Combined treatments of spinosad and chlorpyrifos-methyl for management of resistant psocid pests (Psocoptera: Liposcelididae) of stored grain

The combined efficacy of spinosad and chlorpyrifos-methyl was determined against four storage psocid pests belonging to genus Liposcelis. This research was undertaken because of the increasing importance of these psocids in stored grain and the problem of finding grain protectants to control resistant strains. Firstly, mortality and reproduction were determined for adults exposed to wheat freshly treated with either spinosad (0.5 and 1 mg kg(-1)) or chlorpyrifos-methyl (2.5, 5 and 10 mg kg(-1)) or combinations of spinosad and chlorpyrifos-methyl at 30 degrees C and 70% RH. There were significant effects of application rate of spinosad and chlorpyrifos-methyl, both individually and in combination, on adult mortality and progeny reduction of all four psocids. Liposcelis bostrychophila Badonnel and L. decolor (Pearman) responded similarly, with incomplete control of adults and progeny at both doses of spinosad but complete control in all chlorpyrifos-methyl and combined treatments. In L. entomophila (Enderlein) and L. paeta Pearman, however, complete control of adults and progeny was only achieved in the combined treatments, with the exception of spinosad 0.5 mg kg(-1) plus chlorpyrifos-methyl 2.5 mg kg(-1) against L. entomophila. Next, combinations of spinosad (0.5 and 1 mg kg(-1)) and chlorpyrifos-methyl (2.5, 5 and 10 mg kg(-1)) in bioassays after 0, 1.5 and 3 months storage of treated wheat were evaluated. The best treatment was 1 mg kg(-1) of spinosad plus 10 mg kg(-1) of chlorpyrifos-methyl, providing up to 3 months of protection against infestations of all four Liposcelis spp. on wheat.     

Efficacy of current and potential grain protectant insecticides against a fenitrothion-resistant strain of the sawtoothed grain beetle,Oryzaephilus surinamemis L

The responses of a highly fenitrothion-resistant and a susceptible strain of Oryzaephilus surinamensis L. from Australia were measured against a range of alternative grain protectants using impregnated-paper and treated-grain assays. The R-strain was also highly resistant to carbaryl, moderately resistant to malathion and pirimiphos-methyl and showed low-level resistance to methacrifos, chlorpyrifos-methyl, bioresmethrin, permethrin, cyfluthrin, cypermethrin and dichlorvos. There was marginal resistance to deltamethrin. Methoprene, to which there was no resistance, was the most effective material at suppressing progeny production of the resistant strain. At current commercial application rates chlorpyrifos-methyl was the only compound of those tested that would completely control both adults and larvae of resistant O. surinamensis for a storage period of nine months. Overall, the homogeneity of response of both strains to insecticides using the treated-grain assay was as good as the response from impregnated-paper assays. The latter also tended to exaggerate resistance factors compared to the treated-grain assays.     

Strong resistance to phosphine in the rusty grain beetle,Cryptolestes ferrugineus (Stephens) (Coleoptera: Laemophloeidae): its characterisation,a rapid assay for diagnosis and its distribution in Australia

BACKGROUND: The recent development of very high resistance to phosphine in rusty grain beetle, Cryptolestes ferrugineus (Stephens), seriously threatens stored‐grain biosecurity. The aim was to characterise this resistance, to develop a rapid bioassay for its diagnosis to support pest management and to document the distribution of resistance in Australia in 2007–2011. RESULTS: Bioassays of purified laboratory reference strains and field‐collected samples revealed three phenotypes: susceptible, weakly resistant and strongly resistant. With resistance factors of > 1000 × , resistance to phosphine expressed by the strong resistance phenotype was higher than reported for any stored‐product insect species. The new time‐to‐knockdown assay rapidly and accurately diagnosed each resistance phenotype within 6 h. Although less frequent in western Australia, weak resistance was detected throughout all grain production regions. Strong resistance occurred predominantly in central storages in eastern Australia. CONCLUSION: Resistance to phosphine in the rusty grain beetle is expressed through two identifiable phenotypes: weak and strong. Strong resistance requires urgent changes to current fumigation dosages. The development of a rapid assay for diagnosis of resistance enables the provision of same‐day advice to expedite resistance management decisions. © 2012 Commonwealth of Australia. Published by John Wiley & Sons, Ltd.     

