Resistance of grain storage insects to pesticides

An image of a modern sealable silo

Modern silo equipped with pressure relief valve.

Phosphine resistance

Importance of phosphine as a stored grain fumigant treatment in Australia

Australia´s grain industry relies heavily on phosphine fumigation to meet market requirements for insect free grain. About 80% of Australia´s cereal grains are treated by phosphine fumigation. Alternative treatments are more expensive and some are not as widely accepted by markets.

Prolonging the effective life and availability of phosphine is in the interests of everyone involved in the grains industry, including growers, handlers and merchants.

Importance of insect resistance to phosphine

Resistance is a threat to the future effective use of phosphine. Weak resistance to phosphine has been common in Queensland for a decade. Strong resistance has been identified, and is widespread, but not yet common. Maps are available showing the Australian and eastern Australian sites of resistance to phosphine fumigation has been recorded.

What are the observed trends in phosphine resistance development so far?

The frequency of strong resistance to phosphine is increasing in four of the five major insect pest species. This resistance was first detected in 1997. About 5% of insect populations contain individuals with this strong resistance.

Strong resistance occurs throughout Eastern Australia but has not yet been detected as endemic in the western grain region. The highest level of resistance occurs in the lesser grain borer (Rhyzopertha dominica). The department's Stored Grain Team have based our new recommendation for use of phosphine on this lesser grain borer's resistance.

Several resistant strains of tiny grain pests called psocids have been found, particularly in South Australia.

We expect that the number of resistant insects found will increase rapidly over the next five years. Resistance is likely to appear in all grain growing areas in that time.

The cause of resistance to phosphine

To understand the development of phosphine resistance, we need to understand how phosphine works. In farm grain storages, phosphine is usually applied as aluminium phosphide tablets (e.g. Fostoxin®, Fumitoxin®, X-phos® & other trade names). These react with moisture in the air to release phosphine gas. This can take only a day at high temperatures, or as long as four days at low temperatures. The gas moves around by diffusion and in air currents inside the silo.

The cause of insect resistance to phosphine is the widespread practice of fumigation in unsealed silos, in farm storages, and in merchant and bulk handling storages. This results in frequent exposure of insect populations to sub-lethal dosages, allowing the rare individuals with a new resistance gene to survive treatment and continue breeding, passing on their resistance. Repeat fumigations favour the insects that carry the resistance gene by allowing them to survive, but killing normal, susceptible insects.

Resistance and unsealable (i.e. ´conventional´) silos

As phosphine moves around, it leaks rapidly from silos that are not sealed to be gas-tight (Figure 1). Susceptible adult insects are killed quickly, usually within a day. However, immature insects in the egg and pupal stages are tolerant of phosphine and survive the short exposures to high concentrations of phosphine in unsealed silos (Figure 1). Strongly resistant adults can also survive fumigations in unsealed silos.

Phosphine fumigation in unsealed silos can give the appearance of success by killing susceptible adults. But when strongly resistant insects are present, phosphine fumigation in an unsealed silo will have virtually no effect on the insects. Figure 1 and 2 show the phosphine fumigation in an unsealed and sealed silo .

See Figure 1 - Phosphine fumigation in an unsealed silo

See Figure 2 - Phosphine fumigation in a sealed silo

Resistance and sealable silos

Silos that can be sealed well enough to pass a pressure test will hold a high enough concentration of phosphine for long enough to kill all stages of the insects, including resistant insects. 

Fumigation in sealed silos is a solution to the phosphine resistance problem emerging in Australia.

Slowing development of resistance

You can affect the build-up of resistance by adopting an Integrated Pest Management (IPM) approach to grain insect control by close management of moisture content and temperature of grain in storage - the cooler and drier the grain, the less insects like it. Maintaining strict hygiene standards in grain handling equipment and storage facilities is also a vital component of the pest management program. These non-chemical components greatly reduce the threat from insects and take pressure off both fumigants and protectant insecticides in the overall pest management program. 

