Toxicological, biochemical, and in silico investigations of three trehalase inhibitors for new ways to control aphids.

Insect trehalases have been identified as promising new targets for pest control. These key enzymes are involved in trehalose hydrolysis and plays an important role in insect growth and development. In this contribution, plant and microbial compounds, namely validamycin A, amygdalin, and phloridzin, were evaluated for their effect, through trehalase inhibition, on Acyrthosiphon pisum aphid. The latter is part of the Aphididae family, main pests as phytovirus vectors and being very harmful for crops. Validamycin A was confirmed as an excellent trehalase inhibitor with an half maximal inhibitory concentration and inhibitor constant of 2.2 × 10-7 and 5 × 10-8 M, respectively, with a mortality rate of ~80% on a A. pisum population. Unlike validamycin A, the insect lethal efficacy of amygdalin and phloridzin did not correspond to their trehalase inhibition, probably due to their hydrolysis by insect ß-glucosidases. Our docking studies showed that none of the three compounds can bind to the trehalase active site, unlike their hydrolyzed counterparts, that is, validoxylamine A, phloretin, and prunasin. Validoxylamine A would be by far the best trehalase binder, followed by phloretin and prunasin.

Evaluation of biological activities, and exploration on mechanism of action of matrine-cholesterol derivatives.

To develop new potential pesticides, a series of matrine-cholesterol derivatives were prepared by modifications of two non-food bioactive products matrine and cholesterol. Two N-phenylsulfonylmatrinic esters (5i and 5j) showed the most potent insecticidal activity against Mythimna separata Walker. Two N-benzylmatrinic esters (5e and 5g) exhibited the most promising aphicidal activity against Aphis citricola Van der Goot. Especially compound 5e showed good control effects in the greenhouse against A. citricola. Some interesting results of their structure-activity relationships were also observed. By reverse transcription polymerase chain reaction (RT-PCR) and quantitative real-time polymerase chain reaction (qRT-PCR) analysis of HMG-CoA reductase in apterous adults of A. citricola, it demonstrated that matrine and cholesterol may be the HMG-CoA reductase inhibitors, and the hydroxyl of cholesterol or the lactam ring of matrine may be important for acting with HMG-CoA reductase in A. citricola.

Inhibition of histone acetylation and deacetylation enzymes affects longevity, development, and fecundity in the pea aphid (Acyrthosiphon pisum).

Histone acetylation is an evolutionarily conserved epigenetic mechanism of eukaryotic gene regulation which is tightly controlled by the opposing activities of histone acetyltransferases (HATs) and histone deacetylases (HDACs). In insects, life-history traits such as longevity and fecundity are severely affected by the suppression of HAT/HDAC activity, which can be achieved by RNA-mediated gene silencing or the application of chemical inhibitors. We used both experimental approaches to investigate the effect of HAT/HDAC inhibition in the pea aphid (Acyrthosiphon pisum) a model insect often used to study complex life-history traits. The silencing of HAT genes (kat6b, kat7, and kat14) promoted survival or increased the number of offspring, whereas targeting rpd3 (HDAC) reduced the number of viviparous offspring but increased the number of premature nymphs, suggesting a role in embryogenesis and eclosion. Specific chemical inhibitors of HATs/HDACs showed a remarkably severe impact on life-history traits, reducing survival, delaying development, and limiting the number of offspring. The selective inhibition of HATs and HDACs also had opposing effects on aphid body weight. The suppression of HAT/HDAC activity in aphids by RNA interference or chemical inhibition revealed similarities and differences compared to the reported role of these enzymes in other insects. Our data suggest that gene expression in A. pisum is regulated by multiple HATs/HDACs, as indicated by the fitness costs triggered by inhibitors that suppress several of these enzymes simultaneously. Targeting multiple HATs or HDACs with combined effects on gene regulation could, therefore, be a promising approach to discover novel targets for the management of aphid pests.

Enzymes mediating resistance to chlorpyriphos in Aphis fabae (Homoptera: Aphididae).

