Phosphine Resistance Management Research Articles

Insecticidal effect of diatomaceous earth and pirimiphos-methyl against phosphine-susceptible and phosphine-resistant populations of two stored product beetle species.

In the present study, we evaluated the insecticidal efficacy of diatomaceous earth (DE) and pirimiphos-methyl for the control of phosphine-susceptible and phosphine-resistant populations of Tribolium castaneum (Herbst) and Sitophilus oryzae (L.). Insecticides were applied on wheat or rice at two doses: DE was applied at 1000 and 2000 ppm and pirimiphos-methyl at 1 and 5 ppm. Adult mortality was measured after 7, 14, and 21 days of exposure, and progeny production capacity on the treated substrates was evaluated 65 days later. For T. castaneum, we found that DE, at 2000 ppm, was able to provide 100% control of two of the three populations tested, while for the third population mortality reached only 84%. Similarly, there were differences in mortality levels after exposure to DE-treated grains between the two S. oryzae populations tested. At 1 ppm, pirimiphos-methyl was not effective for any of the T. castaneum populations tested, but complete mortality was recorded for all populations at 5 ppm. In general, populations of S. oryzae were more susceptible than those of T. castaneum, for both commodities. Our data indicate that both insecticides can be used with success in phosphine resistance management programs, but there are populations of a given species that may be less susceptible, which constitutes a preliminary screening essential.

Significant population genetic structuring in Rhyzopertha dominica across Turkey: Biogeographic and practical implications

Rhyzopertha dominica is a key pest of stored grain. Understanding the movement of this beetle on broad geographic scales is crucial, particularly when developing strategies to prevent the spread of phosphine resistance. We assessed population genetic structuring in this pest across Turkey, using a combination of mitochondrial (cytochrome oxidase I) and microsatellite markers. In addition, we screened samples for Wolbachia, as this endosymbiont has previously been suggested to be associated with low mitochondrial genetic diversity in this beetle. Mitochondrial genetic diversity was low, with only six haplotypes identified. The genetic diversity was, however, substantially higher than that previously found in Australia or India, suggesting that R. dominica may have originated in the Middle East. Wolbachia were detected only at a single site, indicating they are not impacting the mitochondrial genetic diversity of R. dominica across Turkey. Microsatellite markers indicated there is significant geographic genetic structuring across Turkey, even among sites less than 100 km apart, suggesting there is little movement of beetles across regions within the country. This contrasts with the significantly higher levels of gene-flow found in Australia and the United States. We suggest that the limited movement of beetles across Turkey may be due to a combination of the historically localised agricultural practices (which limits anthropogenic movement among regions), and the mountainous landscape (which limits active flight among regions). Our results demonstrate that the movement of stored product pests may differ significantly across studies conducted in different countries. As a consequence, phosphine resistance management strategies must incorporate region specific information on the extent of beetle movement.

Genetic diversity and its geographic structure in Sitophilus oryzae (Coleoptera; Curculionidae) across India – implications for managing phosphine resistance

The rice weevil, Sitophilus oryzae, is a serious global pest of stored grains. Fumigation with phosphine gas is the primary control method for S. oryzae, but the indiscriminate and prolonged use of phosphine gas has led to the development of heritable resistance. Developing and implementing an effective phosphine resistance management strategy for S. oryzae relies on an understanding of its genetic diversity and any structuring of that diversity geographically. We therefore sequenced the mitochondrial cytochrome c oxidase subunit I gene from 143 S. oryzae specimens collected from 37 locations across India, and from that assessed the genetic diversity of the species and its phylogeographic structuring. In addition, we compared the genetic diversity in Indian S. oryzae populations (the hypothesised origin of this beetle) to global populations. Genetic diversity was low in Indian S. oryzae, with only eight haplotypes (including two very common haplotypes) identified. The low level of mitochondrial diversity observed in this species appears typical of stored product pests, perhaps suggesting that low mitochondrial diversity is associated with repeated phosphine fumigations, which may eliminate low frequency haplotypes. The genetic diversity of S. oryzae in India is, however, higher than in many other countries, though comparable levels were identified in China. There was no evidence of population genetic structure across India, with most haplotypes found in three of the broad biogeographic regions. This lack of phylogeographic structuring indicates significant gene-flow across India, most likely through the incidental anthropogenic transport of this relatively poor (or reluctant) flyer. The major practical implication is that phosphine resistance management for S. oryzae needs to be dealt with country wide, as populations are not isolated.

