Aberrant splicing of a nicotinic acetylcholine receptor alpha 6 subunit is associated with spinosad tolerance in the thrips predator Orius laevigatus.

Susceptibility to insecticides is one of the limiting factors preventing wider adoption of natural enemies to control insect pest populations. Identification and selective breeding of insecticide tolerant strains of commercially used biological control agents (BCAs) is one of the approaches to overcome this constraint. Although a number of beneficial insects have been selected for increased tolerance to insecticides the molecular mechanisms underpinning these shifts in tolerance are not well characterised. Here we investigated the molecular mechanisms of enhanced tolerance of a lab selected strain of Orius laevigatus (Fieber) to the commonly used biopesticide spinosad. Transcriptomic analysis showed that spinosad tolerance is not a result of overexpressed detoxification genes. Molecular analysis of the target site for spinosyns, the nicotinic acetylcholine receptor (nAChR), revealed increased expression of truncated transcripts of the nAChR α6 subunit in the spinosad selected strain, a mechanism of resistance which was described previously in insect pest species. Collectively, our results demonstrate the mechanisms by which some beneficial biological control agents can evolve insecticide tolerance and will inform the development and deployment of insecticide-tolerant natural enemies in integrated pest management strategies.

Identification and functional characterization of a novel acetyl-CoA carboxylase mutation associated with ketoenol resistance in Bemisia tabaci.

Insecticides of the tetronic/tetramic acid family (cyclic ketoenols) are widely used to control sucking pests such as whiteflies, aphids and mites. They act as inhibitors of acetyl-CoA carboxylase (ACC), a key enzyme for lipid biosynthesis across taxa. While it is well documented that plant ACCs targeted by herbicides have developed resistance associated with mutations at the carboxyltransferase (CT) domain, resistance to ketoenols in invertebrate pests has been previously associated either with metabolic resistance or with non-validated candidate mutations in different ACC domains. A recent study revealed high levels of spiromesifen and spirotetramat resistance in Spanish field populations of the whitefly Bemisia tabaci that was not thought to be associated with metabolic resistance. We confirm the presence of high resistance levels (up to >640-fold) against ketoenol insecticides in both Spanish and Australian B. tabaci strains of the MED and MEAM1 species, respectively. RNAseq analysis revealed the presence of an ACC variant bearing a mutation that results in an amino acid substitution, A2083V, in a highly conserved region of the CT domain. F1 progeny resulting from reciprocal crosses between susceptible and resistant lines are almost fully resistant, suggesting an autosomal dominant mode of inheritance. In order to functionally investigate the contribution of this mutation and other candidate mutations previously reported in resistance phenotypes, we used CRISPR/Cas9 to generate genome modified Drosophila lines. Toxicity bioassays using multiple transgenic fly lines confirmed that A2083V causes high levels of resistance to commercial ketoenols. We therefore developed a pyrosequencing-based diagnostic assay to map the spread of the resistance alleles in field-collected samples from Spain. Our screening confirmed the presence of target-site resistance in numerous field-populations collected in Sevilla, Murcia and Almeria. This emphasizes the importance of implementing appropriate resistance management strategies to prevent or slow the spread of resistance through global whitefly populations.

Sulfoxaflor efficacy in the laboratory against imidacloprid-resistant and susceptible populations of the green peach aphid, Myzus persicae: Impact of the R81T mutation in the nicotinic acetylcholine receptor.

A key to effective insect pest management and insecticide resistance management is to provide growers with a range of new tools as potential alternatives to existing compounds or approaches. Sulfoxaflor (Isoclast™ active) is a new sulfoximine insecticide which is active on a broad range of sap-feeding insects, including species that have reduced susceptibility to currently used insecticides, such as imidacloprid from the neonicotinoid class. Sulfoxaflor (SFX) and imidacloprid (IMI) were tested in laboratory bioassays to compare the susceptibility of field populations of green peach aphid, Myzus persicae (Sulzer), exhibiting varying degrees of resistance involving an alteration (R81T) to the insect nicotinic acetylcholine receptor. The LC50 values for M. persicae exposed to SFX ranged from 0.09 to 1.31 (mg litre-1), whereas when the same populations were exposed to IMI the LC50 values ranged from 0.6 to 76.2 (mg litre-1). M. persicae were significantly more sensitive to SFX as compared to IMI for nine of the 13 populations tested. For M. persicae populations confirmed to be homozygous susceptible (ss) or heterozygous rs) for the R81T point mutation, there was no significant differences in the observed LC50 values for either SFX or IMI relative to the susceptible reference population (15LP1). However, in all M persicae populations that were homozygous (rr) for the R81T point mutation, susceptibility was significantly less to IMI as compared to the reference population with resistance ratios ranging from 22.1 to 63.5-fold. In contrast, only one homozygous resistant population (15MP9) exhibited a statistically significant change in susceptibility (RR = 10-fold) to SFX as compared to the reference population, which was far less than the 56-fold observed for imidacloprid in that same population. Thus, this study indicates there is no specific correlation between the laboratory efficacy of SFX and IMI in field collected populations in Spain displaying varying degrees of resistance to IMI. Furthermore, the presence of target site resistance in M. persicae to IMI, in the form of the R81T mutation, does not a priori translate to a reduction in sensitivity to sulfoxaflor. Consequently, SFX can be an effective tool for use in insect pest management programs for green peach aphid. These data also serve as a baseline reference for green peach aphid sensitivity to SFX prior to commercial uses in Spain.

