Insecticide resistance traits differ among and within host races in Aphis gossypii

Bacterial communities in natural versus pesticide-treated Aphis gossypii populations in North China

The cotton‐melon aphid, Aphis gossypii Glover, is a worldwide‐spreading species, and pesticide‐resistant populations are increasing rapidly. In this study, investigations were performed based on Illumina HiSeq sequencing of the 16S rDNA V4 region for the bacterial communities embodied as intracellular symbionts under natural and in pesticide‐treated populations of A. gossypii. The results revealed that more than 82% of bacterial communities belonged to the phylum Proteobacteria in which the maximum proportion (53.24%) was of the genus Arsenophonus; Hamiltonella composed 22.31; and 1.37% was of the genus Acinetobacter. The relative abundance of Hamiltonella was obvious, vertically transmitted, divided into two groups, and its infection influenced the bacterial communities in A. gossypii. Symbiont density and composition were changed in samples tested on different days. Azadirachtin and phoxim influenced on the composition of bacterial communities. Different biomarkers were used for pesticide‐treated samples with LEfSe results. These findings will increase awareness regarding bacterial communities in naturally occurring populations of A. gossypii and pave the way to study the relationship between symbionts and pesticide resistance.

Genetic diversity of melon aphids Aphis gossypii associated with landscape features

Despite increasing evidence that landscape features strongly influence the abundance and dispersal of insect populations, landscape composition has seldom been explicitly linked to genetic structure. We conducted a genetic study of the melon aphid, Aphis gossypii, in two counties of Beijing, China during spring migration using samples from watermelon. We performed aphid genetic analysis using restriction site associated DNA sequencing (2b-RAD) and investigated the relationship between land cover and the genetic diversity. The percentage area of land cover (cropland, vegetable, orchard, grassland, woodland) was quantified in each particular scale (ranging from 0.5 km to 3 km) and was used as a predictor variable in our generalized linear models. We found a moderate level of genetic differentiation among nine sampled populations. Geographic distance and genetic distance were not significantly associated, indicating that geographic location was not a barrier to migration. These nine populations could be clustered depending on their level of genetic diversity (high and low). The genetic diversity (Shannon's information index) was positively correlated with grassland at the spatial scales of 1 and 2 km and negatively with orchard and vegetable at 0.5 and 1 km. Genetic diversity was best predicted by the grassland + orchard + vegetable model at a spatial scale of 1 km. Based on the method of relative weights, orchard land had the greatest relative importance, followed by grassland and vegetable land, in that order. This study contributes to our understanding of the genetic variation of aphids in agricultural landscapes.

Plant training for induced defense against insect pests: a promising tool for integrated pest management in cotton

Enhancing cotton pest management using plant natural defenses has been described as a promising way to improve the management of crop pests. We here reviewed various studies on cotton growing systems to illustrate how an ancient technique called plant training, which includes plant topping and pruning, may contribute to this goal. Using examples from cotton crops, we show how trained plants can be brought to a state of enhanced defense that causes faster and more robust activation of their defense responses. We revisit the agricultural benefits associated with this technique in cotton crops, with a focus on its potential as a supplementary tool for integrated pest management (IPM). In particular, we examine its role in mediating plant interactions with conspecific neighboring plants, pests and associated natural enemies. We propose a new IPM tool, plant training for induced defense, which involves inducing plant defense through artificial injury. Experimental evidence from various studies shows that cotton training is a promising technique, particularly for smallholders, which can be used as part of an IPM program to significantly reduce insecticide use and to improve productivity in cotton farming. © 2018 Society of Chemical Industry.

Cry1 Bt Susceptibilities of Fall Armyworm (Lepidoptera: Noctuidae) Host Strains

The fall armyworm, Spodoptera frugiperda (Smith; Lepidoptera: Noctuidae), is a highly polyphagous, multivoltine pest of commercial crops including corn (Zea mays L.), cotton (Gossypium spp. L.), rice (Oryza sativa L.), and pasture grasses. Fall armyworm has become a growing concern in agricultural communities across the Americas as field populations in many locales have evolved resistance to several Cry1 toxins derived from the bacterium Bacillus thuringiensis Berliner (Bt). An often overlooked aspect of fall armyworm biology is the existence of two host strains, the 'rice' and 'corn' strains. There has been little research devoted to the characterization of fall armyworm host strains, although there is evidence that the rice and corn-strains may differ in their tolerances to Bt toxins expressed by transgenic plants. In this study, diet-based bioassays were conducted to compare the susceptibilities of one rice-strain, two corn-strains, and one rice-corn hybrid population to Cry1Ab, Cry1Ac, and Cry1F protein. Results indicate that the corn-strains and hybrid populations are more tolerant to the Bt toxins, especially to Cry1F, than the rice-strain population. Results from this study, when combined with existing techniques for host strain identification, may aid in the development of regional insect resistance management programs for fall armyworm.

