Behavioral Responses of Bemisia Tabaci Cryptic Species Med to Three Host Plants and Their Volatiles

Bemisia tabaci Gennadius (Hemiptera: Aleyrodidae) is a worldwide pest that damages more than 900 host plant species. The infestation behavior of this pest is affected by the volatile organic compounds (volatiles) of different plants and their growth stage. We investigated the chemical constituents of the volatiles extracted from three plants (Gossypium hirsutum, Abutilon theophrasti and Ricinus communis) at different growth stages (pre-flowering, florescence and fruiting) and their effects on the behavior of adult B. tabaci. The selectivity studies on three plants showed that the B. tabaci preferred piemarker. The olfactometer studies showed that growth periods of the three plants also affected the preference of B. tabaci. Volatiles of piemarker and cotton plant had different levels of attraction to adults during all stages. Volatile substances released by castor at stage of flowering have a repellent effect on B. tabaci. In the plant VS plant combination ,the adults showed the strongest preference to volatiles from before and during anthesis of piemarker, followed by cotton, and then castor. A total of 23, 22 and 18 compounds were detected from volatiles of piemarker, cotton and castor, respectively, and proportions among the compounds changed during different stages of plant development. The olfactory responses of B. tabaci to volatile compounds showed that linalool and high concentration of leaf acetate had strong trapping effect on this pest, while 1-nonanal had significant repellent effect at high concentration. This study indicates that different plants and their growth stage affects their attractiveness or repellency to B. tabaci adults which is mediated by plant constitutive and dynamic changes. The compounds obtained by analysis screening can be used as potential attractants or repellents to control Mediterranean (MED) B. tabaci.

Olfactory Responses of Trissolcus Mitsukurii to Plants Attacked by Target and Non-target Stink Bugs Suggest Low Risk for Biological Control

In crop systems, successful management of invasive insect herbivores can be achieved through the introduction of exotic biocontrol agents, i.e. parasitoids or predators, having a coevolutionary history with the pest. To avert threats to local biodiversity, recent legislations require a risk assessment for the organism to be released. Evaluation of its ability to exploit, for host location, odours associated with target and non-target species is crucial for a better definition of its ecological host range. Using Y-tube olfactometer in quarantine laboratory, we investigated the ability of the Asiatic egg parasitoid Trissolcus mitsukurii (Hymenoptera: Scelionidae) to exploit odours associated with the global invader Halyomorpha halys (Hemiptera: Pentatomidae) and with non-target stink bugs native to Southern Europe. We demonstrated that T. mitsukurii is attracted by plants exposed to feeding and egg deposition of the coevolved H. halys or the native Nezara viridula, while it is not attracted by physogastric females or eggs alone. Remarkably, T. mitsukurii is repelled by plants bearing eggs of the beneficial stink bug, Arma custos. Our results usefully contribute to a more thorough and nuanced assessment of the potential non-target risks in case of massive parasitoid release.

Olfactory responses of Trissolcus mitsukurii to plants attacked by target and non-target stink bugs suggest low risk for biological control

Field release of classical biocontrol agents requires prior risk assessment to ensure that non-target effects are averted. No-choice and choice bioassays can provide basic information on the physiological host range of the candidate agent. However, other experimental methods, like olfactometer bioassays of host-plant complexes, can depict more realistically the likelihood that the released biocontrol agent localizes non-targets in the field. Halyomorpha halys Stål (Hemiptera: Pentatomidae) is native to Asia and invasive in North America and Europe, where it represents a major threat for crops. The Asiatic egg parasitoid Trissolcus mitsukurii (Ashmead) is adventive in Italy and exhibits high parasitization ability on H. halys. In the light of evaluating T. mitsukurii for H. halys biocontrol, its physiological host range has been already partially explored. Here, we investigated the ability of the parasitoid to exploit odours associated with H. halys or with the following non-targets: Arma custos F., Dolycoris baccarum L., Eurydema ventralis Kolenati, and Nezara viridula L. (all Hemiptera: Pentatomidae). Specifically, using a Y-tube olfactometer, we tested the response of parasitoid females to plants bearing naturally laid eggs, to physogastric females or eggs of the different species. We observed that T. mitsukurii is only attracted by plants challenged by eggs of H. halys or N. viridula, while it is not attracted by physogastric females or eggs alone. Remarkably, T. mitsukurii was almost repelled by plants bearing eggs of the beneficial A. custos. Our results usefully contribute to a more valuable assessment of the potential non-target risks in case of parasitoid release.

