The species, density, and intra-plant distribution of mites on red raspberry (Rubus idaeus L.).

The adoption of the European Green Deal will limit acaricide use in high value crops like raspberry, to be replaced by biological control and other alternative strategies. More basic knowledge on mites in such crops is then necessary, like species, density, and their role as vectors of plant diseases. This study had four aims, focusing on raspberry leaves at northern altitude: (1) identify mite species; (2) study mite population densities; (3) investigate mite intra-plant distribution; (4) investigate co-occurrence of phytophagous mites, raspberry leaf blotch disorder and raspberry leaf blotch virus (RLBV). Four sites in south-eastern Norway were sampled five times. Floricanes from different parts of the sites were collected, taking one leaf from each of the upper, middle, and bottom zones of the cane. Mites were extracted with a washing technique and processed for species identification and RLBV detection. Mites and leaves were tested for RLBV by reverse transcription polymerase chain reaction (RT-PCR) with virus-specific primers. Phytophagous mites, Phyllocoptes gracilis, Tetranychus urticae, and Neotetranychus rubi, and predatory mites, Anystis baccarum and Typhlodromus (Typhlodromus) pyri were identified. All phytophagous mites in cultivated raspberry preferred the upper zone of floricanes, while in non-cultivated raspberry, they preferred the middle zone. The presence of phytophagous mites did not lead to raspberry leaf blotch disorder during this study. RLBV was detected in 1.3% of the sampled plants, none of them with leaf blotch symptoms, and in 4.3% of P. gracilis samples, and in some spider mite samples, implying that Tetranychids could also be vectors of RLBV.

Parental exposure to heat waves improves offspring reproductive investment in Tetranychus urticae (Acari: Tetranychidae), but not in its predator, Phytoseiulus persimilis (Acari: Phytoseiidae).

The more frequent and intense occurrence of heat waves is a challenge for arthropods because their unpredictable incidence requires fast adaptations by the exposed individuals. Phenotypic plasticity within and across generations might be a solution to cope with the detrimental effects of heat waves, especially for fast-developing, small arthropods with limited dispersal abilities. Therefore, we studied whether severe heat may affect the reproduction of a pest species, the spider mite Tetranychus urticae, and its counterpart, the predatory mite Phytoseiulus persimilis. Single offspring females with different parental thermal origins (reared under mild or extreme heat waves) of both species were exposed to mild or extreme heat waves on bean leaves over 10 days, and the oviposition, egg sizes, survival, and escape behavior of the females were evaluated daily. The total losses of predators mainly via escapers were very high compared to prey, which makes a separation between selective and plastic effects on shifted reproductive traits impossible. Predator females laid smaller eggs, while their consumption and oviposition rates were unaffected during extreme heat waves. In comparison, larger prey females fed more and produced more, but smaller, eggs due to within- and trans-generational effects. These advantages for the prey in comparison to its predator when exposed to extreme heat waves during the reproductive phase support the trophic sensitivity hypothesis: higher trophic levels (i.e., the predator) are more sensitive to thermal stress than lower trophic levels (i.e., the prey). Furthermore, the species-specific responses may reflect their lifestyles. The proactive and mobile predator should be selected for behavioral thermoregulation under heat waves via spatiotemporal avoidance of heat-exposed locations rather than relying on physiological adaptations in contrast to the more sessile prey. Whether these findings also influence predator-prey interactions and their population dynamics under heat waves remains an open question.

Resource allocation strategies for survival and reproduction by an invasive pest in response to intermittent fasting.

Intermittent fasting (IF) is a type of dietary restriction that involves fasting periods in intervals, which has been used as a strategy to improve health and extend longevity. Regular fasting is common during the process of biological invasions in nature. Yet, it is not clear how invasive animals adjust their resource allocations to survival and reproduction when periodical starvation occurs. Here, we used Tetranychus ludeni, a haplodiploid spider mite and an important invasive pest of horticultural crops around the world, to investigate the effects of IF on its life history strategies. We show that IF increased the longevity in females but not in males probably because of differences in resource storage, metabolic rate, and mating cost between sexes. In response to IF, females traded off fecundity and egg size but not the number of daughters for longevity gain, suggesting that T. ludeni females can adjust their life history strategies for population survival and growth during invasion process. Eggs produced by fasted females realized the same hatch rate and resultant young had the same survival rate as those by unfasted ones. In addition, IF had transgenerational maternal effects which prolonged offspring development period. We suggest that the longer immature developmental period can increase the body size of resulting adults, compensating egg size loss for offspring fitness. Our findings provide insight into resource allocations as responses to fasting, knowledge of which can be used for evaluation of pest invasions and for management of animal survival and reproduction by dietary regulations.

