Afidopyropen as a potential tool for Potato leafroll virus management in post-neonicotinoid potato production

Potato leafroll virus (PLRV) has been well managed by neonicotinoids since their widespread adoption in the United States, becoming virtually absent from seed and production fields in the Northwest. However, with increasing interest in discontinuing neonicotinoid usage, there is concern that PLRV could enjoy a resurgence in the absence of effective alternative chemistries. We tested the effects of afidopyropen, an insecticide with novel mode of action, on PLRV transmission and the feeding/probing behavior of its primary vector, the green peach aphid (Myzus persicae [Sulzer]). Afidopyropen foliar sprays decreased PLRV transmission by individual green peach aphids relative to water controls: PLRV acquisition from treated potato plants and subsequent transmission was reduced by 89%, and PLRV inoculation by viruliferous aphids to treated potato plants was reduced by 35%. Although electropenetrograph analyses following 4-h recordings of individual aphids on potato plants showed decreases in the total and mean duration of phloem feeding (E) on plants subject to an afidopyropen treatment relative to water, these only trended toward significance. Taken together, these results suggest treatment with afidopyropen can decrease PLRV transmission in potatoes, but that significant changes in feeding/probing might not occur quickly post-exposure. Overall, while the reductions in transmission were not as dramatic as have been observed following neonicotinoid treatments, afidopyropen may be a useful alternative and should be evaluated in field experiments.

Impact of the nicotinic acetylcholine receptor mutation R81T on the response of European Myzus persicae populations to imidacloprid and sulfoxaflor in laboratory and in the field

Sulfoxaflor (Isoclast™ active) is a sulfoximine insecticide that is active on a broad range of sap-feeding insects, including species that exhibit reduced susceptibility to currently available insecticides. Colonies of Myzus persicae (green peach aphid) were established from aphids collected in the field from peach (Prunus persica) and nectarine (Prunus persica var. nucipersica) orchards in France, Italy and Spain. The presence of the nicotinic acetylcholine receptor (nAChR) point mutation R81T was determined for all the colonies. Eight of the 35 colonies collected were susceptible relative to R81T (i.e., R81T absent), three of the colonies were found to be homozygous for R81T while 24 colonies had R81T present in some proportion (heterozygous). Sulfoxaflor and imidacloprid were tested in the laboratory against these M. persicae field colonies, which exhibited a wide range of susceptibilities (sulfoxaflor RR = 0.6 to 61, imidacloprid RR = 0.7 to 986) (resistance ratios, RR) to both insecticides. Although sulfoxaflor was consistently more active than imidacloprid against these field collected M. persicae, there was a statistically significant correlation across all colonies between the RRs for imidacloprid and sulfoxaflor (Pearson's r = 0.939, p < 0.0001). However, when a larger group of the colonies from Spain possessing R81T were analyzed, there was no correlation observed for the RRs between imidacloprid and sulfoxaflor (r = 0.2901, p = 0.3604). Thus, consistent with prior studies, the presence of R81T by itself is not well correlated with altered susceptibility to sulfoxaflor. In field trials, sulfoxaflor (24 and 36 gai/ha) was highly effective (~avg. 88-96% control) against M. persicae, demonstrating similar levels of efficacy as flonicamid (60-70 gai/ha) and spirotetramat (100-180 gai/ha) at 13-15 days after application, in contrast to imidacloprid (110-190 gai/ha) and acetamiprid (50-75 gai/ha) with lower levels of efficacy (~avg. 62-67% control). Consequently, sulfoxaflor is an effective tool for use in insect pest management programs for M. persicae. However, it is recommended that sulfoxaflor be used in the context of an insecticide resistance management program as advocated by the Insecticide Resistance Action Committee involving rotation with insecticides possessing other modes of action (i.e., avoiding rotation with other Group 4 insecticides) to minimize the chances for resistance development and to extend its future utility.

Inter-species mRNA transfer among green peach aphids, dodder parasites, and cucumber host plants

mRNAs are transported within a plant through phloem. Aphids are phloem feeders and dodders (Cuscuta spp.) are parasites which establish phloem connections with host plants. When aphids feed on dodders, whether there is trafficking of mRNAs among aphids, dodders, and host plants and if aphid feeding affects the mRNA transfer between dodders and hosts are unclear. We constructed a green peach aphid (GPA, Myzus persicae)-dodder (Cuscuta australis)-cucumber (Cucumis sativus) tritrophic system by infesting GPAs on C. australis, which parasitized cucumber hosts. We found that GPA feeding activated defense-related phytohormonal and transcriptomic responses in both C. australis and cucumbers and large numbers of mRNAs were found to be transferred between C. australis and cucumbers and between C. australis and GPAs; importantly, GPA feeding on C. australis greatly altered inter-species mobile mRNA profiles. Furthermore, three cucumber mRNAs and three GPA mRNAs could be respectively detected in GPAs and cucumbers. Moreover, our statistical analysis indicated that mRNAs with high abundances and long transcript lengths are likely to be mobile. This study reveals the existence of inter-species and even inter-kingdom mRNA movement among insects, parasitic plants, and parasite hosts, and suggests complex regulation of mRNA trafficking.