Mating, but Not Male Accessory Gland Products, Changes Female Response to Olfactory Cues in Anastrepha Fruit Flies

Copulation and/or ejaculate components can alter female physiological state and female post-mating behavior. The objective of the present study was to determine if copulation and male reproductive accessory gland products (MAGs) modify the behavior of female Anastrepha ludens (Loew) and Anastrepha obliqua (Macquart; Diptera: Tephritidae) in response to two stimuli: male-emitted pheromone and oviposition host volatiles. Olfactometry studies revealed that mated females of both A. ludens and A. obliqua have a stronger response for host volatiles compared to unmated females, which have a stronger response for male pheromone. We also examined olfactory responses of females mated to testectomized males who could transfer MAGs but not sperm. In both species, MAGs alone did not cause the change in the olfactory response observed after copulation, unlike what has been found in Ceratitis capitata (Wiedemann). Females mated to testectomized males responded equally to the male sex pheromone or to host volatiles, thus suggesting that the whole ejaculate is needed to elicit the complete behavioral switch in olfactory response. The function of MAGs is still unknown in these two pests of economic importance. The response for host volatiles by mated females has implications for the development of baits and traps that should preferably attract and target this population.

Avoidance of the Plant Hormone Cis-Jasmone by Aedes aegypti Depends On Mosquito Age in Both Plant and Human Odor Backgrounds

Adults of many mosquito species feed on plants to obtain metabolic energy and to enhance reproduction. Mosquitoes primarily rely on olfaction to locate plants and are known to respond to a range of plant volatiles. We studied the olfactory response of the yellow fever mosquito Aedes aegypti to methyl jasmonate (MeJA) and cis-jasmone (CiJA), volatile compounds originating from the octadecanoid signaling pathway that plays a key role in plant defense against herbivores. Specifically, we investigated how Ae. aegypti of different ages responded to elevated levels of CiJA in two attractive odor contexts, either derived from Lima bean plants or human skin. Aedes aegypti females landed significantly less often on a surface with CiJA and MeJA compared to the solvent control, CiJA exerting a stronger reduction in landing than MeJA. Odor context (plant or human) had no significant main effect on the olfactory responses of Ae. aegypti females to CiJA. Mosquito age significantly affected the olfactory response, older females (7–9 d) responding more strongly to elevated levels of CiJA than young females (1–3 d) in either odor context. Our results show that avoidance of CiJA by Ae. aegypti is independent of odor background, suggesting that jasmonates are inherently aversive cues to these mosquitoes. We propose that avoidance of plants with elevated levels of jasmonates is adaptive to mosquitoes to reduce the risk of encountering predators that is higher on these plants, i.e. by avoiding ‘enemy-dense-space’.

Identification of multiple odorant receptors essential for pyrethrum repellency in Drosophila melanogaster

Pyrethrum extract from dry flowers of Tanacetum cinerariifolium (formally Chrysanthemum cinerariifolium) has been used globally as a popular insect repellent against arthropod pests for thousands of years. However, the mechanistic basis of pyrethrum repellency remains unknown. In this study, we found that pyrethrum spatially repels and activates olfactory responses in Drosophila melanogaster, a genetically tractable model insect, and the closely-related D. suzukii which is a serious invasive fruit crop pest. The discovery of spatial pyrethrum repellency and olfactory response to pyrethrum in D. melanogaster facilitated our identification of four odorant receptors, Or7a, Or42b, Or59b and Or98a that are responsive to pyrethrum. Further analysis showed that the first three Ors are activated by pyrethrins, the major insecticidal components in pyrethrum, whereas Or98a is activated by (E)-β-farnesene (EBF), a sesquiterpene and a minor component in pyrethrum. Importantly, knockout of Or7a, Or59b or Or98a individually abolished fly avoidance to pyrethrum, while knockout of Or42b had no effect, demonstrating that simultaneous activation of Or7a, Or59b and Or98a is required for pyrethrum repellency in D. melanogaster. Our study provides insights into the molecular basis of repellency of one of the most ancient and globally used insect repellents. Identification of pyrethrum-responsive Ors opens the door to develop new synthetic insect repellent mixtures that are highly effective and broad-spectrum.

Optogenetic Manipulation of Olfactory Responses in Transgenic Zebrafish: A Neurobiological and Behavioral Study

Olfaction is an important neural system for survival and fundamental behaviors such as predator avoidance, food finding, memory formation, reproduction, and social communication. However, the neural circuits and pathways associated with the olfactory system in various behaviors are not fully understood. Recent advances in optogenetics, high-resolution in vivo imaging, and reconstructions of neuronal circuits have created new opportunities to understand such neural circuits. Here, we generated a transgenic zebrafish to manipulate olfactory signal optically, expressing the Channelrhodopsin (ChR2) under the control of the olfactory specific promoter, omp. We observed light-induced neuronal activity of olfactory system in the transgenic fish by examining c-fos expression, and a calcium indicator suggesting that blue light stimulation caused activation of olfactory neurons in a non-invasive manner. To examine whether the photo-activation of olfactory sensory neurons affect behavior of zebrafish larvae, we devised a behavioral choice paradigm and tested how zebrafish larvae choose between two conflicting sensory cues, an aversive odor or the naturally preferred phototaxis. We found that when the conflicting cues (the preferred light and aversive odor) were presented together simultaneously, zebrafish larvae swam away from the aversive odor. However, the transgenic fish with photo-activation were insensitive to the aversive odor and exhibited olfactory desensitization upon optical stimulation of ChR2. These results show that an aversive olfactory stimulus can override phototaxis, and that olfaction is important in decision making in zebrafish. This new transgenic model will be useful for the analysis of olfaction related behaviors and for the dissection of underlying neural circuits.