The black bean aphid, Aphis fabae (Homoptera: Aphididae), is a widespread pest that has more than 200 hosts in the world. Insecticide resistance (IR) due to frequent applications is the major limitation in integrated pest management programs. Biochemical resistance is a common type of IR in which the insecticide is detoxified by one or more enzymes of the pest before reaching its target site. In this study, the IR of A. fabae populations to chlorpyrifos was evaluated in two single sprayed fields (fields A and C) and one replicated spraying field (field B) in comparison with a susceptible population (field H) during 2015. After treatments, total protein content and the activity of two detoxifying enzymes, esterases (ESTs) and glutathione S-transferases (GSTs), and acetylcholinesterase (AChE) in the populations were determined. Results clearly showed higher total protein content for the field populations compared to the susceptible population. The total protein content in field B population was significantly more than other populations. The total protein contents in Field A, B and C were 2.81, 2.89 and 1.06-fold more than susceptible strain, respectively. Higher actives of enzymes were observed in fields A, B, and C populations compared to the susceptible population (field H). The highest activity of GSTs and ESTs was observed in the field B population. Taken together, the present study demonstrated a significant IR to chlorpyrifos in the sprayed populations of A. fabae that can be attributed to the higher activity of their detoxification enzymes.

Phenoloxidases are required for the pea aphid's defence against bacterial and fungal infection.

The pea aphid, Acyrthosiphon pisum, has an incomplete immune system compared to those of other insect species; some conserved components and pathways in other species are missing in its genome. As a core component of the insect immune system, prophenoloxidase (PPO) genes are retained in the pea aphid. Early studies have also shown the presence of phenoloxidase activity in specific tissues or cells in the pea aphid and suggested its involvement in response to immune challenges. In this study, we knocked down the expression of PPO genes in the pea aphid using double-stranded RNA-based interference, and quantitative PCR analysis and an enzyme activity assay confirmed our success in the PPO gene knockdown. In bacterial and fungal infection experiments, we observed that the knockdown of PPO resulted in more live bacterial cells and fungal spores in the body of the aphids and higher mortality of the aphids after infection. Our study provides evidence supporting a critical role of PPO in the defence of the pea aphid.

Both farnesyl diphosphate synthase genes are involved in the production of alarm pheromone in the green peach aphid Myzus persicae.

Farnesyl diphosphate synthase (FPPS) catalyzes the formation of FPP, providing the precursor for the biosynthesis of (E)-ß-farnesene (EßF) in plants, but it is unknown if FPPS supplies the precursor for the biosynthesis of EßF, the major component of aphid alarm pheromone, though our previous studies support the hypothesis that EßF is synthesized by the aphid itself. Here, we used two cohorts of the green peach aphid Myzus persicae separately, reared on pepper plant and artificial diet to test the correlations among droplet emission, EßF quantity, and FPPS gene expression. It was found that the proportion of aphids emitting cornicle droplets and the quantity of EßF per milligram of aphid were both significantly different between the two cohorts, which were positively correlated with the expression of the two FPPS genes ( MpFPPS1/ 2) in M. persicae. These results were further confirmed by RNAi-mediated knockdown of MpFPPS1/ 2. Specifically, knockdown of MpFPPS1/ 2 imposed no significant cost on the survival of aphid but remarkably increased the number of offspring per aphid; most importantly, knockdown of MpFPPS1/ 2 significantly reduced the proportion of aphids emitting droplets and the quantity of EßF calculated as per the weight of aphid. Our results suggest that both FPPS genes are involved in the production of EßF in M. persicae and cornicle droplet emission is closely associated with the EßF release in the aphid.

Host Plants Indirectly Influence Plant Virus Transmission by Altering Gut Cysteine Protease Activity of Aphid Vectors.

