Increased fecundity of malathion-specific resistant beetles in absence of insecticide pressure

Despite that resistance frequency is assumed to decline when selective pressure is relaxed, the stability of resistance frequency has been observed in some insects in the absence of insecticide. In the red flour beetle, Tribolium castaneum, the first case of malathion-resistance was reported in the early 1960s. The malathion-specific resistant phenotype has now almost completely replaced the susceptible one in red flour beetle populations. In the present study, several life-history traits that could influence the fitness of the insects were compared between insecticide-susceptible and malathion-specific resistant populations of the red flour beetle. On average, egg fertility and egg-to-adult development time did not differ between susceptible and resistant populations. However, the fecundity of resistant females was greater than that of susceptible ones. Generally, differences in development time between insecticide resistant and susceptible populations are considered as having more effect on fitness than do differences in fecundity. However, the observed increased female fecundity may participate, in combination with the previously observed increased male reproductive success, to the development and the stability of malathion-specific resistance in T. castaneum.     

Purification and characterization of a carboxylesterase involved in malathion-specific resistance from Tribolium castaneum (Coleoptera: Tenebrionidae)

Specific resistance to malathion in a strain of Tribolium castaneum is due to a 44-fold increase in malathion carboxylesterase (MCE) activity relative to a susceptible strain, whereas non-specific esterase levels are slightly lower. Unlike the overproduced esterase of some mosquito and aphid species, MCE in Tribolium castaneum accounts for only a small fraction (0.033-0.045%) of the total extractable protein respectively in resistant and susceptible strains. The enzyme was purified to apparent homogeneity from these two strains and has a similar molecular weight of 62,000. However, preparative isoelectricfocusing indicated that resistant insects possess one MCE with pI of 7.3, while susceptible insects possess a MCE with a pI of 6.6. Purified MCE from both populations had different K(m) and V(m) values for hydrolysis of malathion as well as for alpha-naphthyl acetate. The kinetic analysis suggests that MCE of resistant insects hydrolyses malathion faster than the purified carboxylesterase from susceptible beetles and that this enzyme has greater affinity for malathion than for naphthyl esters. Malathion-specific resistance is due to the presence of a qualitatively different esterase in the resistant strain.     

Insecticide resistance enhances male reproductive success in a beetle

Abstract.— Malathion-specific resistance in the red flour beetle, Tribolium castaneum , is widespread and stable in natural populations even in the absence of pesticide exposure. To understand this stability, both resistant and susceptible males were placed in competition for susceptible female fertilization. Females were then isolated and their progeny was tested for malathion susceptibility. Male reproductive success was estimated for populations originating from different geographic areas and for isogenic strains. In most cases, resistant males had a greater reproductive success rate than susceptibles. The results suggest that male reproductive success is not traded against the selection for malathion resistance, even resistant individuals are at an advantage for this fitness trait. This absence of fitness cost may be the result of postselection of (1) modifier gene which ameliorate the fitness of resistant individuals or (2) nondeleterious resistance gene. Resistant phenotype superiority could be due to increased male mating success, improved ability of resistant males in sperm competition, female mate choice, and/or cryptic female choice of resistance gene(s). The effect of male phenotypic frequency on male reproductive success was also examined. We observed that male fertilization success is frequency dependent and inversely related to their frequency. However, this "rare male" advantage did not counteract the superiority of the resistant males.     

Phosphine resistance in Tribolium castaneum and Rhyzopertha dominica from stored wheat in Oklahoma

Phosphine gas, or hydrogen phosphide (PH3), is the most common insecticide applied to durable stored products worldwide and is routinely used in the United States for treatment of bulk-stored cereal grains and other durable stored products. Research from the late 1980s revealed low frequencies of resistance to various residual grain protectant insecticides and to phosphine in grain insect species collected in Oklahoma. The present work, which used the same previously established discriminating dose bioassays for phosphine toxicity as in the earlier study, evaluated adults of nine different populations of red flour beetle, Tribolium castaneum (Herbst), and five populations of lesser grain borer, Rhyzopertha dominica (F.) collected from different geographic locations in Oklahoma. One additional population for each species was a laboratory susceptible strain. Discriminating dose assays determined eight out of the nine T. castaneum populations, and all five populations of R. dominica, contained phosphine-resistant individuals, and highest resistance frequencies were 94 and 98%, respectively. Dose-response bioassays and logit analyses determined that LC99 values were approximately 3 ppm for susceptible and 377 ppm for resistant T. castaneum, and approximately 2 ppm for susceptible and 3,430 ppm for resistant R. dominica. The most resistant T. castaneum population was 119-fold more resistant than the susceptible strain and the most resistant R. dominica population was over 1,500-fold more resistant. Results suggest a substantial increase in phosphine resistance in these major stored-wheat pests in the past 21 yr, and these levels of resistance to phosphine approach those reported for other stored-grain pest species in other countries.     

