Insecticide resistance in Bemisia tabaci from Cyprus

Abstract A comprehensive study on the Bemisia tabaci (biotype B) resistance to neonicotinoid insecticides imidacloprid, acetamiprid and thiamethoxam, and pyrethroid bifenthrin was conducted in Cyprus. The resistance level to eight field‐collected B. tabaci populations was investigated. The activities of enzymes involved in metabolic detoxification and the frequencies of pyrethroid and organophosphates target site resistance mutations were determined. Moderate to high levels of resistance were detected for imidacloprid (resistance factor [RF] 77–392) and thiamethoxam (RF 50–164) while low resistance levels were observed for acetamiprid (RF 7–12). Uniform responses by the Cypriot whiteflies could be observed against all neonicotinoid insecticides. No cross‐resistance between the neonicotinoids was detected as well as no association with the activity of the P450 microsomal oxidases. Only imidacloprid resistance correlated with carboxylesterase activity. Low to extremely high resistance was observed for insecticide bifenthrin (RF 49–1 243) which was associated with the frequency of the resistant allele in the sodium channel gene but not with the activity of the detoxification enzymes. Finally, the F331W mutation in the acetylcholinesterase enzyme ace1 gene was fixed in all B. tabaci populations from Cyprus.     

Resistance to neonicotinoid insecticides and expression changes of eighteen cytochrome P450 genes in field populations of Bemisia tabaci from Xinjiang,China

The occurrence of Bemisia tabaci poses an increasingly serious threat to cotton and vegetable crops in Xinjiang, China. Currently, neonicotinoid insecticides are commonly used to control the insect, to which resistance is inevitable due to intensive use. However, the resistance status and mechanism of B. tabaci to neonicotinoid insecticides in Xinjiang are poorly understood. Cytochrome P450 monooxygenases represent a key detoxification mechanism in the neonicotinoid resistance of B. tabaci. In this study, the resistance level to imidacloprid and thiamethoxam was investigated using the leaf dipping method in five field populations of B. tabaci from Turpan (TP, two sampling sites), Shache (SC), Hotan (HT) and Yining (YN) in northern and southern Xinjiang. The expression changes of eighteen cytochrome P450 genes from the select B. tabaci populations were determined by real‐time fluorescence quantitative PCR (qPCR). The bioassay revealed that the five populations tested had developed moderate to high levels of resistance to imidacloprid (12.26–46.07‐fold), while the populations remained sensitive to thiamethoxam except for HT, which had a low level of resistance. The qPCR results showed that the expression levels of five P450 genes, CYP4G68, CYP6CM1, CYP303A1‐like, CYP6DZ7 and CYP6DZ4, were significantly higher in some resistant field populations than in the susceptible strain. Resistance to imidacloprid in field populations of B. tabaci might be associated with the increased expression of these five cytochrome P450 genes. The results are useful for further understanding the mechanism of neonicotinoid resistance and will contribute to the management of insecticide‐resistant B. tabaci in Xinjiang.     

Mating behaviour and reproductive output in insecticide‐resistant and ‐susceptible strains of the maize weevil (Sitophilus zeamais)

Insecticide resistance is a broadly recognised and well‐studied management problem resulting from intensive insecticide use, which also provides useful evolutionary models of newly adapted phenotypes to changing environments. Two common assumptions in such models are the existence of fitness costs associated with insecticide resistance, which will place the resistant individuals at a disadvantage in insecticide‐free environments, and the prevalence of random mating among insecticide‐resistant and ‐susceptible individuals. However, cases of insecticide resistance lacking apparent fitness disadvantages do exist impacting the evolution and management of insecticide resistance. Assortative mating, although rarely considered, may also favour the evolution and spread of insecticide resistance. Thus, the possible existence of both conditions in the maize weevil (Sitophilus zeamais), a key pest of stored cereals, led to the assessment of the mating behaviour and reproductive fitness of insecticide‐resistant and ‐susceptible weevil strains and their reciprocal crosses. The patterns of female and male mating choice also were assessed. Although mating behaviour within and between weevil strains was similar without mate choice, mating within the resistant strain led to higher reproductive output than within the susceptible strain; inter‐strain matings led to even higher fertility. Thus, no apparent fitness cost associated with resistance seems to exist in these weevils, favouring the evolution of this phenotype that is further aided by the higher fertility of inter‐strain matings. Mate choice reduced latency to mate and no inter‐strain preference was detected, but female weevils were consistent in their mate selection between 1st and 2nd matings indicating existence of female mating preference among maize weevils. Therefore, if female mate selection comes to favour trait(s) associated with insecticide resistance, higher reproductive fitness will be the outcome of such matings favouring the evolution and spread of insecticide resistance among maize weevil populations reverting into a management concern.     