The green peach aphid, Myzus persicae, is a vector of the Potato leafroll virus (PLRV, Luteoviridae), transmitted exclusively by aphids in a circulative manner. PLRV transmission efficiency was significantly reduced when a clonal lineage of M. persicae was reared on turnip as compared with the weed physalis, and this was a transient effect caused by a host-switch response. A trend of higher PLRV titer in physalis-reared aphids as compared with turnip-reared aphids was observed at 24 h and 72 h after virus acquisition. The major difference in the proteomes of these aphids was the up-regulation of predicted lysosomal enzymes, in particular the cysteine protease cathepsin B (cathB), in aphids reared on turnip. The aphid midgut is the site of PLRV acquisition, and cathB and PLRV localization were starkly different in midguts of the aphids reared on the two host plants. In viruliferous aphids that were reared on turnip, there was near complete colocalization of cathB and PLRV at the cell membranes, which was not observed in physalis-reared aphids. Chemical inhibition of cathB restored the ability of aphids reared on turnip to transmit PLRV in a dose-dependent manner, showing that the increased activity of cathB and other cysteine proteases at the cell membrane indirectly decreased virus transmission by aphids. Understanding how the host plant influences virus transmission by aphids is critical for growers to manage the spread of virus among field crops.

High-voltage electrostatic field-induced oxidative stress: Characterization of the physiological effects in Sitobion avenae (Hemiptera: Aphididae) across multiple generations.

In recent decades, man-made electric fields have greatly increased the intensity of electrostatic fields that are pervasively present in the environment. To better understand the physiological alterations exhibited by herbivorous insects in response to changing electric environments, we determined the activities of anti-oxidative enzymes and the metabolic rate of Sitobion avenae Fabricius (Hemiptera: Aphididae) over multiple generations in response to direct and host-seed exposure to a high-voltage electrostatic field (HVEF) of varying strength for different durations. Under controlled greenhouse conditions, 20-min direct exposure of S. avenae and wheat seeds to a 2- or 4-kV/cm HVEF resulted in significantly increased superoxide dismutase (SOD) activity in the sixth, 11th, 16th, and 21st generations relative to the control activities, whereas significantly decreased SOD activity was detected in the second generation. In addition, the activities of catalase (CAT) and peroxidase (POD) in S. avenae showed significant decreases over multiple generations. We also examined the suppressive effects of the duration of 4-kV/cm treatment on aphid physiology. The results showed that exposure to the 4-kV/cm HVEF for 20 min exerted adverse effects on CAT and POD activities and significantly decreased the metabolic rates of S. avenae, as demonstrated through evaluations of CO2 production rate, and these parameters were not significantly affected by higher HVEF durations. Overall, these findings increase our understanding of plant-pest interactions under novel HVEF environments and provide information that can improve integrated management strategies for S. avenae. Bioelectromagnetics. 40:52-61, 2019. © 2018 Wiley Periodicals, Inc.

Spatiotemporal expression profiling of the farnesyl diphosphate synthase genes in aphids and analysis of their associations with the biosynthesis of alarm pheromone.

The alarm behavior plays a key role in the ecology of aphids, but the site and molecular mechanism for the biosynthesis of aphid alarm pheromone are largely unknown. Farnesyl diphosphate synthase (FPPS) catalyzes the synthesis of FPP, providing the precursor for the alarm pheromone (E)-ß-farnesene (EßF), and we speculate that FPPS is closely associated with the biosynthetic pathway of EßF. We firstly analyzed the spatiotemporal expression of FPPS genes by using quantitative reverse transcription-polymerase chain reaction, showing that they were expressed uninterruptedly from the embryonic stage to adult stage, with an obvious increasing trend from embryo to 4th-instar in the green peach aphid Myzus persicae, but FPPS1 had an overall significantly higher expression level than FPPS2; both FPPS1 and FPPS2 exhibited the highest expression in the cornicle area. This expression pattern was verified in Acyrthosiphon pisum, suggesting that FPPS1 may play a more important role in aphids and the cornicle area is most likely the site for EßF biosynthesis. We thus conducted a quantitative measurement of EßF in M. persicae by gas chromatography-mass spectrometry. The data obtained were used to perform an association analysis with the expression data, revealing that the content of EßF per aphid was significantly correlated with the mean weight per aphid (r = 0.8534, P = 0.0307) and the expression level of FPPS1 (r = 0.9134, P = 0.0109), but not with that of FPPS2 (r = 0.4113, P = 0.4179); the concentration of EßF per milligram of aphid was not correlated with the mean weight per aphid or the expression level of FPPS genes. These data suggest that FPPS1 may play a key role in the biosynthesis of aphid alarm pheromone.