Fitness cost associated with carbosulfan resistance in Aphis gossypii Glover (Hemiptera: Aphididae)

In order to collect basic information for a resistance management program of Aphis gossypii Glover to carbosulfan, studies to evaluate fitness costs associated to this resistance were conducted under laboratory conditions. For this purpose, we studied the stability of carbosulfan resistance in A. gossypii populations with differing initial frequencies of resistance and compared life-history traits of carbosulfan-susceptible and resistant strains. The stability of resistance was followed in three populations with initial frequencies of 20, 50 and 80% of resistant individuals. The susceptibility to carbosulfan was monthly estimated by using the diagnostic concentration of 32 ppm. The biology of carbosulfan-susceptible and resistant strains was evaluated in cotton plants kept under controlled climatic conditions. The resistance to carbosulfan was unstable, that is, there was a significant increase in the susceptibility of different populations of A. gossypii to this insecticide in the absence of selection pressure. Comparison of biological traits between carbosulfan-susceptible and resistant strains showed that there is a fitness cost associated with resistance. The resistant strain showed lower fecundity, shorter reproductive period and lower longevity than the susceptible strain. The net reproductive rate (Ro) was 14.04 and 43.18 and the finite rate of increase (lambda) was 1.25 and 1.30 females/female/day for the resistant and susceptible strains respectively. Therefore, the instability of carbosulfan resistance in A. gossypii can be exploited in resistance management programs.     

赤拟谷盗的磷化氢敏感和抗性品系体内羧酸酯酶的活性比较

本文比较测定了赤拟谷盗Tribolium castaneum(Herbst)的磷化氢抗性(Rf=327)和敏感品系害虫的羧酸酯酶活性,研究了该害虫同一品系不同个体间羧酸酯酶的活性差异,比较了两品系害虫在系列磷化氢浓度下熏蒸24h和6.94×10-2mg/L磷化氢浓度下熏蒸不同时间的羧酸酯酶活性。主要结果为:未熏蒸的抗性害虫幼虫和蛹体内的羧酸酯酶活性分别高于敏感品系的1.37和1.16倍;敏感和抗性害虫同品系内不同个体间羧酸酯酶活性分布频率都存在明显差异,抗性害虫中酶活性大的个体数量占的比例较大;磷化氢浓度分别为0.69×10-2、2.78×10-2、5.56×10-2、8.33×10-2和11.11×10-2mg/L时都可导致敏感害虫羧酸酯酶的活性降低,但活性受抑制的程度不因浓度高低呈相应的增减。浓度分别为5.56×10-2、11.11×10-2、13.89×10-2、20.83×10-2和27.78×10-2mg/L的熏蒸中抗性害虫体内酶活性增加,且活性增高的程度与浓度增减也不呈对应变化。在6.94×10-2mg/L磷化氢浓度下熏蒸不同时间的结果中,敏感害虫的酶活性随时间延长而下降,抗性害虫的活性则随时间延长而增大。研究表明赤拟谷盗对磷化氢的抗性可能与羧酸酯酶的活性增加有关。     

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.     

Fitness costs and morphological change of laboratory‐selected thiamethoxam resistance in the B‐type Bemisia tabaci (Hemiptera: Aleyrodidae)

Thiamethoxam has been used as a key insecticide to control the whitefly, B‐type Bemisia tabaci, for several years in China with no known cases of resistance in field populations. To evaluate the risk of resistance, a field population was collected and resistant strains were developed by exposure to thiamethoxam in the laboratory. After selection for 36 generation, a strain with 60‐fold resistance was successfully identified. Fitness analysis by constructing life tables, demonstrated that resistant B‐type whiteflies had obvious fitness disadvantages in their development and reproduction. The fitness of resistant B‐type whiteflies decreased dramatically, to only one‐half that of the susceptible strain. Some changes in the morphological characteristics of the resistant strain were observed. The lengths of first, second and third instars of the resistant strain were significantly smaller than those of the susceptible strain, and the width of the first and the fourth instars were also significantly smaller than in the susceptible strain. Our results suggest that the B‐type B. tabaci has the potential to develop high resistance to thiamethoxam, and that the resistance changed the morphology of the insects. The slow development of resistance and the lower fitness of resistant B. tabaci strains may result in a quick recovery of sensitivity when the population is no longer in contact with thiamethoxam in the field.     