Insecticide resistance mediated by an exon skipping event.

Many genes increase coding capacity by alternate exon usage. The gene encoding the insect nicotinic acetylcholine receptor (nAChR) α6 subunit, target of the bio-insecticide spinosad, is one example of this and expands protein diversity via alternative splicing of mutually exclusive exons. Here, we show that spinosad resistance in the tomato leaf miner, Tuta absoluta is associated with aberrant regulation of splicing of Taα6 resulting in a novel form of insecticide resistance mediated by exon skipping. Sequencing of the α6 subunit cDNA from spinosad selected and unselected strains of T. absoluta revealed all Taα6 transcripts of the selected strain were devoid of exon 3, with comparison of genomic DNA and mRNA revealing this is a result of exon skipping. Exon skipping cosegregated with spinosad resistance in survival bioassays, and functional characterization of this alteration using modified human nAChR α7, a model of insect α6, demonstrated that exon 3 is essential for receptor function and hence spinosad sensitivity. DNA and RNA sequencing analyses suggested that exon skipping did not result from genetic alterations in intronic or exonic cis-regulatory elements, but rather was associated with a single epigenetic modification downstream of exon 3a, and quantitative changes in the expression of trans-acting proteins that have known roles in the regulation of alternative splicing. Our results demonstrate that the intrinsic capacity of the α6 gene to generate transcript diversity via alternative splicing can be readily exploited during the evolution of resistance and identifies exon skipping as a molecular alteration conferring insecticide resistance.

Cross-resistance and baseline susceptibility of spirotetramat in Frankliniella occidentalis (Thysanoptera: Thripidae).

Western flower thrips, Frankliniella occidentalis (Pergande), is an economically important pest all over the world. New products against thrips are necessary, as there are few effective compounds exhibiting cross-resistance among them. Lethal effects, cross-resistance, and baseline susceptibility to spirotetramat were evaluated in this study. A new bioassay method for testing thrips against spirotetramat was developed. Spirotetramat showed a significant mortality effect on larvae after 7 d of exposure, but a low effect was observed on adults. Baseline results for larval instars showed LC50 values ranging from 11.59 to 49.81 mg AI/liter, with a low natural variability (3.2-fold). Cross-resistance studies showed overlapping confidence limits of the LC50 values for laboratory-selected (against acrinathrin, methiocarb, formetanate, and spinosad) resistant and susceptible strains, and low resistance factors, from 0.5 to 1.9, suggesting no cross-resistance to conventional insecticides previously used. A slight ovicidal effect (21-40% reduction) was also detected. Despite presenting low effects on adults, spirotetramat showed high but slow efficacy on F. occidentalis larvae. Field populations in southeast Spain showed a consistent susceptibility to spirotetramat. Given the scarcity of effective products and the lack of cross-resistance to other insecticides, spirotetramat can be considered as a good chemical tool to control F. occidentalis.

A nicotinic acetylcholine receptor transmembrane point mutation (G275E) associated with resistance to spinosad in Frankliniella occidentalis.

