Organophosphate insecticides and acaricides antagonise bifenazate toxicity through esterase inhibition in Tetranychus urticae

Combined application of entomopathogenic fungi and predatory mites for biological control of Tetranychus urticae on chrysanthemum

BACKGROUND

Most studies on efficacy of fungal pathogens and predatory mites against Tetranychus urticae have been done on individual species in the laboratory. We evaluated fungi and predatory mites separately and together against glasshouse populations of T. urticae on chrysanthemum plants. First, effectiveness of the fungal pathogens Beauveria bassiana (Bb88) and Metarhizium anisopliae (Ma129) was compared; then, effectiveness of the predatory mites Phytoseiulus persimilis and Neoseiulus californicus. Based on the results, N. californicus and isolate Ma129 were selected and evaluated in combination. In all experiments, treatment effects were assessed for eggs and motile stages of T. urticae.

RESULTS

The first experiment showed no significant effect of either fungal isolate on T. urticae populations, except on plants initially infested with 20 mites, where more eggs were found in the control compared to the fungal treatments. In the second experiment, both predatory mites were equally effective at reducing T. urticae populations compared with the control, regardless of initial T. urticae population density. The last experiment demonstrated that populations of T. urticae were reduced most when M. anisopliae (Ma129) and N. californicus were applied together, compared with the control and when each natural enemy was applied separately.

CONCLUSIONS

Metarhizium anisopliae (Ma129) and B. bassiana (Bb88) isolates did not have a significant effect on reducing T. urticae populations. Both predatory mites reduced T. urticae populations, regardless of T. urticae density. Combined application of M. anisopliae (Ma129) and N. californicus were more effective against T. urticae than the control or when each agent was applied separately. © 2024 Society of Chemical Industry.

Biological pest regulation can benefit from diverse predation modes

Increases in agricultural intensity due to anthropogenic demands alongside the need to reduce the reliance on pesticides have resulted in an urgent need for sustainable options for pest control. Biological pest regulation is an alternative strategy that relies on natural predators and is essentially a by-product of successful foraging. Therefore, knowledge of the predator's specific foraging behaviour can significantly improve bioregulation. In this article, we discuss the implications of predators' diverse foraging modes on their efficiency as bioregulators of crop pests using amphibians and reptiles as models. Amphibians and reptiles are promising bioregulators as they are insectivorous, and the diversity in their foraging styles-ambush and active foraging, differing in energy expenditure, movement, cognitive abilities, reliance on cues, response to predatory risk, competition and prey salience-can have specific impacts on pest regulation. We propose the uptake of this concept into strategizing pest management actions. We are now moving towards an era of biological pest regulation, which is the most targeted, economically profitable method with zero negative impact on the ecosystem. Utilizing diverse traits associated with the different foraging modes in vertebrate predators can be a crucial tool in allowing pest management to adapt to the extreme challenges it is facing.

Ferric chloride assisted QuEChERS method for separate detection of bifenazate and bifenazate-diazene in citrus fruits and its field validation

Bifenazate is widely recognized as an effective acaricide for citrus production in various regions. Detecting both the parent compound of bifenazate and its metabolite, bifenazate-diazene, simultaneously can be challenging owing to their tendency to undergo chemical interconversion. Current methods developed for detecting bifenazate or bifenazate-diazene residues often involve lengthy incubation periods and may not effectively separate the two compounds. In this study, we developed a convenient and fast method based on a modified QuEChERS method assisted by oxidants to concurrently detect bifenazate and bifenazate-diazene. Based on preliminary analysis, it appears that ferric chloride has the ability to react with a reducing substance present in citrus, which may prevent the reduction of bifenazate-diazene. The method was validated and applied in a field trial. This work reports a novel strategy to establish a balanced 'neutral' condition to create a potential method for efficient determination of bifenazate acaricide residues in fruit matrices.

