WithTheGrain: Considered fumigation approach needed to avoid long-term resistance

Author: | Date: 02 Jul 2018

Grain storage expert Peter Botta.

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.
Storage CapacityNumber of tablets required
Tonnes WheatCubic Metres
5065100 (1 tin)
100130200 (2 tins)
200260400 (4 tins)
300400600 (6 tins)

Source: Nufarm

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

“The absolute best way to confirm a silo is properly vented is to do a clearance of it (monitoring the gas level),” Mr Botta says.

“It’s not standard practice but it’s probably one of those things we need to be doing more of as an industry because it’s important.

“There are huge changes in on-farm storages so growers need to start looking at what common practice is in other parts of the system like the bulk handling system.

“Bulk handlers would generally always do a clearance to know the grain is coming out at the right threshold and that is as much about the safety of the next person that gets the grain.

“It’s not a contamination issue as such but it’s about ‘is it safe for whoever receives that grain to handle it’.”

The current phosphine gas concentration limit for delivery is 0.3 ppm. Mr Botta says growers had been relied upon to meet this threshold but there had been a rise in instances of the limit being surpassed.

Mr Botta says the issue of insect resistance was a relatively simple equation – fumigation needed to be carried out properly so all life stages of insects were controlled.

He says some farmers would make do with only killing the live adults – which is the delivery requirement – but this could lead to increased resistance further on.

“Fumigation is the only option for eliminating an insect infestation so there has to be a better thought process of how to get set up to do that,” he says.

“If too many people don’t fumigate thoroughly then we may lose phosphine as an option because of resistance.

“It’s up to the growers to ensure that doesn’t happen.”

Looking ahead, growers are urged to invest in gas-tight, sealed silos when exploring new storage options. Technically, a silo is only truly sealed if it passes a five-minute half-life pressure test according to the Australian Standard.

Mr Botta says as an initial step, growers should aim to have sufficient gas-tight storage to accommodate the amount of grain they sell in their biggest delivery month, as a baseline.

“That’s the goal of how much gas tight storage you need so you know you can have grain ready to go, fumigated effectively, and you’re never going to have too little grain to go,” he says.

GRDC research codes: PRB00001

More information

Peter Botta, PBC Consulting, 0417 501 890, pbotta@bigpond.com

Useful resources

GRDC Stored Grain Information Hub – fumigation guide

GRDC GrowNotes – Grain Storage

GRDC Project code: PRB00001