are storing grain in the hope of improving the overall price that they receive. Harvested grain is at its highest quality when it is first loaded in to storage. If the storage environment is not managed correctly the quality will steadily deteriorate. To ensure success, the need for careful planning is essential. Grain insects, end-user requirements and maintaining quality are important issues to consider.

Grain storage expert, Peter Botta (Victoria DPI), says one of the most important things to do is understand the markets you wish to supply and their requirements. This takes careful planning and may mean improving storage facilities. Markets are increasingly demanding grain free of chemical residues. In sealed storage, grain can be fumigated effectively, providing quick, inexpensive and long-lasting insect control without the problem of pesticide residues.

There are many damaging stored grain pests. Some of the most important include the lesser grain borer (Rhyzopertha dominica), the rice weevil (Sitophilus oryzae), the saw toothed grain beetle (Oryzaephilus surinamensis) and the rust red flour beetle (Tribolium casteneum). These pest species are small (between 2-3 mm long) and dark reddish-brown to grey in colour.

The lesser grain borer is a major pest of stored cereal grains and can bore into undamaged kernels of grain at both the adult and larval stages. The rice weevil also attack whole cereal grains and can infest cereal products such as pasta. The saw toothed grain beetle and the rust red flour beetle are important pests, but do not feed on sound grain. Check for these pests in grain residues in harvesting and grain-handling equipment, and in storages.

Generally, grain to be stored for more than six weeks should be treated. Grain can be treated with a protectant when it is added to storage or fumigated in a sealed silo. Most contact protectants give between 3 and 6 months protection, and include fenitrothion, actellic, reldan and IGR. This period is dependent upon the moisture content and temperature of the grain. Too high moisture content and temperatures can lead to the rapid breakdown of protectants and leave grain vulnerable to attack. Always aim to store grain at a moisture content of 12% or less and at a temperature of 25°C or less. This will also help to limit the activity of insects and avoid grain spoilage from moulds and fungi.

Phosphine is an effective and relatively cheap fumigant but must be maintained at a lethal concentration for 7-10 days to kill all insect growth stages. Poor fumigation will result in only the adults being killed, giving the mistaken impression that the fumigation has worked. However, the immature eggs and pupae will not be killed and the infestation will quickly build up again. An added problem with poorly maintained silos is that inadequate fumigation time promotes the selection of resistant individuals. These insects may survive a poor fumigation, and subsequently ‘breed-up’ to become a greater problem in the future. Phosphine resistant insect populations have developed in eastern Australia, largely as a result of poor fumigations.

Peter says prevention is better than cure. It is easier and better to prevent an infestation than to treat an existing one. Any grain spills should be cleaned up immediately wherever they may be, but particularly around the storage area. To help cleaning up, spray out or remove any weeds around the storage area.

Peter says that like any piece of equipment on the farm, sealed silos need to be well maintained to work. Seals should be checked before each filling and replaced if worn or damaged. Always pressure test silos to determine if they are properly sealed.

For further information on stored grain issues click here, or contact Peter Botta (Victoria DPI) on 03 5761 1647 or by email: peter.botta@dpi.vic.gov.au.