The G126S substitution in mitochondrially encoded cytochrome b does not confer bifenazate resistance in the spider mite Tetranychus urticae

Several genetic variants of the cd1- and ef-helices of the Q_o site of mitochondrial cytochrome b have been associated with bifenazate resistance in the spider mite Tetranychus urticae, an important crop pest around the world. Maternal inheritance of bifenazate resistance has provided strong evidence for the involvement of many of these mutations alone or in combination. A number of populations highly resistant to bifenazate were uncovered that carried the G126S substitution in combination with other target-site mutations. This G126S mutation has therefore been investigated in several studies in the context of resistance evolution and the development of diagnostic markers. However, experimental data that link bifenazate resistance with the presence of the G126S mutation without additional cd1- and ef-helices mutations, remain very limited. Here, we genotyped 38 T. urticae field populations for cytochrome b and uncovered nine field populations with a fixed or segregating G126S substitution without other target-site mutations in the conserved cd1- and ef-helices of the cytochrome b Q_o pocket. Toxicity bioassays showed that all nine field populations were very susceptible to bifenazate, providing strong evidence that G126S alone does not confer bifenazate resistance. These findings also implicate that previous T. urticae populations with G126S found to be low to moderately resistant to bifenazate, evolved alternative mechanisms of resistance, and more importantly, that this mutation cannot be used as a molecular diagnostic for bifenazate resistance.

Control efficacy and joint toxicity of thiamethoxam mixed with spirotetramat against the Asian citrus psyllid, Diaphorina citri Kuwayama

BACKGROUND

The Asian citrus psyllid (ACP), Diaphorina citri Kuwayama, is one of the most devastating pests in citrus orchards, and has caused huge economic losses worldwide. Chemical control is the most effective way for psyllid control. Herein, the toxicity of nine insecticides to ACP adults and the joint action of thiamethoxam + spirotetramat were determined by a topical application method in the laboratory; field plot experiments were conducted to evaluate the control efficacy of one self-made thiamethoxam + spirotetramat 40% suspension concentrate (SC) comparing with thiamethoxam 21% SC, spirotetramat 22.4% SC, tolfenpyrad 15% SC and bifenthrin 100 g/L emulsifiable concentrate against ACP using foliar sprays in 2018-2019.

RESULTS

The highest toxicity to ACP adults was achieved by beta-cyfulthrin, bifenthrin, thiamethoxam and acetamiprid, with median lethal doses of 0.247 to 1.382 ng/adult at 24 h after treatment. High toxicity was observed by chlorpyrifos, spirotetramat and tolfenpyrad, but moderate toxicity by pyriproxyfen and buprofezin. For mixutres of thiamethoxam and spirotetramat, a 25:15 mass ratio showed the highest synergistic effect, with a co-toxicity coefficient (CTC) of 246.52; while a 10:30 mass ratio exhibited an additive effect, with a CTC of 109.84. Thiamethoxam + spirotetramat 40% SC at 60-80 mg/kg can effectively control ACP with a control efficacy of 72.92 to 99.29% during 3-30 days. Moreover, foliar sprays of all tested insecticides at the tested rates had no phytotoxic effects on citrus trees.

CONCLUSION

A one-time foliar spray of thiamethoxam + spirotetramat 40% SC at 80 mg/kg could be recommended to control ACP during its infestation period in citrus groves.

Field Efficacies and Joint Actions of Beta-cyfluthrin Mixed With Thiamethoxam or Tolfenpyrad Against Diaphorina citri (Hemiptera: Liviidae)