Influence of concentration, temperature and humidity on the toxicity of phosphine to the strongly phosphine-resistant psocid Liposcelis bostrychophila Badonnel (Psocoptera: Liposcelididae)

BACKGROUND: The psocid Liposcelis bostrychophila Badonnel, is a widespread, significant pest of stored commodities, has developed strong resistance to phosphine, the major grain disinfestant. The aim was to develop effective fumigation protocols to control this resistant pest. RESULTS: Time to population extinction of all life stages (TPE) in days was evaluated at a series of phosphine concentrations and temperatures at two relative humidities. Regression analysis showed that temperature, concentration and relative humidity all contributed significantly to describing TPE (P<0.001, R(2)=0.95), with temperature being the dominant variable, accounting for 74.4% of the variation. Irrespective of phosphine concentration, TPE was longer at lower temperatures and high humidity (70% RH) and shorter at higher temperatures and low humidity (55% RH). At any concentration of phosphine, a combination of higher temperature and lower humidity provides the shortest fumigation period to control resistant L. bostrychophila. For example, 19 and 11 days of fumigation are required at 15 degrees C and 70% RH at 0.1 and 1.0 mg L(-1) of phosphine respectively, whereas only 4 and 2 days are required at 35 degrees C and 55% RH for the same respective concentrations. CONCLUSIONS: The developed fumigation protocols will provide industry with flexibility in application of phosphine.     

Detection and characterisation of strong resistance to phosphine in Brazilian Rhyzopertha dominica (F.) (Coleoptera: Bostrychidae)

As failure to control Rhyzopertha dominica (F.) with phosphine is a common problem in the grain-growing regions of Brazil, a study was undertaken to investigate the frequency, distribution and strength of phosphine resistance in R. dominica in Brazil. Nineteen samples of R. dominica were collected between 1991 and 2003 from central storages where phosphine fumigation had failed to control this species. Insects were cultured without selection until testing in 2005. Each sample was tested for resistance to phosphine on the basis of the response of adults to discriminating concentrations of phosphine (20 and 48 h exposures) and full dose-response assays (48 h exposure). Responses of the Brazilian R. dominica samples were compared with reference susceptible, weak-resistance and strong-resistance strains from Australia in parallel assays. All Brazilian population samples showed resistance to phosphine: five were diagnosed with weak resistance and 14 with strong resistance. Five samples showed levels of resistance similar to the reference strong-resistance strain. A representative highly resistant sample was characterised by exposing mixed-age cultures to a range of constant concentrations of phosphine for various exposure periods. Time to population extinction (TPE) and time to 99.9% suppression of population (LT(99.9)) values of this sample were generally similar to those of the reference strong-resistance strain. For example, at 0.1, 0.5 and 1.0 mg L(-1), LT(99.9) values for BR33 and the reference strong-resistance strain were respectively 21, 6.4 and 3.7 days and 17, 6.2 and 3.8 days. With both strains, doubling phosphine concentrations to 2 mg L(-1) resulted in increased LT(99.9) and TPE. High level and frequency of resistance in all population samples, some of which had been cultured without selection for up to 12 years, suggest little or no fitness deficit associated with phosphine resistance. The present research indicates that widespread phosphine resistance may be developing in Brazil. Fumigation practices should be monitored and resistance management plans implemented to alleviate further resistance development.     

Assessment of the grain protectants chlorpyrifos-methyl plus bioresmethrin,fenitrothion plus (1R)-phenothrin,methacrifos and pirimiphos-methyl plus carbaryl under practical conditions in Australia

Methacrifos (22.5 g t^?1) and the three protectant combinations chlorpyrifos-methyl (10 g t^?1) plus bioresmethrin (1 g t^?1), fenitrothion (12 g t^?1) plus (1R)-phenothrin (2 g t^?1) and pirimiphos-methyl (4 g t^?1) plus carbaryl (8 g t^?1) were each applied to grain that was stored in at least 15 silos. Grain temperature and levels of protectant were regularly monitored, and samples from 12 storages using each treatment were taken for laboratory assays against Rhyzopertha dominica and Tribolium castaneum. Grain condition did not deteriorate during storage. Grain remained free of insects in 60 of the 63 storages treated; partial failure in the other 3 storages was attributed to low or irregular levels of protectant. The mean and range of residue values of all protectants were recorded as a function of time and the mean observed values corresponded to predicted values. In laboratory bioassay, the order of effectiveness against T. castaneum was methacrifos > chlorpyrifos-methyl plus bioresmethrin > fenitrothion plus (1R)-phenothrin = pirimiphos-methyl plus carbaryl. The order of effectiveness against R. dominica was carbaryl plus pirimiphos-methyl = (1R)-phenothrin plus fenitrothion > methacrifos > bioresmethrin plus chlorpyrifos-methyl.     

