Killing storage pests without mercy

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Author: Andrew Ridley, Philip Burrill and Pat Collins, Queensland Department of Agriculture and Fisheries

Take home message

Results of trial fumigations conducted in 1,400 t silos to test the capability of these large storages have led the following conclusions:

  • Recirculation greatly facilitates the distribution of gas in large silos
  • Fumigation in large silos without recirculation results in much lower concentration in the base of the silo.
  • Peak concentration of phosphine typically occur between day 4 and 6 and decline for the rest of the fumigation.
  • The current pressure half-life standard (AS2628) of 5 minutes is appropriate for large silos and is vital for effective fumigation.
  • Fumigations are likely to fail where there are points of gas / fresh air leaks in a silo. Pressure testing prior to fumigation is a vital step in identifying and locating gas leaks.
  • Strongly phosphine resistant rusty grain beetle can only be controlled by extending fumigation time beyond the minimum label recommendation (of 20 d for blankets) or by implementing active recirculation.

Introduction

There are very few options available to growers to control storage pests when an infestation has been detected. Phosphine, sold in solid formulation of aluminium phosphide (AlP) under the trade names such as phostoxin® or fumitoxin® is by far the most common disinfestation treatment for stored grain.

The label was first written in the 1970’s for relatively small silos and other storages. A significant number of growers are now investing in large capacity (e.g. 1,500 t), flat bottom silos for storing grain on farm. We do not know whether the label directions are appropriate for these larger storages.

Coupled with this uncertainty is the development of strong phosphine resistance in the rusty grain beetle. The resistant populations of the rusty grain beetle, found at a number of sites in eastern Australia, are significantly harder to control than other pests and label rates may need to be updated.

Fan forced recirculation of gas in large silos helps to distribute phosphine and has been advised for some time. Recirculation is not a requirement on the current label but may be a cost effective way to perform better fumigations.

The aim of this trial was to answer the following questions:

  • Can strongly resistant rusty grain beetle be controlled in large farm silos?
  • Is the current Australian Standard (AS2628 – 5 min pressure half-life) for silo gas-tightness appropriate for large silos?
  • What concentrations of phosphine are achieved under passive gas distribution and to what extent does that lengthen the fumigation?
  • Do large silos need recirculation for effective fumigation?
  • What is an acceptable recirculation air flow rate and system design for large silos?

Two silos, labelled A and B, were fumigated at label rates. The phosphine in silo A was dispersed by natural means (passive fumigation). The gas in silo B was recirculated (active fumigation) for the first five days of the fumigation. Phosphine concentrations were monitored at four centre sampling points (headspace and at 9, 5, and 1 m above the floor) and at three points around the base wall (North, 120° and 240°) of each silo. Silo A had a Pressure Half Life (PHL) of 7 minutes and 35 seconds and silo B had a PHL of 2 minutes and 10 seconds. Both silos were leaking air at the silo base entry door during the pressure tests indicating a location for potential gas loss and dilution of gas with fresh air from outside.

Figure 1. Phosphine concentrations measured in silo A (passive fumigation). The silo had a pressure half-life of 7 minutes and 30 seconds. The dosage (concentration x time) required to control phosphine-resistant lesser grain borer is indicted by the blue box and for phosphine-resistant rusty grain beetle by the red box.

Figure 1. Phosphine concentrations measured in silo A (passive fumigation). The silo had a pressure half-life of 7 minutes and 30 seconds. The dosage (concentration x time) required to control phosphine-resistant lesser grain borer is indicted by the blue box and for phosphine-resistant rusty grain beetle by the red box.

Figure 2. Phosphine concentrations measured in silo B (active fumigation). A recirculation system with an air-flow rate of 0.013 L/s/t was fitted to the silo and was run for the first five days of the fumigation. The silo had a below standard pressure half-life of 2 minutes 10 seconds. The dosage (concentration x time) required to control phosphine-resistant lesser grain borer is indicted by the blue box and for phosphine-resistant rusty grain beetle by the red boxes.  Two alternative strategies to meet the required dose to control phosphine-resistant rusty grain beetle are shown. That is, a higher concentration, shorter exposure period and a lower concentration, longer exposure period.