Pyrethroid resistance associated with M918 L mutation and detoxifying metabolism in Aphis gossypii from Bt cotton growing regions of China

BACKGROUND

The cotton aphid, Aphis gossypii (Glover), is a destructive pest that affects numerous crops throughout the world. Pyrethroid resistance has become endemic in A. gossypii populations in the cotton growing areas of China due to wide- spread application of insecticides. To assess the extent of pyrethroid resistance, bioassays were conducted on field populations collected from several cotton cultivation areas from 2010 to 2015. The frequency of a known resistance-associated sodium channel mutation (M918 L) in A. gossypii was evaluated and the bioassay of bifenthrin with or without the synergist was performed to illuminate the mechanisms underlying resistance to pyrethroids.

RESULTS

The field populations exhibited very high levels of resistance to both beta-cypermethrin and deltamethrin. Pretreatment with synergists, DEF and PBO, significantly increased the toxicity of bifenthrin to cotton aphid populations collected from Bt cotton fields in China. Further, 96.8-100% of individuals with the M918 L mutation (including both RR and RS individuals) were observed in various populations, and only 2.8-3.2% of individuals with wild-type homozygotes (SS) were detected.

CONCLUSION

The mutation M918 L in the voltage-gated sodium channel along with detoxifying metabolism was contributed to the pyrethroid resistance in the field populations of Aphis gossypii from cotton growing regions of China. And insecticides with different modes of action should be recommended for the control of A. gossypii in the future. © 2017 Society of Chemical Industry.

Transcriptional responses of detoxification genes to four plant allelochemicals in Aphis gossypii

Aphis gossypii Glover (Hemiptera: Aphididae) can damage a variety of agricultural crops, so it is very important for cotton aphids to evolve adaptive mechanisms to various allelochemicals from host plants. Our results aim to provide a fundamental and rich resource for exploring aphid functional genes in A. gossypii. A transcriptome data set and five expression profile data sets of A. gossypii samples were analyzed by Illumina sequencing platform. In total, 53,763,866 reads were assembled into 1,963,516 contigs and 28,555 unigenes. Compared with the control, 619 genes were significantly up- or downregulated in the treatment group by 2-tridecanone. There were 516, 509, and 717 of differential expression genes in tannic acid, quercetin, and gossypol treatment groups, respectively. Furthermore, there were 4 of 54 putative cytochrome P450 genes and 1 of 7 putative carboxylesterases downregulated in all treatment groups by four plant allelochemicals. When aphids fed on 2-tridecanone, tannic acid, and quercetin, only one P450 gene was upregulated. These results show that plant allelochemical stress can induce differential gene expression in A. gossypii. The differential response information of gene expression based on a large-scale sequence would be useful to reveal molecular mechanisms of adaptation for A. gossypii to plant allelochemicals.

Characterization of the voltage-gated sodium channel of the Asian citrus psyllid, Diaphorina citri

The Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Liviidae), is an important insect pest of citrus. It is the vector of 'Candidatus' Liberibacter asiaticus, a phloem-limited bacterium that infects citrus, resulting in the disease Huanglongbing (HLB). Disease management relies heavily on suppression of D. citri populations with insecticides, including pyrethroids. In recent annual surveys to monitor insecticide resistance, reduced susceptibility to fenpropathrin was identified in several field populations of D. citri. The primary target of pyrethroids is the voltage-gated sodium channel (VGSC). The VGSC is prone to target-site insensitivity because of mutations that either reduce pyrethroid binding and/or alter gating kinetics. These mutations, known as knockdown resistance or kdr, have been reported in a wide diversity of arthropod species. Alternative splicing, in combination with kdr mutations, has been also associated with reduced pyrethroid efficacy. Here we report the molecular characterization of the VGSC in D. citri along with a survey of alternative splicing across developmental stages of this species. Previous studies demonstrated that D. citri has an exquisite enzymatic arsenal to detoxify insecticides resulting in reduced efficacy. The results from the current investigation demonstrate that target-site insensitivity is also a potential basis for insecticide resistance to pyrethroids in D. citri. The VGSC sequence and its molecular characterization should facilitate early elucidation of the underlying cause of an established case of resistance to pyrethroids. This is the first characterization of a VGSC from a hemipteran to this level of detail, with the majority of the previous studies on dipterans and lepidopterans.