Pest categorisation of Eotetranychus sexmaculatus.

The EFSA Panel on Plant Health performed a pest categorisation of Eotetranychus sexmaculatus (Acari: Tetranychidae), the six-spotted spider mite, for the EU. The mite is native to North America and has spread to Asia and Oceania. It is not known to occur in the EU. The species is not listed in Annex II of Commission Implementing Regulation (EU) 2019/2072. E. sexmaculatus feeds on more than 50 hosts in 20 botanical families and can be a serious pest of important crops in the EU such as citrus (Citrus spp.), avocado (Persea americana), grapevine (Vitis spp.) and ornamentals such as Ficus spp. and Rosa spp. In California and New Zealand, the mite can breed continuously on evergreen hosts such as avocados and citrus, growing slowly during the winter and faster during the summer. Dry weather conditions hamper its development. Plants for planting, fruit, cut branches and cut flowers provide potential pathways for entry into the EU. Some host plants for planting are prohibited from entering the EU while others require a phytosanitary certificate, as do cut branches and cut flowers. In the warmer parts of southern EU Member States, climatic conditions and host plant availability are conducive for establishment and spread. The introduction of E. sexmaculatus is expected to have an economic impact in the EU through the reduction in yield, quality and commercial value of citrus and avocado production. Additional damage on other host plants, including ornamentals, under EU environmental conditions and cropping practices cannot be ruled out. Phytosanitary measures are available to reduce the likelihood of entry and spread. E. sexmaculatus satisfies with no key uncertainties the criteria that are within the remit of EFSA to assess for it to be regarded as a potential Union quarantine pest.

Reproducing during Heat Waves: Influence of Juvenile and Adult Environment on Fecundity of a Pest Mite and Its Predator.

The thermal history of arthropod predators and their prey may affect their reproductive performance during heat waves. Thus, a matching juvenile and adult environment should be beneficial as it enables the individuals to acclimate to extreme conditions. Prey fecundity, however, is also affected by a second stressor, namely predation risk. Here, we assessed the impact of extreme and mild heat waves on the reproductive output of acclimated (juvenile and adult heat wave conditions are matching) and non-acclimated females of the biocontrol agent Phytoseiulus persimilis, a predatory mite, and its herbivorous prey, the two-spotted spider mite Tetranychus urticae, on bean leaves. Their escape and oviposition rates and egg sizes were recorded over 10 days. Additionally, ovipositing prey females were exposed to predator cues and heat waves. Acclimation changed the escape rates and egg sizes of both species, whereas fecundity was only influenced by the adult thermal environment via increased egg numbers under extreme heat waves. Acclimation reduced predator and prey escape rates, which were higher for the predator. Pooled over acclimation, both species deposited more but smaller eggs under extreme heat waves. Acclimation dampened this effect in prey eggs, whereas acclimation resulted in smaller female eggs of the predator. Prey deposited larger male and female eggs. Predator cues reduced prey oviposition, but the effect was small compared to the large increase gained under extreme heat waves. We argue that the success of predators in controlling spider mites during heat waves mainly depends on the fates of escaping predators. A permanent absence of predators may result in the numerical dominance of prey.

Predator- and killed prey-induced fears bear significant cost to an invasive spider mite: Implications in pest management.