Figure 2. Phosphine concentrations measured in silo B (active fumigation). A recirculation system with an air-flow rate of 0.013 L/s/t was fitted to the silo and was run for the first five days of the fumigation. The silo had a below standard pressure half-life of 2 minutes 10 seconds. The dosage (concentration x time) required to control phosphine-resistant lesser grain borer is indicted by the blue box and for phosphine-resistant rusty grain beetle by the red boxes.  Two alternative strategies to meet the required dose to control phosphine-resistant rusty grain beetle are shown. That is, a higher concentration, shorter exposure period and a lower concentration, longer exposure period.

Conclusions

  • For phosphine fumigations, strongly phosphine resistant rusty grain beetle can only be controlled by extending fumigation time beyond the minimum label recommendation (of 20 d for blankets) or by implementing active recirculation in gas-tight, sealable silo (AS2628)
  • The current pressure half-life standard (AS2628) of 5 minutes is suitable for large silos
  • Fumigation without recirculation requires a fumigation period of over 30 days
  • Recirculation significantly shortened the fumigation period required to 14 days
  • The label recommendations for solid formulations of phosphine must be updated to allow effective control of strongly resistant rusty grain beetle
  • Should label rate fumigations with phosphine fail, and rusty grain beetle is identified, consider an alternative treatments such as sulfuryl fluoride (Profume®)

Based on these conclusions, options for updating the label to ensure control of phosphine resistant rusty grain beetle include:

  1. Increase application rate to maintain current fumigation period of 20 days for passive fumigations
  2. Keep current application rate but extend the passive fumigation period possibly past 30 days
  3. Keep the current application rate but mandate active recirculation, and maintain or possibly reduce the fumigation period
  4. Increase the application rate, mandate active recirculation and reduce the fumigation period

Increasing the application rate (option 1) may be possible but would require APVMA approval and may require significant industry input to undertake residue testing etc. Increasing fumigation period (option 2) is viable but fumigations may become too long to be practical. This option is heavily reliant on silos being sealed to the Australian standard of a 5 min pressure half-life. Mandating recirculation (option 3) would require a small capital cost to retrofit silos. Increasing the application rate in conjunction with active fumigation (option 4) could reduce fumigation times to a week or less.

A number of issues would need to be resolved if any changes are to be made to the label:

Increase application rate

  • Residue testing
  • WHS provisions

Increase fumigation time

  • Fumigating partially filled silos
  • Fumigating highly sorptive commodities such as canola

Active recirculation

  • Minimum flow rates
  • Fan run times

Measuring the level of silo gas-tightness

Pressure tests were carried out on silos A and B before the fumigation and at the end of the fumigation before venting to measure silo gas-tightness. Silos were sealed and pressurised using a cordless leaf blower. Internal pressure was measured using a digital manometer (Exotech HD755) connected to the plumbing of the pressure relief valve which comes from the headspace down the side of the silo.

 

Figure 3. Pressure loss from silo A demonstrates that pressure is lost at a fast rate at higher pressures compared to lower pressures. The rate of pressure loss slows down as the pressure gets closer to atmospheric. This is why it is important to conduct pressure half-life tests using the industry AS2628 standard test method, 250 to 125 Pa.
Figure 3
. Pressure loss from silo A demonstrates that pressure is lost at a fast rate at higher pressures compared to lower pressures. The rate of pressure loss slows down as the pressure gets closer to atmospheric. This is why it is important to conduct pressure half-life tests using the industry AS2628 standard test method, 250 to 125 Pa.

Recirculation system fitted to silo B

A tube was connected to the pressure relief downpipe to 0.37Kw power fan (F370 Downfield, Toowoomba) positioned between the two aeration ducting trenches of the silo (Figure 4). A two way splitter was fitted to the end of a PVC pipe and two 50 mm tubes of equal length was connected to the silo aeration ducting using standard plumbing fittings. Valves (50mm) made it possible to seal the silo at the aeration ducting and isolate the fan for removal. (The short length of white PCV pipe (ID 0.15 m) was fitted to the output side of the fan for the purpose of measuring air flows during the trial.)

Figure 4. Philip Burrill (DAF Qld) measuring air-flow in the recirculation system. For easy to follow details on how to measure air-flow in silos see https://storedgrain.com.au/testing-aeration/

Figure 4. Philip Burrill (DAF Qld) measuring air-flow in the recirculation system. For easy to follow details on how to measure air-flow in silos see this link to the Stored Grain website on Testing Aeration.