Problems of development of resistance to phosphine by insect pests of stored grains1

The FAO global survey of pesticide sensitivity of stored grain pests in 1972–73 showed an overall incidence of phosphine resistance of 9.7% in insect strains from 82 countries, especially in Tribolium spp. and Rhyzopertha dominica. The levels of resistance increased under laboratory selection. Generally, eggs and pupae were found to be more tolerant, and fumigation doses have to be designed with these stages in mind. With increasing worldwide use of phosphine, resistance has become more frequent and occurs at a higher level and in other species (Cryptolestes ferrugineus, Oryzaephilus surinamensis). Studies of fumigation under different conditions in Bangladesh showed that common procedures, without use of sheets, led to chronic underdosing, creating ideal conditions for the selection of resistant strains. Such strains are very likely to be introduced into the EPPO region and established there. While inspection and control procedures in ports could limit this risk, these are costly and impractical. The problem is best tackled at source, by supporting projects aimed at improving the conditions under which grain is stored and fumigated in developing countries.     

Pseudomonas-induced defence molecules in rice plants against leaffolder (Cnaphalocrocis medinalis) pest

Fluorescent pseudomonad strains Pf1, TDK1 and PY15 individually and in combination were evaluated against leaffolder, Cnaphalocrocis medinalis Guen., in rice under in vitro, glasshouse and field conditions. Among the various treatments used, a combination of Pf1, TDK1 and PY15 strains effectively reduced the incidence of leaffolder pest in rice plants to an extent comparable with chlorpyrifos-methyl. In addition, morphogenesis of the insect pest in all stages, larval, pupal and adult, was greatly affected by a combination of Pseudomonas Pf1, TDK1 and PY15 strains. Further, the induction of defence-related molecules was demonstrated. An increased accumulation of defence molecules such as chitinase and proteinase inhibitors was observed with a combined Pf1, TDK1 and PY15 treatment compared with all other treatments. Western blot analysis of chitinase revealed the extra induction of 18, 28 and 35 kDa isoforms in rice plants treated with a mixture of Pf1, TDK1 and PY15 strains against leaffolder pest. The study revealed that a combination of fluorescent pseudomonad strains affects the development of leaffolder pest by inducing defence molecules in rice plants which in turn enhance resistance to leaffolder attack.     

纯磷化氢熏蒸对桃小食心虫的毒力作用

桃小食心虫在我国温带水果主产区广泛分布,严重威胁水果出口贸易。为明确纯磷化氢熏蒸处理桃小食心虫的可能性,本文系统研究了纯磷化氢在不同温度条件下对桃小食心虫的毒力。耐受性分析结果表明,桃小食心虫各虫态对磷化氢的耐受力有很大差别,1~2龄幼虫耐受性最弱,5龄老熟幼虫耐受性最强。室内毒力试验结果表明,25℃,0.14~0.70mg/L磷化氢熏蒸5龄老熟幼虫81~199h或15℃,0.28~0.84 mg/L磷化氢熏蒸136~166h,桃小食心虫死亡率可达99%;在25℃和15℃下,试虫达到一定死亡率的气体浓度、时间乘积(CT)关系式分别为k=C0.5 T和k=C0.2 T,说明在本试验所取浓度范围内,处理时间是影响磷化氢对桃小食心虫老熟幼虫毒力作用的主要因素,相同条件下高温比低温更有利于磷化氢熏蒸杀虫。本文首次对不同温度条件下纯磷化氢熏蒸桃小食心虫的毒力作用进行了系统研究,明确了纯磷化氢熏蒸技术在水果携带桃小食心虫的检疫处理中极具应用前景。     

不同书虱品系的磷化氢半数击倒时间与抗性相关性研究

参照FAO推荐方法测定了磷化氢对嗜卷书虱(Liposcelis bostrychophila(Badonnel))2个品系、嗜虫书虱(L.entomophila(Enderlein))3个品系和小眼书虱(L.paeta(Pearman))3个品系等共8个不同来源书虱的毒力和抗性系数Rf,测定了磷化氢浓度为2mg/mL条件下供试各书虱品系的半数击倒时间KT50,分析了其LC50值与KT50值的相关性。结果为:3种书虱的8个品系对磷化氢的抗性差异明显,高抗性的嗜虫书虱的磷化氢抗性比敏感的嗜卷书虱高1000倍以上,各品系书虱的KT50值与Rf值呈正相关,相应LC50值之对数与KT50值的对数呈直线相关。结果表明,磷化氢对书虱成虫的半数击倒时间KT50值可以用来判断书虱抗性大小,并可用于快速判断抗性以指导现场熏蒸。     