BACKGROUND: The success of biological control using predators is normally assumed to be achieved through direct predation. Yet it is largely unknown how the predator- and killed prey-induced stress to prey may contribute to biological control effectiveness. Here, we investigate variations in life-history traits and offspring fitness of the spider mite Tetranychus ludeni in response to cues from the predatory mite Phytoseiulus persimilis and killed T. ludeni, providing knowledge for evaluation of the nonconsumptive contribution to the biological control of T. ludeni and for future development of novel spider mite control measures using these cues. RESULTS: Cues from predators and killed prey shortened longevity by 23-25% and oviposition period by 35-40%, and reduced fecundity by 31-37% in T. ludeni females. These cues significantly reduced the intrinsic rate of increase (rm ) and net population growth rate (R0 ), and extended time to double the population size (Dt ). Predator cues significantly delayed lifetime production of daughters. Mothers exposed to predator cues laid significantly smaller eggs and their offspring developed significantly more slowly but these eggs had significantly higher hatch rate. CONCLUSION: Predator- and killed prey-induced fears significantly lower the fitness of T. ludeni, suggesting that these nonconsumptive effects can contribute to the effectiveness of biological control to a great extent. Our study provides critical information for evaluation of biological control effectiveness using predators and paves the way for identification of chemical odors from the predator and killed prey, and development of new materials and methods for the control of spider mite pests. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Trans- and Within-Generational Developmental Plasticity May Benefit the Prey but Not Its Predator during Heat Waves.

Theoretically, parents can adjust vital offspring traits to the irregular and rapid occurrence of heat waves via developmental plasticity. However, the direction and strength of such trait modifications are often species-specific. Here, we investigated within-generational plasticity (WGP) and trans-generational plasticity (TGP) effects induced by heat waves during the offspring development of the predator Phytoseiulus persimilis and its herbivorous prey, the spider mite Tetranychus urticae, to assess plastic developmental modifications. Single offspring individuals with different parental thermal origin (reared under mild or extreme heat waves) of both species were exposed to mild or extreme heat waves until adulthood, and food consumption, age and size at maturity were recorded. The offspring traits were influenced by within-generational plasticity (WGP), trans-generational plasticity (TGP), non-plastic trans-generational effects (TGE) and/or their interactions. When exposed to extreme heat waves, both species speeded up development (exclusively WGP), consumed more (due to the fact of WGP but also to TGP in prey females and to non-plastic TGE in predator males), and predator females got smaller (non-plastic TGE and WGP), whereas prey males and females were equally sized irrespective of their origin, because TGE, WGP and TGP acted in opposite directions. The body sizes of predator males were insensitive to parental and offspring heat wave conditions. Species comparisons indicated stronger reductions in the developmental time and reduced female predator-prey body size ratios in favor of the prey under extreme heat waves. Further investigations are needed to evaluate, whether trait modifications result in lowered suppression success of the predator on its prey under heat waves or not.

Sampling approaches of the Hindustan citrus mite (Schizotetranychus hindustanicus) in Brazilian citrus orchards and climatic factors affecting its population dynamics.

The Hindustan citrus mite, Schizotetranychus hindustanicus (Acari: Tetranychidae), is an invasive pest in South America and constitutes a threat to Brazilian citriculture. This study aimed to determine the contribution of weather variables to the seasonal abundance of S. hindustanicus and the best sampling scheme (sampling variable and unit) for this mite. Populations of S. hindustanicus were monitored monthly in an orange orchard for 31 months in Roraima state, Brazil. Eggs, mobile stages, and the symptoms caused by S. hindustanicus were sampled in different combinations of canopy quadrants, vertical tree strata, and branch sections. The optimal sampling variable and sampling unit for S. hindustanicus scouting were determined according to fidelity and precision criteria. Rainfall and high air temperature were the main factors reducing S. hindustanicus populations. The most suitable variable for S. hindustanicus sampling was egg count. The optimal sampling unit was a leaf collected in the central section of branches located in the middle tree stratum. In addition, the sample should be taken from the southwest quadrant. Collectively, this study adds to the understanding of S. hindustanicus population dynamics and provides a sampling scheme for better management of this pest.

Molecular identification and phylogenetic analysis of spider mites (Prostigmata: Tetranychidae) of Turkey.