Sublethal and transgenerational effects of sulfoxaflor on the biological traits of the cotton aphid, Aphis gossypii Glover (Hemiptera: Aphididae)

Sulfoxaflor is a novel insecticide belonging to sulfoximine chemical class that can be used to control sap-feeding insects, notably Aphis gossypii Glover. In addition to its acute toxicity, it is also important to consider the possible sublethal effects when establishing a comprehensive understanding of the toxicity of a new insecticide. We assessed the effects of a low lethal concentration (LC₂₅) of sulfoxaflor on biological parameters of A. gossypii adults (F0) and subsequent transgenerational effects, i.e., on the progeny (F1 generation). The data were analyzed using an age-stage life table procedure. The results showed that the longevity and fecundity were not significantly affected by the LC₂₅ of sulfoxaflor in the F0 or F1 generations. In addition, no significant differences were observed on the developmental time of each instar, the adult pre-oviposition period, and on the longevity of F1 individuals. However, the duration of their pre-adult stage and total pre-oviposition period, as well as their mean generation time were significantly increased. These observed effects affected aphid demographic traits; the survival rate, the intrinsic rate of increase (r _i ), the finite rate of increase (λ), the net reproductive rate (R₀), and the gross reproduction rate (GRR) of the F1 individuals (i.e., from F0 mothers) were significantly lower compared to the control. Our results showed that sublethal effects of sulfoxaflor significantly slowed down A. gossypii population growth; they indicated that effects of sulfoxaflor might be increased (beyond lethal effect) through sublethal effects when concentrations decreased in sulfoxaflor-treated areas after initial application in field.

Cycloxaprid: A novel cis-nitromethylene neonicotinoid insecticide to control imidacloprid-resistant cotton aphid (Aphis gossypii)

Imidacloprid is a nicotinic acetylcholine receptor (nAChR) agonist with potent insecticidal activity. However, resistance to imidacloprid is a significant threat and has been identified in several pest species. Cycloxaprid with cis-configuration is a novel neonicotinoid insecticide, which shows high activity against imidacloprid-resistant pests. The LC50 of imidacloprid against the resistant Aphis gossypii was 14.33mgL(-1) while it was only 0.70mgL(-1) for the susceptible population, giving a resistance ratio of 20.47. In this imidacloprid-resistant population, a point mutation (R81T) located in the loop D region of the nAChR β1 subunit was found out. But this point mutation did not decrease the activity of cycloxaprid against A. gossypii. The LC50 of cycloxaprid was 1.05 and 1.36mgL(-1) for the imidacloprid-susceptible and imidacloprid-resistant populations, respectively. In addition, cycloxaprid provided better efficacies against resistant A. gossypii than imidacloprid in the fields. Although cycloxaprid was highly toxic to A. gossypii, it showed high selective activity between A. gossypii and its predominant natural enemies, Harmonia axyridis and Chrysoperla sinica. These results demonstrate that cycloxaprid is a promising insecticide against imidacloprid-resistant A. gossypii and suitable for the integrated pest management.

