Our best defence against storage insects needs protection, explain Peter Hughes and Ken Bullen
Limiting the use of phosphine to sealed storages only is essential to prolonging the useful life of this vital grain storage fumigant. About 80 per cent of Australia"s cereal grains is treated by phosphine fumigation - most markets require grain with no live insects - and alternatives to this fumigant are limited. in some crops phosphine is also the only treatment registered for insect control in stored grains.
While insecticide sprays can be an alternative, some markets do not accept the residues from those sprays.
Methyl bromide fumigant is being phased out because it affects the ozone layer, and carbon dioxide is much more expensive than phosphine. Alternative fumigants are being developed, but they are not yet registered and may not be as easy to apply or as cost-effective as phosphine.
Therefore, limiting correctly applied phosphine to sealed storages and is the only way to economically control all insects regardless of resistance level or growth stage. Also, sealed storages ensure that all phosphine remains in the storage and does not leak into surrounding areas.
However, resistance to phosphine is increasing, with resistant insects having been found throughout the Australian grain belt. The higher the level of resistance, the higher the concentration of phosphine and/or the longer the fumigation period required to control all the life stages of the insect (figure 1).
The highest doses in the unsealed silo trial in figure 2 would achieve the following:
Such ineffective control is obviously unacceptable to today"s graingrowers and exposes them to the risk of grain rejection.
Sealed silos will maintain concentrations of phosphine for the required time only if they are adequately maintained and meet the pressure test requirement (pressure half-life of three minutes is the industry standard).
Figure 3 Application rates for phosphine tablets in storages of various sizes
Note: Table refers to use rate for most common tablet formulation (1gm of phosphine per tablet)
GRDC Research Code DAQ00028
For more information: Ken Bullen and Peter Hughes, QDPI&F, Toowoomba, 07 4688 1200
North, South, West