The dispersal flight of Tribolium castaneum – A field test of laboratory generated predictions

The reasons for Tribolium castaneum undertaking dispersal flight in the field are poorly known. The research reported here was designed to test, in the field, the predictions generated from our previous laboratory study about the dispersal flight of T. castaneum beetles. The results of that laboratory study suggested that younger beetles with large bodies and higher fat content tend to fly more frequently, and female flyers are likely to live longer with a higher lifetime fecundity relative to those beetles that remain. These predictions were tested by sampling beetles flying within storage and at various distances (20 and 300 m) from a large population infesting stored cotton seed. Beetles were also collected directly from the infested cotton seed as a representation of ‘resident beetles’. Of the adults collected, a subset was weighed and measured, and others were tested for their fecundity, fat content and lipofuscin levels. The survival and total fecundity of females (until day 105 post-emigration) was statistically similar across the treatments, but access to males later in life (i.e. after day 105), to a subset of females from each treatment, increased their fecundity (except for those trapped at 300 m). The majority of females that were caught dispersing are presumed to be relatively young, multiply mated, and produced numerous offspring (mean = 307.3) in the absence of males. Fat content was significantly higher across flying beetles (both sexes) trapped at 2 m (within the shed) compared to flying beetles trapped at longer distances. Flight propensity was evidently not influenced by sex, weight or body size. The lipofuscin intensities of flyers was generally significantly higher than those of resident males and females, although a reliable estimate of their age would add significant value to spatio-temporal studies in these beetles. These findings have implications for management of phosphine resistance.

Susceptibility of Tribolium castaneum to phosphine in China and functions of cytochrome P450s in phosphine resistance

Worldwide application of phosphine fumigation has caused the development of resistance especially in Tribolium castaneum during the past 30 years. The understanding of how phosphine is metabolized by insects will benefit the management of phosphine resistance in stored-grain pests. In this study, ten field populations of T. castaneum were collected from different cities in China. Bioassays showed that XD, CD, ZB and GD populations were strongly resistant to phosphine with resistant ratios (RRs) higher than 160, followed by resistant populations QH and HK and four susceptible populations FX, CD, NX and TL. Enzyme activity assays showed that cytochrome P450 activities in resistant populations were higher than susceptible populations and P450 activities were up-regulated in phosphine-resistant T. castaneum after exposure to phosphine. Moreover, CYP345A subfamily members, CYP345A1 and CYP345A2, were significantly overexpressed in resistant populations. After phosphine treatment, the expression levels of CYP345A1 and CYP345A2 were up-regulated. Moreover, RNAi via dsRNA injection was effective to knock down the expression levels of CYP345A1 and CYP345A2 and resulted in an increase of the susceptibility to phosphine in T. castaneum, respectively. Overall, higher P450 enzyme activity and overexpression of CYP345A subfamily genes likely contribute to phosphine resistance in T. castaneum. The results will provide a foundation for developing better strategies to control stored-grain pests.

Susceptibility of phosphine-resistant cigarette beetles to various insecticides: Poster

Management of phosphine resistance in the cigarette beetle Lasioderma serricorne (F.) has become a topic of great interest to the tobacco industry in recent years. Effective use of contact insecticides with modes of action different from that of phosphine can be a key element in preventing or delaying the evolution of phosphine resistance. This study was conducted to ascertain whether five insecticides selected from three mode-of-action classes (fenitrothion, pirimiphos-methyl, permethrin, bifenthrin, and spinosad) can be incorporated into a phosphine-resistance management strategy. Specifically, we examined the contact efficacy of the insecticides to a phosphine-susceptible strain and six resistant strains (38–184-fold in resistance ratio based on LC50). Susceptibility to organophosphates (fenitrothion, pirimiphos-methyl) and spinosad was not significantly different between phosphine-susceptible and phosphine-resistant strains (within 2.3-fold resistance ratio). The absence of the cross-resistance between these insecticides and phosphine makes them ideal for resistancemanagement programmes. However, high resistance to synthetic pyrethroids (>145-fold for permethrin and >1697-fold for bifenthrin) was found in three of six phosphine-resistant strains. Based on these results, synthetic pyrethroids cannot be recommended as insecticides of primary choice.