Expression patterns, mutation detection and RNA interference of Rhopalosiphum padi voltage-gated sodium channel genes

The voltage-gated sodium channel (VGSC) is the target of sodium-channel-blocking insecticides. Traditionally, animals were thought to have only one VGSC gene comprising a α-subunit with four homologous domains (DI–DIV). The present study showed that Rhopalosiphum padi, an economically important crop pest, owned a unique heterodimeric VGSC (H1 and H2 subunits) encoded by two genes (Rpvgsc1 and Rpvgsc2), which is unusual in insects and other animals. The open reading frame (ORF) of Rpvgsc1 consisted 1150 amino acids, and the ORF of Rpvgsc2 had 957 amino acids. Rpvgsc1 showed 64.1% amino acid identity to DI–DII of Drosophila melanogaster VGSC and Rpvgsc2 showed 64.0% amino acid identity to DIII–DIV of D. melanogaster VGSC. A M918L mutation previously reported in pyrethroids-resistant strains of other insects was found in the IIS4-S6 region of R. padi field sample. The two R. padi VGSC genes were expressed at all developmental stages and showed similar expression patterns after treatment with beta-cypermethrin. Knockdown of Rpvgsc1 or Rpvgsc2 caused significant reduction in mortality rate of R. padi after exposure to beta-cypermethrin. These findings suggest that the two R. padi VGSC genes are both functional genes.

Insight into the durability of plant resistance to aphids from a demo-genetic study of Aphis gossypii in melon crops

Resistance breakdown has been observed following the deployment of plant cultivars resistant to pests. Assessing the durability of a resistance requires long-term experiments at least at a regional scale. We collected such data for melon resistance conferred by the Vat gene cluster to melon aphids. We examined landscape-level populations of Aphis gossypii collected in 2004-2015, from melon-producing regions with and without the deployment of Vat resistance and with different climates. We conducted demo-genetic analyses of the aphid populations on Vat and non-Vat plants during the cropping seasons. The Vat resistance decreased the density of aphid populations in all areas and changed the genetic structure and composition of these populations. Two bottlenecks were identified in the dynamics of adapted clones, due to the low levels of production of dispersal morphs and winter extinction. Our results suggest that (i) Vat resistance will not be durable in the Lesser Antilles, where no bottleneck affected the dynamics of adapted clones, (ii) Vat resistance will be durable in south-west France, where both bottlenecks affected the dynamics of adapted clones and (iii) Vat resistance will be less durable in south-east France, where only one of the two bottlenecks was observed.

Multiple Mutations on the Second Acetylcholinesterase Gene Associated With Dimethoate Resistance in the Melon Aphid, Aphis gossypii (Hemiptera: Aphididae)

The melon aphid, Aphis gossypii Glover (Hemiptera: Aphididae), is an important cosmopolitan and extremely polyphagous species capable of causing direct and indirect damage to various crops. Insecticide resistance in melon aphids is of particular concern. To determine the basis of resistance, organophosphate (OP)-resistant strains of A. gossypii were obtained by continuous selection with dimethoate in the laboratory, and resistance mechanisms were investigated along with susceptible strains. Three resistant strains LKR-1, LKR-2, and LKR-3 exhibiting 270-, 243-, and 210-fold resistance obtained after 30 generations of selection with dimethoate, respectively, were utilized in this study. The role of acetylcholinesterase (AChE), a target enzyme for OPs and carbamates (CMs), was investigated. AChE enzyme assay revealed that there was no significant change in the activities of AChE in resistant and susceptible strains. However, AChE inhibitory assay showed that 50% of the enzyme activity in resistant strains was inhibited at significantly higher concentration of dimethoate (131.87, 158.65, and 99.29 µmolL(−1)) as compared with susceptible strains (1.75 and 2.01 µmolL(−1)), indicating AChE insensitivity owing to altered AChE. Molecular diagnostic tool polymerase chain reaction-restriction fragment length polymorphism revealed the existence of two consistent non-synonymous point mutations, single-nucleotide polymorphism, viz., A302S (equivalent to A201 in Torpedo californica Ayres) and S431F (equivalent to F331 in T. californica), in the AChE gene Ace2 of resistant strains. Further, cloning and sequencing of a partial fragment of Ace2 (897 bp) gene from susceptible and resistant strains revealed an additional novel mutation G221A in resistant strains, LKR-1 and LKR-2. Susceptible Ace2 genes shared 99.6 and 98.9% identity at the nucleic acid and amino acid levels with resistant ones, respectively. Functional analysis of these point mutations was assessed by in silico docking studies using the modeled wild-type and naturally mutated AChE2. Computational analysis showed that the conformational changes in AChE2 active site due to structural gene substitutions (A302S, S431F, and G221A) significantly reduced the level of ligand (OP-dimethoate, omethoate, and CM-pirimicarb) binding, suggesting that they are potentially associated with resistance development. These results unambiguously suggested that multiple mutations located in the enzyme active site are responsible for AChE insensitivity to dimethoate and are likely the molecular basis for dimethoate resistance in these selected field populations of A. gossypii.