Molecular characterization and gene silencing of Laccase 1 in the grain aphid, Sitobion avenae.

Laccase 1 (Lac1), a polyphenol oxidase, has been proposed to be involved in insect iron metabolism and immunity responses. However, little information is available on the roles of Lac 1 in insect-plant interactions. The grain aphid Sitobion avenae is one of the most destructive pests of cereal, directly drawing phloem sap and transmitting viruses. In the present study, we first cloned the open reading frame (ORF) of Lac 1 from S. avenae, and the putative protein sequence was predicted to have a carboxyl-terminal transmembrane domain. We found that SaLac1 had higher expression levels in the fourth and adult stages using reverse transcription real-time quantitative PCR (RT-qPCR). SaLac 1 was highly expressed in the salivary gland and midgut and also in wingless compared with winged morphs. After feeding on aphid-resistant wheat with a high total phenol content, the expression level of SaLac 1 increased significantly. RNA interference (RNAi) by oral feeding successfully inhibited the transcript levels of SaLac 1, and the knockdown of Lac 1 significantly decreased the survival rate of S. avenae on aphid-resistant wheat. Our study demonstrated that S. avenae Lac1 was involved in the detoxification of phenolic compounds in wheat and was essential for the aphid to adapt to resistant plants.

Expression profile changes of cytochrome P450 genes between thiamethoxam susceptible and resistant strains of Aphis gossypii Glover.

Cytochrome P450 monooxygenases represent a key detoxification mechanism in neonicotinoids resistance in Aphis gossypii Glover. Synergism analysis has indicates that P450s are involved in thiamethoxam resistance. In this study, expression changes in the transcripts of P450 genes were determined in thiamethoxam-susceptible and thiamethoxam-resistant strains. Nine P450 genes in CYP3 clade were significantly overexpressed in the resistant strain (especially CYP6CY14, which was increased 17.67-fold) compared with the susceptible strain. Transcripts of ecdysone synthesis-related P450 genes, including CYP302A1, CYP306A1, CYP307A1 and CYP315A1, were up-regulated in the resistant strain, which may accelerate molting hormone production. The ecdysone response genes (ecdysone receptor (EcR), ultra-spiracle (USP) and Broad-complex protein (Br-C)) were overexpressed in the resistant strain. RNA interference (RNAi) targeting CYP6CY14 significantly increased the sensitivity of the resistant aphid to thiamethoxam. The results of the present study indicate the possible involvement of these P450 genes in thiamethoxam resistance. Our findings may facilitate further work to validate the roles of these P450s in thiamethoxam resistance based on heterologous expression, and show that screening the expression changes in P450 genes can reveal the impact of thiamethoxam on ecdysone synthesis-related P450 genes. These results are useful for understanding the mechanism of thiamethoxam resistance and will contribute to the management of insecticide-resistant cotton aphids in China.

Contribution of cytochrome P450 monooxygenase CYP380C6 to spirotetramat resistance in Aphis gossypii Glover.

