Growers are being reminded that the responsibility of thorough fumigation rests on them to ensure safe grain handling and prevent associated concerns like insect resistance to fumigants like phosphine.
PCB Consulting director Peter Botta says on-farm storage, phosphine application, ventilation and withholding periods are all vital aspects in ensuring proper fumigation and are practices which should be treated with the utmost respect.
Ensuring silos are gas-tight is the first step in effective fumigation, according to Mr Botta. This means the gas will be held in the silo for the required time of fumigation and at the concentration needed to control all life cycles of an insect – the egg, larva, pupa and adult.
A five-minute half-life pressure test is recommended on new silos, with three minutes deemed sufficient on older silos.
A half-life test is conducted by ensuring the oil levels are 25 millimetres apart before starting a timer. The time taken for oil to drop from 25mm apart to 12mm apart must be no less than five minutes.
“If it doesn’t meet that half-life of a minimum of three minutes, ideally five minutes, then you can never be sure you’re going to have a successful fumigation that’s going to control the life cycle of the insects,” Mr Botta says.
“If you’re not reaching five minutes, then there won’t be successful fumigation. Growers may find they are controlling the adults but not controlling the other life stages, which leads to the issue of increasing insect resistance to phosphine from not having proper fumigation.
“We talk a lot about resistance but essentially the issue is the system. If it’s not gas tight then thorough fumigation won’t be achieved.”
Concentration of phosphine and time are the critical factors in fumigation according to Mr Botta, who says under-dosing in the search of a quick turnaround and something as simple as not reading label directions were other mistakes often made during the process.
Working out the number of tablets, or belts, to apply should be done by working out the volume of the silo (or looking at the identification plate near the base of the silo), and then calculating the dose as per the label.
Alternatively, it is recommended that for every 100 tonnes wheat equivalent of grain storage, 200 phosphine tablets (two tins) be applied (Table 1).
It is important for growers to treat the entire storage volume, regardless of whether it is a quarter full, half full, or completely full of grain.
Table 1: Phosphine application rates for stored grain.
Tablets required Tonnes Wheat Cubic Metres
1 1.3 2
50 65 100 (1 tin)
100 130 200 (2 tins)
200 260 400 (4 tins)
300 400 600 (6 tins)
Mr Botta says tablets should be distributed evenly in a tray to ensure they react with the moisture in the storage atmosphere and liberate the gas.
It is recommended not to heap them so as to avoid broken down tablets covering unreacted tablets.
If using blankets or belts, hang these in the silo head space or roll them out flat on the top of the grain so air can pass around them freely as the gas dissipates.
Phosphine is a highly toxic gas with potentially fatal consequences if handled incorrectly.
As a minimum requirement, labels instruct the use of eye protection, elbow-length PVC gloves and a full-face piece respirator with a combined dust and gas cartridge or supplied air respirator.
To control pests at all life stages and prevent insect resistance, phosphine gas concentration needs to reach 300 parts per million (ppm) for seven days (when grain is above 25°C) or 200 ppm for 10 days (when grain is between 15–25°C). (Figure 1)
Insects are less active in cooler grain, so require longer exposer to the gas to receive a lethal dose.
Mr Botta says current ventilation periods range between one to five days, depending on the system.
The withholding period of two days means effective fumigation can range from 10 to 17 days depending on the system in place (Figure 1).
Figure 1: Phosphine fumigation period