Insecticide resistance in Bemisia tabaci Gennadius (Homoptera: Aleyrodidae) and Anopheles gambiae Giles (Diptera: Culicidae) could compromise the sustainability of malaria vector control strategies in West Africa

Insecticides from the organophosphate (OP) and pyrethroid (PY) chemical families, have respectively, been in use for 50 and 30 years in West Africa, mainly against agricultural pests, but also against vectors of human disease. The selection pressure, with practically the same molecules year after year (mainly on cotton), has caused insecticide resistance in pest populations such as Bemisia tabaci, vector of harmful phytoviruses on vegetables. The evolution toward insecticide resistance in malaria vectors such as Anopheles gambiae sensus lato (s.l.) is probably related to the current use of these insecticides in agriculture. Thus, successful pest and vector control in West Africa requires an investigation of insect susceptibility, in relation to the identification of species and sub species, such as molecular forms or biotypes. Identification of knock down resistance (kdr) and acetylcholinesterase gene (Ace1) mutations modifying insecticide targets in individual insects and measure of enzymes activity typically involved in insecticide metabolism (oxidase, esterase and glutathion-S-transferase) are indispensable in understanding the mechanisms of resistance. Insecticide resistance is a good example in which genotype-phenotype links have been made successfully. Insecticides used in agriculture continue to select new resistant populations of B. tabaci that could be from different biotype vectors of plant viruses. As well, the evolution of insecticide resistance in An. gambiae threatens the management of malaria vectors in West Africa. It raises the question of priority in the use of insecticides in health and/or agriculture, and more generally, the question of sustainability of crop protection and vector control strategies in the region. Here, we review the susceptibility tests, biochemical and molecular assays data for B. tabaci, a major pest in cotton and vegetable crops, and An. gambiae, main vector of malaria. The data reviewed was collected in Benin and Burkina Faso between 2008 and 2010 under the Corus 6015 research program. This review aims to show: (i) the insecticide resistance in B. tabaci as well as in An. gambiae; and (ii) due to this, the impact of selection of resistant populations on malaria vector control strategies. Some measures that could be beneficial for crop protection and vector control strategies in West Africa are proposed.

Multiple origins of pyrethroid insecticide resistance across the species complex of a nontarget aquatic crustacean, Hyalella azteca

Use of pesticides can have substantial nonlethal impacts on nontarget species, including driving evolutionary change, often with unknown consequences for species, ecosystems, and society. Hyalella azteca, a species complex of North American freshwater amphipods, is widely used for toxicity testing of water and sediment and has frequently shown toxicity due to pyrethroid pesticides. We demonstrate that 10 populations, 3 from laboratory cultures and 7 from California water bodies, differed by at least 550-fold in sensitivity to pyrethroids. The populations sorted into four phylogenetic groups consistent with species-level divergence. By sequencing the primary pyrethroid target site, the voltage-gated sodium channel, we show that point mutations and their spread in natural populations were responsible for differences in pyrethroid sensitivity. At least one population had both mutant and WT alleles, suggesting ongoing evolution of resistance. Although nonresistant H. azteca were susceptible to the typical neurotoxic effects of pyrethroids, gene expression analysis suggests the mode of action in resistant H. azteca was not neurotoxicity but was oxidative stress sustained only at considerably higher pyrethroid concentrations. The finding that a nontarget aquatic species has acquired resistance to pesticides used only on terrestrial pests is troubling evidence of the impact of chronic pesticide transport from land-based applications into aquatic systems. Our findings have far-reaching implications for continued uncritical use of H. azteca as a principal species for monitoring and environmental policy decisions.

Evidence of superclones in Australian cotton aphid Aphis gossypii Glover (Aphididae: Hemiptera)

BACKGROUND

Aphis gossypii is an important pest of cotton that has developed resistance to many chemicals used for its control. Any lack of understanding of its genetic structure, resistance status and host plant specialisation hampers effective management.

