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Grain Storage – Fumigation Strategies for the Future

Sealed Silos will Save Phosphine

Authors: Peter Hughes and Ken Bullen, Plant Science, Toowoomba, Q..

Introduction

Phosphine is vital to the grains industry’s continued ability to deliver insect free grain. Most markets require grain with no live insects. To meet this market need, an estimated 80% of Australia's cereal grains are treated by phosphine fumigation. Phosphine is the only treatment registered for insect control in stored oilseeds and pulses. Using phosphine only in sealed storages, is the best way to slow resistance and keep it as a useful grain fumigant for many years to come.

Alternatives to phosphine are limited

While insecticide sprays on stored grain are an alternative to fumigation, some export markets do not accept the residues from those sprays. Methyl bromide fumigant is being phased out due to effects on the ozone layer, while carbon dioxide treatment 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 nearly as cost effective as phosphine.

$ealed $ilos will $ave Phosphine

Application of phosphine in sealed storages is the only way to economically control all insects regardless of resistance level or growth stage. In addition, sealed storages ensure that all phosphine remains in the storage and doesn’t leak into surrounding work areas where human health may be placed at risk.

Resistance to phosphine is increasing

Insects can be classified as susceptible, moderately resistant and strongly resistant to phosphine. Moderate resistance is now widespread and strongly resistant insects have been identified throughout the Australian grain belt and are becoming more frequent each year. High resistance levels require higher concentrations of phosphine and / or much longer fumigation periods to control all the life stages of the insect (Figures1 and 2).

Figure 1 Fumigation days required to control all life stages at 25oC and 217ppm Phosphine.

(Note: Strong resistance for Rice weevil is calculated from Chinese strains)

Figure 2: Fumigation days to control all life stages at 25oC,

at various phosphine concentrations.

The message here: Unsealed storages do not retain enough phosphine for long enough to provide the required level of control (Figures 3 and 4).

Figure 3: Unsealed silo fumigation using a high label rate

of phosphine applied by admixture with the grain.

The situation depicted in the above trial, is typical of what happens in many on-farm situations where a grower fumigates in an unsealed or poorly sealed storage. The highest doses measured would at best achieve only the following substandard results:

·  10% survival for resistant adults,

·  90% survival of eggs for resistant Rice Weevil,

·  100% survival of eggs for resistant Red Flour Beetle,

·  20 % survival of eggs for resistant Lessor Grain Borer,

·  80% survival of larvae for resistant Rice Weevil,

·  25% survival of larvae for resistant Red Flour Beetle and

·  5 % survival of larvae for resistant Lessor Grain Borer

Sometimes the above level of insect control may get you through a sample inspection – however it is the start of a nightmare for the downstream handler or exporter and places stresses on our industry we can all ill afford.

Figure 4: Different sealing levels and the effect on

phosphine concentration over time.

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 3 minutes). Each line on Figure 4 represents only one phosphine sensor at one depth in the silo. In this silo, there was only one of the sensors that showed any phosphine at all in the ‘No Seal’ treatment and that is the line depicted for ‘No Seal’.

In a silo that is able to maintain a pressure half-life for about 3 minutes, the phosphine concentrations will be maintained for much longer. In the fumigation shown in figure 5, sensors at varying depths in the grain bulk record phosphine concentration. While the concentration of the phosphine was greatest higher in the silo, all sensors recorded concentrations that were sufficiently high for long enough to achieve a good fumigation.

Figure 5: Sealed silo with an adequate pressure test.

Figure 6: Same sealed silo but with a small leak.

The same silo is shown in figure 6, with the only difference being that 4 pieces of thin clothes line were laid out under the hatch before it was closed, thus creating a small leak. The impact on how the silo held phosphine levels was dramatic and devastating. The difference would mean a failed fumigation.

Phosphine application checklist:

o  Follow label recommendations for each phosphine product.

Products vary in amount of phosphine released / tablet)

o  Effective fumigation with phosphine needs 1.5 grams of phosphine / m3 of total silo capacity

That is, apply the same number of tablets – whether the silo is full or partly full

Table 1: Application rates for phosphine tablets in storages of various sizes

Storage capacity / Number of tablets required (approx)
Tonnes wheat / Bushels / Cubic metres
20 / 730 / 27 / 40
50 / 1830 / 65 / 100
100 / 3660 / 130 / 200
300 / 11000 / 400 / 600

Note: Table refers to use rate for most tablet formulation (1gm of phosphine per tablet)

·  Use in sealed storages only.

·  Recommended Minimum exposure period:

7 days when grain temp is above 25°C; 10 days at 15° - 25°C; insects are very hard to kill at temperatures below 15°C.

·  Ventilation period: 24 hrs with fans, up to 5 days without fans.

·  Withholding period: 2 days after the ventilation period (human or stock feed).

·  Total fumigation time: minimum period at 25°C (fumigation to use) is 7 days exposure + 1 day ventilation with fans + 2 days withholding = 10 days.

·  Min. allowable time without fans and at below 25°C is 17 days.

·  Spread tablets thinly on tray - to avoid fire risk.

·  Never add water to tablets - fire risk.

·  Never fumigate grains in transport - it is illegal and very dangerous.

Phosphine should be used in an integrated management program. This includes other control methods such as:

·  Aeration cooling to reduce insect activity.

·  Protectant insecticides.

·  Structural treatments and hygiene.

Phosphine is a vital tool to the grain industry. If you abuse it – you will lose it!

Further information

If you require further information, please see:

·  DPI&F website www.dpi.qld.gov.au/fieldcrops/3947.html

·  Sealed Silos Save – A 2-page advice sheet on pressure testing silos can be found on the GRDC website at http://www.grdc.com.au/growers/as/

·  DPI&F Call Centre open from 8.00am to 6.00pm Monday to Friday (telephone 13 25 23 for the cost of a local call within Queensland; interstate callers 07 3404 6999) or email

·  Or, contact one of the National Grain Storage Extension Team

Qld / Peter Hughes or Ken Bullen / 07 4688 1200
Qld / Philip Burrill / 07 4660 3620
Vic. / Peter Botta / 03 5761 1647
SA / Peter Fulwood / 08 8568 6422
WA / Chris Newman / 08 9366 2309
NSW / John Cameron / 02 9482 4930

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