The Potential of Semiochemicals for Control ofPhorodon humuli(Homoptera: Aphididae)

Exogenous Application of Methyl Salicylate Induces Defence in Brassica against Peach Potato Aphid Myzus persicae

Plants use a variety of secondary metabolites to defend themselves against herbivore insects. Methyl salicylate (MeSA) is a natural plant-derived compound that has been used as a plant defence elicitor and a herbivore repellent on several crop plants. The aim of this study was to investigate the effect of MeSA treatment of Brassica rapa subsp. chinensis ('Hanakan' pak choi) on its interactions with peach potato aphids, Myzus persicae, and their natural enemy, Diaeretiella rapae. For this, we selected two concentrations of MeSA (75 mg/L and 100 mg/L). Our results showed that aphid performance was significantly reduced on plants treated with MeSA (100 mg/L). In a cage bioassay, the MeSA (100 mg/L)-treated plants showed lower adult survival and larviposition. Similarly, the MeSA (100 mg/L)-treated plants had a significantly lower aphid settlement in a settlement bioassay. In contrast, the M. persicae aphids did not show any significant difference between the MeSA (75 mg/L)-treated and control plants. In a parasitoid foraging bioassay, the parasitoid D. rapae also did not show any significant difference in the time spent on MeSA-treated and control plants. A volatile analysis showed that the MeSA treatment induced a significant change in volatile emissions, as high numbers of volatile compounds were detected from the MeSA-treated plants. Our results showed that MeSA has potential to induce defence in Brassica against M. persicae and can be utilised in developing sustainable approaches for the management of peach potato aphids.

Methyl Salicylate Can Benefit Ornamental Pest Control, and Does Not Alter Per Capita Predator Consumption at Close-Range

Methyl salicylate (MeSA) is an herbivore-induced plant volatile widely tested for attracting natural enemies for pest control. MeSA is commercially sold as slow-release lures or as a spray. While MeSA application has increased the abundance of natural enemies in numerous food crops, its ability to reduce pests for crop protection is not as frequently demonstrated. Our first objective was to test MeSA lures in ornamental fields where few studies have been done, and monitor natural enemies, pests, and crop protection. A 2-year study in spruce container yards revealed more aphid parasitoids (Pseudopraon sp.), fewer aphids (Mindarus obliquus) on shoot tips, and less shoot tip damage in MeSA plots during the first year. A 2-year study in red maple fields revealed more predatory lady beetles and rove beetles, and parasitic Ceraphronidae, Diapriidae, and Chalcidoidea in one or both years with MeSA. Fewer pest thrips were also captured in MeSA plots, though it is not clear whether this was due to enhanced predation or reduced colonization. Maple growth as measured by stem diameter change did not differ with MeSA use. A 2-year study examining predation on sentinel Halyomorpha halys eggs in various mature ornamental stock blocks found no increase in predation except for 1 month, though green lacewings, lady beetles, and predatory thrips occurred more in MeSA plots in the first year. While MeSA is expected to enhance biological control by herding in natural enemies, the impacts that applied volatiles have on predator efficiency is mostly unknown. Thus, our second objective examined how volatiles would impact feeding rates at close-range. Adult carabid Pterostichus melanarius, adult coccinellids Coccinella septempunctata and Harmonia axyridis, and larval lacewing Chrysoperla rufilabris consumed their prey at similar rates in the presence/absence of MeSA when food was presented directly in a 28 cm2 or 30 ml arena, or when foraging in a 520 cm2 outdoor soil arena or 946 ml arena with aphids on leaves.

