Phosphine resistance

摘要

FUMIGANTS PHOSPHINE RESISTANCE M. Q. Chaudhry from the Central Science Laboratory (CSL) of the UK Ministry of Agriculture Fisheries and Food discusses a growing threat to an ideal fumigant The ease of application and residue-safe nature made phosphine a popular choice which replaced virtually all other residue-producing fumigants over the years. Methyl bromide which has been the only other alternative until now is also due for a phase out under the Montreal Protocol due to effects on the ozone layer. It seems likely that phosphine will soon be the only major fumigant available for safe disinfestation of stored commodities. The future availability and efficacy of phosphine is very important for most developing countries in particular where the mode of storage is in bags which can only be feasibly treated with fumigant gases.Phosphine fumigation should be carried out under the following conditions l Trained staff to carry out fumigations l Acceptable standard of gas-tightness of the area under fumigation l Appropriately-timed application of optimal doses of phosphine and maintenance of the exposure over a minimum required length of time l Monitoring of gas concentrations at regular intervals to ensure maintenance of effective levels l Post-fumigation assessment of the effectiveness of each treatment l Integration with other methods (e.g. surface treatment with residual grain protectants or provision of a physical barrier) to reduce the risk of re-infestation during subsequent storage.Phosphine an ideal fumigant Rarely has there been a fumigant gas with such ideal properties that it has remained in use for almost half a century. This certainly applies to phosphine which is one of the most widely used fumigants in the world. Food-grains and other commodities are stored globally as part of food security until the next harvest. An essential component of their safe storage is the management of insects and other pests for which fumigant gases are heavily relied upon in many countries. Phosphine is particularly suited for use in the tropics where a 5ndash;7 day exposure at an effective concentration provides complete control of all insect pests. In temperate regions longer exposures (up to 3 weeks) may be required to achieve similar effects.Phosphine is used for disinfestation of stored seed- and food-grains high-value commodities such as coffee beans and tobacco dried fruits and certain processed foodstuffs. Commodities are fumigated in a variety of storage buildings during transit (e.g. in ship holds) or in plastic sheet enclosures. Phosphine is also used for disinfestation of empty buildings and food processing facilities such as flour mills. The following features make phosphine an ideal fumigant l Effective against all respiring pest species l Does not leave toxic residues in treated communities l Does not harm viability of seeds l Generated in situ from solid metal phosphide products on exposure to moist air l Rapid diffusion in air means a recirculation system is not required during normal fumigations The rust-red grain beetle (Cryptolestes ferrugineus) ndash; one of the major storage pests in the UK.Grain infested with the rust-red flour beetle (Tribolium castaneum) Pest ic ide Outlook ndash; June 2000 88 FUMIGANTS 1991; Chaudhry and Price 1992). These radicals can wreak a havoc to vital proteins and enzymes resulting in insecticidal action of phosphine. Studies on grain beetles however did not reveal any evidence to suggest that R (resistant) insects have a better biochemical defense against oxyradicals (e.g. radical-mopping enzymes) than their S (susceptible) counterparts (Chaudhry and Price 1992). A remarkable feature of phosphine resistance is that it can be selected relatively easily from field populations of most stored product insects which suggests that the frequency of resistance gene(s) is relatively high.This leaves very little room for inadequate fumigations without the risk of selecting resistant individuals from insect populations. Phosphine resistance is also quite stable and R insects do not appear to suffer a biological cost of resistance. The level of resistance in most of our laboratory-maintained insect cultures has not changed over many years even though they were not maintained under the selection pressure of phosphine. The most peculiar feature of phosphine resistance however is the ability of R insects to absorb very little phosphine compared to their S counterparts. This phenomenon has been consistently found in all R populations of different stored product beetles tested at CSL some of them collected from different continents.In fact the uptake of phosphine by some live R beetles has been found to be even lower than that passively absorbed by dead insects; suggesting that resistance mechanism involves lsquo;active exclusionrsquo; of phosphine (Price 1984). Even a similar uptake of phosphine that killed all S beetles caused only minor effects in their R counterparts (Chaudhry and Price 1990) indicating the presence of an additional detoxification process. Conventional genetic studies have also indicated that more than one (possibly two) genes are involved in phosphine resistance. Thus very high levels of resistance found in some insect populations may be because of multigenic R individuals.The overall evidence suggests that either much of the respired phosphine by R insects never reaches the target site and is detoxified elsewhere or the target site is insensitive to phosphine. Many examples of target site insensitivity in insects to insecticides are known. Cyanide insensitive respiration in some insect species is also well known. Studies on other insecticide target sites have shown that only a few point mutations in the relevant gene cause structural changes in target site protein making it less reactive to insecticide(s). Research into isolation and characterisation of genes that encode for phosphine target site in stored product insects is currently in progress at CSL. Development of phosphine resistance Ironically the popular use of phosphine also led to its misuse over the years.The residue-safe nature of phosphine demands that fumigations are carried out effectively. The absence of residual toxicity means that any surviving insects from an inadequate fumigation can re-infest commodities negating the whole operation and necessitating refumigations. An imperative for effective use of any fumigant gas is lsquo;gastightnessrsquo; of the fumigated space but this is often ignored. Lack of trained staff and inadequate management in many tropical regions has led to repeated fumigations in leaky structures where phosphine gas was rapidly lost during treatments. This practice over the years contributed to selection of resistant insect populations.Since first detection in the 1970s (Champ and Dyte 1976) the global scale and spread of phosphine resistance has been phenomenal which in part is due to trade in commodities and poor quarantine measures in some countries. Phosphine resistance is now known to be present in at least 11 species of stored-product insects in 45 countries (Table 1) and the list is growing. In some parts of the world (especially Asia and Africa) insect populations with very high levels of resistance have been reported (Mills 1983; Taylor and Halliday 1986; Taylor 1989; Sayaboc et al. 1998) pointing to the possibility of future control failures. In the UK where phosphine usage forms only a minor part of the total commodity treatment (MAFF Pesticide Usage Survey Reports 1994/95) a survey of grain stores and animal feed mills has revealed evidence of phosphine resistance in three species of grain beetles (Prickett 1994).An example of the use of phosphine fumigation in a developing country is outlined in the following article by Dr Rajendran of the Central Food Technological Research Institute in India; it illustrates some of the problems which can lead to the spread of phosphine resistance. Table 1. Species of stored product insects reported to be resistant to phosphine Sitophilus oryzae Sitophilus granarius Rhyzopertha dominica Tribolium castaneum Tribolium confusum Oryzaephilus surinamensis Trogoderma granarium Cryptolestes ferrigineus Cadra cautella Plodia interpunctella Lasioderma serricorne rice weevil granary weevil lesser grain borer rust-red flour beetle confused flour beetle Saw-toothed grain beetle Khapra beetle flat grain beetle almond moth Indian meal moth cigarette beetle Countering resistance The insecticidal action of phosphine differs from other fumigants in that the relationship of its dosage with concentration and time of exposure is not linear.It is well known that extended exposures to phosphine are much more effective against insects than higher concentrations alone. In fact the latter can induce a state of lsquo;narcosisrsquo; in most species of stored product insects which may protect them from toxic effects. Studies have shown that R insects can survive high concentrations of phosphine over a certain length of Action and inaction of phosphine Despite a number of studies into the mechanism of phosphine resistance the exact biochemical basis of this intriguing phenomenon remains to be elucidated.Phosphine is believed to disrupt normal oxygen metabolism in insects causing production of highly deleterious lsquo;oxyradicalsrsquo; and other intermediates (Bolter and Chefurka 1990; Lam et al. 8 9 Pes ti c ide Out look ndash; June 2000 A piece of tubuar PE sheet folded at both ends to form an enclosure for a small stack of bagged grain. This inexpensive enclosure has proved very effective in small scale phosphine fumigation of farm-stored grain in the tropics (Chaudhry et al. 1989). 