The Asian citrus psyllid, Diaphorina citri Kuwayama, is the most serious pest of citrus because it is a vector for the highly destructive citrus greening disease (huanglongbing, HLB). Currently, insecticide applications are being used widely to control psyllid populations, thereby suppressing the spread of HLB. In the present study, topical application bioassays were performed to detect the joint actions of beta-cyfluthrin and thiamethoxam or tolfenpyrad against D. citri adults in the laboratory. In 2019, a field plot experiment was conducted to evaluate the control efficacies of beta-cyfluthrin+thiamethoxam 22% capsule suspension and beta-cyfluthrin+tolfenpyrad 30% microemulsion against D. citri using foliar sprays. For the former, a 9:13 mass ratio had the highest synergistic effect, with a cotoxicity coefficient of 188.64. For the latter, a 5:25 mass ratio had the highest synergistic effect, with a cotoxicity coefficient of 153.94. A one-time foliar spray of the former at 30-40 mg/kg or of the latter at 40-60 mg/kg effectively controlled D. citri, with control efficacies varying from 80.1 to 99.4% or 80.4 to 100.0%, during the 3-30 d after treatment, respectively. Moreover, field observations indicated that these foliar sprays at the tested rates had no negative effects on citrus trees. Thus, foliar sprays of beta-cyfluthrin+thiamethoxam or beta-cyfluthrin+tolfenpyrad under the given conditions may control D. citri.

Characterization of an Intradiol Ring-Cleavage Dioxygenase Gene Associated With Abamectin Resistance in Tetranychus cinnabarinus (Acari: Tetranychidae)

Tetranychus cinnabarinus (Boisduval), i.e., carmine spider mite, is a worldwide pest that can cause serious damage to plants. Problems of resistance have arisen since abamectin usage in the control of T. cinnabarinus. Unfortunately, there are only limited data on the extent of this problem. To understand the development of abamectin resistance in the carmine spider mite, we prokaryotically expressed an intradiol ring-cleavage dioxygenase (ID-RCD) gene sequence, TcID-RCD1, which had a significant upregulated expression of over 7.7 times in an abamectin-resistant strain (AbR) when compared with that of a susceptible strain (SS). The crude enzyme activity also indicated that the AbR had a higher activity than that exhibited in SS. When susceptible individuals were treated with abamectin, TcID-RCD1 was also overexpressed. Furthermore, using the RNA interference (RNAi) technique, TcID-RCD1 was successfully knocked down, with the expression level decreasing significantly to approximately 39% in the SS strain compared with the control. And the mortality of mites feeding on dsTcID-RCD1 increased significantly when treated with abamectin. These results strongly suggest that TcID-RCD1 is involved in abamectin resistance in T. cinnabarinus.

Downregulation of carboxylesterase contributes to cyflumetofen resistance in Tetranychus cinnabarinus (Boisduval)

BACKGROUND

Increased expression or point mutations of carboxyl/cholinesterases (CCEs) have been involved in many cases of insecticide and acaricide resistance. However, it has been only rarely documented that downregulation of CCE genes is associated with resistance, although many insecticides and acaricides need hydrolytic activation in vivo. Previously, expression analysis of a laboratory-selected cyflumetofen-resistant strain of Tetranychus cinnabarinus indicated that resistance was associated with increased expression of a CCE gene of TcCCE04, but also the downregulation of two CCE genes, TcCCE12 and TcCCE23.

RESULTS

Synergism experiments revealed the importance of ester hydrolysis in cyflumetofen toxicity, because treatment with S,S,S-tributylphosphorotrithioate (DEF) caused strong inhibition of cyflumetofen hydrolysis, in both the susceptible and resistant strains. Moreover, silencing expression of TcCCE12 and TcCCE23 via RNAi further decreased the susceptibility of mites to cyflumetofen significantly, suggesting that downregulated CCE genes could be involved in cyflumetofen resistance. In addition, it was shown that recombinant TcCCE12 protein could hydrolyze cyflumetofen effectively.

CONCLUSION

Decreased esterase activity via downregulation of specific CCE genes most likely contributes to cyflumetofen resistance by decreased activation of cyflumetofen to its active metabolite. Mixtures of cyflumetofen and esterase-inhibition acaricides (e.g. organophosphates or carbamates) should be avoided in field applications. © 2019 Society of Chemical Industry.