Recent developments in the global application of ECO2FUME ® and VAPORPH3OS ® phosphine fumigants:

ECO2FUME® (2% phosphine, 98% CO2 by weight) and VAPORPH3OS® (99.3% phosphine average by weight) are cylinderized gas formulations of phosphine that have achieved significant growth in commercial applications for the disinfestation of food and non-food commodities in the last two decades. The expansion in the global application of these two cylinderized phosphine fumigants is driven by increasing concern for safety, efficacy, unreacted powdered residue and disposal associated with aluminium phosphide tablets, which are promoted as alternatives to methyl bromide, and the concern of insect resistance to phosphine in both developed and developing countries. This paper describes recent developments in the global application of ECO2FUME® and VAPORPH3OS® in terms of commercial in-transit fumigation of grains and logs in ships, fumigation of export distiller’s dried grain with solubles (DDGS) in containers and shiphold, best practices in the management of phosphine resistance of insects in grains, and establishment and application of quarantine and preshipment (QPS) phosphine fumigation protocols for selected fresh fruits, vegetables, dried fruits and cut flowers as an alternative to methyl bromide. The growing issue of powdered residue from unspent aluminium phosphine tablets and the use of cylinderized phosphine as an effective solution are discussed.

A high-throughput system used to determine frequency and distribution of phosphine resistance across large geographical regions.

Next-generation sequencing can enable genetic surveys of large numbers of individuals. We developed a genotyping-by-sequencing assay for detecting strong phosphine resistance alleles in the dihydrolipoamide dehydrogenase (dld) gene of Rhyzopertha dominica populations. The assay can estimate the distribution and frequency of resistance variants in thousands of individual insects in a single run. We analysed 1435 individual insects collected over a 1-year period from 59 grain-storage sites including farms (n=29) and central storages (n=30) across eastern Australia. Resistance alleles were detected in 49% of samples, 38% of farms and 60% of central storages. Although multiple alleles were detected, only two resistance variants (P49S and K142E) were widespread and each appeared to have a distinct but overlapping geographical distribution. The type of structure in which the grain is stored had a strong effect on resistance allele frequency. We observed higher frequencies of resistance alleles in bunker storages at central sites compared with other storage types. This contributed to the higher frequencies of resistance alleles in bulk-handling facilities relative to farms. The discovery of a storage structure that predisposes insects to resistance highlights the utility of our high-throughput assay system for improvement of phosphine resistance management practices. © 2018 Society of Chemical Industry.

Progression of phosphine resistance in susceptible Tribolium castaneum (Herbst) populations under different immigration regimes and selection pressures.

Insecticide resistance is an escalating global issue for a wide variety of agriculturally important pests. The genetic basis and biochemical mechanisms of resistance are well characterized in some systems, but little is known about the ecological aspects of insecticide resistance. We therefore designed a laboratory experiment to quantify the progression of phosphine resistance in Tribolium castaneum populations subject to different immigration regimes and selection pressures. Mated resistant females were added to originally susceptible populations under two distinct migration rates, and in addition, half of the populations in each migration treatment were exposed to selection pressures from phosphine fumigation. The progression of phosphine resistance was assessed by screening beetles for the resistance allele at rph2. Phosphine resistance increased slowly in the low migration treatment and in the absence of selection, as expected. But at the higher migration rate, the increase in frequency of the resistance allele was lower than predicted. These outcomes result from the high levels of polyandry known in T. castaneum females in the laboratory, because most of the Generation 1 offspring (86%) were heterozygous for the rph2 allele, probably because resistant immigrant females mated again on arrival. Phosphine resistance was not fixed by fumigation as predicted, perhaps because susceptible gametes and eggs survived fumigation within resistant females. In terms of phosphine resistance progression in populations exposed to selection, the effect of fumigation negated the difference in migration rates. These results demonstrate how species‐specific traits relating to the mating system may shape the progression of insecticide resistance within populations, and they have broad implications for the management of phosphine resistance in T. castaneum in the field. ​We specify and discuss how these mating system attributes need to be accounted for when developing guidelines for resistance management.

