Practical and modelling studies on the use of modified atmospheres for insect and mite control in grain stores

Abstract

This research project covered a new area for the use of modified atmospheres (MA) known as storage life protection. MAs for storage life protection are aimed at the weak points in an insect's life cycle and are less extreme than those used to disinfest grain, and as a result have an economic advantage in their operation. Burner gas from controlled propane combustion was chosen as the candidate MA. The project had three aspects. The first involved laboratory tests at 20oC and 25oC on two species of insects, the grain weevil (Sitophilus granarius) and the rust-red grain beetle (Cryptolestes ferrugineus), and two species of mites, Acarus siro and Tyrophagus longior. All these are well-known pests of grain and at various stages in their life cycle show a high level of tolerance to MAs and are therefore difficult to control.

The second aspect involved field trials of this MA supported by the third aspect of the project, modelling using computational fluid dynamics. This was used to help augment the practical knowledge of MA application at CSL and thereby establish the optimal strategy for gas application. The operational cost of using burner gas as the MA for storage life protection was then assessed.

Laboratory results

1) There is a hierarchy of O2 content of the MA required to prevent breeding:

5% for the grain weevil (S. granarius) at 20oC and A. siro at 25oC.

4% for the rust-red grain beetle (C. ferrugineus) and the grain weevil (S. granarius) at 25oC, and A. siro and T. longior at 20 and 25oC.

3% for the rust-red grain beetle (C. ferrugineus) at 20oC.

The target for burner gas for storage life protection is 4 - 5% O2 for most pests.

2) Economy of the system for storage life protection can be improved with the addition of CO2. Complete cessation of population growth:

a) The grain weevil (S. granarius): 5% O2 and 10% CO2 at 20 and 25oC.

b) The rust-red grain beetle (C. ferrugineus): 5% O2 and 20% CO2 at 20oC alone.

c) Acarus siro: 20oC - 6% O2 and 20% CO2. 25oC - 6% O2 and 10% CO2.

f) Tyrophagus longior: 6% O2 and 10% CO2 at 20 and 25oC.

Computational modelling has simulated the interactions between external conditions and internal gas flows in bins, and predicted the O2 concentrations in the grain bulk. The predictions show that under windy conditions the pressures created by MA injection are unable to prevent air penetration into tower and flat stores despite high MA injection rates.

Guidelines and properties for effective MA application to bulk grain

a) Minimise leakage into the grain bulk through the structure.

b) Continuous gas flow system applied to the most gas-tight end of the structure.

c) The system is capable of functioning in storage facilities that are not designed specifically for modified atmospheres.

d) The modelling simulations have shown that MA as a control strategy is robust enough to survive intermittent climatic weather conditions which are especially prevalent during the winter storage season.

e) Previous studies have shown that it is possible to hold MAs of 1% oxygen within bins. This study has shown that the same restraints apply when attempting to hold a bin under 5% oxygen.

Propane gas consumption for production of a 5% oxygen atmosphere is reduced by 25% compared to production of an atmosphere with less than 1% oxygen, the level required to kill the most tolerant life stages within a month at 20oC or above.