Interaction of spirodiclofen with insecticides for the control of Brevipalpus yothersi in citrus

BACKGROUND

The mite Brevipalpus yothersi (Acari: Tenuipalpidae) is of great importance in citriculture because it is a vector of the disease citrus leprosis. The frequency of application of a combination of pesticides has increased because of an increase in the occurrence of economically important pests of citrus. However, the combined effect of acaricide and other insecticides targeting B. yothersi is unknown. The objective of this study was to evaluate the effect of a combination of spirodiclofen and other insecticides on the control of B. yothersi.

RESULTS

Analysis using spirodiclofen alone resulted in 97.22% efficiency in the control of B. yothersi on day 7 after application. A combination of spirodiclofen with either phosmet or imidacloprid resulted in 55% and 59% efficiency, respectively. A reduction in the efficiency of mite control by up to 42% was observed when the mites were exposed to a combination of acaricide with other insecticides.

CONCLUSIONS

Application of a combination of spirodiclofen with phosmet or imidacloprid compromises the efficiency of acaricide in the control of B. yothersi. © 2018 Society of Chemical Industry.

The relative contribution of target-site mutations in complex acaricide resistant phenotypes as assessed by marker assisted backcrossing in Tetranychus urticae

The mechanisms underlying insecticide and acaricide resistance in insects and mites are often complex, including additive effects of target-site insensitivity, increased metabolism and transport. The extent to which target-site resistance mutations contribute to the resistance phenotype is, however, not well studied. Here, we used marker-assisted backcrossing to create 30 congenic lines carrying nine mutations (alone, or in combination in a few cases) associated with resistance to avermectins, pyrethroids, mite growth inhibitors and mitochondrial complex III inhibitors (QoI) in a polyphagous arthropod pest, the spider mite Tetranychus urticae. Toxicity tests revealed that mutations in the voltage-gated sodium channel, chitin synthase 1 and cytochrome b confer high levels of resistance and, when fixed in a population, these mutations alone can result in field failure of acaricide treatment. In contrast, although we confirmed the implication of mutations in glutamate-gated chloride channels in abamectin and milbemectin insensitivity, these mutations do not lead to the high resistance levels that are often reported in abamectin resistant strains of T. urticae. Overall, this study functionally validates reported target-site resistance mutations in T. urticae, by uncoupling them from additional mechanisms, allowing to finally investigate the strength of the conferred phenotype in vivo.

Silencing NADPH-cytochrome P450 reductase results in reduced acaricide resistance in Tetranychus cinnabarinus (Boisduval)

Cytochrome P450 monooxygenases (P450s) are involved in metabolic resistance to insecticides and require NADPH cytochrome P450 reductase (CPR) to transfer electrons when they catalyze oxidation reactions. The carmine spider mite, Tetranychus cinnabarinus is an important pest mite of crop and vegetable plants worldwide, and its resistance to acaricides has quickly developed. However, the role of CPR on the formation of acaricide-resistance in T. cinnabarinus is still unclear. In this study, a full-length cDNA encoding CPR was cloned and characterized from T. cinnabarinus (designated TcCPR). TcCPR expression was detectable in all developmental stages of T. cinnabarinus, but it’s much lower in eggs. TcCPR was up-regulated and more inducible with fenpropathrin treatment in the fenpropathrin-resistant (FeR) strain compared with the susceptible SS strain. Feeding of double-strand RNA was effective in silencing the transcription of TcCPR in T. cinnabarinus, which resulted in decreasing the activity of P450s and increasing the susceptibility to fenpropathrin in the FeR strain but not in the susceptible strain. The current results provide first evidence that the down-regulation of TcCPR contributed to an increase of the susceptibility to fenpropathrin in resistant mites. TcCPR could be considered as a novel target for the development of new pesticides.