Chlorpyrifos-methyl plus bioresmethrin; Methacrifos; Pirimiphos-methyl plus bioresmethrin; and synergised bioresmethrin as grain protectants for wheat

Field trials with various pesticide combinations were carried out on bulk wheat in commercial silos in Queensland, South Australia and Western Australia. Laboratory bioassays on samples of treated grain at intervals over 8 months using malathion-susceptible and malathion-resistant strains established the following orders of efficacy: against Sitophilus oryzae (L.), chlorpyrifos-methyl 10 mg kg^?1 + bioresmethrin 1 mg kg^?1 = methacrifos 15 mg kg^?1 in aerated storage > pirimiphos-methyl 4 or 6 mg kg^?1 + bioresmethrin 1 mg kg^?1 = bioresmethrin 4 mg kg^?1 + piperonyl butoxide 16 mg kg^?1; against Rhyzopertha dominica (F.), bioresmethrin 4 mg kg^?1 + piperonyl butoxide 16 mg kg^?1 > methacrifos 15 mg kg^?1 > chlorpyrifos-methyl 10 mg kg^?1 + bioresmethrin 1 mg kg^?1 = pirimiphos-methyl 4 or 6 mg kg^?1 + bioresmethrin 1 mg kg^?1. All treatments completely prevented production of progeny in Sitophilus granarius (L.), Tribolium castaneum (Herbst), T. confusum Jackquelin du Val and Oryzaephilus surinamensis (L.). The biological efficacy of methacrifos was greater and the rate of degradation lower in aerated than in non-aerated storage. Residue levels of all compounds were determined chemically and were below proposed international residue levels to be considered by the Codex Alimentarius Commission.     

Influence of phosphine on hatching of Cryptolestes ferrugineus (Coleoptera: Cucujidae), Lasioderma serricorne (Coleoptera: Anobiidae) and Oryzaephilus surinamensis (Coleoptera: Silvanidae)

The hatching and mortality response of 0- to 48-h-old eggs of field strains of the stored-product insects Cryptolestes ferrugineus (Stephens), Lasioderma serricorne (F) and Oryzaephilus surinamensis (L) following phosphine fumigation for 24, 48 or 120 h at 27 (+/- 2) degrees C was investigated. Hatching was delayed and reduced in the first few days in a phosphine-resistant strain of C ferrugineus that was treated with 2.0-7.0 mg litre(-1) doses for 48 h (5-80% mortality) and with 1.0-2.0 mg litre(-1) for 120 h (44-84% mortality). In both the exposures there were significant increases in hatching on later days when compared with the corresponding controls. Developmental delay was, however, not evident in susceptible strains of C ferrugineus, L serricorne and O surinamensis that were exposed to phosphine for 24 h.     

磷化氢熏蒸南洋臀纹粉蚧的效果及对火龙果品质的影响

本文在25℃条件下,利用磷化氢熏蒸南洋臀纹粉蚧以及越南进口红心、白心火龙果4 h,研究了磷化氢熏蒸处理对南洋臀纹粉蚧的杀灭效果以及对火龙果采后储藏品质的影响。耐受性比较试验结果表明,不同发育阶段的南洋臀纹粉蚧对磷化氢的耐受性为雌成虫>3龄若虫=2龄若虫>1龄若虫。毒力分析试验表明,使用1.70 mg/L磷化氢熏蒸雌成虫4 h可在95%置信区间下达到99.996 8%的死亡率。品质试验结果表明,使用0.55～2.21 mg/L磷化氢熏蒸处理对两种火龙果冷藏和货架期间的色泽、硬度、可溶性糖、酸度均无显著影响,但在熏蒸后短期内会提高呼吸强度,随着储存时间延长,呼吸强度恢复正常。这些结果显示,磷化氢熏蒸用于进口火龙果携带南洋臀纹粉蚧的检疫处理可达到理想效果,在25℃使用1.66～2.21 mg/L磷化氢熏蒸4 h可作为口岸检疫熏蒸的技术指标。     