The family Tetranychidae includes many agriculturally important species known as spider mites. Their morphological identification is quite difficult due to the tiny size of their taxonomic characters and the requirement for high-level expertise. This may lead to pest misidentification and thus failure in pest management. DNA-based species identification seems to offer an alternative solution to overcome these issues. In the present study, two common molecular markers-Cytochrome oxidase subunit I (COI) and Internal transcribed spacer 2 (ITS2)-were used to identify 10 spider mite species from Turkey. Furthermore, genetic distances for several of them were assessed. Panonychus ulmi and Bryobia kissophila had the lowest (1.1%) and highest (4.5%) intra-specific genetic distances, respectively. In addition, integrative taxonomy allowed to identify Eotetranychus quercicola in Turkey as a new record. The sequences herein obtained will allow rapid species identification using molecular techniques and will contribute to resolve the phylogenetic history of spider mites.

Intradiol ring cleavage dioxygenases from herbivorous spider mites as a new detoxification enzyme family in animals.

BACKGROUND: Generalist herbivores such as the two-spotted spider mite Tetranychus urticae thrive on a wide variety of plants and can rapidly adapt to novel hosts. What traits enable polyphagous herbivores to cope with the diversity of secondary metabolites in their variable plant diet is unclear. Genome sequencing of T. urticae revealed the presence of 17 genes that code for secreted proteins with strong homology to "intradiol ring cleavage dioxygenases (DOGs)" from bacteria and fungi, and phylogenetic analyses show that they have been acquired by horizontal gene transfer from fungi. In bacteria and fungi, DOGs have been well characterized and cleave aromatic rings in catecholic compounds between adjacent hydroxyl groups. Such compounds are found in high amounts in solanaceous plants like tomato, where they protect against herbivory. To better understand the role of this gene family in spider mites, we used a multi-disciplinary approach to functionally characterize the various T. urticae DOG genes. RESULTS: We confirmed that DOG genes were present in the T. urticae genome and performed a phylogenetic reconstruction using transcriptomic and genomic data to advance our understanding of the evolutionary history of spider mite DOG genes. We found that DOG expression differed between mites from different plant hosts and was induced in response to jasmonic acid defense signaling. In consonance with a presumed role in detoxification, expression was localized in the mite's gut region. Silencing selected DOGs expression by dsRNA injection reduced the mites' survival rate on tomato, further supporting a role in mitigating the plant defense response. Recombinant purified DOGs displayed a broad substrate promiscuity, cleaving a surprisingly wide array of aromatic plant metabolites, greatly exceeding the metabolic capacity of previously characterized microbial DOGs. CONCLUSION: Our findings suggest that the laterally acquired spider mite DOGs function as detoxification enzymes in the gut, disarming plant metabolites before they reach toxic levels. We provide experimental evidence to support the hypothesis that this proliferated gene family in T. urticae is causally linked to its ability to feed on an extremely wide range of host plants.

A Method to Measure the Damage Caused by Cell-Sucking Herbivores.

The damage that herbivores inflict on plants is a key component of their interaction. Several methods have been proposed to quantify the damage caused by chewing insects, but such methods are not very successful when the damage is inflicted by a cell-sucking organism. Here, we present a protocol that allows a non-destructive quantification of the damage inflicted by cell-sucking arthropods, robustly filtering out leaf vascular structures that might be mistakenly classified as damage in many plant species. The protocol is set for the laboratory environment and uses Fiji and ilastik, two free software packages.

Combination of target site mutation and associated CYPs confers high-level resistance to pyridaben in Tetranychus urticae.