Rapid Screening of Myzus persicae (Green Peach Aphid) RNAi Targets Using Tobacco Rattle Virus

Plant-mediated RNA interference (RNAi) can be used to reduce the growth of insect pests, including Myzus persicae (green peach aphid), a prolific pest of numerous dicot crop species. In one approach, viruses that have been engineered to carry an aphid gene fragment are used to infect plants and thereby silence target gene expression in the aphids feeding on these plants, a process called virus-induced gene silencing, or VIGS. Tobacco Rattle Virus (TRV) in the model plant, Nicotiana benthamiana, was the first of many VIGS systems that have been developed for different plant species. In this chapter, we describe a method for silencing M. persicae gene expression using an established TRV-VIGS vector that infects and spreads in N. benthamiana. The two parts of the TRV genome, RNA1 and RNA2, have been cloned into Agrobacterium T-DNA vectors for initiation of plant infections. The RNA2 construct is modified with a Gateway-compatible cloning site to allow insertion of aphid genes. When feeding on TRV-infected N. benthamiana plants, aphids ingest dsRNAs that silence specific target genes. TRV-VIGS of aphid genes allows rapid identification of essential gene targets that can be used for the control of M. persicae by this and other RNAi methods.

Functional Characterization of Target Genes Associated with Insecticide Resistance of the Green Peach Aphid, Myzus persicae

Identifying genes responsive to insecticide treatment is the first step towards understanding the mechanism(s) of insecticide resistance and the development of effective insecticides against economic insect pests such as the Green peach aphid (GPA). Functional and Reverse Genetics approaches such as the RNA interference (RNAi) technology can be used to assess the possible involvement of genes whose expression is associated with an insecticide treatment. For GPA, this can be done by comparing the behavior and development of the insect following RNAi of a putative gene associated with insecticide treatment and exposure of the RNAi-treated insects to lethal doses of insecticides. In a case where knockdown of a gene or genes increases the susceptibility of RNAi-treated populations compared to controls, the target gene may have a direct role in the development of resistance to the insecticide or the gene may be involved in other metabolic processes that may be required for resilience against the insecticide.

Aphid estrogen-related receptor controls glycolytic gene expression and fecundity

Aphids, the major insect pests of agricultural crops, reproduce sexually and asexually depending upon environmental factors such as the photoperiod and temperature. Nuclear receptors, a unique family of ligand-dependent transcription factors, control insect development and growth including morphogenesis, molting, and metamorphosis. However, the structural features and biological functions of the aphid estrogen-related receptor (ERR) are largely unknown. Here, we cloned full-length cDNA encoding the ERR in the green peach aphid, Myzus persicae, (Sulzer) (Hemiptera: Aphididae) (MpERR) and demonstrated that the MpERR modulated glycolytic gene expression and aphid fecundity. The phylogenetic analysis revealed that the MpERR originated in a unique evolutionary lineage distinct from those of hemipteran insects. Moreover, the AF-2 domain of the MpERR conferred nuclear localization and transcriptional activity. The overexpression of the MpERR significantly upregulated the gene expression of rate-limiting enzymes involved in glycolysis such as phosphofructokinase and pyruvate kinase by directly binding to ERR-response elements in their promoters. Moreover, ERR-deficient viviparous female aphids showed decreased glycolytic gene expression and produced fewer offspring. These results suggest that the aphid ERR plays a pivotal role in glycolytic transcriptional control and fecundity.

Impact of Turnip yellows virus infection on seed yield of an open-pollinated and hybrid canola cultivar when inoculated at different growth stages

Turnip yellows virus (TuYV; family Luteoviridae, genus Polerovirus) is the most economically damaging virus infecting canola (Brassica napus) in the south-west Australian grainbelt. However, the impact of TuYV infection at different growth stages on canola seed yield has not been examined. This information is vital for implementing targeted management strategies. Four glasshouse experiments were conducted to examine seed yield losses incurred by an open-pollinated (ATR Bonito) and hybrid (Hyola® 404RR) canola cultivar when aphid-inoculated with TuYV at GS12 (two leaves unfolded), GS17 (seven leaves unfolded), GS30 (beginning of stem elongation) and GS65 (full flowering). When inoculated at GS12 and GS17, cv. Bonito plants incurred 30 % and 36 % seed yield losses, respectively, compared to healthy plants. Similarly, cv. 404RR incurred 41 % and 26 % seed yield losses at GS12 and GS17, respectively. However, when inoculated at GS30, whilst cv. Bonito plants incurred a 26 % seed yield loss, cv. 404RR incurred no significant loss. Neither cultivar incurred seed yield losses from inoculation at GS65. Additional information was collected from these experiments to improve sampling protocols to enhance TuYV detection, with a molecular and serological technique. When canola plants were at pre-flowering growth stages, TuYV was reliably detected 7-14 days after inoculation (DAI) in the youngest leaf. Once flowering had begun, TuYV was consistently detected 7-14 DAI in petals and flower buds. In contrast, regardless of growth stage, testing the oldest leaf regularly resulted in delayed detection or false negatives. Information generated in this study helps to quantify the value of management strategies targeted at preventing TuYV spread in pre-flowering canola crops and ultimately increase the efficiency of resource use.