Evaluation of airborne methyl salicylate for improved conservation biological control of two-spotted spider mite and hop aphid in Oregon hop yards

The use of synthetic herbivore-induced plant volatiles (HIPV) to attract natural enemies has received interest as a tool to enhance conservation biological control (CBC). Methyl salicylate (MeSA) is a HIPV that is attractive to several key predators of two-spotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae), and hop aphid, Phorodon humuli (Schrank) (Homoptera: Aphididae). A 2-year study was conducted to evaluate the recommended commercial use of MeSA in hop yards in Oregon. Slow-release MeSA dispensers were stapled to supporting poles in 0.5 ha plots and these plots were compared to a paired non-treated plot on each of three farms in 2008 and 2009. Across both years, there was a trend for reduced (range 40-91%) mean seasonal numbers of T. urticae in five of the six MeSA-baited plots. Stethorus spp., key spider mite predators, tended to be more numerous in MeSA-baited plots compared to control plots on a given farm. Mean seasonal densities of hop aphid and other natural enemies (e.g., Orius spp. and Anystis spp.) were similar between MeSA-treated and control plots. Variability among farms in suppression of two-spotted spider mites and attraction of Stethorus spp. suggests that the use of MeSA to enhance CBC of spider mites in commercial hop yards may be influenced by site-specific factors related to the agroecology of individual farms or seasonal effects that require further investigation. The current study also suggests that CBC of hop aphid with MeSA in this environment may be unsatisfactory.

Effect of methyl salicylate-based lures on beneficial and pest arthropods in strawberry

Methyl salicylate (MeSA) is a common herbivore-induced plant volatile that, when applied to crops, has the potential to enhance natural enemy abundance and pest control. The impacts of MeSA in strawberry were unknown and examined in the spring and midsummer period. Strawberry plots contained no lures (control) or two 30-d MeSA lures (Predalure) in the center: one lure 0.61 m aboveground over a sticky trap, and one lure on a plant near the ground. Arthropod abundance was monitored at the point source, 5 m and 10 m away from lures over 31 d with white sticky traps, pitfall traps, and leaf inspection. Twenty-seven and nine comparisons were made among beneficial and pest arthropods, respectively. Overall positive responses were found among Chrysopidae in July-August 2008 and Orius tristicolor (White) in May-June 2009 to MeSA based on sticky traps. Chrysopidae showed attraction to the point source, but not at 5 m and 10 m. Ground-dwelling predators collected in pitfall traps such as Araneae, the carabid beetles, Pterostichus melanarius (Illiger), and Nebria brevicollis (Fabricius) did not respond. Increased abundance of six natural enemy groups appeared on various dates between 3 and 24 d after placement of lures in the field based on leaf inspection and sticky traps. Conversely, fewer Coccinellidae were captured on sticky traps on days 0-3, and fewer natural enemies were observed on leaves on day 28 in MeSA plots. MeSA did not increase nor decrease pest abundance.

Genome annotation and comparative analyses of the odorant-binding proteins and chemosensory proteins in the pea aphid Acyrthosiphon pisum

Odorant-binding proteins (OBPs) and chemosensory proteins (CSPs) are two families of small water-soluble proteins, abundant in the aqueous fluid surrounding olfactory receptor neurons in insect antennae. OBPs are involved in the first step of olfactory signal transduction, carrying airborne semiochemicals to the odorant receptors and can be classified into three groups: Classic OBPs, Plus-C OBPs and Atypical OBPs. Here, we identified and annotated genes encoding putative OBPs and CSPs in the pea aphid Acyrthosiphon pisum using bioinformatics. This identified genes encoding 13 Classic and two Plus-C OBPs and 13 CSPs. Homologous OBP sequences were also identified in nine other aphid species, allowing us to compare OBPs across several aphid and non-aphid species. We show that, although OBP sequences are divergent within a species and between different orders, there is a high similarity between orthologs within a range of aphid species. Furthermore, the phylogenetic relationships between OBP orthologs reflect the divergence of aphid evolution lineages. Our results support the 'birth-and-death' model as the major mechanism explaining aphid OBP sequence evolution, with the main force acting on the evolution being purifying selection.