Susceptibility of laboratory and field strains of four stored-product insect species to spinosad

Two field strains of the Indianmeal moth, Plodia interpunctella (Hübner); red flour beetle, Tribolium castaneum (Herbst); and lesser grain borer, Rhyzopertha dominica (F.), and one field strain of the rusty grain beetle, Cryptolestes ferrugineus (Stephens), were collected from hard red winter wheat stored on farms in northeastern Kansas. Fifty eggs of P. interpunctella and 25 beetle adults of each species were exposed to 100 g of untreated wheat or wheat treated with various rates of spinosad, to determine susceptibility of the field and corresponding insecticide-susceptible laboratory strains. Mortality of beetle adults and P. interpunctella larvae was assessed after 7 and 21 d postinfestation, respectively. Field strains of P. interpunctella, C. ferrugineus, and T. castaneum were less susceptible to spinosad than the corresponding laboratory strains. The LD50 and LD95 values for P. interpunctella and C. ferrugineus field strains were 1.7-2.5 times greater than values for corresponding laboratory strains. Adults of both laboratory and field strains of T. castaneum were tolerant to spinosad, resulting in <88% mortality at 8 mg/kg. The LD50 and LD95 values for the field strains of T. castaneum were 2.0-7.5 times greater compared with similar values for the laboratory strain. The field and laboratory strains of R. dominica were highly susceptible to spinosad, and one of the field strains was relatively less susceptible to spinosad than the laboratory strain. Our results confirm a range of biological variability in field populations, which is consistent with findings for other compounds, and underscores the need to adopt resistance management programs with stored grain insect pests. The baseline data generated on the susceptibility of the four insect species to spinosad will be useful for monitoring resistance development and for setting field rates.     

Relative developmental and reproductive fitness associated with pyrethroid resistance in the major southern African malaria vector, Anopheles funestus

The effect of pyrethroid resistance on the fitness of a laboratory strain of Anopheles funestus originating from southern Mozambique was evaluated by comparing the developmental and reproductive characteristics of a pyrethroid resistant strain with an insecticide susceptible strain. Fitness was evaluated in terms of fecundity, fertility, egg production, developmental time and life stage progression and survival. Of the eggs laid by females of the resistant strain, 81.5% hatched while only 66.9% were recorded in the susceptible strain. The time from egg hatch to adult emergence was longer for the resistant strain (15.9 days) than the susceptible strain (15.2 days). A significantly higher proportion of eggs from the resistant strain (61.6%) survived to adulthood compared with those of the susceptible strain (49%). Fecundity and larval and pupal survival did not differ significantly between strains. Of spermathecae dissected from females of the resistant strain, 56.8% were fertilized compared to 52.6% from the susceptible strain. The proportion of females that successfully produced eggs was 43.3% and 23.3% for the resistant and susceptible strains respectively. Complete failure of larval hatch was recorded in 28.6% of susceptible strain families compared to 7.7% of resistant families. Our results show that pyrethroid resistance in southern African An. funestus does not incur any loss of fitness under laboratory conditions. These results suggest that the removal of pyrethroid insecticide selection pressure may not lead to a regression of resistance alleles in pyrethroid resistant An. funestus populations in southern Africa.     

Inheritance of resistance to pyriproxyfen in Bemisia tabaci (Hemiptera: Aleyrodidae) males and females (B biotype)

We evaluated effects of the insect growth regulator pyriproxyfen on Bemisia tabaci (Gennadius) (B biotype) (Hemiptera: Aleyrodidae) males and females in laboratory bioassays. Insects were treated with pyriproxyfen as either eggs or nymphs. In all tests, the LC50 for a laboratory-selected resistant strain was at least 620 times greater than for an unselected susceptible strain. When insects were treated as eggs, survival did not differ between males and females of either strain. When insects were treated as nymphs, survival did not differ between susceptible males and susceptible females, but resistant males had higher mortality than resistant females. The dominance of resistance decreased as pyriproxyfen concentration increased. Resistance was partially or completely dominant at the lowest concentration tested and completely recessive at the highest concentration tested. Hybrid female progeny from reciprocal crosses between the susceptible and resistant strains responded alike in bioassays; thus, maternal effects were not evident. Rapid evolution of resistance to pyriproxyfen could occur if individuals in field populations had resistance with traits similar to those of the laboratory-selected strain examined here.     