烟粉虱对拟除虫菊酯杀虫剂的抗性机理

通过增效剂生物测定、生化分析以及钠离子通道基因ⅡS4-6 cDNA片段的RT-PCR扩增,探讨了烟粉虱Bemisia tabaci（Gennadius）对拟除虫菊酯杀虫剂的抗性机理。结果表明：对于采自田间的6个烟粉虱抗性品系,磷酸三苯酯（TPP）和胡椒基丁醚（PBO）对氯氰菊酯、溴氰菊酯、氯氟氰菊酯和甲氰菊酯均有显著的增效作用,而DEM对4种拟除虫菊酯杀虫剂均无明显的增效作用。烟粉虱抗性品系的α-NA羧酸酯酶和β-NA羧酸酯酶活性分别是敏感品系的2.16～2.65倍和1.22～1.41倍,抗性品系的谷胱甘肽S转移酶活性与敏感品系没有差异,表明羧酸酯酶和多功能氧化酶在烟粉虱对拟除虫菊酯类杀虫剂的抗性中具有重要的作用,而谷胱甘肽S转移酶与抗性无关。通过RT-PCR克隆了6个烟粉虱田间抗性品系的钠离子通道结构域ⅡS4-6 cDNA片段的序列（420 bp）,发现与敏感品系相比,有2个位点发生突变,分别为L925I突变和I917V突变,L925I突变在所有6个烟粉虱田间抗性种群中均有发生,该位点突变已被证实与拟除虫菊酯类杀虫剂密切相关,表明神经不敏感性可能是烟粉虱对拟除虫菊酯产生抗性的另一个重要因子。     

Assessment of cross-resistance potential to neonicotinoid insecticides in Bemisia tabaci (Hemiptera: Aleyrodidae)

Laboratory bioassays were carried out with four neonicotinoid insecticides on multiple strains of Bemisia tabaci (Gennadius) to evaluate resistance and cross-resistance patterns. Three imidacloprid-resistant strains and field populations from three different locations in the southwestern USA were compared in systemic uptake bioassays with acetamiprid, dinotefuran, imidacloprid and thiamethoxam. An imidacloprid-resistant strain (IM-R) with 120-fold resistance originally collected from Imperial Valley, California, did not show cross-resistance to acetamiprid, dinotefuran or thiamethoxam. The Guatemala-resistant strain (GU-R) that was also highly resistant to imidacloprid (RR=109-fold) showed low levels of cross-resistance when bioassayed with acetamiprid and thiamethoxam. However, dinotefuran was more toxic than either imidacloprid or thiamethoxam to both IM-R and GU-R strains as indicated by low LC50s. By contrast, a Q-biotype Spanish-resistant strain (SQ-R) of B. tabaci highly resistant to imidacloprid demonstrated high cross-resistance to the two related neonicotinoids. Field populations from Imperial Valley (California), Maricopa and Yuma (Arizona), showed variable susceptibility to imidacloprid (LC50s ranging from 3.39 to 115 microg ml(-1)) but did not exhibit cross-resistance to the three neonicotinoids suggesting that all three compounds would be effective in managing whiteflies. Yuma populations were the most susceptible to imidacloprid. Dinotefuran was the most toxic of the four neonicotinoids against field populations. Although differences in binding at the target site and metabolic pathways may influence the variability in cross-resistance patterns among whitefly populations, comparison of whitefly responses from various geographic regions to the four neonicotinoids indicates the importance of ecological and operational factors on development of cross-resistance to the neonicotinoids.     