Pyridaben is a mitochondrial electron transport complex I inhibitor. The H110R mutation in the PSST subunit has been reported as a major factor in pyridaben resistance in the two-spotted spider mite, Tetranychus urticae. However, backcross experiments revealed that the mutant PSST alone conferred only moderate resistance. In contrast, inhibition of cytochrome P450 (CYP) markedly reduces resistance levels in a number of highly resistant strains. It was reported previously that maternal factors contributed to the inheritance of pyridaben resistance in the egg stage, but the underlying mechanisms have yet to be elucidated. Here, we studied the combined effects of the PSST H110R mutation and candidate CYPs, as metabolic resistance factors, on pyridaben resistance in T. urticae. We found that the maternal effects of inheritance of resistance in the egg stage were associated with CYP activity. Analysis of differential gene expression by RNA-seq identified CYP392A3 as a candidate causal factor for the high resistance level. Congenic strains, where the alleles of both PSST and CYP392A3 were derived from a resistant strain (RR_i; i = 1 or 2) and a susceptible strain (SS_i) in a common susceptible genetic background, were constructed by marker-assisted backcrossing. RR_i showed upregulation of CYP392A3 and high resistance levels (LC50 > 10,000 mg L-1), while SS_i had LC50 < 10 mg L-1. To disentangle the individual effects of PSST and CYP392A3 alleles, we also attempted to uncouple these genes in RR_i. We conclude that given the variation in LC50 values and expression levels of CYP392A3 in the congenic and uncoupled strains, it is likely that the high pyridaben resistance levels are due to a synergistic or cumulative effect of the combination of mutant PSST and associated CYPs, including CYP392A3, but other yet to be discovered factors cannot be excluded.

Heat waves affect prey and predators differently via developmental plasticity: who may benefit most from global warming?

BACKGROUND: Climate warming is considered to affect the characteristics of heat waves by increasing their duration, frequency and intensity, which can have dramatic consequences for ectothermic arthropods. However, arthropods may respond to heat waves via plastic modifications, which could differently affect a predator and its prey. We examined this assumption using prominent counterparts in biological control, the predatory mite Phytoseiulus persimilis and its prey, the spider mite Tetranychus urticae. Individuals of both species were separately exposed to mild and extreme heat waves during their juvenile development. RESULTS: Both species developed faster during extreme heat waves, but the proportional increase of the developmental rates was higher in the prey. Independent of sex, P. persimilis reached smaller size at maturity under extreme heat waves, whereas the body size modifications were sex-dependent in T. urticae: males became smaller, but females were able to maintain their size. CONCLUSIONS: An accelerated development may result in the reduction of the exposure time of susceptible juvenile stages to heat waves and prey stages to predators. Plastic size adjustments caused a shift in the female predator-prey body size ratio in favor of the prey, which may lead to higher heat resistance and reduced predation risk for prey females under extreme heat waves. In conclusion, our findings indicate that species-specific shifts in age and size at maturity may result in lower suppression efficacy of the predator P. persimilis against its prey T. urticae with severe consequences for biological control of spider mites, if global warming continues.

Mite fauna on transgenic soybean in an experimental station in western Bahia, Brazil

The aim of this study was to identify mite fauna associated with soybean crops and to report new species of the Monsoy 8349 IPRO variety in the municipality of Luís Eduardo Magalhães, western region of Bahia. Samplings were performed in an area with transgenic soybeans, subdivided into three treatments: T01, with no use of agricultural pesticides; T02, complete package of pesticides, except for acaricides; and T03, complete package of pesticides including acaricides. Twenty plants were selected per treatment at each sampling; one apical, one median, and one basal leaf were collected from each plant, totaling 60 leaves per treatment. A total of 1,292 mites were found, belonging to three families, five genera, and six species. The major phytophagous mite species found were Mononychellus planki McGregor and Tetranychus urticae Koch, while the major predatory mites found were Neoseiulus transversus Denmark & Muma and Euseius concordis Chant.

Pest categorisation of Oligonychus mangiferus.

The EFSA Panel on Plant Health performed a pest categorisation of the mango red spider mite, Oligonychus mangiferus (Rahman and Sapra, 1940) (Acari: Prostigmata: Tetranychidae), for the territory of the EU. This species is not included in the EU Commission Implementing Regulation 2019/2072. The polyphagous mite feeds on more than 50 host plants including tropical fruits such as mangoes and avocados and temperate fruits such as pears, peaches, pomegranates and grapes. It occurs in Africa, Asia, Oceania and South America. O. mangiferus is multivoltine and can develop throughout the year as long as temperatures are above 12°C. The species produces dense webs on the above-ground parts of their host plants, where all stages of development (egg, larva, nymph and adult) can be found. This species is considered a pest of mango, grapevine, lychee and pomegranate, mostly in areas with hotter climates than those occurring in the EU. Potential entry pathways for O. mangiferus include plants for planting with foliage, fruit and cut flowers. Plants for planting of a few hosts (i.e. Pinus, Prunus, Pyrus, Rosa, Vitis and Arecaceae) are banned from entering into the EU from countries where O. mangiferus is known to occur and can be considered as closed entry pathways. However, other plants for planting, as well as the fruit and the cut flowers pathways remain open. There are no EU records of interception. Should O. mangiferus enter the EU, the ample availability of hosts and the climatic conditions in the EU would most probably allow this species to successfully establish and spread, at least in southern MSs, where economic impact in different fruit production (e.g. pomegranate, mango and grape) is anticipated. O. mangiferus satisfies the criteria that are within the remit of EFSA to assess for this species to be regarded as a potential Union quarantine pest.