Genetically-induced susceptibility to malathion in Tribolium castaneum despite selection for resistance

Immigration of malathion-susceptible males into resistant populations of Tribolium castaneum considerably increased mortality despite selection for resistance every generation. It is suggested that this method may prevent the appearance of resistance with continuous use of chemicals. Its use with warehouse populations of stored-products insects seems to be feasible.

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Genetische verlangsamung der entwicklung von resistenz gegen malathion bei Tribolium castaneum Trots selektion

Zusammenfassung Die Einwanderung von Malathion-sensiblen Männchen in resistenten Populationen von Tribolium castaneum erhöhte die Mortalität beträchtlich, trotzdem eine Selektion auf Resistenz in jeder Generation erfolgte. Es wird vermutet, dass diese Methode das Auftreten von Resistenz bei dauernder Insektizidanwendung verhindern könnte. Die Anwendung in Lagerhauspopulationen von Vorratsschädlingen erscheint möglich.

     

Carboxyesterase degradation of malathion in vitro by susceptible and resistant strains of Tribolium castaneum (Herbst) (Coleoptera, Tenebrionidae)

Malathion monoacid was found to be the major metabolite of malathion produced in vitro by homogenates of susceptible and resistant Tribolium castaneum. Slightly increased production of this metabolite was detected in a malathion specific resistant strain. In all strains, monoacid formation was inhibited by triphenyl phosphate. Partial purification of the enzymes responsible failed to indicate a significant increase in enzyme levels in the resistant strain.     

Influence of phosphine resistance genes on flight propensity and resource location in Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae): the landscape for selection

Phosphine resistance in Tribolium castaneum Herbst (Coleoptera: Tenebrionidae) has evolved through changes to enzymes involved in basic metabolic pathways. These changes impose metabolic stress and could affect energy‐demanding behaviours. We therefore tested whether phosphine resistance alleles impact the movement of these insects in their quest for new resources. We measured walking and flight parameters of four T. castaneum genotypes: (1) a field‐derived population, (2) a laboratory cultured, phosphine‐susceptible reference strain, (3) a laboratory cultured, phosphine‐resistant reference strain, and (4) a resistant introgressed strain that is almost identical genetically to the susceptible population. The temporal pattern of flight was identical across all populations, but resistant beetles took flight significantly less, walked more slowly, and located resources less successfully than did susceptible beetles. Also, the field‐derived beetles (proved not to be carrying resistance genes) walked significantly faster and more directly towards food resources, and had a higher propensity for flight when compared to the susceptible laboratory beetles. These negative effects suggest survival of beetles with the resistance alleles will be compromised should they leave phosphine application sites. The field for selection therefore extends beyond the site at which phosphine fumigant imposed its effect, and other mutations are also likely to be affected in this way.     

Stability of spinosad resistance in Frankliniella occidentalis (Pergande) under laboratory conditions

The stability of spinosad resistance in western flower thrips (WFT), Frankliniella occidentalis (Pergande), populations with differing initial frequencies of resistance was studied in laboratory conditions. The stability of resistance was assessed in bimonthly residual bioassays in five populations with initial frequencies of 100, 75, 50, 25 and 0% of resistant individuals. There were no consistent changes in susceptibility of the susceptible strain after eight months without insecticide pressure. In the resistant strain, very highly resistant to spinosad (RF50>23,000-fold), resistance was maintained up to eight months without further exposure to spinosad. In the absence of any immigration of susceptible genes into the population, resistance was stable. In the case of the population with different initial frequency of resistant thrips, spinosad resistance declined significantly two months later in the absence of selection pressure. With successive generations, these strains did not change significantly in sensitivity. Spinosad resistance in F. occidentalis declined significantly in the absence of selection pressure and the presence of susceptible WFT. These results suggest that spinosad resistance probably is unstable under field conditions, primarily due to the immigration of susceptible WFT. Factors influencing stability or reversion of spinosad resistance are discussed.     

