Resistance to insect pest : Stored product pests have long been a problem in food processing plants, warehouses and storage structures (bins/silos).
Stored product insect infestations deteriorate commodities both qualitatively and quantitatively by decreasing the nutritional value or causing mold, leading to a reduction in overall food quality.
Resistance to pesticide
Conventional pesticides are being used as the major tools for stored grain and food protection. Many conventional pesticides
Chemical insecticides
Including fumigants, disinfectants and grain protectants, are essential components of grain insect pest management systems.
Worldwide, the fumigant phosphine is by far the most important insect control treatment for stored grain. However, resistance to this fumigant has developed in major pest species in many regions threatening its continued viability.
The incidence of resistance to residual grain protectants is widespread. Populations of major pest species have developed resistance to organophosphates, pyrethroids, carbamates and other agents such as methoprene and Bacillus thuringiensis.
In some regions, the situation is precarious with insect populations containing multiple resistances leaving no effective protectant options available. Because grain protection chemicals are a rare resource, the ability to manage or reduce the impact of resistance is a priority.
Effective management relies on early detection which can only be achieved with a routine monitoring system and a research capability to estimate the impact of resistance. Development of effective strategies requires an understanding of the grain storage system, the ecology of the pest insects, Insect resistance, chemical, fumigant, phosphine,
Protectants
Protectants, which are applied as liquids or dust directly to the grain stream are designed to provide long term protection. They are currently used on about 25 % of the grain in Australia. There is a range of protectant chemicals with various efficacious.
However, none will control all species, so a mixture of two is applied to the grain. Malathion was the first “protectant”. It came into widespread use in Australia in mid-1960.
Malathion
It provided only about 12 years of widespread use before being abandoned, with the incidence of resistance increasing from zero to virtually 100 % in that time. In the same era, resistance to malathion became widespread internationally and occurred in many species (Champ and Dyte 1976).
In most species economic resistance to malathion did not extend to other potential protectants (Champ and Dyte, 1976) so that a range of organic phosphorothioate materials, including fenitrothion, chlorpyrifos-methyl, and pirimiphos-methyl, was introduced to replace malathion.
The exception was R. Dominica. Resistance to malathion in this species was so strong that no other chemically similar protectant could be used against this pest.
However, this resistance did not extend to an insecticide group called “pyrethroids” and one or more of these has been used against this species since the loss of malathion. The pyrethroid, bioresmethrin, was used successfully for about 12 years in Australia until resistance was first detected in 1990 (Collins et al., 1993).
Incidence of resistance was patchy at first, but a strong increase in the frequency of resistance is evident for the last 10 years (this includes a period where deltamethrin has replaced bio resmethrin).
For example, detection of resistance to pyrethroids in the north-eastern grain belt, where data have been collected systematically, has increased steadily from a few percents in 1996 to greater than 50 % in the last few seasons.
As R. Dominica is the most significant pest and the most difficult to control, a second protectant, the juvenile hormone analog methoprene, was developed from another chemical group and introduced in about 1994.
This material has the advantage of also being effective against T. castaneum and very effective against Oryzaephilus surinamensis (L.). Resistance to this material was detected in R. Dominica only two years later with the frequency continuing to increase steadily over the last 10 years.
The organo phosphorous
Protectants were more successful, and although economic resistance has been detected to these chemicals in T. castaneum and Sitophilus oryzae (L.), these incidences remain rare. However, only a few years after their introduction, populations of O. surinamensis, an insect previously considered a minor pest, were detected with resistance to fenitrothion (Heather and Wilson, 1983).
Since that time, this resistance has become widespread in eastern Australia and broadened to include all registered OP protectants. (Collins and Wilson, 1987). Disinfectant Under this heading I include dichlorvos.
This chemical is a highly volatile organophosphate that is applied in the same way as a grain protectant. However, it does not provide residual protection to the grain as it has a short half-life. Resistance to dichlorvos is very common in R. Dominica but not detected in other pest species.
Fumigants
Are applied as gases and penetrate the grain mass to exert control of insect populations. Because of various problems with other fumigants, phosphine is by far the dominant material used to protect grain world-wide. Phosphine is unique and the likelihood of finding a replacement that is as cheap, effective (when properly applied), easy to use and accepted by markets as a residue-free treatment, is extremely remote.
Although phosphine has been used by the Australian grain industry since the mid-1950 s it was generally regarded as a back-up for the grain protectants.
However, from the 1980 s, both domestic and international markets began to increasingly require nil or very low chemical residues on grain. Phosphine was the only viable replacement for grain protectants and consequently, its use increased dramatically through the 1990s.
Phosphine
Is now the primary insect control tool used on 70-80 % of the grain. Concomitant with the increased use of phosphine was an increase in the frequency of resistance in all five major target pests. Resistance at that time was referred to as weak or moderate and was not a major concern for the industry.
However, from 1997, there appeared a quantitative change in resistance levels in four of the five major species. This change occurred in each species when the frequency of weak resistance reached about 80 %.
Genetic analysis
Revealed that moderate resistance in R. Dominica was controlled by one major gene and that it was this gene plus the selection of a second resistance gene that produced the new “Strong resistance” phenotype (Schlipalius et al., 2002). The strong resistance phenotype has been detected in all states of eastern Australia and all sectors of the grains value chain.
For example, not only is the cost of fumigant increased, but silos also need to be of a high standard to maintain the gas concentrations required, and 9th International Working Conference on Stored Product Protection 280 logistics of grain handling are compromised.
There is also further evidence from China (Wang et al., 2006) that populations of other pest species will survive long fumigations – up to 21 days at reasonably high concentrations.
The danger for Australia is that major increases in resistance, similar to those experienced overseas, will evolve with the consequence that phosphine will be either ineffective or almost useless because effective fumigations will require such long fumigation periods and very high concentrations of gas.
This concern has motivated the Australian grain industry to develop a national phosphine resistance strategy in consultation with researchers.
Managing resistance to phosphine
The Australian strategy has four parts:
- A national monitoring program to provide early detection of resistance and strategic and tactical information.
- A research capability to estimate the impact of resistance and to develop effective strategies to combat resistance.
- An extension network to promote fumigation best management practice.
- A resistance management implementation plan.
The plan because the grain industry operates within fairly strict regulatory, operational and market-driven boundaries these constraints had to be recognized and incorporated into the Australian phosphine resistance management plan. Nevertheless, the strategy is based on the best scientific information on insect pest biology and resistance selection available.
The strategy is based on tactics to
- Reduce selection pressure.
- Destroy resistant insects. Selection pressure is reduced by limiting the number of fumigations applied to any one parcel of grain to three per year and by better application of non-chemical options such as cooling and hygiene. Resistant populations are destroyed by “making every fumigation count” i.e. by only Pest Resistance to Pesticides and Control Measures 281 using approved rates of phosphine that are researched and known to control resistant insects, and by using alternative fumigants or protectants where available.
Authors: 1Faheem Shoukat, 2Zohaib Afzal
1Department of Entomology, MNS- University of Agriculture, Multan, Punjab, Pakistan.
2Department of Soil Science, MNS- University of Agriculture, Multan, Punjab, Pakistan.