The cytochrome P450 monooxygenases play a key role in detoxification mechanism for spirotetramat resistance in Aphis gossypii Glover. However, only one P450 genes (CYP6DA2), among thirty-five P450 genes identified from Aphis gossypii transcriptome database, has been reported to play important role in spirotetramat resistance in previous resistance level until now. In this study, after the confirmation of the rise of resistance level and important roles of P450s in spirotetramat resistance by the synergism analysis, the gene expression changes were determined for P450 genes in spirotetramat susceptible and resistant strains. Compared with the susceptible strain, CYP6CY4, CYP6CY14, CYP6CY18 and CYP6DC1 in CYP3 Clade were up-regulated in resistant nymphs, with the CYP6CY14, CYP6CY4, CYP6DC1, and CYP6CY18 increased to 2.54-, 1.51-, 1.31- and 1.29-fold, respectively. Eight genes in CYP3 Clade, three genes in CYP4 Clade and one gene in Mito Clade were down-regulated. In resistant adult aphids, CYP380C6 in CYP4 Clade, CYP353B1 in CYP2 Clade, and CYP307A1 in Mito Clade were up-regulated under spirotetramat stress, with the CYP380C6, CYP353B1 and CYP307A1 increased to 2.89-, 1.91-, and 1.38-fold, respectively. In contrast, the other P450 genes were almost down-regulated, especially these P450 genes in CYP3 Clade, CYP4 Clade and Mito Clade. RNA interference of CYP380C6 significantly increased the sensitivity of the resistant adults and nymphs to spirotetramat, while suppression of CYP6CY14 could not increase the toxicity of spirotetramat. These results indicate the possible involvement of the CYP380C6 genes in spirotetramat resistance at present very high resistance levels. Screening the expression changes of P450 genes under different spirotetramat resistance levels in the genome-scale will provide an overall view on the possible metabolic factors in the resistance development. The results may facilitate further work to validate the roles of P450 in spirotetramat resistance with heterologous expression.

Novel mutations and expression changes of acetyl-coenzyme A carboxylase are associated with spirotetramat resistance in Aphis gossypii Glover.

Acetyl-coenzyme A carboxylase (ACC) catalyses the carboxylation of acetyl-coenzyme A (acetyl-CoA) to produce malonyl-CoA during the de novo synthesis of fatty acids. Spirotetramat, an inhibitor of ACC, is widely used to control a range of sucking insects, including the Aphis gossypii. In the present study, Reverse transcription quantitative real-time PCR (RT-qPCR) results demonstrated that ACC was significantly overexpressed in a laboratory-selected spirotetramat-resistant strain compared with the susceptible strain. ACC RNA interference significantly suppressed fecundity and led to cuticle formation deficiencies in resistant adults and nymphs compared with the control. The full-length ACC gene was sequenced from both resistant and susceptible cotton aphids, and a strong association was found between spirotetramat resistance and 14 amino acid substitutions in the biotin carboxylase domain and carboxyl transferase domain of the ACC gene. Furthermore, ACC activity was higher in resistant aphids than in the susceptible strain, and ACC in the resistant aphids exhibited significant insensitivity to spirotetramat and spirotetramat-enol. The results indicate that the overexpressed insensitive (mutated) ACC target played an important role in the high levels of spirotetramat resistance observed here. This association of amino acid substitution with resistance is the first report of a potential target site mechanism affecting spirotetramat in the cotton aphid.

Functional characterization of carboxylesterase gene mutations involved in Aphis gossypii resistance to organophosphate insecticides.

Carboxylesterases (CarEs) play an important role in detoxifying insecticides in insects. Over-expression and structural modification of CarEs have been implicated in the development of organophosphate (OP) insecticide resistance in insects. A previous study identified four nonsynonymous mutations (resulting in four amino acid residue substitutions) in the open reading frame of the carboxylesterase gene of resistant cotton aphids compared to the omethoate susceptible strain, which has possibly influenced the development of resistance to omethoate (a systemic OP insecticide). The current study further characterized the function of these mutations, both alone and in combination, in the hydrolysis of OP insecticides. The metabolism results suggest that the combination of four mutations, mainly existing in the laboratory-selected OP-resistant cotton aphid population, increased the OP hydrolase activity (approximately twofold) at the cost of detectable carboxylesterase activity. The functional studies of single or multiple mutations suggest the positive effect of H104R, A128V and T333P on the acquisition of OP hydrolase activity, especially the combination of H104R with A128V or T333P. K484R substitution decreased both the OP hydrolase activity and the CarE activity, indicating that this mutation primarily drives the negative effect on the acquisition of OP hydrolase activity amongst these four mutations in the resistant strain. The modelling and docking results are basically consistent with the metabolic results, which strongly suggest that the structural gene modification is the molecular basis for the OP resistance in this laboratory-selected cotton aphid strain.