Baseline susceptibility of Bemisia tabaci B biotype (Hemiptera: Aleyrodidae) populations from California and Arizona to spiromesifen

Susceptibility to spiromesifen, a tetronic acid derivative, was determined for three imidacloprid-resistant strains and 12 geographically discrete natural populations of Bemisia tabaci (Gennadius) (=Bemisia argentifolii Bellows & Perring) (Hemiptera: Aleyrodidae) from California and Arizona by laboratory bioassays. Newly emerged first instars were sprayed with aqueous serial dilutions of spiromesifen and evaluated for toxicity to establish baseline susceptibility data. Interpopulation variability in susceptibility to spiromesifen was observed among the natural populations of whiteflies up to 29-fold; however, there was only 30-fold difference in susceptibility among natural and resistant populations tested. In general, spiromesifen was quite toxic to first instars across most of their geographic range, with LC50 values ranging from 0.210 to 6.08 microg (AI)/ml. The magnitude of variation was smaller among the three-resistant strains. These results suggest that the observed variability reflect natural variation in spiromesifen susceptibility among all the test populations, possibly due to previous exposure to insecticides at each location. The effectiveness of spiromesifen also was evaluated against all immature stages of whiteflies from three field and two resistant strains. Spiromesifen was significantly more active against early instars of whiteflies based on lower LC50 values recorded compared with the fourth instars. Spiromesifen was effective against the resistant strains including a Q-biotype of B. tabaci from Spain, which is highly resistant to neonicotinoids. Results of this study indicate absence of cross-resistance between spiromesifen and more commonly used neonicotinoids. Our findings suggest that spiromesifen should be considered an ideal candidate for whitefly resistance management programs in rotation with other effective chemistries.     

Deltamethrin‐induced feeding plasticity in pyrethroid‐susceptible and ‐resistant strains of the maize weevil,Sitophilus zeamais

Phenotypic plasticity contributes to the adaptative evolution of populations exposed to new or altered environments. Feeding plasticity is a component of phenotypic plasticity not usually considered in insect strains adapted to insecticide‐altered environments, but which may either accentuate or mitigate insecticide resistance. This is a concern in the pyrethroid‐resistant strains of the maize weevil Sitophilus zeamais Motsch. (Col., Curculionidae), and the reason for this study. A pyrethroid‐susceptible and two pyrethroid‐resistant strains of maize weevil were subjected to free‐choice and no‐choice tests with maize grains sprayed with increasing doses of the pyrethroid, deltamethrin. The insects from the pyrethroid‐resistant strains exhibited higher feeding avoidance with increased deltamethrin doses than insects from the susceptible strain when subjected to free‐choice tests. The strains of maize weevil physiologically resistant to pyrethroids were also behaviourally resistant to deltamethrin – an additional management concern. The resistant strains avoid deltamethrin‐sprayed grains and are less nutritionally affected by this compound, with divergent responses from the susceptible strain with increased doses of deltamethrin. Furthermore, the higher relative growth rate and consequently higher efficiency of food conversion observed in the insecticide‐resistant strains were significant even without insecticide exposure, indicating that these traits are stimulus‐independent and may persist even without further insecticide selection, potentially limiting the options available for their management.     

Can insecticide mixtures be considered to surmount neonicotinoid resistance in Bemisia tabaci?