Expression characteristics of two novel cytochrome P450 genes involved in fenpropathrin resistance in Tetranychus cinnabarinus (Boisduval)

The carmine spider mite, Tetranychus cinnabarinus, which is also considered as the red form of Tetranychus urticae, is one of the most serious mite pests on crops. It is capable of rapidly developing resistance to acaricides, and has caused difficulty in controlling. However, the resistance mechanism of this mite remains unclear at molecular level. As a member of main detoxification enzymes, cytochrome P450 monooxygenases (CYPs or P450s) play important roles in the development of acaricide resistance in arthropods. In this study, two novel P450 genes (CYP389B1 and CYP392A26) were identified and characterized from T. cinnabarinus. The opening reading frames (ORFs) of CYP389B1 and CYP392A26 contained 1545 and 1488 nucleotides, which encode 514 and 495 amino acids, respectively. Phylogenetic analysis showed that CYP389B1 and CYP392A26 were most closely related to CYP389B1 and CYP392A4 from T. urticae, respectively. When treated with piperonyl butoxide (PBO), an inhibitor of P450s, the resistance ratio of fenpropathrin-resistant (FeR) strain decreased from 101- to 75-fold, which suggested a correlation between P450 and fenpropathrin-resistance in T. cinnabarinus. Furthermore, constitutive over-expressions of CYP389B1 and CYP392A26 were detected in FeR strain. Meanwhile, the expressions of CYP389B1 and CYP392A26 were inducible in FeR strain after treatment in 6, 12 and 24 h with LC30 of fenpropathrin; especially, the expression of CYP392A26 increased to a markedly high level (20.88-fold higher than in the control) after treatment in 6 h. However, there was no significant difference between treatment and control in susceptible strain. Furthermore, stage specific expression profiles of these two genes did not show significant difference among developing stages, except for eggs, in which the mRNA levels were quite low. The results indicate that CYP389B1 and CYP392A26 were involved in the fenpropathrin-resistance in T. cinnabarinus, and the expression of CYP392A26 was more sensitive to fenpropathrin stress. These findings provide clues for further elucidating the function and regulation mechanism of these two cytochrome P450 genes in T. cinnabarinus.

Cross-resistance between cyenopyrafen and pyridaben in the twospotted spider mite Tetranychus urticae (Acari: Tetranychidae)

BACKGROUND

Cyenopyrafen is an inhibitor of complex II of the mitochondrial electron transport chain. It has a molecular structure that shares some common features with frequently used complex I inhibitors such as pyridaben. To evaluate whether this similarity in structure poses a cross-resistance risk that might complicate resistance management, we selected for pyridaben and cyenopyrafen resistance in the laboratory and characterized resistance.

RESULTS

The selection for cyenopyrafen conferred cross-resistance to pyridaben and vice versa. Resistance towards these both acaricides was incompletely dominant in adult females. However, in eggs maternal effects were observed in pyridaben resistance, but not in the cyenopyrafen-resistance (completely dominant). In the cyenopyrafen resistant strain, the LC50 of eggs remained lower than the commercially recommended concentration. The common detoxification mechanisms by cytochrome P450 was involved in resistance to these acaricides. Carboxyl esterases were also involved in cyenopyrafen resistance as a major factor.

CONCLUSIONS

Although cross-resistance suggests that pyridaben resistance would confer cyenopyrafen cross-resistance, susceptibility in eggs functions to delay the development of cyenopyrafen resistance.

Resistance selection and biochemical mechanism of resistance against cyflumetofen in Tetranychus cinnabarinus (Boisduval)

The carmine spider mite, Tetranychus cinnabarinus is an important crop and vegetable plants pest mite. As a novel acaricide, cyflumetofen is effective against Tetranychus and Panonychus mites, but its risk and biochemical mechanism of resistance in mites is not clear. In this study, the resistance against cyflumetofen was selected and its biochemical mechanisms were studied in T. cinnabarinus. After selection the susceptibility and resistance against cyflumetofen in T. cinnabarinus, the final resistance ratio reached 21.33 at LC50 (CyR-43/CyS). All the collected field populations showed low resistance against cyflumetofen, although it had never been used in China. The activity of detoxifying enzymes CarE, MFO and GSTs were significantly increased in the final selected resistance strain (CyR-43), especially that for GSTs increased more than 7-folds after selection. The resistance against cyflumetofen developed slowly when selected from the susceptible strain in laboratory, but the resistant genes already existed in field populations, and the GSTs was the most important detoxifying enzyme conferring resistance against cyflumetofen in T. cinnabarinus. These results would provide the valuable information for designing appropriate strategies for the practical application of cyflumetofen in the field and delaying resistance development.