RSULTS

Eight microsatellite markers were genotyped for a collection of Australian A. gossypii field isolates from 55 plant species from major Australian cotton-producing regions. The aphid's pirimicarb resistance status linked to the ACE1 (acetylcholinesterase) S431F mutation was determined by PCR-RFLP. Overall, the genetic diversity was low and there were only 13 multilocus genotype (MLG) groups found in a total of 936 aphids, suggesting asexual reproduction. Three MLGs (Aust-01, Aust-02 and Aust-04) represented 78% of all aphids tested. MLGs Aust-01 (41%) and Aust-02 (18%) were linked to the ACE1 S431F mutation and found on cotton and a range of hosts. Aust-04 (19%) hosted mainly on cotton (but also Asteraceae and Malvaceae) was predominantly susceptible to pirimicarb. Given their abundance and widespread occurrence, these three clones were considered to be superclones.

CONCLUSION

The study demonstrated that any strategy to control A. gossypii and manage pirimicarb resistance should target A. gossypii strains of all MLG types residing on any plant species and not just cotton

Diversity and Convergence of Sodium Channel Mutations Involved in Resistance to Pyrethroids

Pyrethroid insecticides target voltage-gated sodium channels, which are critical for electrical signaling in the nervous system. The intensive use of pyrethroids in controlling arthropod pests and disease vectors has led to many instances of pyrethroid resistance around the globe. In the past two decades, studies have identified a large number of sodium channel mutations that are associated with resistance to pyrethroids. The purpose of this review is to summarize both common and unique sodium channel mutations that have been identified in arthropod pests of importance to agriculture or human health. Identification of these mutations provides valuable molecular markers for resistance monitoring in the field and helped the discovery of the elusive pyrethroid receptor site(s) on the sodium channel.

What do spring migrants reveal about sex and host selection in the melon aphid?

Background

Host plants exert considerable selective pressure on aphids because the plants constitute their feeding, mating and oviposition sites. Therefore, host specialisation in aphids evolves through selection of the behavioural and chemical mechanisms of host-plant location and recognition, and through metabolic adaptation to the phloem content of the host plant. How these adaptive traits evolve in an aphid species depends on the complexity of the annual life cycle of that species. The purpose of this field study was to determine how winged spring-migrant populations contribute to the evolution and maintenance of host specialisation in Aphis gossypii through host-plant choice and acceptance. We also assessed whether host-specialised genotypes corresponded exclusively to anholocyclic lineages regardless of the environmental conditions.

Results

The spring populations of cotton-melon aphids visiting newly planted melon crops exhibited an unexpectedly high level of genetic diversity that contrasted with the very low diversity characterising the host-specialised populations of this aphid species. This study illustrated in natura host-plant-selection pressure by showing the great differences in genetic diversity between the spring-migrant populations (alate aphids) and the melon-infesting populations (the apterous offspring of the alate aphids). Moreover, an analysis of the genetic composition of these alate and apterous populations in four geographic regions suggested differences in life-history strategies, such as host choice and reproductive mode, and questioned the common assertion that A. gossypii is an anholocyclic species throughout its distribution area, including Europe.

Conclusions

Our results clearly demonstrate that the melon plant acts as a selective filter against the reproduction of non-specialised individuals. We showed that olfactory cues are unlikely to be decisive in natura for host recognition by spring-migrant aphid populations that are not specialised on Cucurbitaceae. The agroecosystem structure and history of the four studied regions may have partially shaped the genetic structure of the spring-migrant populations of A. gossypii. Cucurbitaceae-specialised genotypes corresponded exclusively to anholocyclic lineages, regardless of the environmental conditions. However, some genotypes that were genetically close to the host-specialised genotypes and some genotypes that probably originated from wild plants had never been previously sampled; both were holocylic.

