8.4 Application and timing of insecticide treatments in storage

Insecticides, including fumigant treatments, are chemical methods for controlling storage insects. The most common methods of application have included:

Admixture of insecticidal dusts with the produce before loading it into the sack. Mixing was carried out in various ways, such as shovel mixing on a tarpaulin or, for large-scale operations, mixing in dust formulations in rotating drums or on conveyor belts. 

However, these techniques are likely to give rise to potential health hazards and are no longer recommended (except for seed treatments where they can be highly efficient).

Applying liquid insecticide sprays or dusts to successive layers of sacks as the stack is built. Spraying or dusting successive layers of sacks with insecticides was considered less likely to build up residues, but is not always effective and is no longer recommended.

Enclosing a fumigant with the sacks under a gas-proof sheet. This is usually the most effective method of insect control and when used correctly, is safe and least likely to lead to residue problems. Phosphine (phostoxin) is a toxic gas that is generated from sachets containing metal phosphides. 

It is slowly released among bags covered by a gas-proof sheet: which is held down by “sand snakes” or similar weights. With phosphine, the covered stack is typically left for between 5 and 16 days, and then opened up to allow the gas to escape. 

The time depends on the temperature and the commodity, but is never less than 96 hours (whereas methyl bromide was popular because it was effective in less than 3 days). 

The Federation of Cocoa Commerce Statement of Best Practice provides further details of procedures.

Introduction of fogs into enclosed spaces such as containers. The application of insecticides (e.g. synergized pyrethroids) using thermal foggers is primarily designed to kill flying insects such as warehouse moths that might escape or hatch inside containers.

An issue that may be overlooked is the treatment of the wooden pallets on which cocoa sacks are stored - especially for the control of termites. 

Termite insecticides are often, out of necessity, persistent and toxic and have included chemicals such as chlorpyrifos and fipronil, together with other now obsolete organochlorines. 

It is now thought that some high residue incidents in produce have arisen from indiscriminate treatment of pallets, and that greater care must be taken in future.