Evaluation of Artemia franciscana Cysts to Improve Diets for Mass Rearing Stethorus gilvifrons, a Predator of Tetranychus turkestani.

Stethorus gilvifrons is an acarophagous coccinellid distributed in the Mediterranean region and could potentially be mass-reared for the augmentative biological control of Tetranychus turkestani and related species on crop plants. The hypothesis that brine shrimp Artemia franciscana cysts can improve diets for rearing of S. gilvifrons was tested in laboratory experiments. The diet treatments included A. franciscana cysts (D1), A. franciscana cysts plus a vitamin B complex (D2), A. franciscana cysts plus date palm pollen (D3), and A. franciscana cysts plus date palm pollen and Ephestia kuehniella eggs (D4). The results indicated that D1 did not support immature development. D2 supported egg-larval development but not pupal-adult development. Both D3 and D4 supported development to the adult stage and reproduction. However, D4 was the most effective diet, determined by observations of S. gilvifrons oviposition behavior and fecundity. A life table analysis corroborated these results; an intrinsic rate of increase, net and gross reproductive rates, and mean generation time were best for S. gilvifrons fed D4 rather than D3. A mixed diet composed of A. franciscana cysts, date palm pollen, and E. kuehniella eggs can be used to mass rear S. gilvifrons.

Complementary description of Panonychus caricae Hatzinikolis, 1984, with the resurrection of the genus Sasanychus Ehara, 1978 (Acari, Prostigmata, Tetranychidae).

A complementary description of Panonychus caricae Hatzinikolis, 1984, is presented based on the morphology of adult female and male individuals collected from fig trees (Ficus sp., Moraceae) in Greece. Morphological differences between Panonychus caricae and two closely related species, Panonychus ulmi (Koch, 1836) and Panonychus hadzhibejliae (Reck, 1947), are discussed. Panonychus caricae can be separated from two other Panonychus species using the length of the female dorsal setae in combination with the ratio between the length of female dorsal opisthosomal setae f2 and h1, and the ratio between the length of dorsal setae sc1 and h1. A phylogenetic maximum likelihood tree was constructed based on the cytochrome c oxidase subunit I (COI) gene of mitochondrial DNA (mtDNA) from 10 species of the subgenus Panonychus s.str. (including the re-described species P. caricae) and the only two species of the subgenus Sasanychus. The phylogenetic tree indicates that these 12 species are clearly separated from each other. The two subgenera, Panonychus s.str. and Sasanychus, comprise strongly supported monophyletic clades with 98% bootstrap values. The convergence of molecular and morphological data (dorsal setae set on tubercles or not, number of tactile setae on tibiae I and II, and patterns of the dorsocentral striae) suggests that Sasanychus should not be classified under the genus Panonychus. Consequently, molecular and morphological evidence supports the resurrection of the genus Sasanychus, which contains two species, S. akitanus (Ehara) and S. pusillus Ehara Gotoh, as distinct from Panonychus. A key to the world species of Panonychus and Sasanychus is also provided.

Molecular characterization of Cardinium, Rickettsia, Spiroplasma and Wolbachia in mite species from citrus orchards.