Cross-resistance risk of the novel complex II inhibitors cyenopyrafen and cyflumetofen in resistant strains of the two-spotted spider mite Tetranychus urticae

BACKGROUND

Cyflumetofen and cyenopyrafen are novel acaricides acting as complex II inhibitors. This new mode of action is extremely useful for devising efficient resistance management strategies for mite control. The authors determined the cross-resistance risk of both compounds, using a collection of well-characterised resistant strains of Tetranychus urticae, and also selected for cyflumetofen resistance in the laboratory.

RESULTS

Cross-resistance to cyflumetofen and cyenopyrafen was detected in field strains, with LC50 values exceeding the registered field dose. Synergism experiments suggested that P450 monooxygenases are involved in resistance, and that the activation mechanism of the two compounds most likely differs. Laboratory selection with cyflumetofen resulted in a highly resistant T. urticae strain that displayed negative cross-resistance to cyenopyrafen.

CONCLUSIONS

The cross-resistance risk of cyflumetofen and cyenopyrafen documented in this study needs to be integrated in resistance management strategies, especially in regions or crops with a history of frequent acaricide applications, in order to safeguard the efficacy of these compounds with a valuable new mode of action.

The sodium channel gene in Tetranychus cinnabarinus (Boisduval): identification and expression analysis of a mutation associated with pyrethroid resistance

BACKGROUND

The carmine spider mite (CSM), Tetranychus cinnabarinus, is the most harmful mite pest of various crops and vegetable plants. Pyrethroid insecticide fenpropathrin has been used to control insects and mites worldwide, but CSM has developed resistance to this compound.

RESULTS

Three synergists together eliminated about 50% resistance against fenpropathrin in the CSM. A point mutation was identified from the sodium channel gene of fenpropathrin-resistant CSM (FeR) by comparing cDNA sequences between FeR and susceptible (S) sodium channel genes, which caused a phenylalanine (F) to isoleucine (I) change at amino acid 1538 position in IIIS6 of the sodium channel and has been proven to confer strong resistance to pyrethroid in other species. The mRNA expression of the sodium channel gene in the FeR and abamectin-resistant strain (AbR), which was included as a control, were both relatively lower than in the S.

CONCLUSION

These results demonstrate that a mutation (F1538I) is present in the sodium channel gene in FeR of CSM, likely playing an important role in fenpropathrin resistance in T. cinnabarinus, but that decrease in the abundance of sodium channel did not confer this resistance. The F1538I mutation could be used as a molecular marker for detecting kdr resistance in Arachnida populations.

Acaricide resistance mechanisms in the two-spotted spider mite Tetranychus urticae and other important Acari: a review

The two-spotted spider mite Tetranychus urticae Koch is one of the economically most important pests in a wide range of outdoor and protected crops worldwide. Its control has been and still is largely based on the use of insecticides and acaricides. However, due to its short life cycle, abundant progeny and arrhenotokous reproduction, it is able to develop resistance to these compounds very rapidly. As a consequence, it has the dubious reputation to be the"most resistant species" in terms of the total number of pesticides to which populations have become resistant, and its control has become problematic in many areas worldwide. Insecticide and acaricide resistance has also been reported in the ectoparasite Sarcoptes scabiei, the causative organism of scabies, and other economically important Acari, such as the Southern cattle tick Rhipicephalus microplus, one of the biggest arthropod threats to livestock, and the parasitic mite Varroa destructor, a major economic burden for beekeepers worldwide. Although resistance research in Acari has not kept pace with that in insects, a number of studies on the molecular mechanisms responsible for the resistant phenotype has been conducted recently. In this review, state-of-the-art information on T. urticae resistance, supplemented with data on other important Acari has been brought together. Considerable attention is given to the underlying resistance mechanisms that have been elucidated at the molecular level. The incidence of bifenazate resistance in T. urticae is expanded as an insecticide resistance evolutionary paradigm in arthropods.