Aphid pheromones

Aphids are the main insect pests of agricultural crops in temperate regions causing major economic losses. Although broad-spectrum insecticides are available for control, alternative and more targeted methods are needed due to insecticide resistance and increasing environmental pressures. An alternative control method for aphids is to exploit their pheromones, which have been extensively studied in recent years. For example, aphids release alarm pheromones in response to natural enemy attack and these could be used to deter aphids from the crops. Sex pheromones have also been identified which could be used to interfere males locating conspecific females (oviparae), as well as for manipulating natural enemies. Several hypotheses relating to how species integrity is maintained via the aphid sex pheromone have been proposed. The composition and behavioral activity of these pheromones, and how their use could be implemented in integrated pest management systems to control aphids, is discussed.

Female-induced increase of host-plant volatiles enhance specific attraction of aphid male Dysaphis plantaginea (Homoptera: Aphididae) to the sex pheromone

All aphid species studied so far share the same sex pheromone components, nepetalactol and nepetalactone. Variation by different enantiomers and blends of the two components released by different aphid species are limited and can only partially explain species-specific attraction of males to females. While some host-plant odours are known to enhance specific attraction of aphid species, herbivore-induced plant volatiles that synergise attractiveness to the sex pheromone are unknown. Here, we demonstrate that for the host-alternating rosy apple aphid (Dysaphis plantaginea (Passerini)) specificity of attraction of males to females is triggered by female-induced tree odours in combination with a 1:8 ratio of (4aS,7S,7aR)-nepetalactone and (1R,4aS,7S,7aR)-nepetalactol. Female aphid infestation induces increased release of four esters (hexyl butyrate, (E)-2-hexenyl butyrate, (Z)-3-hexenyl 3-methylbutyrate and hexyl 2-methylbutyrate) from apple leaves. Two different combinations of three esters applied in a 1:1:1 ratio increase the number of male D. plantaginea and decrease the number of other aphid species caught in water traps in the presence of the pheromone components. The ester blend alone was not attractive. Combination of the pheromone blend with each single ester was not increasing attraction of male D. plantaginea. The demonstration that sexual aphid species use herbivore-induced plant volatiles as a species-specific attractant for mate finding adds a new dimension to our understanding of insect species using or manipulating chemical cues of host plants for orientation.

Structure, ratios and patterns of release in the sex pheromone of an aphid, Dysaphis plantaginea

Insect communication is primarily via chemicals. In Aphidinae aphids, the structure and ratio of iridoid (monoterpenoid) chemicals are known to be important components of the sex pheromone. However, for enhanced species specificity, it has been suggested that release of sex pheromone might be restricted to a narrow time period within the diel cycle. Here, we determine the structure, ratios and release patterns of iridoid chemicals produced by a serious global pest, the rosy apple aphid, Dysaphis plantaginea. Volatiles were collected from batches of oviparae (sexual females) and chemicals identified by gas chromatography, mass-spectrometry and microscale NMR spectroscopy.(1R,4aS,7S,7aR)-Nepetalactol and(4aS,7S,7aR)-nepetalactone were detected in a 3.7:1 ratio. To investigate timing of release, we constructed a sequential sampling device that allowed volatile chemicals to be captured hourly from 95 same-aged oviparae over 20 consecutive days. Release patterns of the two sex pheromone components show that D. plantaginea oviparae release high levels of the two components during photophase and low levels during scotophase. Release of the two components increased significantly during the first 3 h of photophase and thereafter remained at a high level until the onset of scotophase. The ratio of(1R,4aS,7S,7aR)-nepetalactol to(4aS,7S,7aR)-nepetalactone released did not change significantly between days two to 14 of the adult stadium, but from the 15th day onward there was a significant decrease in the relative amount of(1R,4aS,7S,7aR)-nepetalactol. Pheromone release was greatest on the eighth day of the adult stadium, with up to 8.4 ng of pheromone released per ovipara per hour. This is the first report on the full structural identification and ratios of volatile iridoid components collected from D. plantaginea oviparae and is also the most detailed temporal study on sex pheromone release from any aphid species. The lack of a temporally narrow and distinct period of very high sex pheromone release suggests that alternative mechanisms or factors for species recognition and isolation may be important. Findings are discussed broadly in relation to the biology of the aphid.