Fitness costs of Cry1F resistance in fall armyworm,Spodoptera frugiperda

Transgenic corn, Zea mays L., expressing the Bacillus thuringiensis Berliner (Bt) protein Cry1F has been registered for Spodoptera frugiperda (J. E. Smith) control since 2003 in the USA. Unexpected damage to Cry1F corn was reported in 2006 in Puerto Rico, and Cry1F resistance in S. frugiperda from Puerto Rico was documented. The study of fitness costs associated with insect resistance to Bt insecticidal proteins is important for understanding resistance evolution and for evaluating resistance management practices used to mitigate resistance to transgenic corn. Currently, no studies have addressed the fitness costs associated with Cry1F resistance in S. frugiperda. In this study, susceptible and resistant strains with similar genetic background and their reciprocal crosses were used to estimate Cry1F resistance fitness costs. Comparisons between life‐history traits and population growth rates of homozygous susceptible, heterozygous and homozygous resistant S. frugiperda were used to determine whether the resistance is associated with fitness costs. Major fitness costs were not apparent in either heterozygotes or homozygous resistant insects. However, there was a slight indication of hybrid vigour in the heterozygotes. Additionally, two lines in which the frequency of the resistant alleles was fixed at 0.5 were followed for seven generations, after which the frequency of resistant alleles slightly decreased in both lines. The lack of strong fitness costs associated with Cry1F resistance in S. frugiperda indicates that initial allele frequencies may be higher than expected in field populations and will tend to remain stable in field populations in the absence of selection pressure (e.g. Puerto Rico).     

Variability in diapause propensity within populations of a temperate insect species: interactions between insecticide resistance genes and photoperiodism

Temperate insects generally use day length as a reliable cue for long-term seasonal changes in their environment. Significant variation in photoperiodism between and within populations is thought to be associated with genetic variation resulting from local adaptation. In this study, we investigated whether genetic variation associated with selection for insecticide resistance may be a source of divergence in the photoperiodic timing of diapause through pleiotropic interactions. Critical photoperiods for diapause induction were estimated in one susceptible and two insecticide-resistant homozygous strains of the codling moth Cydia pomonella , as well as in their reciprocal F₁ progeny. Diapause responses to naturally decreasing day length were subsequently followed in the laboratory strains and in two field populations of C. pomonella in south-eastern France. We found higher critical photoperiods for diapause induction in homozygous resistant individuals than in both homozygous susceptible and heterozygous ones. This partly explained the significantly earlier timings of diapause found in homozygous resistant individuals of both laboratory and field populations of C. pomonella under natural photoperiods, relative to those found in both homozygous susceptible and heterozygous ones. We assume that adaptive genetic changes associated with selection for insecticide resistance may generate substantial variation in the seasonal timing of diapause, an essential ecological feature of the fitness of insecticide resistance genes in this species.  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 83 , 341–351.     

Lack of fitness costs of insecticide resistance in the western flower thrips (Thysanoptera: Thripidae)

The fitness costs of spinosad and acrinathrin resistance was investigated in the western flower thrips, Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae). Fitness studies were conducted on susceptible and resistant strains of F. occidentalis. Resistant females were significantly more fecund (number of eggs per female) than susceptible females. The hatching rate (fertility) for both susceptible and acrinathrin-resistant strains was significantly lower than in the spinosad-resistant strain. Mean developmental time from egg to adult did not differ between thrips populations. Similarly, female longevity did not differ between populations. These data suggest that lack of fitness costs related to insecticide resistance may accelerate the development of insecticide resistance in populations of F. occidentalis from southeastern Spain.     

Pleiotropy of insecticide resistance in the codling moth, Cydia pomonella

The codling moth, Cydia pomonella (L.) (Lepidoptera: Tortricidae), has developed resistance to various insecticides. Relative fitness of one susceptible strain (Sv) and two strains selected for resistance to diflubenzuron (Rt) and deltamethrin (Rv), respectively, was measured in the absence of insecticide selection pressure. Mating rate, fecundity, fertility, developmental time, fifth instar weight, and adult longevity were compared. Both resistant strains were less fecund and fertile, developed more slowly, weighed less, and had shorter life-spans than the susceptible strain. These results indicate that biological constraints are associated with insecticide resistance in the codling moth. We also found that fitness estimates of the Rv strain did not differ statistically from those of the Rt strain. Enhanced mixed-function oxidase and glutathione-S-transferase activities have been shown to be involved in insecticide resistance in both Rt and Rv strains. This suggests that the fitness cost described in both resistant strains was mainly associated to metabolic resistance. The impact of such deleterious pleiotropy of insecticide resistance in C. pomonella in terms of resistance management in the field is discussed.