Functional characterization of two acetylcholinesterase genes in the brown citrus aphid, Aphis (Toxoptera) citricidus (Kirkaldy), using heterologous expression and RNA interference.

Acetylcholinesterase (AChE) is the primary target of organophosphate- and carbamate-based insecticides. We sequenced the full-length cDNAs of two AChE genes from the brown citrus aphid Aphis (Toxoptera) citricidus (Kirkaldy). These two genes, Tcace1 and Tcace2, which encode TcAChE1 and TcAChE2, respectively, had a shared amino acid identity of 29% and were highly similar to other insect ace1 and ace2 genes, respectively, having specific functional motifs. Potential differences in enzymatic function were characterized by the heterologous expression of the two genes using a baculovirus system in Sf9 insect cells. Both of the recombinant AChEs had high specific activities for three typical substrates, acetylthiocholine iodide, butyrylthiocholine iodide, and propinylthiocholine iodide. TcAChE1 had a lower Michaelis-Menten constant value and a higher maximal reaction velocity than recombinant TcAChE2, indicating a higher affinity for substrates and greater catalytic efficiency, respectively. Bioassays showed a greater sensitivity of recombinant TcAChE1 to the 10 tested insecticides. Silencing of Tcace1 and Tcace2 by RNA interference significantly increased the susceptibility of A. citricidus to malathion and carbaryl; however, silencing Tcace1 resulted in a higher mortality rate than silencing Tcace2. Additionally, the specific enzyme activity decreased more after silencing Tcace1 than after silencing Tcace2. Thus, TcAChE1 plays a major role in postsynaptic neurotransmission in A. citricidus.

Monitoring insecticide resistance and diagnostics of resistance mechanisms in the green peach aphid, Myzus persicae (Sulzer) (Hemiptera: Aphididae) in China.

Myzus persicae (Sulzer) is one of the most serious agricultural pests in China, and management strategies mainly rely on insecticidal treatment. To evaluate the resistance of field populations of M. persicae to seven insecticides, we assessed the susceptibility of 11 field populations collected from eight provinces in China using leaf-dip bioassays. Toxicity assays showed that M. persicae field populations have developed several levels of resistance to each tested insecticide. For pyrethroids, the field populations have developed a high level of resistance to ß-cypermethrin and cypermethrin, while the resistance to bifenthrin is still low. The resistance ratios of field populations to imidacloprid ranged from 1.48 to 52.36, and eight populations have developed moderate to high resistance. Resistance to acetamiprid is low, and only two populations have a moderate level of resistance. Most of the field populations of M. persicae developed moderate to high resistance to methomyl and omethoate. To investigate potential resistance mechanisms, we analyzed the enzyme activity of carboxylesterases, the type of amplified esterase genes, as well as the kdr (L1014F) mutation. All of the field populations exhibited a higher esterase activity compared to the laboratory susceptible strain. An amplified FE4, as well as the L1014F mutation, were also found in all of our experimental field populations. These results provide valuable insight into the current status of insecticide resistance and will prove to be a valuable resource in designing appropriate resistance management strategies for M. persicae in China.

Identification, characterization and functional analysis of a chitin synthase gene in the brown citrus aphid, Toxoptera citricida (Hemiptera, Aphididae).

