Practical management of high level phosphine resistance in Rusty grain beetle
Author: Philip Burrill, DAF Qld. | Date: 24 Jul 2015
Take home message
Rusty grain beetle (RGB) is one of three Flat grain beetle species seen in Australian stored grain. If flat grain beetles survive a fumigation using phosphine, it is important to send the live insects to a laboratory for resistance testing. ProFume® may be recommended if it is the strong resistant strain of RGB.
Consider using this simple “farm storage checklist” that works towards establishing best practice and building a professional, mutually beneficial relationship with your key grain buyers
To achieve reliable grain quality and pest control results, use an integrated strategy of good storage hygiene, monthly monitoring / recording, aeration cooling and appropriate use of protectants and fumigation when required.
A. Integrated pest control
The rusty grain beetle (RGB), (Cryptolestes ferrugineus), is one of three flat grain beetle species found in stored grain throughout Australia. They are a small (2mm long), fast moving beetle with long antennae that readily flies. Sieving grain or using insect probe traps is usually required to detect this pest as it hides and avoids exposure on the grain surface. See Figure 1.
In 2007 a strong phosphine resistant strain of RGB was identified which is now present in the northern and southern grain production regions of Australia. ProFume, (sulfuryl fluoride), may be required to control an infestation of this resistant RGB.
To reduce the risk of RGB and other common storage pests developing resistance, causing grain quality problems and delays in marketing, combine the strategies listed below.
Regular clean up of grain residues in empty storages and grain handling equipment significantly reduces the number of breeding sites for insect pests. For storages and equipment, physically clean &/or water wash out grain residues and dust on a sunny day, then consider using a diatomaceous earth (DE) structural treatment. e.g. Dryacide®
Fit aeration fans to storages and use them to reduce grain temperature. Cooling grain either slows or stops the insects breeding life cycle. Rust red flour beetle stops breeding at 20 °C; lesser grain borer stops at 18 °C; rusty grain beetle (RGB) stops at 17 °C, and all insects stop breeding below 15 °C. Maximum breeding rates for most storage pests occurs at the typical warm grain temperatures during harvest of around 30 °C.
Aerate grain as soon as it is put into storage. Grain temperatures of less than 23 °C in summer and less than 15°C in winter are achievable targets. For reliable results use a good quality automatic controller to run fans, as they select air with the best temperature and humidity available.
Never ‘store and forget’: Sample, sieve and check insect probe traps in your grain at least once per month. Identify pests and keep monthly storage records, including any grain treatments or fumigations. See “On-farm storage Checklist” section.
Grain protectants are applied to clean grain, typically at harvest time while auguring grain into storage. They are not registered for use on grain that is already infested with insect pests. Misuse of these products results in poor adult insect control and selection of resistant insects.
For cereal grains, consider applying grain protectants such as Conserve On-farm® (Dow AgroScience), or K-Obiol® (Bayer) to planting seed in storage, grain held for stock feed on-farm, or grain held for extended periods in sheds or other non-sealable storage where fumigation is not possible.
Prior to using any treatment, always check the label, then discuss with potential grain buyers, as some markets do not accept grain treatments.
As a resistance management strategy, after two consecutive years of using Conserve On-farm (chlorpyrifos-methyl, S-methoprene, spinosad), rotate with one year of the Bayer product K-Obiol (deltamethrin + piperonyl butoxide).
Storage choices / fumigation
Many older silos were not designed to be sealed gas-tight for the purpose of fumigation. Fit these silos with aeration fans and do a good job with hygiene. When buying new silos, look for a quality design, easy to clean, sealable and fitted with aeration fans.
Ask the silo manufacturer if the silo meets the Australian Standard (AS 2628) for sealable silos (i.e. passes a pressure test). Aim to have at least two sealable, aerated silos on your farm. This allows you to achieve an effective fumigation of any infested grain. As a general rule, only leave a silo sealed for the actual time required for the fumigation period of 7 to 20 days.
Table 1. Major pests of stored grain
Lesser grain borer
Rust red flour beetle
Sawtoothed grain beetle
Flat grain beetles &
Figure 1. Rusty grain beetle (RGB) Cryptolestes ferrugineus
B. Diatomaceous earth (DE) - storage hygiene treatments
DE is one of the few products available as a silo structural treatment suitable for all cereal grains, pulses and oilseeds. By using DE, rather than insecticides like fenitrothion, the problem of unwanted chemical residues on oilseeds and pulses being detected by domestic and export grain buyers is minimised.