Behavioral and physiological responses induced by ozone in five Brazilian populations of Rhyzopertha dominica

This study evaluated locomotory and respiratory responses induced by ozone at a concentration of 500 ppm in five Brazilian populations of Rhyzopertha dominica (Coleoptera: Bostrichidae). Toxicity and body mass were also assessed to establish their relationship with behavioral patterns. The results indicated that none of the evaluated populations of Rhyzopertha dominica showed resistance to ozone. No significant correlations were observed between ozone toxicity and locomotory behavioral patterns. Moreover, no significant correlations were found between ozone toxicity and the respiratory rate of Rhyzopertha dominica. Ozone is a potential alternative for phosphine resistance management, and its rapid degradation constitutes an advantage for the environment.

An analysis of trends, frequencies and factors influencing the development of resistance to phosphine in the red flour beetle Tribolium castaneum (Herbst) in Australia

Although resistance to phosphine, the key disinfestant used worldwide in the stored grain environment has been an ongoing industry issue, studies on its trend over large geographic region and over long period of time is very limited. In this study, we critically analysed 20 years' phosphine resistance diagnosis data for the red flour beetle Tribolium castaneum (Herbst) stored in the Australian Grain Insect Resistance Database. Resistance diagnosis on a staggering 6336 samples, along with information on storage types and treatment history was interrogated to establish trends and frequencies of resistance development in this species and factors that may have contributed towards these resistance occurences. Using descriptive statistics, linear and trend analysis and a well established Bayesian hurdle model, we determined that strong resistance in T. castaneum was significantly more prevalent in quarantine intereceptions than in central storages and on farms. The strong resistance incidences had been confined to eastern states of Queensland, New South Wales, Victoria and South Australia, whereas it is yet to be detected in the state of Western Australia. We could not establish any significant correlation between the strong resistance development and any commodity or treatments. After an initial increasing trend in incidences since the first detection of strong resistance in 1997 in this species, the frequency was stabilised during 2001–08; after which there had been an upward trend since 2009 till the last survey in 2013. The conclusions derived from this analyses highlighted the importance of a resistance monitoring program with relevant information being used in Australia as the basis for ongoing and future phosphine resistance management strategies. This research may also proved valuable towards devising similar strategies in overseas countries with phosphine resistance problems.

Phosphine resistance does not confer cross-resistance to sulfuryl fluoride in four major stored grain insect pests.

Susceptibility to phosphine (PH3 ) and sulfuryl fluoride (SF) and cross-resistance to SF were evaluated in two life stages (eggs and adults) of key grain insect pests, Rhyzopertha dominca (F.), Sitophilus oryzae (L.), Cryptolestes ferrugineus (Stephens), and Tribolium castaneum (Herbst). This study was performed with an aim to integrate SF into phosphine resistance management programmes in Australia. Characterisation of susceptibility and resistance to phosphine in eggs and adults showed that C. ferrugineus was the most tolerant as well as resistant species. Mortality responses of eggs and adults to SF at 25 °C revealed T. castaneum to be the most tolerant species followed by S. oryzae, C. ferrugineus and R. dominica. A high dose range of SF, 50.8-62.2 mg L-1 over 48 h, representing c (concentration) × t (time) products of 2438-2985 gh m-3 , was required for complete control of eggs of T. castaneum, whereas eggs of the least tolerant R. dominca required only 630 gh m-3 for 48 h (13.13 mg L-1 ). Mortality response of eggs and adults of phosphine-resistant strains to SF in all four species confirmed the lack of cross-resistance to SF. Our research concludes that phosphine resistance does not confer cross-resistance to SF in grain insect pests irrespective of the variation in levels of tolerance to SF itself or resistance to phosphine in their egg and adult stages. While our study confirms that SF has potential as a 'phosphine resistance breaker', the observed higher tolerance in eggs stresses the importance of developing SF fumigation protocols with longer exposure periods. © 2016 Society of Chemical Industry.