Chitin synthase (CHS) is a crucial enzyme involved in the final step of the insect chitin biosynthetic pathway. In this study, we cloned the full-length cDNA sequence of a chitin synthase gene (TCiCHS) from the brown citrus aphid, Toxoptera citricida, an important citrus pest and the main vector of citrus tristeza virus worldwide. TCiCHS was expressed during the entire lifecycle and in all insect tissues examined. Expression was highest in first-second-instar nymphs, nymph-adult transitions and in the abdomen (6.7-fold higher than head). Embryos had a higher expression level than the integument. Fourth-instar nymphs were exposed to 5 and 500 mg/l concentrations of the chitin synthesis inhibitor diflubenzuron (DFB) for 48 h and had the highest mortality at the 500 mg/l concentration. The mRNA expression levels of TCiCHS were significantly enhanced upon the exposure of nymphs to both low and high DFB concentrations. Silencing of TCiCHS occurred through plant-mediated double-stranded RNA (dsRNA) feeding. Most dsRNA-fed nymphs were unable to moult to the next stage, and the expression of TCiCHS decreased 48% compared with controls. These results demonstrate that TCiCHS plays an important role in nymph to adult development, is possibly help identify molecular targets for To. citricida control.

Antioxidant Flavonols and Phenolic Compounds from Atraphaxis frutescens and Their Inhibitory Activities against Insect Phenoloxidase and Mushroom Tyrosinase.

Chemical investigation of the aerial parts of Atraphaxis frutescens resulted in the isolation of five 7-methoxyflavonols with pyrogallol B-ring moieties (1-5), a fisetinidol glucoside (13), and a benzyl glycoside (18), together with 26 known compounds including flavonoids, phenylpropanoid amides, anthraquinone glycosides, lignans, and a benzyl derivative. The principal chemical structural feature of the isolated compounds was either a pyrogallol or catechol B-ring moiety, and they showed potent 1,1-diphenyl-2-picrylhydrazyl radical scavenging activities. To assess the effects of these antioxidants on biological enzymes, their inhibitory effects against an insect phenoloxidase and a mushroom tyrosinase were evaluated. This study indicated that insect phenoloxidase was inhibited by phenylpropanoid amides and that mushroom tyrosinase was inhibited by the characteristic 7-methoxyflavonol 3-O-rhamnopyranosides.

SUPPRESSING A PEROXIDASE GENE REDUCES SURVIVAL IN THE WHEAT APHID Sitobion avenae.

Peroxidases (POXs) make up a large superfamily of enzymes that act in a wide range of biological mechanisms, including maintaining appropriate redox balances within cells, among other actions. In this study, we cloned a sequence that encodes a POX protein, SaPOX, from wheat aphids, Sitobion avenae. Amino acid sequence alignment showed the SaPOX sequence was conserved with POXs from other insect species. SaPOX mRNA accumulations were present in all nymphal and adult stages, at higher levels during the first and second instar, and lower during later stages in the life cycle. Ingestion of dsRNA specific to POX led to reduced SaPOX mRNA accumulation. Sitobion avenae nymphs continuously exposed to dietary dsPOX via an artificial diet led to reduced survival rate and ecdysis index. We infer that POX is important to maintain the growth and development of S. avenae.

Sublethal effects of imidacloprid on the fecundity, longevity, and enzyme activity of Sitobion avenae (Fabricius) and Rhopalosiphum padi (Linnaeus).

The aphid species Sitobion avenae and Rhopalosiphum padi are the most important pests in wheat growing regions of many countries. In this study, we investigated the sublethal effects of imidacloprid on fecundity, longevity, and enzyme activity in both aphid species by comparing 3-h exposure for one or three generations. Our results indicated that 3-h exposure to sublethal doses of imidacloprid for one generation had no discernible effect on the survival, fecundity, longevity, or enzyme activity levels of aphids. However, when pulse exposures to imidacloprid were sustained over three generations, both fecundity and longevity were significantly decreased in both S. avenae and R. padi. Interestingly, the fecundity of R. padi had almost recovered by the F5 generation, but its longevity was still deleteriously affected. These results indicated that R. padi laid eggs in shorter time lags and has a more fast resilience. The change in reproduction behavior may be a phenomenon of R. padi to compensate its early death. If this is stable for the next generation, it means that the next generation is more competitive than unexposed populations, which could be the reason underlying population outbreaks that occur after longer-term exposure to an insecticide. This laboratory-based study highlights the sublethal effects of imidacloprid on the longevity and fecundity of descendants and provides an empirical basis from which to consider management decisions for chemical control in the field.