Cotton whitefly, Bemisia tabaci is an important polyphagous pest worldwide. It is exposed to various chemical insecticides throughout the year, resulting in the rapid development of insecticide resistance. Mixtures of insecticides with distinct modes of action could enhance the toxicity of chemicals more effectively than sequences or rotations in resistant pest populations. Bioassays were conducted to study the efficacy of mixtures of neonicotinoid and ketoenol insecticides at different ratios against a laboratory susceptible (Lab-WB) and a neonicotinoid resistant (TMX-SEL) strain of B. tabaci Asia I. The results showed that mixtures of imidacloprid, acetamiprid, thiamethoxam or dinotefuran with spiromesifen at 1:1, 1:10 and 1:20 ratios and of imidacloprid, thiamethoxam or dinotefuran with spirotetramat at 1:1 ratio significantly increased (p < 0.05) toxicity to neonicotinoids in TMX-SEL strain. The combination indices of each tested neonicotinoids + ketoenols at 1:1 ratio and of acetamiprid + spiromesifen, and imidacloprid or dinotefuran + spirotetramat at 1:10 ratio for TMX-SEL strain were significantly below 1, suggesting synergistic interactions. The inhibitors PBO and DEF largely overcame resistance to the tested neonicotinoids, while none of the synergists significantly restored the susceptibility of B. tabaci to ketoenols. Increased activities of P450 monooxygenase and esterase were observed in TMX-SEL strain with an elevated 2.76 and 1.32-fold, respectively. Mixtures of neonicotinoids with spiromesifen or spirotetramat at a 1:1 ratio could be used to restore the neonicotinoid susceptibility in B. tabaci.     

Fitness of Bt‐resistant cabbage loopers on Bt cotton plants

Development of resistance to the insecticidal toxins from Bacillus thuringiensis (Bt) in insects is the major threat to the continued success of transgenic Bt crops in agriculture. The fitness of Bt‐resistant insects on Bt and non‐Bt plants is a key parameter that determines the development of Bt resistance in insect populations. In this study, a comprehensive analysis of the fitness of Bt‐resistant Trichoplusia ni strains on Bt cotton leaves was conducted. The Bt‐resistant T. ni strains carried two genetically independent mechanisms of resistance to Bt toxins Cry1Ac and Cry2Ab. The effects of the two resistance mechanisms, individually and in combination, on the fitness of the T. ni strains on conventional non‐Bt cotton and on transgenic Bt cotton leaves expressing a single‐toxin Cry1Ac (Bollgard I) or two Bt toxins Cry1Ac and Cry2Ab (Bollgard II) were examined. The presence of Bt toxins in plants reduced the fitness of resistant insects, indicated by decreased net reproductive rate (R₀) and intrinsic rate of increase (r). The reduction in fitness in resistant T. ni on Bollgard II leaves was greater than that on Bollgard I leaves. A 12.4‐day asynchrony of adult emergence between the susceptible T. ni grown on non‐Bt cotton leaves and the dual‐toxin‐resistant T. ni on Bollgard II leaves was observed. Therefore, multitoxin Bt plants not only reduce the probability for T. ni to develop resistance but also strongly reduce the fitness of resistant insects feeding on the plants.     

Relative fitness of Cry1A-resistant and -susceptible Helicoverpa armigera (Lepidoptera: Noctuidae) on conventional and transgenic cotton

Glasshouse and laboratory experiments were conducted to evaluate the relative fitness of Cry1A-susceptible and laboratory-selected resistant strains of Helicoverpa armigera (Hübner). Life history parameters of H. armigera larvae feeding on young cotton plants showed a significant developmental delay of up to 7 d for the resistant strain compared with the susceptible strain on non-Bacillus thuringiensis (Bt) cotton. This fitness cost was not evident on artificial diet. There was no developmental delay in the F1 hybrid progeny from the reciprocal backcross of the resistant and susceptible strains, indicating that the fitness cost is recessive. In two cohorts tested, survival to pupation of resistant larvae on Bt cotton expressing Cry1Ac was 54 and 51% lower than on non-Bt cotton, whereas all susceptible and F1 larvae tested on Cry1Ac cotton were killed. Mortality of susceptible larvae occurred in the first or second instar, whereas the F1 larvae were able to develop to later instars before dying, demonstrating that resistance is incompletely recessive. The intrinsic rate of increase was reduced by >50% in the resistant strain on Cry1Ac cotton compared with the susceptible strain on non-Bt cotton. There was a significant reduction in the survival of postdiapausal adults from the resistant strain and the F1 strains, indicating that there is a nonrecessive overwintering cost associated with Cry1A resistance in H. armigera.     