Interactions Between Host-plant Volatiles and the Sex Pheromones of the Bird Cherry-oat Aphid, Rhopalosiphum padi and the Damson-hop Aphid, Phorodon humuli

The bird cherry-oat aphid, Rhopalosiphum padi (L.), and the damson-hop aphid, Phorodon humuli (Schrank), migrate at the same time of year and colonize closely related Prunus spp. as primary hosts, but utilize (1R,4aS,7S,7aR)-nepetalactol and (1RS,4aR,7S,7aS)-nepetalactol, respectively, as sex pheromones. Interactions between these sex pheromones and benzaldehyde and methyl salicylate, plant volatiles common to primary hosts of both species, were investigated to assess whether they confer reproductive isolation between these species. Female autumn migrants (gynoparae) and males of these two species were caught in the field with water traps baited with their respective sex pheromones. Rhopalosiphum padi gynoparae and males also responded positively to benzaldehyde. Release of either benzaldehyde or methyl salicylate with the conspecific sex pheromone increased catches of both species of aphid. However, releasing both plant volatiles with the sex pheromone of R. padi increased catches of gynoparae and males, but reduced those with the sex pheromone of P. humuli. These results support the hypothesis that specific plant volatiles synergize responses of autumn migrating aphids to their sex pheromone. Because these interactions are species-specific, they may be important in allowing males to discriminate between conspecific sexual females (oviparae) and those of other aphid species.

Concepts and applications of trap cropping in pest management

Interest in trap cropping, a traditional tool of pest management, has increased considerably in recent years. In this review we propose a broader definition of trap cropping that encompasses the inherent characteristics of the trap crop plants themselves as well as the strategies associated with their deployment. Inherent characteristics of a trap crop may include not only natural differential attractiveness for oviposition and feeding, but also other attributes that enable the trap crop plants to serve as a sink for insects or the pathogens they vector. Successful deployment of trap crops within a landscape depends on the inherent characteristics of the trap crop and the higher value crop, the spatial and temporal characteristics of each, the behavior and movement patterns of insect pests, and the agronomic and economic requirements of the production system. Thus, trap cropping is more knowledge-intensive than many other forms of pest management. We review recent references on trap cropping, classify them according to their modalities and level of implementation, and provide a synthesis of the factors that influence the success of trap cropping. Last, we provide a list of recommendations and guidelines that should prove helpful in moving trap cropping forward to its full potential.

Methyl salicylate, a soybean aphid-induced plant volatile attractive to the predator Coccinella septempunctata

Induced volatiles provide a signal to foraging predatory insects about the location of their prey. In Iowa, early in the growing season of soybean, Glycine max, many predacious seven-spotted lady beetles, Coccinella septempunctata, were observed on plants with heavy infestations of soybean aphid, Aphis glycines. We studied whether the attraction of this beetle is caused by the release of specific volatile compounds of soybean plants infested by aphids. Volatile compounds emitted by soybean plants infested by aphids were compared with those of undamaged, uninfested, and artificially damaged plants. Gas chromatography-mass spectrometry analyses revealed consistent differences in the profiles of volatile compounds between aphid-infested soybean plants and undamaged ones. Significantly more methyl salicylate was released from infested plants at both the V1 and V2 plant growth stages. However, release patterns of two other induced plant volatiles, (D)-limonene and (E,E)-alpha-farnesene, differed between the two plant growth stages. Gas chromatographic-electroantennographic detection of volatile extracts from infested soybean plants showed that methyl salicylate elicited significant electrophysiological responses in C. septempunctata. In field tests, traps baited with methyl salicylate were highly attractive to adult C. septempunctata, whereas 2-phenylethanol was most attractive to the lacewing Chrysoperla carnea and syrphid flies. Another common lady beetle, the multicolored Asian lady beetle, Harmonia axyridis, showed no preference for the compounds. These results indicate that C. septempunctata may use methyl salicylate as the olfactory cue for prey location. We also tested the attractiveness of some selected soybean volatiles to alate soybean aphids in the field, and results showed that traps baited with benzaldehyde caught significantly higher numbers of aphids.