At the start of a recent wheat harvest on the Darling Downs, over 1000 lesser grain borers were found in the first 30 kg of wheat passing through the header. If the header had been cleaned down and treated with DE after the previous season’s sorghum harvest, this early infestation problem may have been avoided. Any warm, sheltered location with grain residues is a good breeding site for storage pests. The best hygiene results come from physically cleaning out residues, followed by a DE treatment.
“Clean as you go”, for empty storages and grain handling equipment significantly reduces the number of pest breeding sites on farm. During most of the year, particularly in the warmer months, storage pests fly looking for fresh grain to infest.
DE (amorphous silica) is an inert dust made from the fossil remains of diatom skeletons, which are known to have insecticidal properties. Diatoms are a type of green or brown algae that grows in fresh-water lakes and marine estuaries. There are many thousands of species of diatoms. The skeleton of each species has a unique size and shape. DE products produced from the various mine sites in Australia and overseas have different properties. The way DE is processed also affects its properties. It should therefore be of no surprise that of the several commercial DE products available to growers in Australia, there is a significant difference in their efficacy against storage insect pests.
The DAF Qld. Postharvest research team in Brisbane tested the efficacy of four commercial DE products, plus lime in the laboratory in 2013 and 2014. The most significant difference between products, was their efficacy against the very common, rust red flour beetle (Tribolium castaneum), with only two products showing acceptable results. Product performance from best to worst was – Dryacide®, Permaguard®, Cut’n’dry®, Absorbacide® and AgLime®. See Figure 2.
Figure 2. Efficacy of four DE products and AgLime over three weeks against Rust Red Flour beetles
(P.Burrill, V.Byrne, A.Ridley 2014)
DE dust particles kill insects by absorbing some of the waxy layer covering the insect’s body. Damage to this outer ‘water-proofing wax layer’ leads to desiccation and eventual death over a number of days. Research in the USA indicates DE’s impact on the insect’s body is both abrasion and desiccation.
Therefore, DE works best in low humidity conditions. Stored cereal grains, such as wheat, must be no greater than 12% moisture content, or below 60% relative humidity for DE to be effective.
While the Dryacide® label allows for it’s use as both a structural surface treatment and a grain treatment (1 kg DE / tonne grain), in most circumstances, the main focus for DE’s use should be for structural treatment.
The problem for grain treatments at the label rate of 1 kg DE / tonne of grain is that it affects grain flow characteristics and therefore handling. It also has an impact on grain ‘angle of repose’. For this reason most bulk handling companies (e.g. Graincorp etc.) and other grain buyers do not accept bulk grain treated with DE. Presence of DE in grain can also impact on milling quality and there may be other specific market objections to DE dust application to bulk grain. Always check before applying it to grain. Current retail cost for DE ranges from $6 - $10 per kg., making it an expensive treatment for bulk grain.
Structural treatment – storages & grain handling equipment
DE can be applied to structures and equipment either dry or added to water to form a slurry. The rate for dry dust application (straight out of the bag) is 2 g/m², which is equivalent to 1 kg of DE over 500 m². A Blow-Vac gun is one option for application.
DE dust can also be mixed into water at 120 g dust / litre of water, applied at a rate of 5 litres per 100 m² giving an application of approx. 6 g /m² dry basis to the storage structure.
The label specifies a flat fan nozzle with at least 5 litres/minute flow rating, so an XR11015VK ceramic nozzle would be a suitable choice. The ceramic will withstand the abrasion much longer than a plastic nozzle. At 3 bar this nozzle will give you 5.9 L/min.
In summary, for all storages and equipment, physically clean &/or water wash, all grain residues and dust, then consider using a DE structural treatment to deal with any remaining pests.
C. Profume® fumigation - when & how to use
ProFume® gas fumigant (998g/kg Sulfuryl fluoride) is currently registered in Australia for the control of insect pests in stored cereal grains, baled hay, dried fruits, nuts and various other uses as specified on the current label.
It was registered for use in Australia in 2007 by Dow AgroSciences, about the same time as the strong resistant strain of rusty grain beetle (RGB) was identified at bulk handler’s depots in Australia.