Phosphine Resistance in Adult and Immature Life Stages of Tribolium castaneum (Coleoptera: Tenebrionidae) and Plodia interpunctella (Lepidoptera: Pyralidae) Populations in California.

Phosphine resistance in stored-product insects occurs worldwide and is a major challenge to continued effective use of this fumigant. We determined resistance frequencies and levels of resistance in Tribolium castaneum and Plodia interpunctella collected from California almond storage and processing facilities. Discriminating doses of phosphine were established for eggs and larvae of P. interpunctella and eggs of T. castaneum using laboratory susceptible strains of the two species. For T. castaneum and P. interpunctella eggs, discriminating doses were 62.4 and 107.8 ppm, respectively, over a 3-d fumigation period, and for P. interpunctella larvae, discriminating dose was 98.7 ppm over a 20-h fumigation period. Discriminating dose tests on adults and eggs showed that 4 out of 11 T. castaneum populations tested had resistance frequencies that ranged from 42 to 100% for adults and 54 to 100% for eggs. LC99 values for the susceptible and the most resistant adults of T. castaneum were 7.4 and 356.9 ppm over 3 d, respectively. LC99 values for T. castaneum eggs were 51.5 and 653.9 ppm, respectively. Based on adult data, the most resistant T. castaneum beetle population was 49× more resistant than the susceptible strain. Phosphine resistance frequencies in P. interpunctella eggs ranged from 4 to 20%. Results show phosphine resistance is present in both species in California. Future research will investigate phosphine resistance over a wider geographic area. In addition, the history of pest management practices in facilities where insects tested in this study originated will be determined in order to develop phosphine resistance management strategies for California almond storage and processing facilities.

Resistance to phosphine in Sitophilus oryzae in Australia: A national analysis of trends and frequencies over time and geographical spread

In Australia, along with many other parts of the world, fumigation with phosphine is a vital component in controlling stored grain insect pests. However, resistance is a factor that may limit the continued efficacy of this fumigant. While strong resistance to phosphine has been identified and characterised, very little information is available on the causes of its development and spread. Data obtained from a unique national resistance monitoring and management program were analysed, using Bayesian hurdle modelling, to determine which factors may be responsible. Fumigation in unsealed storages, combined with a high frequency of weak resistance, were found to be the main criteria that led to the development of strong resistance in Sitophilus oryzae. Independent development, rather than gene flow via migration, appears to be primarily responsible for the geographic incidence of strong resistance to phosphine in S. oryzae. This information can now be utilised to direct resources and education into those areas at high risk and to refine phosphine resistance management strategies.

Phosphine resistance in Australian Cryptolestes species (Coleoptera: Laemophloeidae): perspectives from mitochondrial DNA cytochrome oxidase I analysis.

The flat grain beetle (FGB) species Cryptolestes ferrugineus, C. pusillus, C. pusilloides and C. turcicus are major stored-product pests worldwide, of which the first three are present in Australia. C. ferrugineus is also a species with high phosphine resistance status in various countries. Morphological identification of Cryptolestes species is difficult and represents an additional barrier to effective management of phosphine resistance in FGBs. Mitochondrial DNA cytochrome oxidase I (mtDNA COI) gene characterisation enabled differentiation of the four major FGB pest species through direct sequence comparison, and enabled the development of a PCR-RFLP method for rapid species differentiation. We detected two mtDNA haplotypes (Cunk-01, 02) present at low frequencies with an average nucleotide divergence rate of 0.079 ± 0.011 (SE) from C. pusillus. This nucleotide divergence rate is similar to that between C. ferrugineus and C. pusilloides (0.088 ± 0.012). Male and female genitalia morphologies of the Cunk-02 individuals indicated they were consistent with C. pusillus, yet DNA sequence analyses suggested species-level divergence. The mtDNA COI gene of phosphine-bioassayed, lab-reared F1 generation survivors supported the presence of strong phosphine resistance in C. ferrugineus, but unexpectedly also in C. pusilloides and C. pusillus F1 survivors. We demonstrated the utility of molecular DNA techniques for differentiating closely related insect species, and its usefulness in assisting the management of pest insect species. The likely presence of a cryptic C. pusillus species in Australia and the possible development of strong phosphine resistance in Australian FGB pest species require further investigation. © 2014 Society of Chemical Industry.