溴虫氟苯双酰胺对小菜蛾的毒力及相关酶活性的影响

溴虫氟苯双酰胺是一种全新的双酰胺类杀虫剂,主要用于防治鳞翅目、鞘翅目、白蚁以及蚊蝇等害虫。为了明确溴虫氟苯双酰胺对小菜蛾的生物活性及解毒酶活性的影响,本研究选择氯虫苯甲酰胺为对照药剂,采用叶片药膜法测定了溴虫氟苯双酰胺对小菜蛾的生物活性,并测定了亚致死剂量(LC20、LC40)的溴虫氟苯双酰胺和氯虫苯甲酰胺对小菜蛾多功能氧化酶、谷胱甘肽S-转移酶和羧酸酯酶活性的影响。结果表明,溴虫氟苯双酰胺与氯虫苯甲酰胺对小菜蛾的生物活性相当,两者对小菜蛾的LC50分别为0.042和0.056μg/mL。与氯虫苯甲酰胺相似,亚致死剂量的溴虫氟苯双酰胺能够诱导小菜蛾体内羧酸酯酶活性升高,但溴虫氟苯双酰胺对多功能氧化酶和谷胱甘肽S-转移酶活性没有显著影响。     

Organophosphorus and synergised synthetic pyrethroid insecticides as grain protectants for stored sorghum

A field experiment was carried out on bulk sorghum stored for 26 weeks in concrete silos in South Queensland. No natural infestation occurred. Laboratory bioassays of treated grain, in which malathion-resistant strains of insects were added to grain samples, indicated that all the treatments were generally effective. Deltamethrin (2mg kg⁻¹)+piperonyl butoxide (8mg kg⁻¹), fenitrothion (12mg kg⁻¹)+fenvalerate (1 mg kg⁻¹)+piperonyl butoxide (8mg kg⁻¹), and fenitrothion (12mg kg⁻¹)+phenothrin (2mg kg⁻¹)+piperonyl butoxide (8mg kg⁻¹) controlled typical malathion-resistant strains of Sitophilus oryzae (L.), Rhyzopertha dominica (F.), Tribolium castaneum (Herbst) and Ephestia cautella (Walker). Pirimiphos-methyl (6mg kg⁻¹)+permethrin (1mg kg⁻¹)+piperonyl butoxide (8mg kg⁻¹) allowed some survival of adults and progeny production by S. oryzae after 12 weeks, and by one strain of R. dominica throughout. Chemical assays established that the residues and rates of breakdown of these grain protectants on sorghum conformed to the general pattern on other cereal grains. Residues at the conclusion of the experiment were below the individual Maximum Residue Limits recommended by the Codex Alimentarius Commission.     

Impact of Research on Grain Storage Practices in Canada1

Cold winters, a well-developed bulk handling system, regular inspections of elevator premises, stored grain and cargo ships, and the application of pest control measures when necessary, minimize storage problems in Canadian grain. Research has played an important role in identifying the organisms and circumstances which cause infestations of stored grain and in developing and evaluating environmental, physical and chemical methods for preventing and controlling infestations. This information enables commercial grain organizations and government regulatory agencies to provide high quality grain for domestic consumption and export markets.     

Effect of phosphine dose on sorption in wheat

BACKGROUND: In spite of the extensive use of phosphine fumigation around the world to control insects in stored grain, and the knowledge that grain sorbs phosphine, the effect of concentration on sorption has not been quantified. A laboratory study was undertaken, therefore, to investigate the effect of phosphine dose on sorption in wheat. Wheat was added to glass flasks to achieve filling ratios of 0.25-0.95, and the flasks were sealed and injected with phosphine at 0.1-1.5 mg L(-1) based on flask volume. Phosphine concentration was monitored for 8 days at 25 degrees C and 55% RH. RESULTS: When sorption occurred, phosphine concentration declined with time and was approximately first order, i.e. the data fitted an exponential decay equation. Percentage sorption per day was directly proportional to filling ratio, and was negatively correlated with dose for any given filling ratio. Based on the results, a tenfold increase in dose would result in a halving of the sorption constant and the percentage daily loss. Wheat was less sorptive if it was fumigated for a second time. CONCLUSIONS: The results have implications for the use of phosphine for control of insects in stored wheat. This study shows that dose is a factor that must be considered when trying to understand the impact of sorption on phosphine concentration, and that there appears to be a limit to the capacity of wheat to sorb phosphine.     