Tetranychidae spider mites are considered key citrus pests in some production areas, especially Tetranychus urticae Koch. Over the past decades, pesticide overuse seems to have promoted T. urticae population selection in citrus orchards. However, the microbiota has also been pointed out as a plausible explanation for population structure or plant host specialisation observed in several arthropod species. In this work, we have determined the incidence of Cardinium, Rickettsia, Spiroplasma and Wolbachia as representatives of major distorter bacteria genera in Aplonobia histricina (Berlese), Eutetranychus banksi (McGregor), Eutetranychus orientalis (Klein), Panonychus citri (McGregor), Tetranychus evansi Baker and Pritchard, Tetranychus turkestani Ugarov and Nikolskii, and T. urticae populations from Spanish citrus orchards. Only Wolbachia was detected by PCR. The multilocus alignment approach and phylogenetic inference indicated that all detected Wolbachia belong to supergroup B. The deep analysis of each 16S rDNA, ftsZ and wsp gene sequences allowed identifying several phylogenetically different Wolbachia sequences. It probably indicates the presence of several different races or strains, all of them belonging to supergroup B. The wsp sequence typing analysis unveiled the presence of the two already identified alleles (61 and 370) and allowed to contribute with five new alleles, supporting the presence of different but related B-races in the studied mite populations. The results are discussed and related to T. urticae population structure, previously observed in Spanish citrus orchards.

QTL mapping using microsatellite linkage reveals target-site mutations associated with high levels of resistance against three mitochondrial complex II inhibitors in Tetranychus urticae.

The acaricides cyflumetofen, cyenopyrafen, and pyflubumide act as inhibitors of the mitochondrial electron transport system at complex II (succinate dehydrogenase; SDH), a new mode of action in arthropods. The development and mechanisms of low-level resistance against cyenopyrafen and cyflumetofen have been previously reported in Tetranychus urticae. In the present study, we investigated high levels of resistance against three SDH inhibitors in T. urticae field populations and clarify the genetic basis of resistance using quantitative trait locus (QTL) analysis. First, we constructed a microsatellite linkage map comprising 64 markers assembled into three linkage groups (LGs) with total length of 683.8 cM and average marker spacing of 11.03 cM. We then used the linkage map to perform QTL mapping, and identified significant QTLs contributing to resistance to cyflumetofen (one QTL on LG1), cyenopyrafen (one QTL on LG3), and pyflubumide (two QTLs on LG1 and LG3). The QTL peaks on LG1 for cyflumetofen and pyflubumide overlapped and included the SdhB locus. For cyenopyrafen resistance, the QTLs on LG3 included the SdhC locus. For cyflumetofen resistance, we found an I260T mutation in SdhB. For pyflubumide and cyenopyrafen resistance, we detected I260V and S56L substitutions in SdhB and SdhC, respectively, by direct sequencing. Both I260 in SdhB and S56 in SdhC were present in highly conserved regions of the ubiquinone binding site formed at the interface among SdhB, SdhC, and SdhD. Mutations at these positions have been implicated in resistance against fungicides that act as Sdh inhibitors in various pathogens. Therefore, we consider these mutations to be target-site resistance mutations for these acaricidal SDH inhibitors.

Inter- and intraspecific variation of spider mite susceptibility to fungal infections: Implications for the long-term success of biological control.

Spider mites are severe pests of several annual and perennial crops worldwide, often causing important economic damages. As rapid evolution of pesticide resistance in this group hampers the efficiency of chemical control, alternative control strategies, such as the use of entomopathogenic fungi, are being developed. However, while several studies have focused on the evaluation of the control potential of different fungal species and/or isolates as well as their compatibility with other control methods (e.g., predators or chemical pesticides), knowledge on the extent of inter- and intraspecific variation in spider mite susceptibility to fungal infection is as yet incipient. Here, we measured the mortality induced by two generalist fungi, Beauveria bassiana and Metarhizium brunneum, in 12 spider mite populations belonging to different Tetranychus species: T. evansi, T. ludeni, and T. urticae (green and red form), within a full factorial experiment. We found that spider mite species differed in their susceptibility to infection by both fungal species. Moreover, we also found important intraspecific variation for this trait. These results draw caution on the development of single strains as biocontrol agents. Indeed, the high level of intraspecific variation suggests that (a) the one-size-fits-all strategy may fail to control spider mite populations and (b) hosts resistance to infection may evolve at a rapid pace. Finally, we propose future directions to better understand this system and improve the long-term success of spider mite control strategies based on entomopathogenic fungi.