See Stored grain management guide

Use phosphine appropriately. It is simply impossible with an unsealed storage to ensure adequate phosphine concentration and exposure time for killing all stages of the insect pest´s life cycle. Some of these are concealed within the individual grains and are thus less susceptible to phosphine, unless it´s used properly. There is also no chance of killing resistant insects in an unsealed silo.

Even though the grain industry has to cope with strong phosphine resistance, Australian research has shown that resistant insect populations can be controlled in sealed storages. Investment in sealable, aeratable storages is essential for today´s grain grower who is storing more grain for longer periods on-farm.

Our researchers must strive to identify and test new chemical treatments.

Limit insect populations by appropriate management of grain moisture and temperature using aeration and/or hot air drying while in storage.

See Aeration for Cooling and Drying

Carbon dioxide can be used as an alternative fumigant to control phosphine resistant insects but it costs 5-10 times as much as phosphine, and also requires a sealed silo. Alternative control options should be used in unsealed silos. These could include aeration cooling and/or protectant insecticides to prevent or slow the development of insect infestations, and spraying grain with dichlorvos to kill insects in the grain.

What does the future hold?

Based on our Australian and overseas experience, resistances can be expected to become more serious and widespread.

  • Scientists are trying to improve their understanding of the mechanism of resistance development using the latest ´biotech´ techniques. With grain industry support through GRDC, University of Queensland scientists are searching for a ´rapid test´ for phosphine resistance.
  • We´d like to retain phosphine as a principal fumigant for the foreseeable future because of its low cost, its ease of use and its acceptance by all markets i.e. there is no ready alternative.
  • We will see wider investment by the grain industry in sealable, aeratable silos. This is aimed at improving the effectiveness of phosphine useage. Some Australian silo manufacturers have already reported a marked increase in demand for such silos from grain growers in recent years.
  • The Australian grain industry will adopt a ´Food Production´ ethos in the management of grain production and marketing. There is a widespread trend overseas towards ´identity preservation´ and Quality Assurance (QA).
  • We will continue to investigate and develop new fumigants (and protectant insecticides) for industry adoption. For example, a new fumigant, ´Vapormate´® (ethyl formate and CO2 mix developed by CSIRO), is now available from BOC.

See Stored grain management guide

There are other fumigants also under investigation, but these too are expected to be more expensive and more complex to apply than phosphine products currently in widespread use in Australia.

Protectant insecticide resistance

Importance of protectant insecticides in the Australian grain industry

Resistance of grain insects to protectant insecticides is becoming a major problem in Australia. Mixtures of protectants are recommended because no single protectant will give long-term protection against all resistant species. Protect insecticides are being used less frequently each season in Australia. It is estimated that only about 5% of export grain in Queensland and Northern New South Wales is treated, very little in southern States and no export grain is treated in Western Australia. However, grain kept for planting seed is commonly treated with protectants.

Which species are resistant to commonly used protectant insecticides?

In Table 1 we have classified the resistance status of the five common beetle pests of stored cereal grain as:

  • common - at least 30% of strains tested include resistant individuals; alternative insecticides or other control measures should be used to control this species
  • increasing - between 5% and 30% of strains tested include resistant individuals; alternative insecticides or other control measures should be used to control this species if the insecticide has not given effective control at the storage site
  • rare - less than 5% of strains tested include resistant individuals; rotation with alternative insecticides may prolong the useful life of the insecticide
  • none - resistant individuals have not been detected in any strain
  • not effective - the species was never controlled by the insecticide.

See Modern sealable silo equipped with pressure relief valve

See Map 3 - Sites showing resistance to protectant insecticides

Table 1. Status of resistance in Australia to registered grain treatments in the common insect and mite pests of stored grain in 2007
Protectant insecticide Sawtooth grain beetle Lesser grain borer Rice weevil Red rust flour beetle Common moth species Mites Psocids
Pimimiphos-methyl e.g. Actellic® Resistance common Not effective Resistance rare No known resistance Not effective Not effective against some species
Fenitrothion e.g. Fenitrothion 1000®
Chlorpyrifos-methyl e.g. Reldan®
Methoprene e.g. IGR®, Diacon® No known resistance Resistance common Not effective No known resistance Not effective Not effective against any species
Dichlorvos e.g. DDVP500® Resistance common* No known resistance No known resistance Resistance status untested
Spinosad e.g. Conserve® Registered in USA, and registration being sought in Australia Not effective No known resistance Not effective No known resistance No known resistance when used with chlorpyrifos-methyl
Diatomaceous earth e.g. Absorba-Cide®, Dryacide® No known resistances, when used in grains drier than 13% moisture