Field‐level effects of preventative management tactics on soybean aphids (Aphis glycines Matsumura) and their predators

A 2‐year field experiment was conducted in northern Illinois to evaluate the effects of host plant resistance and an insecticidal seed treatment (thiamethoxam) on soybean aphids, Aphis glycines Matsumura and their predators. Densities of soybean aphids varied between the 2 years of the experiment. During both years, resistant plants experienced fewer cumulative aphid days than susceptible plants. Populations of soybean aphids on resistant plants rarely exceeded the economic injury level of 250 soybean aphids per plant. The use of thiamethoxam reduced cumulative aphid days in 2007, but not in 2008. Although soybean aphids reached densities that were sufficient to cause yield‐loss for untreated and susceptible plants, no yield‐benefit was associated with using the two management tactics in either year. This latter finding suggests that densities of soybean aphids need to be greater and sustained for a longer period of time than what we observed if the two management tactics are expected to provide a yield‐benefit. Monitoring natural enemies revealed that densities of key aphidophagous predators were relatively unaffected by host plant resistance or thiamethoxam; the effect of these management tactics on densities of predators, as well as the effectiveness of the method used to sample predators, is discussed.     

Abamectin resistance in strains of vegetable leafminer,Liriomyza sativae (Diptera: Agromyzidae) is linked to elevated glutathione S‐transferase activity

Abamectin resistance was selected in the vegetable leafminer, Liriomyza sativae (Blanchard) (Diptera: Agromyzidae) under laboratory conditions, and cross‐resistance patterns and possible resistance mechanisms in the abamectin‐resistant strains (AL‐R, AF‐R) were investigated. Compared with the susceptible strain (SS), strain AL‐R displayed 39‐fold resistance to abamectin after 20 selection cycles during 25 generations, and strain AF‐R exhibited 59‐fold resistance to abamectin after 16 selection cycles during 22 generations. No cross‐resistance to cyromazine was found in both abamectin‐resistant strains. However, we failed to select for cyromazine resistance in L. sativae under laboratory conditions by conducting 17 selection cycles during 22 generations. However, moderate levels of cross‐resistance to abamectin (6–9 fold) were observed in strains which received cyromazine treatments. Biochemical analysis showed that glutathione S‐transferase (GST) activity in both abamectin‐resistant strains (AL‐R, AF‐R) was significantly higher than in the susceptible strain (SS), suggesting metabolically driven resistance to abamectinin L. sativae. Recommendations of mixtures or rotation of cyromazine and abamectin should be considered carefully, as consecutive cyromazine treatments may select for low‐level cross‐resistance to abamectin.     

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.     

Biotic performances of thelytokous and arrhenotokous strains of <Emphasis Type="Italic">Thrips tabaci</Emphasis> (Thysanoptera: Thripidae) showing resistance to cypermethrin

Two distinct reproductive types of Thrips tabaci Lindeman, one thelytokous and one arrhenotokous, have been shown to develop target-insensitive cypermethrin resistance conferred by sodium channel mutation (T929I). In this study, we examined the relation between cypermethrin resistance and biotic performances of these reproductive types. Among the T. tabaci strains examined, resistant arrhenotokous strains became adults more quickly after hatching than resistant thelytokous strains. Female adults of resistant thelytokous strains exhibited shorter longevity and oviposited fewer eggs than those of susceptible strains of the same reproductive type. Resistant arrhenotokous strains exhibited similar longevity and fecundity to susceptible thelytokous strains. We further examined the relation between reproductive types and T929I using a total of 85 strains. All 53 arrhenotokous strains encoding T929I were judged to be cypermethrin resistant. Among 32 thelytokous strains with T929I, only four were regarded as cypermethrin resistant. Based on the results, we discuss possible mechanisms accounting for the recent prominence of arrhenotoky T. tabaci in Japan.     