Acetylcholinesterase point mutations in European strains of Tetranychus urticae (Acari: Tetranychidae) resistant to organophosphates

BACKGROUND

In Tetranychus urticae Koch, acetylcholinesterase insensitivity is often involved in organophosphate (OP) and carbamate (CARB) resistance. By combining toxicological, biochemical and molecular data from three reference laboratory and three OP selected strains (OP strains), the AChE1 mutations associated with resistance in T. urticae were characterised.

RESULTS

The resistance ratios of the OP strains varied from 9 to 43 for pirimiphos-methyl, from 78 to 586 for chlorpyrifos, from 8 to 333 for methomyl and from 137 to 4164 for dimethoate. The insecticide concentration needed to inhibit 50% of the AChE1 activity was, in the OP strains, at least 2.7, 55, 58 and 31 times higher for the OP pirimiphos-methyl, chlorpyrifos oxon, paraoxon and omethoate respectively, and 87 times higher for the CARB carbaryl. By comparing the AChE1 sequence, four amino acid substitutions were detected in the OP strains: (1) F331W (Torpedo numbering) in all the three OP strains; (2) T280A found in the three OP strains but not in all clones; (3) G328A, found in two OP strains; (4) A201S found in only one OP strain.

CONCLUSIONS

Four AChE1 mutations were found in resistant strains of T. urticae, and three of them, F331W, G328A and A201S, are possibly involved in resistance to OP and CARB insecticides. Among them, F331W is probably the most important and the most common in T. urticae. It can be easily detected by the diagnostic PCR-RLFP assay developed in this study.

Susceptibility of an organophosphate resistant strain of the two-spotted spider mite (Tetranychus urticae) to mixtures of bifenazate with organophosphate and carbamate insecticides

Bifenazate, a new and frequently used carbazate, is a pro-acaricide which needs to be activated by carboxylesterases. We evaluated the possible antagonism of organophosphate and carbamate insecticides on bifenazate toxicity in Tetranychus urticae applied in mixtures. Two organophosphate resistant strains were used (WI and MR-VL) and several organophosphate (chlorpyrifos, azinphosmethyl and phosmet) and carbamate (carbaryl and methomyl) insecticides were evaluated. Mixing chlorpyrifos with bifenazate decreased bifenazate toxicity in both tested strains. However, in the strain with a higher esterase activity, antagonism decreased after 2 days. Of all other tested chemicals, only methomyl displayed an antagonistic effect 1 day after treatment. These findings indicate that mixing organophosphate and carbamate insecticides with bifenazate may inhibit bifenazate efficacy under field conditions, especially when resistant strains are present.

Identification of pyrethroid resistance associated mutations in the para sodium channel of the two-spotted spider mite Tetranychus urticae (Acari: Tetranychidae)

We investigated pyrethroid resistance mechanisms in Tetranychus urticae strains from Greece. Combined bioassay, biochemical and synergistic data indicated that although P450 mono-oxygenase activities were associated with the trait, target site insensitivity was the major resistance component. A 3.3 kb cDNA fragment of the T. urticae para sodium channel gene encompassing segment 4 of domain II to segment 6 of domain IV was obtained by a degenerate PCR strategy. The T. urticae sequence showed highest identity (56%) to the scabies mite, Sarcoptes scabiei, and was phylogenetically classified within the divergent group of Arachnida. Comparison of resistant and susceptible strains identified the point mutation F1538I in segment 6 of domain III, which is known to confer strong resistance to pyrethroids, along with a second mutation (A1215D) in the intracellular linker connecting domains II and III with an unknown role. Three transcripts were identified corresponding to the k and l alternative exons. The mode of inheritance of resistance was confirmed as incompletely recessive, which is consistent with a target site mechanism for pyrethroids.