Effectiveness of Sulfuryl Fluoride Fumigation for the Control of Phosphine-Resistant Grain Insects Infesting Stored Wheat.

A field experiment was conducted in eight 13.6-MT steel bins containing 6.8 MT each of wheat to assess efficacy of sulfuryl fluoride or SF fumigant to control phosphine-resistant and susceptible Rhyzopertha dominica (F.) and Tribolium castaneum (Herbst). Approximately 400 adults of each type of beetle were added to each bin. Additionally, muslin bags containing immature stages and adults, with their respective diets, were also placed in bins. Four bins were fumigated with SF and others were untreated control bins. The SF dosages in treated bins ranged from 1,196–1,467 mg-h/liter. Mortality of adults in each bag was assessed 5 d postfumigation; diet minus adults was incubated in a jar, and number of adults counted after 8 wk. No significant change occurred in number of insect-damaged kernels in SF-treated bins. In trier samples from SF-treated bins, R. dominica numbers declined from 24 prefumigation to 0 at 3- and 6-wk postfumigation; T. castaneum numbers were unchanged. In WBII traps from SF-treated bins, numbers R. dominica and T. castaneum declined from 25 and 33, respectively, prefumigation to 0 or near 0 at 3- and 6-wk postfumigation. Mortalities of resistant and susceptible adult R. dominica, and adult and large larvae of T. castaneum in SF-treated bags was 100%. For all four types of beetles, adult numbers in jars associated with SF-treated bins were 0 or near 0. Results show SF is effective against all life stages of phosphine-resistant R. dominica and T. castaneum, and can be used for phosphine resistance management.

Studies on Use of TNAU- UV Light Trap for Management of Phosphine Resistance in Lasioderma serricorne (F.) (Coleoptera:Anobiidae) in Turmeric Warehouses

Lasioderma serricorne (F.) commonly called cigarette beetle is a cosmopolitan, polyphagous and major insect pest of stored turmeric that is known to be resistant to the phosphine fumigation. Practical studies were carried out to assess the efficacy of TNAU UV-Light Trap in mass trapping of L. serricorne during the pre and post fumigation periods and its role in phosphine resistance management in L. serricorne in turmeric warehouses. Adult beetles collected from UV light traps before and after fumigation were tested through bioassay on the basis of the response of adults to discriminating concentration of 0.07mg L -1 for 24 h exposure period using modified FAO method (FAO, 1975). The results of the present study showed that, the mean number of adults caught per trap/day before and after fumigation ranged from 52.7 to 83.5 and 9.6 to 18.2 beetles per light trap, respectively. The bioassay results showed that, the phosphine resistance level in L. serricorne collected from UV- Light traps ten days before and after fumigation ranged from 52.85 to 65.37 and 68.24 to 76.53 per cent, respectively. Further, the beetles caught in large numbers in light traps during the post fumigation period were found to be more resistant to phosphine than the pre fumigation population. Hence, UV- Light trap can be used as one of the resistance management strategy for phosphine resistance in L. serricorne in turmeric warehouses since mass trapping of L. serricorne was recorded in TNAU- UV Light trap during pre and post fumigation periods.

Status of resistance to phosphine in insect pests of stored products in Vietnam

In response to numerous reports of failures to control insect pests of stored products with phosphine in Vietnam, a national survey for resistance to this key fumigant was undertaken in 2009–2011. Data from a more limited survey undertaken by the authors in 2002 in northern Vietnam are also presented. Samples collected in the 2002 survey (Sitophilus oryzae, n=8; Tribolium castaneum, n=8) were tested using a full dose- response assay, while for the 2009–11 survey, F1 generations were tested for resistance with two discriminating dosages of phosphine to detect frequency of weak and strong resistance phenotypes. Compared with a susceptible reference strain, in 2002, resistance to phosphine was indicated in six T. castaneum samples but only two of S. oryzae. Resistance factor, however, did not exceed 2.8-fold in T. castaneum and 1.7 in S. oryzae indicating relatively low frequency and weak expression of resistance. In 2009–11 survey, 176 samples were collected from a range of food and feed storages along the supply chain and from all major regions of Vietnam (125 sites). Rhyzopertha dominica and S. oryzae were the most common species found infesting stored commodities. Resistance was detected at high frequency in all the species. Weak and strong resistance phenotype frequencies were, respectively: Cryptolestes ferrugineus (37 and 58%, n=19), R. dominica (1.5 and 97%, n=65), S. oryzae (34 and 59%, n=82) and T. castaneum (70 and 30%, n=10). Strong resistance phenotype was detected in all the major regions and all parts of the supply chain but frequency was the highest in central storages and animal feed establishments. The increase in frequency and strength of resistance to phosphine in the eight years between the two surveys has been rapid and dramatic. The survey demonstrates the threat of resistance to grain protection in Vietnam and highlights the need for training of fumigators, and the development and adoption of phosphine resistance management tactics nationally.