Acknowledgements

The research was part of the project PBCRC3150 “An integrated approach to manage and resistance to phosphine in stored grain” supported by the PBCRC of which the GRDC is a partner. Trial fumigations were conducted at Balarang Lands (Weemelah) owned and operated by Jason and Lisa Orchin. We thank them for their support. The authors wish to thank Peter Hobday from AgriStorage and Logistics for assistance conducting the trial.

Contact details

Andrew Ridley
Department of Agriculture and Fisheries, Queensland
EcoSciences Precinct, Boggo Road, Dutton Park
Mb: 0491 215 268
Ph: 07 3255 4442
Email: andrew.ridley@daf.qld.gov.au

Western Region Grain Storage Pest Control Guide

Grain Storage monitoring

The tolerance for live pests in grain sold off farm is nil. With growers increasing the amount of grain stored on farm, an integrated approach to pest control is crucial.

KEY POINTS

  • Effective grain hygiene and aeration cooling can overcome 85 per cent of pest problems.
  • When fumigation is needed it must be carried out in pressure-tested, sealed silos.
  • Monitor stored grain monthly for moisture, temperature and pests.

Prevention is better than cure

The combination of meticulous grain hygiene plus well-managed aeration cooling generally overcomes 85 per cent of storage pest problems. For grain storage, three key factors provide significant gains for both grain storage pest control and grain quality – hygiene, aeration cooling and correct fumigation.

Attack early: Managing grain storage pests starts before grain enters the storage with grain hygiene and structural treatments.

Hygiene

The first grain harvested is often at the greatest risk of early insect infestation due to contamination. One on-farm test found more than 1000 lesser grain borers in the first 40 litres of wheat passing through the harvester. Remove grain residues from empty storages and grain handling equipment, including harvesters, field bins, augers and silos to ensure an uncontaminated start for new-season grain. Clean equipment by blowing or hosing out residues and dust and then consider a structural treatment (see Table 2, page 2). Remove and discard any grain left in hoppers and bags from the grain storage site so it doesn’t provide a habitat for pests during the off season.

Aeration cooling

Freshly-harvested grain usually has a temperature around 30°C, which is an ideal breeding temperature for storage pests (see Table 1). Studies have shown that rust-red flour beetles stop breeding at 20°C, lesser grain borer at 18°C and below 15°C all storage pests stop breeding.

Aim for grain temperatures of less than 23°C during summer and less than 15°C during winter. When placing grain into storage, run aeration fans continuously for the first 2-3 days to push the first cooling front through the grain and to create uniform moisture conditions. Then run the fans during the coolest 9-12 hours per day for the next 3-5 days. This will push a second cooling front through the grain bulk.

Aeration cooling generally only requires air-fl ow rates of 2-4 litres per second per tonne. Finally the grain requires approximately 50 hours of appropriate quality air each fortnight during storage. Use an aeration controller that will perform the cooling process at the right time and continue to aerate the grain selecting the coolest air to run fans. An effective aeration controller will also ensure fans don’t operate when the relative humidity is higher than 85 per cent, which can re-wet and damage grain if operated for extended periods.

TABLE 1 THE EFFECT OF GRAIN TEMPERATURE AND MOISTURE ON STORED GRAIN INSECT AND MOULD DEVELOPMENT

Ineffective fumigation

Fumigation with phosphine is a common component of many integrated pest control strategies. Taking fumigation shortcuts may kill enough adult insects in grain so it passes delivery standards, but the repercussions of such practices are detrimental to the grains industry. Poor fumigation techniques fail to kill pests at all life cycle stages, so while some adults may die, grain will soon be reinfested again as soon as larvae and eggs develop. What’s worse, every time a poor fumigation is carried out, insects with some resistance survive, and pass the resistance gene into their progeny making control more diffi cult in the future.

Effective fumigation

Using the right type of storage is the first and most important step towards an effective fumigation. Only use fumigants, like phosphine, in a pressure-tested, sealed silo. Research shows that fumigating in a storage that is anything less than pressure sealed doesn’t achieve a high enough concentration of fumigant for a long enough period to kill pests at all life cycle stages. For effective phosphine fumigation, a minimum of 300 parts per million (ppm) gas concentration for seven days or 200ppm for 10 days is required. Fumigation trials in silos with small leaks demonstrated that phosphine levels are as low as 3ppm close to the leaks. The rest of the silo also suffers from reduced gas levels.