1997) may also reduce operator costs. The use of very low levels of phosphine however has the risk of selecting resistant insects in countries where multigenic resistance is present The options for controlling phosphine resistance with another fumigant are scarce.Methyl bromide (whilst available) may be very useful in controlling resistance before switching back to the use of phosphine. An IPM approach incorporating physical methods (cooling heating) or controlled atmospheres (reduced oxygen with high levels of carbon dioxide or nitrogen) can be applicable in certain situations. There are few new fumigants at different stages of development e.g. carbonyl sulfide (Banks et al. 1993) sulfuryl fluoride (Reichmuth et al. 1997) and methyl phosphine (Chaudhry et al. 1997). The latter has been found to be effective against phosphine resistant insects.It however remains to be seen whether any of the new potential fumigants is a suitable substitute for phosphine. FUMIGANTS A model bag stack in polyethylene sheet enclosure for indoor fumigation with phosphine in the tropics. For outdoor application the enclosure is further covered under heavy tarpaulin sheets and tied with ropes to prevent wind damage to PE sheet and loss of the fumigant gas (photo courtesy of Grain Storage Research Laboratory Karachi Pakistan). time but not extended exposures to even a relatively low concentration. It appears that the toxic effects of phosphine accumulate slowly in R insects and that the resistance mechanism is overwhelmed during long exposure periods.The main strategy to achieve maximum effectiveness of phosphine against all life stages of insect pests (including R ones) must therefore aim to extend exposure periods rather than increasing the dose unnecessarily. Despite the high levels of resistance found in some insect species a combination of optimal doses of phosphine and extended exposures can still provide effective control of insect infestations. A detailed study has shown that exposure to at least 0.33 mg lndash;1 (240 ppm) phosphine for 14 days in the tropics is sufficient to completely control all stored product insects including those highly resistant to phosphine (Mahmood et al. 1991). The length of exposures to phosphine can however be extended in a cost-effective manner only if the area to be fumigated is reasonably gastight.Rendering storage buildings leak-proof or constructing new ones to a high gastightness standard could be prohibitively expensive for most developing countries where phosphine resistance is also widespread. Through appropriate strategies however it should be possible to recover the cost of improving fumigation practices in terms of subsequent savings on the cost of fumigant and preservation of grain quality and quantity. An effective way of extending phosphine exposures is by fumigating commodities in a gas-proof plastic sheet enclosure (Graver and Annis 1994). Fumigation of relatively dry grain with phosphine and subsequent storage in polyethylene sheet enclosure to avoid re-infestation has been recommended to prolong the duration of insect-free storage (Ahmed et al.1987; Chaudhry et al. 1989). It is also possible to extend phosphine exposure in a semigastight situation although the cost of fumigant will increase with the degree of leakage. For example the normal dose of phosphine may be divided into multiple applications at appropriate time intervals to maintain an adequate level throughout the exposure period. Alternatively the top-up amount of phosphine may be supplied from a cylinder to compensate for the loss through leakage. Other automated lsquo;flow-throughrsquo; systems which maintain or recirculate low levels of phosphine over long periods (Winks and Russell References Ahmed H.; Ahmed M.; Ahmed A. (1987) Protection of bagged grains by government agencies in Pakistan- a scrutiny of current practices and recommendations for improvements.Technical Report No. 3 Grain Storage Research laboratory Pakistan Pesti c ide Outlook ndash; June 2000 90 Agricultural Research Council University of Karachi Karachi Pakistan. Banks H. J.; Desmarchelier F. J. M.; Ren Y. (1993) Carbonyl sulfide fumigant and method of fumigation Patent WO 93/13659. Bolter C. J.; Chefurka W. (1990) The effect of phosphine treatment on superoxide dismutase catalase and peroxidase in the granary weevil Sitophilus granarius. Pesticide Biochemistry and Physiology 36 52ndash;60. Champ B. R.; Dyte C. E. (1976) Report on the FAO global survey of pesticide susceptibility of stored grain pests. FAO Plant Protection and Production Services No.5 FAO Rome 297 pp. Chaudhry M. Q. (1997) A review of the mechanisms involved in the action of phosphine as an insecticide and phosphineresistance in stored-product insects Pesticide Science 49 213ndash;228. Chaudhry M. Q.; MacNicoll A. D.; Mills K. A.; Price N. R. (1997) The potential of