Protocol for semi‐automatic identification of whiteflies Bemisia tabaci and Trialeurodes vaporariorum on yellow sticky traps

Yellow sticky traps (YSTs) are commonly used in greenhouse crops to monitor flying pest species. Whiteflies like Trialeurodes vaporariorum (Westwood) (Hemiptera: Aleyrodidae) and Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) are typically monitored using YSTs in tomato and sweet pepper crops. By counting the whiteflies on a YST, growers get an idea of the pests density in space and time in the greenhouse and can take pest control measurements accordingly. The downside is that manual counting of whiteflies on a YST is very time‐consuming and thus costly. A protocol to semi‐automate counting and identification of whiteflies on YSTs using image analysis software was developed to speed up the monitoring process. Bemisia tabaci is on average smaller than T. vaporariorum and by discriminating by size based on the amount of pixels in digital images, ratios of both species in a mixed population on YSTs could be estimated accurately. At low densities, the countings of different YSTs should be pooled till a 200 density threshold is reached in order to get accurate ratio estimates of both species. This study provides a protocol to reliably count and identify whiteflies semi‐automatically on standardized pictures. More research is required to develop alternative techniques to make standardized pictures in the field (e.g., with smartphone).     

Novaluron (Rimon), a novel IGR: potency and cross-resistance

The potency of novaluron on laboratory susceptible and field strains of S. littoralis resembles that of chlorfluazuron and both compounds are about 20-fold more potent than teflubenzuron. No appreciable resistance to novaluron or chlorfluazuron was observed in a field strain of Spodoptera littoralis collected from cucumber field in the central part of Israel. On the other hand, the field strain showed a mild resistance of about 4-fold to teflubenzuron as compared to the laboratory susceptible strain. A very resistant colony of Bemisia tabaci to pyriproxyfen (1,200- to 2,000-fold) showed no appreciable cross-resistance to novaluron. Two field colonies of B. tabaci pressurized with acetamiprid or thiamethoxam for 22 generations, resulting in a 30- to 50-fold resistance to acetamiprid and thiamethoxam, has no cross-resistance to novaluron. The above results are of special interest, indicating a possible alternation between novaluron, pyriproxyfen, and neonicotinoids in insecticide-resistance management programs aiming at preventing resistance development to these novel groups of insecticides against important pests such as whitefly and lepidopteran species.     

Effects of thiamethoxam and lambda‐cyhalothrin on spermatogenesis of Euschistus heros (Heteroptera: Pentatomidae)

Euschistus heros (Fabricius, 1798) is one of the most harmful insect pests damaging Brazilian soybean crops and has become a major problem due to its high population density and resistance to insecticides. Currently, there are no data on whether alterations of testicular morphology and chromosomal behavior are associated with the resistance mechanisms related to the action of insecticides. This study integrated analyses of the testicular morphology, meiocyte cell division, and chromosomal structure and behavior in the process of spermatogenesis in E. heros. We compared these features among wild‐caught individuals, insecticide‐susceptible and ‐resistant strains. The resistant strain was established through a selection experiment exposing the bugs to insecticides (thiamethoxam + lambda‐cyhalothrin) for 15 generations. No differences were detected in the examined features among the three groups of experimental individuals: the testis comprised six lobes, with the fifth lobe thinner than the others; the karyotype was 2n = 14 (12 + XY), with no evident changes in chromosomal breakage, rearrangement or behavior in the meiosis; and abundant spermatozoa were observed in all testicular lobes. Thus, any effects of the long‐term (15 generations) experimental selection by exposure to the insecticides were not detected on the male germinal tissue and chromosomes, suggesting the irrelevancy of the examined features to insecticide‐resistance mechanisms in E. heros.