High levels of resistance to spinosad, a macrocyclic lactone insecticide, have been reported previously in western flower thrips, Frankliniella occidentalis, an economically important insect pest of vegetables, fruit and ornamental crops. We have cloned the nicotinic acetylcholine receptor (nAChR) α6 subunit from F. occidentalis (Foα6) and compared the nucleotide sequence of Foα6 from susceptible and spinosad-resistant insect populations (MLFOM and R1S respectively). A single nucleotide change has been identified in Foα6, resulting in the replacement of a glycine (G) residue in susceptible insects with a glutamic acid (E) in resistant insects. The resistance-associated mutation (G275E) is predicted to lie at the top of the third α-helical transmembrane domain of Foα6. Although there is no direct evidence identifying the location of the spinosad binding site, the analogous amino acid in the C. elegans glutamate-gated chloride channel lies in close proximity (4.4 Å) to the known binding site of ivermectin, another macrocyclic lactone pesticide. The functional consequences of the resistance-associated mutation have been examined in the human nAChR α7 subunit. Introduction of an analogous (A272E) mutation in α7 abolishes the modulatory effects of spinosad whilst having no significant effect upon activation by acetylcholine, consistent with spinosad having an allosteric mechanism of action.

Overexpression of the UDP-glycosyltransferase UGT34A23 confers resistance to the diamide insecticide chlorantraniliprole in the tomato leafminer, Tuta absoluta.

The tomato leafminer, Tuta absoluta, is an invasive crop pest that has evolved resistance to many of the insecticides used for its control. To facilitate the investigation of the underpinning mechanisms of resistance in this species we generated a contiguous genome assembly using long-read sequencing data. We leveraged this genomic resource to investigate the genetic basis of resistance to the diamide insecticide chlorantraniliprole in Spanish strains of T. absoluta that exhibit high levels of resistance to this insecticide. Transcriptomic analyses revealed that, in these strains, resistance is not associated with previously reported target-site mutations in the diamide target-site, the ryanodine receptor, but rather is associated with the marked overexpression (20- to >100-fold) of a gene encoding a UDP-glycosyltransferase (UGT). Functional expression of this UGT, UGT34A23, via ectopic expression in Drosophila melanogaster demonstrated that it confers strong and significant resistance in vivo. The genomic resources generated in this study provide a powerful resource for further research on T. absoluta. Our findings on the mechanisms underpinning resistance to chlorantraniliprole will inform the development of sustainable management strategies for this important pest.

Effect of the Amount of Ephestia kuehniella Eggs for Rearing on Development, Survival, and Reproduction of Orius laevigatus.

Orius laevigatus is a key tool for the success of augmentative biological control programs in protected crops. This biological control agent is mass-reared feeding on eggs of Ephestia kuehniella. However, this factitious prey is expensive, accounting for a significant percentage of the rearing costs. Therefore, there is a need to optimize the amount of Ephestia eggs needed per individual leading to a cost reduction, which in turn will favor biological control adoption. This study investigated the effect of the amount of Ephestia eggs provided on the developmental and reproductive fitness of O. laevigatus. At least a daily supply of 1 and 3 Ephestia eggs was needed for optimal development of the first two nymphal instars, respectively, although for maximum survival, 1 egg was enough for both instars. For subsequent development until adulthood, a minimum of 8 eggs per day were needed to fully support growth, but only 3 eggs for optimal survival. Similarly, male body size was also maximized by feeding 8 eggs, but for maximum female body size 10 eggs per day were required. Oviposition rate of females increased with the daily number of Ephestia eggs provided, until a plateau was reached at 8 eggs/day. Benefits and savings for industrial production of O. laevigatus are discussed.

Inheritance and Biological Characterization of an Orange-nymph Mutant in Orius laevigatus (Hemiptera: Anthocoridae).

A mutation showing a distinct orange color in the nymph stages was found in Orius laevigatus (Fieber) (Hemiptera: Anthocoridae), a key biological control agent in protected crops, used to control small pests, especially thrips. A laboratory strain carrying this body color mutation ambar was established. Genetic analysis determined that the mutation (ambar) was controlled by a single autosomal recessive allele. Some biological and ecological characteristics of this orange strain were compared to a normal population. Longevity, fecundity and fertility were similar in both populations, but immature survival, development rate, body size, starvation tolerance and predation capacity were inferior in the orange strain. The utility of the orange mutant as a visible marker for biological and ecological studies of this important biological control agent is discussed.

Effect of Parental Age and Mating Status on Reproductive Performance of Orius laevigatus (Hemiptera: Anthocoridae).