methylphosphine as a fumigant for the control of phosphine-resistant strains of four species of storedproduct insects In Proc. International Conference on Controlled Atmosphere and Fumigation in Stored Products Nicosia Cyprus 21ndash;26 April 1996 pp 45ndash;57. Chaudhry M. Q.; Price N. R. (1992) Comparison of the oxidant damage induced by phosphine and the uptake and tracheal exchange of 32P-radiolabelled phosphine in the susceptible and resistant strains of Rhyzopertha dominica (F.) (Coleoptera Bostrychidae) Pesticide Biochemistry and Physiology 42 167ndash;179.Chaudhry M. Q.; Price N. R. (1990) Insect mortality at doses of phosphine which produce equal-uptake in susceptible and resistant strains of Rhyzopertha dominica (F.) (Coleoptera Bostrychidae) Journal of Stored Product Research 26(2) 101ndash;107. Chaudhry M.Q. Ahmed H. and Anwar M. (1989) Development of an airtight polyethylene enclosure for integrated pest management of grains stored at farm level in Pakistan. Tropical Science (London) 29 177ndash;87. Graver J. van S.; Annis P. S. (1994) Suggested recommendations for the fumigation of grain in the ASEAN Region Part 3. Phosphine fumigation of bag-stacks sealed in plastic enclosures an operations manual ASEAN Food Handling Bureau (AFHB) Kuala Lumpur Malaysia 79 pp.Lam W. W.; Toia R. F.; Casida J. E. (1991) Oxidatively initiated phosphorylation reactions of phosphine. Journal of Agricultural and Food Chemistry 39 2274ndash;2278. EVER THOUGHT OF WRITING AN ARTICLE FOR PESTICIDE OUTLOOK? The Editor would welcome articles for inclusion in Pesticide Outlook. The aim of the journal is to publish readable up-to-date interesting articles for a wide audience which should be understandable without any assumed specialist knowledge. They should have an introduction a few sections of ldquo;meatrdquo; arranged to give a logical flow of argument and end with a conclusion summing things up and pointing the way forward. Articles can range in length from 500ndash;2000 words.Photographs diagrams tables etc. are welcomed to increase the visual appeal of the article. Please note that contributions are refereed by two members of our Editorial Board and so publication is not guaranteed. A small honorarium is paid on publication. Please send manuscripts to Hamish Kidd Pesticide Outlook The Royal Society of Chemistry Thomas Graham House Science Park Milton Road Cambridge CB4 0WF. FAX +44 (0)1223 420247; email KIDDH@RSC.ORG. FUMIGANTS Mahmood T.; Ahmed M. S.; Javed M. A.; Iqbal M. (1991) Determination of phosphine dosage for the control of resistant stored grain insect pests in Pakistan in Proc. Bulk Wheat Handling and Storage Conference Lahore Pakistan June 17ndash;19 1991 pp 155ndash;169 Kansas State University Food and Feed Grain Institute Kansas USA.Mills K. A. (1983) Resistance to the fumigant hydrogen phosphide in some stored-product species associated with repeated inadequate treatments Mitteilungen der Deutschen Gesellschaft fur Allgemeine und Angewandte Entomologie 4(1/3) 98ndash;101. Price N. R. (1984) Active exclusion of phosphine as a mechanism of resistance in Rhyzopertha dominica (F.) (Coleoptera Bostrychidae). Journal of Stored Product Research 20 163ndash;168. Prickett A. J. (1994) Animal Feed Mills 1992; England and Wales; Pest Management Parts 1 and 2 Report No 54 Central Science Laboratory Sand Hutton York UK. Reichmuth C.; Scholler M.; Dugast L. F.; Drinkall M. J. (1997) On the efficacy of sulfuryl fluoride against stored-product pest moths and beetles In Proceedings of the International Conference on Controlled Atmosphere and Fumigation in Stored Products Nicosia Cyprus 21ndash;26 April 1996 pp 17ndash;23.Sayaboc P. D.; Gibe A. J. G.; Caliboso F. M. (1998) Resistance of Rhizopertha dominica (F.) (Coleoptera Bostrychidae) to phosphine in the Philippines Philippine Entomologist 12(1) 91ndash;95. Taylor R. W. D. (1989) Phosphinemdash; a major fumigant at risk International Pest Control 31 10ndash;14. Taylor R.W.D. and Halliday D. (1986) The geographical spread of resistance to phosphine by coleopterous pests of stored products. in Proceedings ldquo;British Crop Protection Conference Pests and Diseasesrdquo; Brighton UK 1986 pp. 607ndash;613. Winks R. G.; Russell G. F. (1997) Active fumigation systems better ways to fumigate grain in Proceedings of the International Conference on Controlled Atmosphere and Fumigation in Stored Products Nicosia Cyprus 21-26 April 1996 pp 293ndash;303. Qasim Chaudhry has been involved in research into mechanisms of insecticide and fumigant resistance in insects at the Central Science Laboratory of the Ministry of Agriculture Fisheries and Food at Sand Hutton York UK. Of particular interest are the study of biochemical and genetic factors involved in resistance and development of strategies to manage resistant pest species. 9 1 Pes tic ide Out look ndash; June 2000