Electroantennogram responses of the three migratory forms of the damson-hop aphid, Phorodon humuli, to aphid pheromones and plant volatiles

Electroantennogram (EAG) responses were recorded from alate fundatrigeniae (spring migrants), gynoparae (the winged female form that produces sexual females) and males, the three migratory forms of the damson-hop aphid, Phorodon humuli (Schrank). EAG responses of gynoparae and males showed typical dose response characteristics to (E)-2-hexenal, (-)-R-carvone, hexanenitrile and (1RS,4aR,7S,7aS)-nepetalactol, the sex pheromone of this species. The 34 plant volatiles elicited broadly similar EAG response profiles in the three migratory forms. Green leaf volatiles produced large responses in all forms; however, the relative order of responsiveness varied. EAG responses to isomers of the monoterpene carvone differed between forms, with males being most, and spring migrants least, responsive. The hop-plant volatile and aphid alarm pheromone, (E)-beta-farnesene, evoked similar EAG responses in all forms. By contrast, males were most responsive to the three sex pheromone components, (-)-(4aS,7S,7aR)-nepetalactol, (+)-(4aS,7S,7aR)-nepetalactone and (1RS,4aR,7S,7aS)-nepetalactol. Males were no more responsive to their own sex pheromone, (1RS,4aR,7S,7aS)-nepetalactol, than to the other aphid sex pheromone components tested. Spring migrants and gynoparae also responded to the three sex pheromone components. This study indicates that migratory forms of P. humuli detect a wide range of volatile compounds, and that they are equally well-adapted for the detection of volatiles associated with host and non-host plants and with other species of aphid.

Electroantennogram and behavioural responses of different forms of the bird cherry-oat aphid, Rhopalosiphum padi, to sex pheromone and a plant volatile

Winged (alate) virginoparae were induced in the laboratory by crowding the bird cherry-oat aphid, Rhopalosiphum padi (L.) under long-day conditions. Males and gynoparae (the winged female form that produces sexual females) were induced by short days. Electroantennogram (EAG) and behavioural responses were investigated in each of the three forms to two aphid sex pheromone components, (-)-(4aS,7S,7aR)-nepetalactol and (+)-(4aS,7S,7aR)-nepetalactone, and benzaldehyde, a volatile which is released by the winter host plant. All three compounds elicited EAG responses with the males showing the highest sensitivity to each compound. Both the nepetalactol and the nepetalactone elicited larger EAG responses in gynoparae than in the winged virginoparae but antennae from virginoparae were more responsive to benzaldehyde. Although the nepetalactone is not a sex pheromone component in R. padi the EAG responses were similar to those evoked by the nepetalactol, the sex pheromone, in all three aphid forms. In a linear-track olfactometer, significantly more male R. padi moved into air containing nepetalactol, nepetalactone or benzaldehyde than into a simultaneous choice of clean air (i.e. attraction) but nepetalactol was more attractive than nepetalactone. Males, however, showed no response to a mixture of nepetalactol and nepetalactone. Gynoparae were attracted only to the nepetalactol but were less sensitive than the males and showed no response to the nepetalactone or benzaldehyde. In contrast, alate virginoparae showed no behavioural responses to any of the compounds. The present study supports the idea that the male R. padi utilise both sex pheromone and benzaldehyde for mate/host-plant location in autumn. It also demonstrates, for the first time, polyphenic differences in the olfactory responses at the peripheral level between the two female forms. Such differences impact on the life-cycle strategy where winged virginoparae move between graminaceous summer host plants while gynoparae move from the summer hosts to the bird cherry, winter host. The latter move appears to be assisted by the sex pheromone released by sexual females, already present on that host, acting as an aggregation pheromone.