Achieve effective fumigation by placing the correct phosphine rates (as directed on the label) onto a tray and hanging it in the top of a pressure-tested, sealed silo or into a ground level application system if the silo is fitted with recirculation. After fumigation, ventilate grain for a minimum of one day with aeration fans running, or five days if no fans are fitted. A minimum withholding period of two days is required after ventilation before grain can be used for human consumption or stock feed. The total time needed for fumigating is 10-17 days.

As a general rule, only keep a silo sealed while carrying out the fumigation (for example, one to two weeks). If grain moisture content is low (8-12%) the silo can remain sealed after fumigating but regular monitoring is essential to check for insect infestation and moisture migration to the head space.

TABLE 2 RESISTANCE AND EFFICACY GUIDE FOR STORED GRAIN INSECTS 2010 – CEREAL GRAINS SEPTEMBER 2010 (WESTERN GRAIN PRODUCTION REGIONS).

Check regularly: Monitor stored grain at least monthly, including sampling from the top of the storage, if it can be done safely, or with a pitfall trap.Monitoring

When grain is put into storage it needs monitoring just like it does when it’s in the paddock – regularly.Check stored grain at least monthly, taking samples from the bottom, and if safe, the top of the storage.

Things to monitor:

  • Insect pests
  • Grain temperature
  • Grain moisture content
  • Grain quality and germination

Storage choices

When buying a new silo, buy a quality, sealable silo fi tted with aeration and check with the manufacturer that it meets the Australian Standard for sealable silos (AS2628). Experience has shown that at least two sealable, aerated silos on farm provide the option for an effective fumigation and delivery program. Many older silos are not designed to be sealed and cannot be used for fumigation, however retro-fitting aeration can reduce insect multiplication through grain cooling.

Seed held on farm (cereals — wheat, barley, oats)

Seed that is dry, cool and sound (not weather damaged) will remain viable for longer. In well-managed storage, germination percentages can be expected to reduce by only 5 per cent after six months. To achieve this, keep grain moisture content below 12%. Grain temperature also has a major impact on germination. Aim for grain temperatures of 20°C and below in seed storage by using aeration cooling (with auto control). Wheat at 12 per cent moisture content stored at 30-35°C (unaerated grain temperature) will reduce germination percentages and seedling vigour when stored over a long period. Position small seed silos in the shade or paint them reflective white to assist keeping grain cool. WA growers can treat seed with a grain protectant combined with a dyed grain fungicide in combination with aeration cooling to maximise insect control.

Pulse and oilseeds

Insect control options are limited for stored pulses and oilseeds. Aeration and phosphine fumigation are the main methods and controlled atmosphere (inert gasses such as carbon dioxide or nitrogen) may be an option. The effectiveness of phosphine fumigation on oilseeds is often reduced due to phosphine sorption during treatment.Monitoring gas concentrations with a gas monitor is essential to ensure the correct concentration is achieved for the correct length of time. Use sound grain hygiene in combination with aeration cooling to reduce insect activity. Small seed-size grains, such as canola, may need larger-capacity aeration fans to combat the greater amount of back pressure in the storage. Always store these grains at their recommended grain moisture content level.

PHOSPHINE RESISTANCE IS WIDESPREAD – PLAN, MONITOR AND CONTROL FOR CLEAN GRAIN

  • Dispose of grain residues and seed gradings. Clean empty storages and grain handling equipment, including harvesters, field bins and augers.
  • Sieve stored grain for the presence of insects at least monthly, or use pitfall traps. Also check grain temperature and moisture.
  • If grain temperature has been kept below 15°C by aeration, live insect numbers are likely to be low.
  • Sample grain three weeks before sale to allow time for any treatment.
  • For effective fumigations, pressure test sealable silos at least once a year to identify any leaks and ensure rubber seals are maintained.
  • Phosphine fumigation typically requires 7 to 10 days in a gas-tight sealed silo. When completed, open silo top with care, ventilate using aeration fan for one day; if not aerated, open silo top and ventilate for five days. The minimum withholding period is then two days after ventilation is completed. The total time needed for fumigation is therefore 10-17 days.
  • Sieve a half-litre sample onto a white tray. Hold tray in sunlight to warm for 20 to 30 seconds to encourage insect movement.
  • If live insects are found, identify them and fumigate in a gas-tight silo according to the label.
  • Take care when climbing silos to sample grain for insects and wear a safety harness. Sample from the base, and if safe, take

PHOSPHINE RESISTANCE – NATIONAL SITUATION