In-field capable loop-mediated isothermal amplification detection of Turnip yellows virus in plants and its principal aphid vector Myzus persicae

Widespread Turnip yellows virus (TuYV) infection causes severe seed yield and quality losses in rapeseed (Brassica napus) crops grown in broadacre agricultural systems worldwide. Current TuYV detection protocols are expensive and time consuming, and can have poor specificity and sensitivity. Typically, they are used as a diagnostic tool to test already symptomatic plants, limiting their practical value to reactive disease management. To improve diagnostic services so that they provide earlier, cheaper, faster, more specific and sensitive TuYV detection, novel and innovative protocols that utilise new technology are required. A reverse transcription loop-mediated isothermal amplification (RT-LAMP) assay was developed to detect TuYV in crude and total RNA extractions of leaf material and its principal aphid vector Myzus persicae. The assay was based on a set of six primers, highly sensitive and specific to TuYV, derived from a TuYV isolate originating from the south-west Australian grainbelt. TuYV was readily detected in 1 in 100 dilutions of (i) infected to uninfected leaf material, and (ii) viruliferous to non-viruliferous M. persicae. Furthermore, detection was successful in a majority of aphids stored for at least 8 weeks in various trapping and storage substances, including 30% ethylene glycol, sticky trap glue and 70% ethanol. This RT-LAMP assay protocol enables quicker and cheaper diagnosis for TuYV than currently adopted laboratory-based diagnostic techniques. Ultimately, it has the potential for earlier in-field TuYV detection in combination with aphid trapping surveillance programs.

Sequential acquisition of Potato virus Y strains by Myzus persicae favors the transmission of the emerging recombinant strains

In the past decade recombinant strains of Potato virus Y (PVY) have overtaken the ordinary strain, PVY^O, as the predominant viruses affecting the US seed potato crop. Aphids may be a contributing factor in the emergence of the recombinant strains, but studies indicate that differences in transmission efficiency of individual PVY strains either from single or mixed infections, although variable, are not generally significant. Multiple strains of PVY are present in all potato production areas and common in many potato fields. Therefore, it is likely that individual alate aphids moving through a potato field will sequentially encounter multiple strains as they "taste test" multiple potato plants while looking for a suitable host. This study examined the transmission likelihood and efficiency of three common PVY strains when acquired sequentially by individual aphids. Green peach aphids (Myzus persicae, Sulzer) were allowed a 2-3min acquisition access period (AAP) on potato leaves infected with PVY^O, PVY^N:O or PVY^NTN, followed by another 2-3min AAP on a second potato leaf infected with a different PVY strain before being transferred to healthy potato seedlings for a 24h inoculation access period. All possible combinations of the three strains were tested. Strain-specific infection of the recipient plants was determined by TAS-ELISA and RT-PCR 3-4wk post-inoculation. The recombinant strains, PVY^N:O and PVY^NTN, were transmitted more efficiently than PVY^O when they were sequentially acquired regardless of the order acquired. PVY^N:O and PVY^NTN were transmitted with similar efficiencies when they were sequentially acquired regardless of the order. The recombinant strains appear to preferentially bind to the aphid stylet over PVY^O or they may be preferentially released during inoculation. This may contribute to the increased incidence of the recombinant strains over PVY^O in fields or production regions where multiple PVY strains are detected.

Biological Control of Aphid Using Fungal Culture and Culture Filtrates of Beauveria bassiana

Aphids are one of the most destructive pests in crop production such as pepper, cucumber, and eggplants. The importance of entomopathogenic fungi as alternative pest control agents is increasing. Conidia of entomopathogenic fungi are influenced by environmental conditions, such as temperature and relative humidity, and cause slow and fluctuating mortality. These factors have prevented wider application and use of biocontrol agents. For investigation of means of mitigation of such problems, we conducted bioassays with 47 fungal culture filtrates in order to evaluate the potential of secondary metabolites produced by entomopathogenic fungi for use in aphid control. Among 47 culture filtrates cultured potato dextrose broth, filtrate of Beauveria bassiana Bb08 showed the highest mortality (78%) against green peach aphid three days after treatments. Filtrate of Bb08 cultured in Adamek's medium showed higher toxicity as 100% to third instar nymphs of the aphid compared with seven other filtrates cultured in different broths amended with colloidal chitin or oil. The culture filtrates and fungal cultures from media amended with colloidal chitin or oil had lower control efficacies than filtrates without these additives in three different media. These results indicate that the fungal culture fluid or culture filtrate of B. bassiana Bb08 cultured in Adamek's medium has potential for development as a mycopesticide for aphid control.