The reproductive potential of biological control agents (BCAs) is crucial for efficient mass-rearing and field performance, and it all begins with mating. Fecundity can be strongly influenced by intrinsic conditions, such as female age and, often neglected, male age and mating status. However, little is known about the impact of parental status at mating on female reproductive outcomes in BCAs. Orius laevigatus (Fieber) (Hemiptera: Anthocoridae) is widely used to control thrips in protected crops. We evaluated how many consecutive females a male could successfully mate and the effect on a female's reproductive output. In addition, we studied the effects of male and female age on mating. In the multiple mating experiment, the males showed a high capacity to fertilize females successively, not reducing fecundity until the sixth mated female. In the delayed mating experiment, copulation duration and fecundity increased with male age but decreased with female age. In contrast, fertility followed an opposite pattern, increasing with female age but decreasing with male age. However, fecundity gains outweighed fertility declines in both sexes. Therefore, reproductive capacity is increased when mating newly emerged females with males a few days old. The implications of our results for mass rearing and field performance are discussed.

Selection for resistance to pyrethroids in the predator Orius laevigatus.

BACKGROUND: Insecticide resistance in the natural enemies of pest species is a very desirable trait. It allows better integration of biological control with the synthetic and natural compounds applied to manage certain pests, especially secondary pest outbreaks. Insecticide resistance in predatory insects has been documented for lady beetles and lacewings, but intriguingly no cases of field-evolved resistance have been noted in any heteropteran predator. In this work, we first explored the variation in susceptibility to pyrethroids in wild and commercial populations of Orius laevigatus (Fieber). Second, we exploited this genetic variation to artificially select a strain of O. laevigatus resistant to pyrethroids. RESULTS: We found significant variation [median lethal dose (LC50 ) 1.6-77.0 mg L-1 ] in susceptibility to pyrethroids in wild populations of the heteropteran predator O. laevigatus, with a baseline LC50 value of 14.6 mg L-1 . We successfully selected a strain of O. laevigatus highly resistant to pyrethroids (LC50 = 1059.9 mg L-1 ). In addition, such resistance was expressed in every instar, particularly in the last nymphal stages, increasing the resilience of the whole population present in the crop facing pyrethroid application. CONCLUSION: The level of resistance achieved may be sufficient to allow survival of adults and nymphs of O. laevigatus exposed to the maximum field rate of several pyrethroids and natural pyrethrins, widely used to control a number of pests in organic and integrated pest management crops. Therefore, this strain resistant to pyrethrins and pyrethroids would improve the resilience of biocontrol protocols, which is an urgent requirement for wider adoption of biological control. © 2021 Society of Chemical Industry.

Insights into the origin of the invasive populations of Trioza erytreae in Europe using microsatellite markers and mtDNA barcoding approaches.

The African citrus psyllid Trioza erytreae is one of the major threats to citrus industry as the vector of the incurable disease known as huanglongbing (HLB) or citrus greening. The psyllid invaded the northwest of the Iberian Peninsula 6 years ago. The invasion alarmed citrus growers in the Mediterranean basin, the largest citrus producing area in Europe, which is still free of HLB. Before our study, no research had been carried out on the genetic diversity of T. erytreae populations that have invaded the Iberian Peninsula and the archipelagos of the Macaronesia (Madeira and the Canary Islands). In this study, combining microsatellites markers and mtDNA barcoding analysis, we characterize the genetic diversity, structure and maternal relationship of these new invasive populations of T. erytreae and those from Africa. Our results suggest that the outbreaks of T. erytreae in the Iberian Peninsula may have derived from the Canary Islands. The populations of T. erytreae that invaded Macaronesia and the Iberian Peninsula are likely to have originated from southern Africa. We anticipate our results to be a starting point for tracking the spread of this invasive pest outside of Africa and to be important for optimizing contingency and eradication plans in newly invaded and free areas.

Humans Share More Preferences for Floral Phenotypes With Pollinators Than With Pests.

Studies on the selection of floral traits usually consider pollinators and sometimes herbivores. However, humans also exert selection on floral traits of ornamental plants. We compared the preferences of bumblebees (Bombus terrestris), thrips (Frankliniella occidentalis), and humans for flowers of snapdragon. From a cross of two species, Antirrhinum majus and Antirrhinum linkianum, we selected four Recombinant Inbred Lines (RILs). We characterised scent emission from whole flowers and stamens, pollen content and viability, trichome density, floral shape, size and colour of floral parts. We tested the preferences of bumblebees, thrips, and humans for whole flowers, floral scent bouquets, stamen scent, and individual scent compounds. Humans and bumblebees showed preferences for parental species, whereas thrips preferred RILs. Colour and floral scent, in combination with other floral traits, seem relevant phenotypes for all organisms. Remarkably, visual traits override scent cues for bumblebees, although, scent is an important trait when bumblebees cannot see the flowers, and methyl benzoate was identified as a key attractant for them. The evolutionary trajectory of flowers is the result of multiple floral traits interacting with different organisms with different habits and modes of interaction.