Potential of piperonyl butoxide‐synergised pyrethrins against psocids (Psocoptera: Liposcelididae) for stored‐grain protection

BACKGROUND: Piperonyl butoxide (PB)‐synergised natural pyrethrins (pyrethrin:PB ratio 1:4) were evaluated both as a grain protectant and a disinfestant against four Liposcelidid psocids: Liposcelis bostrychophila Badonnel, L. entomophila (Enderlein), L. decolor (Pearman) and L. paeta Pearman. These are key storage pests in Australia that are difficult to control with the registered grain protectants and are increasingly being reported as pests of stored products in other countries. Firstly, mortality and reproduction of adults were determined in wheat freshly treated at 0.0, 0.75, 1.5, 3 and 6 mg kg^?1 of pyrethrins + PB (1:4) at 30 ± 1 °C and 70 ± 2% RH. Next, wheat treated at 0.0, 1.5, 3 and 6 mg kg^?1 of pyrethrins + PB (1:4) was stored at 30 ± 1 °C and 70 ± 2% RH and mortality and reproduction of psocids were assessed after 0, 1.5, 3 and 4.5 months of storage. Finally, the potential of synergised pyrethrins as a disinfestant was assessed by establishing time to endpoint mortality for adult psocids exposed to wheat treated at 3 and 6 mg kg^?1 of synergised pyrethrins after 0, 3, 6, 9 and 12 h of exposure. RESULTS: Synergised pyrethrins at 6 mg kg^?1 provided 3 months of protection against all four Liposcelis spp., and at this rate complete adult mortality of these psocids can be achieved within 6 h of exposure. CONCLUSION: Piperonyl butoxide‐synergised pyrethrins have excellent potential both as a grain protectant and as a disinfestant against Liposcelidid psocids. Copyright © State of Queensland, Department of Employment, Economic Development and Innovation, 2010. Published by John Wiley and Sons, Ltd.     

Phosphine-induced walking response of the lesser grain borer (Rhyzopertha dominica)

BACKGROUND: In spite of the intensive worldwide use of phosphine against stored-product insects, its potential sublethal effects on targeted insect species is seldom recognised and may compromise the efficacy of this fumigant, particularly against phosphine-resistant insects. Therefore, the behavioural response of three populations of the lesser grain borer Rhyzopertha dominica (Coleoptera: Bostrichidae) to sublethal phosphine exposure was assessed. RESULTS: Concentration-mortality bioassays with phosphine confirmed the resistance status of the insect populations studied, with levels of phosphine resistance of 40.8-fold and 85.7-fold compared with the susceptible population. Regarding walking behaviour, determined using a digital video-tracking system, the phosphine-resistant populations were less active than the susceptible population. In addition, sublethal phosphine exposure decreased the walking activity of all three populations. CONCLUSION: Phosphine exposure decreased walking ativity. Such reduced walking activity is likely to lower the respiration rate, thereby minimising phosphine uptake. As higher phosphine resistance was also associated with reduced walking activity, phosphine-resistant populations may resist phosphine fumigation even more efficiently on account of this behavioural trait, further aiding their physiological resistance, and should be a matter of concern. Copyright © 2012 Society of Chemical Industry     

Effects of time and concentration on mortality of phosphine-resistant Sitophilus oryzae (L) fumigated with phosphine

The effects of exposure period and phosphine concentration on mortality of susceptible and resistant Sitophilus oryzae (L) were investigated. Although S oryzae is one of the world's most serious pests of stored grain there are few data on the practical significance of phosphine resistance in this species. The strains investigated were an Australian susceptible strain, a homozygous resistant strain exhibiting a level of resistance common in Australia and an unselected field strain from China with a much stronger resistance. Fumigations were carried out at 25 degrees C on adults and mixed-age cultures. For adults of all three strains and mixed-age cultures of the susceptible and resistant Australian strains, the relationship between concentration and time could be described by equations of the form Cnt = k. In all cases n < 1, indicating that time was a more important variable than concentration. In all fumigations of adults the resistant strains were harder to kill than the susceptible strain. However, in fumigations of mixed-age cultures, which contained the tolerant pupal stage, the difference between susceptible and resistant strains was more pronounced at lower concentrations than higher concentrations. For example, at 0.02 mg litre-1 the estimated LT99.9 for mixed-age cultures of the Australian resistant strain (27 days) is 3.4 times that of the susceptible strain (8 days), but at 1 mg litre-1 there is no difference between the two strains (4 days). Limited data on the Chinese resistant strain supported this finding. Twenty-three days exposure at 0.02 mg litre-1 had no effect on mixed-age cultures of this strain, but there were no survivors after 5 days exposure to 1 mg litre-1.