In recent months (July 2015) Dow AgroSciences, has sold the Profume product business to Douglas Products LLC based in the USA. However “A-Gas Rural”, the South Australian based company will continue to act as the product handling / distribution company
Key points for ProFume - Sulfuryl fluoride (SF):
- ProFume, a cylinderised gas, can only be use by approved, licensed fumigators. Contact “A-Gas Rural”
- Is only registered for cereal grains, - NOT for pulse or oilseeds
- Requires a gas-tight, sealable storage to maintain gas concentrations for the required fumigation time
- SF fumigations with grain temperatures below 20°C give poor insect control results
- Fumigation time ( 7 days) is critical to ensure control of the egg and pupae stages of storage pests
Prior to the recent (2007) appearance of strong phosphine resistant RGB all the major grain storage pests in Australia were controlled using a phosphine dose rate target that produced at least 300 ppm for 7 days with grain temperatures of 25 °C. This is typically achieved using the current label does rate in gas-tight sealable silos that meet the Australian standard AS2628.
However to control this strong phosphine resistant RGB, research has shown it would now require a concentration of 360 ppm phosphine gas for a minimum of 27 days. This may be too much to ask of many good quality sealable storages. Using ProFume (SF) is the logical choice, once strong phosphine resistant RGB has been identified in a post phosphine fumigation situation.
Fortunately, research has also shown that SF (ProFume) will control this strong resistant RGB. It is important however to be aware it will normally require 7 day fumigations at 25 °C or higher grain temperatures to provide reliable control of all the hard to kill egg stage of our major storage pests. See Figure 3.
Figure 3. Sulfuryl fluoride (SF) - time and concentration required for control of four major storage pests; rust-red flour beetle (blue), rusty grain beetle (red), lesser grain borer (green), rice weevil (purple) (R.Kaur & M.Nayak 2014)
Currently the ProFume gas product costs about $31 per kg. With a standard dose of 35g/m³, this would use 4.4 kg for a silo that can hold 100 tonne of wheat (125 m³). So Profume product cost is about $1.40 per tonne.
To this cost, we also need to add travel and application costs required by the licensed fumigator.
This typically brings total ProFume treatment costs into the range of $4 to $7 per tonne.
Phosphine tablet product costs growers about 40c per tonne. To this add application labour costs.
ProFume and resistance management
Despite ProFume’s relatively high cost, it is very valuable to the Australian grain industry for its key role in dealing with the strong phosphine resistant rusty grain beetle (RGB) (Cryptolestes ferrugineus).
It is important we use ProFume (SF) correctly to maintain it’s efficacy in the grains industry, which includes this important function as a resistant management tool to rotate with phosphine gas.
Use of Profume in situations that result in “under-dosing” insects in poorly sealed storages, or using short fumigation exposure times, will only see insects quickly develop resistance to yet another valuable product.
D. Checklist for on-farm storage – AAA score
Most producers have developed mutually beneficial business relationships with one or more grain buyers / traders/ end users. It is in the interest of growers, buyers and the industry as a whole to see ‘on-farm storage facilities’ well managed and as a profitable part of the farm business.
The aim of the a checklist is to assist grain producers to move towards ‘best practice for their farm storages facilities’. In doing so, individual producers build a reputation as reliable suppliers of good quality, insect free grain. Over time, this builds confidence and profitability for both producer and grain buyer. For the Australian industry, there are cost savings for everyone along the value chain as grain moves to domestic & export markets.
A “Stored grain checklist” is available from the stored grain website at www.storedgrain.com.au or contact Philip Burrill on firstname.lastname@example.org or 0427 696 500 and he will be happy to email it to you.
With a copy of the electronic word document, grain growers and others can add or delete items to suit their own storage facilities and grain trading situations.
The author would like to thank members of the Department of Agriculture & Fisheries Qld., Postharvest grain protection team based at the EcoSciences Precinct, Brisbane. Research results presented were derived from experiments undertaken by Valerie Byrne and Raman Kaur, along with valuable input from Andrew Ridley, Greg Daglish, Pat Collins, Manoj Nayak. The author is also grateful for the continued support of GRDC and the significant contribution from growers with on-farm trials, comments and suggestions.
Department of Agriculture and Fisheries (DAF Qld.)
Mb: 0427 696 500
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