Global patterns in genomic diversity underpinning the evolution of insecticide resistance in the aphid crop pest Myzus persicae.

The aphid Myzus persicae is a destructive agricultural pest that displays an exceptional ability to develop resistance to both natural and synthetic insecticides. To investigate the evolution of resistance in this species we generated a chromosome-scale genome assembly and living panel of >110 fully sequenced globally sampled clonal lines. Our analyses reveal a remarkable diversity of resistance mutations segregating in global populations of M. persicae. We show that the emergence and spread of these mechanisms is influenced by host-plant associations, uncovering the widespread co-option of a host-plant adaptation that also offers resistance against synthetic insecticides. We identify both the repeated evolution of independent resistance mutations at the same locus, and multiple instances of the evolution of novel resistance mechanisms against key insecticides. Our findings provide fundamental insights into the genomic responses of global insect populations to strong selective forces, and hold practical relevance for the control of pests and parasites.

Challenges facing arthropod biological control: identifying traits for genetic improvement of predators in protected crops.

Biological control is an efficient pest control method but there are still limitations that are hindering its wider adoption. Genetic improvement of biological control agents (BCAs) can help to overcome these constraints, but the choice of key attributes for better performance that need to be selected is still an open question. Several characteristics have been suggested but the harsh reality is that selective breeding of BCAs has received a lot of attention but resulted in very little progress. Identifying the appropriate traits to be prioritized may be the first step to reverse this situation. In our opinion, the best way is to look at the factors limiting the performance of key BCAs, especially generalist predators (pesticide compatibility, prey-density dependence, non-suitable crops, and extreme environmental conditions), and according to these challenges, to choose the attributes that would allow BCAs to overcome those limitations. The benefits of selection for higher resistance to toxins, whether artificially applied (pesticides) or plant produced (plant defenses); increased fitness when feeding on non-prey food (supplemented or plant-derived); and better adaptation to extreme temperature and humidity are discussed. In conclusion, genetic improvement of BCAs can bring about new opportunities to biocontrol industry and users to enhance biocontrol resilience. © 2020 Society of Chemical Industry.

Spiromesifen and spirotetramat resistance in field populations of Bemisia tabaci Gennadius in Spain.

BACKGROUND: Spiromesifen and spirotetramat are novel insecticides belonging to the chemical class of tetronic and tetramic acid derivatives. Both compounds have proven very effective against field populations of Bemisia tabaci around the world. However, several growers have recently reported control failures in Spain. Therefore, we studied the resistance level to these insecticides in field populations reporting control failures. In addition, we further selected a spiromesifen-resistant strain to study the mechanisms involved and the cross-resistance pattern. RESULTS: All the new field populations collected were significantly more resistant to spiromesifen than the susceptible population, confirming the presence of resistance. Several populations showing high levels of resistance to spiromesifen (>10 000-fold), exhibited cross-resistance to spirotetramat, but resistance ratios were much lower (130-fold). The spiromesifen laboratory-selected strain was very resistant to spiromesifen (LC50 > 30 000 mg L-1 ) and spirotetramat (LC50 = 368.1 mg L-1 ), but lacks any cross-resistance to other insecticides, thus providing options for resistance management. None of the synergists tested significantly restored the susceptibility of B. tabaci to either spiromesifen or spirotetramat. CONCLUSION: This is the first report of resistance to spiromesifen and spirotetramat in B. tabaci, and such high levels of resistance have not been reported before in any field collected pest. Our results suggest that enhanced detoxification does not critically contribute to resistance to ketoenols in B. tabaci. The obvious lack of a metabolic resistance mechanism either suggests a target-site resistance mechanism or a metabolic mechanism insensitive to the synergists tested. © 2018 Society of Chemical Industry.

The evolution of multiple-insecticide resistance in UK populations of tomato leafminer, Tuta absoluta.