Olfactory Response of Mahanarva spectabilis (Distant, 1909) (Hemiptera: Cercopidae) to Volatile Aqueous Extracts of Plant Origin Applied to Elephant Grass Plants (Pennisetum purpureum Schum)

In this study, we evaluate the olfactory responses of Mahanarva spectabilis adults to aqueous extracts from the following non-host plants for the pest insect: garlic (Allium sativum L.), rue (Ruta graveolens L.), cinnamon (Cinnamomum verum J. Presl), lemongrass (Cymbopogon citratus Stapf.), clove (Syzygium aromaticum L.), star anise (Illicium verum Hook.f), eucalyptus (Eucalyptus globulus Labill.), tobacco (Nicotiana tabacum L.), and thyme (Thymus vulgaris L.) applied to the host plant, elephant grass (Pennisetum purpureum Schum.). The bioassays were performed using a Y olfactometer, and the combinations of the plant extracts applied to the host plant were tested against fresh air and against the host plant without the extract. The results show that the extracts of tobacco and star anise were non-attractive to M. spectabilis adults. The extracts from the remaining plants did not alter the response of M. spectabilis to the host plant.

Avoidance of the plant hormone cis-jasmone by Aedes aegypti depends on mosquito age in both plant and human odor backgrounds

Adults of many mosquito species feed on plants to obtain metabolic energy and to enhance reproduction. Mosquitoes primarily rely on olfaction to locate plants and are known to respond to a range of plant volatiles. We studied the olfactory response of the yellow fever mosquito Aedes aegypti to methyl jasmonate (MeJA) and cis-jasmone (CiJA), volatile compounds originating from the octadecanoid signaling pathway that plays a key role in plant defense against herbivores. Specifically, we investigated how Ae. aegypti of different ages responded to elevated levels of CiJA in two attractive odor contexts, either derived from Lima bean plants or from human skin. Aedes aegypti females landed significantly less often on a surface with CiJA and MeJA compared to the solvent control, CiJA exerting a stronger reduction in landing than MeJA. Odor context (plant or human) had no significant main effect on the olfactory responses of Ae. aegypti females to CiJA. Mosquito age significantly affected the olfactory response, older females (7–9 d) responding more strongly to elevated levels of CiJA than young females (1–3 d) in either odor context. Our results show that avoidance of CiJA by Ae. aegypti is independent of odor background, suggesting that jasmonates are inherently aversive cues to these mosquitoes. We propose that avoidance of plants with elevated levels of jasmonates is adaptive to mosquitoes to reduce the risk of encountering predators that is higher on these plants, i.e. by avoiding ‘enemy-dense-space’.

Exploring the Kairomone-Based Foraging Behaviour of Natural Enemies to Enhance Biological Control: A Review

Kairomones are chemical signals that mediate interspecific interactions beneficial to organisms that detect the cues. These attractants can be individual compounds or mixtures of herbivore-induced plant volatiles (HIPVs) or herbivore chemicals such as pheromones, i.e., chemicals mediating intraspecific communication between herbivores. Natural enemies eavesdrop on kairomones during their foraging behaviour, i.e., location of oviposition sites and feeding resources in nature. Kairomone mixtures are likely to elicit stronger olfactory responses in natural enemies than single kairomones. Kairomone-based lures are used to enhance biological control strategies via the attraction and retention of natural enemies to reduce insect pest populations and crop damage in an environmentally friendly way. In this review, we focus on ways to improve the efficiency of kairomone use in crop fields. First, we highlight kairomone sources in tri-trophic systems and discuss how these attractants are used by natural enemies searching for hosts or prey. Then we summarise examples of field application of kairomones (pheromones vs. HIPVs) in recruiting natural enemies. We highlight the need for future field studies to focus on the application of kairomone blends rather than single kairomones which currently dominate the literature on field attractants for natural enemies. We further discuss ways for improving kairomone use through attract and reward technique, olfactory associative learning, and optimisation of kairomone lure formulations. Finally, we discuss why the effectiveness of kairomone use for enhancing biological control strategies should move from demonstration of increase in the number of attracted natural enemies, to reducing pest populations and crop damage below economic threshold levels and increasing crop yield.