Climate effects on life cycle variation and population genetic architecture of the black bean aphid, Aphis fabae

Aphid species may exhibit different reproductive modes ranging from cyclical to obligate parthenogenesis. The distribution of life cycle variation in aphids is generally determined by ecological forces, mainly climate, because only sexually produced diapausing eggs can survive harsh winters or periods of absence of suitable host plants. Aphids are thus interesting models to investigate intrinsic and environmental factors shaping the competition among sexual and asexual lineages. We conducted a Europe-wide sampling of black bean aphids, Aphis fabae, and combined population genetic analyses based on microsatellite data with an experimental determination of life cycle strategies. Aphids were collected from broad beans (Vicia faba) as well as some Chenopodiaceae, but we detected no genetic differentiation between aphids from different host plants. Consistent with model predictions, life cycle variation was related to climate, with aphids from areas with cold winters investing more in sexual reproduction than aphids from areas with mild winters. Accordingly, only populations from mild areas exhibited a clear genetic signature of clonal reproduction. These differences arise despite substantial gene flow over large distances, which was evident from a very low geographic population structure and a lack of isolation-by-distance among 18 sites across distances of more than 1000 km. There was virtually no genetic differentiation between aphids with different reproductive modes, suggesting that new asexual lineages are formed continuously. Indeed, a surprising number of A. fabae genotypes even from colder climates produced some parthenogenetic offspring under simulated winter conditions. From this we predict that a shift to predominantly asexual reproduction could take place rapidly under climate warming.

Uncommon associations in target resistance among French populations of Myzus persicae from oilseed rape crops

Within the framework of a molecular exploration of target resistance in populations of Myzus persicae on oilseed rapes in France, (1) the S431F mutation (coding gene ace2), although previously reckoned to be rare, revealed to be frequent, (2) M918L (phenotypically characterised) and L932F (both on para) were found for the first time in M. persicae, and (3) a linkage was revealed between M918L and S431F. While until recently populations developing on French oilseed rapes were dominated by genotypes possessing pyrethroid target resistance and esterase overproduction, to date a different type of dominating genotype, equipped with carbamate and pyrethroid target resistance, seems to be invading such fields.

Insecticide use and competition shape the genetic diversity of the aphid Aphis gossypii in a cotton-growing landscape

Field populations of the cotton aphid, Aphis gossypii Glover, are structured into geographically widespread host races. In the cotton-producing regions of West and Central Africa (WCA), two genotypes have been repeatedly detected within the cotton host race, one of which (Burk1) is prevalent (>90%) and resistant to several insecticides, as opposed to the second one (Ivo). Here, we conducted whole plant and field cage experiments to test hypotheses for such low genetic diversity, including selection from insecticide treatments, interclonal competition and adaptation to host plant, or climatic conditions. To assess the genetic diversity of immigrant aphids, alatae were trapped and collected on cotton and relay host plants (okra and roselle) in the early cropping season. Individuals were genotyped at eight specific microsatellite loci and characterized by a multilocus genotype (MLG). When independently transferred from cotton (Gossypium hirustum L.) leaf discs to whole plants (G. hirsutum and G. arboreum, roselle and okra), Ivo and Burk1 performed equally well. When concurrently transferred from cotton leaf discs to the same plant species, Ivo performed better than Burk1, indicating that competition favoured Ivo. This was also the case on G. hirsutum growing outdoors. Conversely, Burk1 prevailed when cotton plants were sprayed with insecticides. In experiments where aphids were allowed to move to neighbouring plants, Burk1 was better represented than Ivo on low-populated plants, suggesting that dispersal may be a way to avoid competition on crowded plants. Most cotton aphids collected on cotton or relay host plants in the early cropping season were Burk1 (>90%), indicating high dispersal ability and, probably reflecting high frequency on host plants from which they dispersed. In the agricultural landscape of WCA, the use of broad-range insecticides on both cotton and relay host plants has led to the prevalence of one genotype of A. gossypii resistant to different classes of insecticides. Deployment of widespread and integrated pest management strategies are needed to restore cotton aphid control.

Weak spatial and temporal population genetic structure in the rosy apple aphid, Dysaphis plantaginea, in French apple orchards

We used eight microsatellite loci and a set of 20 aphid samples to investigate the spatial and temporal genetic structure of rosy apple aphid populations from 13 apple orchards situated in four different regions in France. Genetic variability was very similar between orchard populations and between winged populations collected before sexual reproduction in the fall and populations collected from colonies in the spring. A very small proportion of individuals (∼2%) had identical multilocus genotypes. Genetic differentiation between orchards was low (F(ST)<0.026), with significant differentiation observed only between orchards from different regions, but no isolation by distance was detected. These results are consistent with high levels of genetic mixing in holocyclic Dysaphis plantaginae populations (host alternation through migration and sexual reproduction). These findings concerning the adaptation of the rosy apple aphid have potential consequences for pest management.