BACKGROUND: The tomato leafminer, Tuta absoluta, is an economically important pest of tomatoes in Europe, Africa, Asia and South America. In the UK this species is controlled using an integrated pest management (IPM) programme which incorporates the insecticides spinosad and chlorantraniliprole. In response to UK grower concerns of loss of efficacy of these compounds at certain sites, insecticide bioassays were performed on five populations collected from four commercial glasshouses and potential mechanisms of resistance investigated. RESULTS: We observed high levels of resistance to spinosad in four of the strains, and in two of these tolerance to chlorantraniliprole. Selection of one of these strains with chlorantraniliprole rapidly resulted in a line exhibiting potent resistance to this compound. Sequencing of messenger RNA encoding the nicotinic acetylcholine receptor (nAChR) α6 subunit, target of spinosad, revealed Taα6 transcripts in the spinosad-resistant strains that lack exon 4 and encode a highly truncated protein, or contain a triplet deletion in the predicted first transmembrane domain resulting in the loss of a highly conserved amino acid. Sequencing of the ryanodine receptor gene, encoding the target of diamide insecticides, of the chlorantraniliprole-selected line revealed an amino acid substitution (G4903V) that has been previously linked to diamide resistance in populations of T. absoluta in the Mediterranean and South America. CONCLUSION: Taken together our results reveal emerging resistance in UK populations of T. absoluta to two of the most important insecticides used as part of IPM, with significant implications for the control of this species in the UK. © 2019 Society of Chemical Industry.

Insecticide resistance management strategies against the western flower thrips, Frankliniella occidentalis.

Western flower thrips (WFT), Frankliniella occidentalis (Pergande), is an economically important pest of a wide range of crops grown throughout the world. Insecticide resistance has been documented in many populations of WFT. Biological and behavioural characteristics and pest management practices that promote insecticide resistance are discussed. In addition, an overview is provided of the development of insecticide resistance in F. occidentalis populations and the resistance mechanisms involved. Owing to widespread resistance to most conventional insecticides, a new approach to insecticide resistance management (IRM) of F. occidentalis is needed. The IRM strategy proposed consists of two parts. Firstly, a general strategy to minimise the use of insecticides in order to reduce selection pressure. Secondly, a strategy designed to avoid selection of resistance mechanisms, considering cross-resistance patterns and resistance mechanisms.

Age-specific expression of resistance to a neonicotinoid insecticide in the whitefly Bemisia tabaci.

Neonicotinoid insecticides retain a crucial role within many chemical and integrated control strategies for the tobacco whitefly, Bemisia tabaci Gennadius, in spite of the establishment of potent and widespread resistance in many areas. Metabolic resistance mechanisms mediated by overexpression of P450-dependent monooxygenases have been implicated in neonicotinoid resistance in the two most prevalent B. tabaci biotypes. Further characterisation of resistance to the neonicotinoid imidacloprid in populations of both these B- and Q-types is reported.Expression of resistance to imidacloprid was age specific in B- and Q-type strains of B. tabaci. The highest observed resistance ratio at LC(50) expressed in prepupal nymphs was 13, compared with at least 580 in their adult counterparts. For all strains, resistance expressed in immatures was not sufficiently potent to compromise recommended imidacloprid application rates.Targeting neonicotinoids towards immature life stages of B. tabaci may circumvent the protection conferred by current mechanisms of resistance, simultaneously reducing the selection pressures imposed. However, such tactics may enhance the expression of existing resistance mechanisms in immatures, or promote the establishment of novel ones expressed in all life stages.

Inheritance of resistance to acrinathrin in Frankliniella occidentalis (Thysanoptera: Thripidae).

BACKGROUND: The western flower thrips (WFT), Frankliniella occidentalis (Pergande), is an economically important pest. The genetic basis of acrinathrin resistance was investigated in WFT. RESULTS: The resistant strain, selected in the laboratory for acrinathrin resistance from a pool of thrips populations collected in Almeria (south-eastern Spain), showed a high resistance to acrinathrin (43-fold based on LC(50) values) compared with the laboratory susceptible strain. Mortality data from reciprocal crosses of resistant and susceptible thrips indicated that resistance was autosomal and not influenced by maternal effects. Analysis of probit lines from the parental strains and reciprocal crosses showed that resistance was expressed as a codominant trait. To determine the number of genes involved, a direct test of monogenic inheritance based on the backcrosses suggested that resistance to acrinathrin was probably controlled by one locus. Another approach, which was based on phenotypic variances, showed n(E), or the minimum number of freely segregating genetic factors for the resistant strain, to be 0.79. CONCLUSION: The results showed that acrinathrin resistance in WFT was autosomal and not influenced by maternal effects, and was expressed as a codominant trait, probably controlled by one locus.