*The higher dose rate of dichlorvos is reasonably effective against resistant lesser grain borer, but withholding period increases from 7 days for low dose rate to 28 days for the high dose rate.

Resistance to protectant insecticides is of major concern in two beetle pests of grain in Queensland:

  1. Lesser grain borer is resistant to all the organophosphorus (OP) insecticides fenitrothion, Reldan, Actellic and dichlorvos, and a few cases of resistance to IGR/Diacon have been found. Thus, lesser grain borer is resistant to some degree to almost all registered grain treatments. It is expected that a new insecticide, spinosad, (trade name: Conserve®) will be released for control of lesser grain borer in Australia in 2011.
  2. Sawtoothed grain beetle is also resistant to some extent, to the organophosphorus insecticides fenitrothion, Reldan and Actellic. Methoprene, (IGR/Diacon) is still effective against this species.

See Identification of insect pests

Two very tiny pests of stored grain, mites and psocids, are also difficult to kill using protectants. Mites and psocids are naturally tolerant of the protectant insecticides. DEEDI research has shown that a combination of chlorpyrifos-methyl and spinosad e.g. Reldan® + Conserve®, at full label rates of each, will effectively control most psocid species. 

See Psocid pests (or booklice ) and Mite pests

Do you have resistant insects?

Resistance should be suspected when insects appear before the end of the storage period claimed on the label, and the protectant was applied evenly at the label rate, especially if insects of one type only are involved.

If this occurs, use different insecticides for future treatments. For a full summary of insecticides approved for use in Queensland for treatment of grains and grain handling and storage equipment, see Stored grain management guide .

However, don´t jump to the conclusion that every control failure is caused by resistance - most are not. Usually the reason for control failure is a problem with the dose rate or application technique.

If you suspect resistance to insecticide treatments please send live insect specimens to Dr Pat Collins, Stored Grain, Entomology, DEEDI, 80 Meiers Road, Indooroopilly, Qld, 4068. Please include all your contact details and details of previous treatments with the insect specimens. This is a free service which is intended to improve management of resistance.

Slowing development of resistance

You can affect the build-up of resistance by the control methods you use.

Repeated use of the same chemicals will speed up resistance development. Rotation between alternative chemicals may slow resistance. Fortunately, lesser grain borer will be manageable through use of spinosad in the foreseeable future, despite development of increased resistance to methoprene. We also recommend amorphous silica / diatomaceous earth powder (eg, Dryacide®, Absorba-Cide®, Perma-Guard D10®) instead of chemical insecticides for treating silo surfaces and storage areas after cleanup because this reduces selection for resistance to the chemicals.

Selection for resistance can also be minimised by limiting the use of protectant chemicals. Use the non-chemical control methods such as improved hygiene in grain storage and handling facilities and cooling with aeration to slow insect population growth.

New chemicals

Over the past 30 years, resistance in stored grain pests has been overcome by registration of new groups of chemicals that are effective against the resistant insects. Researchers continue to search and evaluate new groups of chemicals for insecticides which are both effective against the current range of resistant insects and safe for use on grain. Testing and registration of new chemicals is time consuming and expensive. Recent research has highlighted the efficacy of spinosad (Conserve®) as a protectant insecticide for control of lesser grain borer in stored grain.


Contact us or the National Grain Storage Extension Team:

Queensland Philip Burrill: 07 4660 3620

Victoria Peter Botta: 03 5761 1647

Western Australia Chris Newman: 08 9366 2309

Further Information

Last updated 18 November 2010