Efficacies of Spinosad and a Combination of Chlorpyrifos-Methyl and Deltamethrin Against Phosphine-Resistant <I>Rhyzopertha dominica</I> (Coleoptera: Bostrichidae) and <I>Tribolium castaneum</I> (Coleoptera: Tenebrionidae) on Wheat

Highly phosphine-resistant populations of Rhyzopertha dominica (F.) (Coleoptera: Bostrichidae) and Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae) have recently been found in Oklahoma grain storage facilities. These findings necessitate development of a phosphine resistance management strategy to ensure continued effective use of phosphine. Therefore, we investigated the efficacies of two grain insecticides, namely, spinosad applied at label rate of 1 ppm and a mixture of chlorpyrifos-methyl and deltamethrin applied at label rates of 3 and 0.5 ppm, respectively, against highly phosphine-resistant R. dominica and T. castaneum. Adult mortality and progeny production suppression of spinosad- or chlorpyrifos-methyl + deltamethrin mixture-treated wheat that had been stored for 2, 84, 168, 252, and 336 d posttreatment were assessed. We found that both spinosad and chlorpyrifos-methyl + deltamethrin were effective against phosphine-resistant R. dominica and caused 83-100% mortality and also caused total progeny production suppression for all storage periods. Spinosad was not effective against phosphine-resistant T. castaneum; the highest mortality observed was only 3% for all the storage periods. Chlorpyrifos-methyl + deltamethrin was effective against phosphine-resistant T. castaneum only in treated wheat stored for 2 and 84 d, where it caused 93-99% mortality. However, chlorpyrifos-methyl + deltamethrin was effective and achieved total suppression of progeny production in T. castaneum for all the storage periods. Spinosad was not as effective as chlorpyrifos-methyl + deltamethrin mixture at suppressing progeny production of phosphine-resistant T. castaneum. These two insecticides can be used in a phosphine resistance management strategy for R. dominica and T. castaneum in the United States.

Phosphine Resistance in the Rust Red Flour Beetle, Tribolium castaneum (Coleoptera: Tenebrionidae): Inheritance, Gene Interactions and Fitness Costs

The recent emergence of heritable high level resistance to phosphine in stored grain pests is a serious concern among major grain growing countries around the world. Here we describe the genetics of phosphine resistance in the rust red flour beetle Tribolium castaneum (Herbst), a pest of stored grain as well as a genetic model organism. We investigated three field collected strains of T. castaneum viz., susceptible (QTC4), weakly resistant (QTC1012) and strongly resistant (QTC931) to phosphine. The dose-mortality responses of their test- and inter-cross progeny revealed that most resistance was conferred by a single major resistance gene in the weakly (3.2×) resistant strain. This gene was also found in the strongly resistant (431×) strain, together with a second major resistance gene and additional minor factors. The second major gene by itself confers only 12–20× resistance, suggesting that a strong synergistic epistatic interaction between the genes is responsible for the high level of resistance (431×) observed in the strongly resistant strain. Phosphine resistance is not sex linked and is inherited as an incompletely recessive, autosomal trait. The analysis of the phenotypic fitness response of a population derived from a single pair inter-strain cross between the susceptible and strongly resistant strains indicated the changes in the level of response in the strong resistance phenotype; however this effect was not consistent and apparently masked by the genetic background of the weakly resistant strain. The results from this work will inform phosphine resistance management strategies and provide a basis for the identification of the resistance genes.