Scaling the interactive effects of attractive and repellent odours for insect search behaviour

Bacterial volatiles elicit differential olfactory responses in insect species from the same and different trophic levels

Insect communities consist of species from several trophic levels that have to forage for suitable resources among and within larger patches of nonresources. To locate their resources, insects use diverse stimuli, including olfactory, visual, acoustic, tactile and gustatory cues. While most research has focused on cues derived from plants and other insects, there is mounting evidence that insects also respond to volatile organic compounds (VOCs) emitted by microorganisms. However, to date little is known about how the olfactory response of insects within and across different trophic levels is affected by bacterial VOCs. In this study, we used Y-tube bioassays and chemical analysis of VOCs to assess how VOCs emitted by bacteria affect the olfactory response of insects of the same and different trophic levels. Experiments were performed using two aphid species (Amphorophora idaei Börner and Myzus persicae var. nicotianae Blackman), three primary parasitoid species (Aphidius colemani Viereck, A. ervi Haliday, and A. matricariae Viereck), and two hyperparasitoid species (Asaphes suspensus Nees and Dendrocerus aphidum Rondani). Olfactory responses were evaluated for three bacterial strains (Bacillus pumilus ST18.16/133, Curtobacterium sp. ST18.16/085, and Staphylococcus saprophyticus ST18.16/160) that were isolated from the habitat of the insects. Results revealed that insects from all trophic levels responded to bacterial volatiles, but olfactory responses varied between and within trophic levels. All bacteria produced the same set of volatile compounds, but often in different relative concentrations. For 11 of these volatiles we found contrasting correlations between their concentration and the behavior of the primary parasitoids and hyperparasitoids. Furthermore, olfactometer experiments on three of these compounds confirmed the contrasting olfactory responses of primary parasitoids and hyperparasitoids. The potential of these findings for the development of novel semiochemical-based strategies to improve biological aphid control has been discussed.

The Response of Susceptible and Pyrethroid-Resistant Blattella germanica (Dyctioptera: Blattellidae) to Shelter-Associated Cues

In this work, it was studied the role of faeces in the location and permanence in a shelter in susceptible and pyrethroid-resistant individuals of Blattella germanica (Linnaeus 1767). Additionally, the effect of different concentrations of palmitic acid on the modulation of these behaviours was tested. A shelter constituted by a square cardboard structure was offered to susceptible as well as to resistant specimens. The shelter bases were treated with faecal extracts obtained from susceptible or resistant cockroaches, or with solutions of palmitic acid. The behaviour of susceptible as well as resistant specimens was analysed using infrared videography software. Susceptible's faecal extract attracted both specimens since the time spent by cockroaches to locate the treated shelters was lower, whereas the faecal extract from resistant insects did not elicit any effect on both strains. Faecal extracts showed an arrestant effect on both strains, suggested by the time spent inside the shelter that was significantly higher in their presence. On the other hand, treatment with palmitic acid produced an attractant or a repellent effect depending on the concentration and strain. The tested lower concentration was attractant to susceptible insects, but did not produce any effect on resistant ones. In addition, the higher concentrations did not produce any effect on susceptible individuals, but resulted repellent for resistant ones. Palmitic acid did not produce an arrestant effect on the strains as there was not an increase in time spent inside the shelter in the presence of this substance. An increase in the number of visits to the shelter and to the periphery was also observed in shelters treated with the faecal extract and with the lower concentration of palmitic acid. These results show that compounds of the susceptible faeces were attractant to cockroaches of both strains, while faecal extracts from resistant insects were not. Moreover, a dual effect of palmitic acid was observed, being attractant at low concentrations and repellent as concentration increased. Additionally, a difference in the concentration threshold at which the effect of this substance changes was observed between strains.

Effect of Metabolic Changes in Aphis gossypii-Damaged Cotton Plants on Oviposition Preference and Larval Development of Subsequent Helicoverpa armigera

Aphis gossypii and Helicoverpa armigera are two important agricultural pests in cotton plants. However, whether early colonization of A. gossypii affects subsequent H. armigera is unknown. We implemented ecological experiments to reveal that A. gossypii-damaged cotton plants [Bacillus thuringiensis (Bt) and non-Bt] had a significant avoidance effect on the oviposition preference of H. armigera adults. However, A. gossypii-damaged cotton plants (non-Bt) increased the weight and pupation rate and reduced the mortality of H. armigera larvae. Transcriptomic and metabolomic analyses showed that 13 and 9 genes were significantly upregulated to be involved in salicylic acid (SA) and indole acetic acid (IAA) biosynthesis, and SA and IAA contents were significantly increased, respectively. However, 15 genes involved in jasmonic acid (JA) biosynthesis were significantly downregulated as a result of the antagonism of SA and JA. Moreover, there was significant upregulation in multiple genes involved in the biosynthesis of l-histidine, fructose, maltotetraose, melezitose, lecithin, stearidonic acid, and mannitol, in which metabolites were confirmed to promote the growth and development of H. armigera. Our study is a reference for investigating the evolutionary relationships and provides insights into implementing effective insect biocontrol between H. armigera and A. gossypii.

Feeding Behaviour of a Pyrethroid-Resistant Strain of the German Cockroach Blattella germanica (Linnaeus, 1767)

Many authors report biological differences among insecticide-resistant pest species, mainly associated with parameters of life history, but only a few studies approach differences at the behavioural level. Feeding behaviour in Blattella germanica (L.) is modulated by the detection of chemical volatiles emitted from food sources in order to be located a long-median distance, and also by the physicochemical properties of food. This work aimed to study the differences in the feeding behaviour of a susceptible and a pyrethroid-resistant strain of B. germanica, in the location and exploitation of a food source. Resistant males showed a lower performance in the feeding behaviour compared to susceptible males. Particularly, the time taken to locate the source was significantly higher in the resistant individuals, suggesting a lower capacity in the detection of food odours. In addition, although the Intake rate was negatively related to the stiffness of food for both strains, resistant individuals showed a lower intake rate compared to susceptible ones, when exploiting a food source of the same stiffness. In a control context, the phenotypic characteristics associated to feeding of pyrethroid-resistant individuals could induce a behavioural resistance mechanism caused by a reduction in the amount of toxic gel bait ingested. If that is the case, resistance of this strain would not be associated to physiological changes affecting the toxicity of the active principle of gel baits, but to a lower efficacy of these products against resistant individuals due to less amount of gel consumed, generating control problems of this pest.

Repellency of Wild Oregano Plant Volatiles, Plectranthus Amboinicus, and Their Essential Oils to the Silverleaf Whitefly, Bemisia Tabaci, on Tomato

The Bemisia tabaci (Gennadius) whitefly is a major economically damaging pest of many crops such as tomato (Solanum lycopersicum L.). Pesticides are widely used to control B. tabaci while the use of aromatic plants is an alternative control method. The aim of this study was to assess the B.tabaci repellent effect of wild oregano, Plectranthus amboinicus (Lour.) Spreng, a widespread aromatic plant in the West Indies. We tested three origins of wild oregano, including northern, central, and southern Martinique (French West Indies). Our results showed that all essential oils of wild oregano had either masking properties or were true repellents-the mean percentage of whiteflies present in the upper part of the still-air olfactometer was 1.3- to 1.9-fold lower than in the controls. The ethanolic solution of volatile organic compounds of wild oregano from southern Martinique also had a true repellent effect-the mean percentage of whiteflies present in the upper part of the still-air olfactometer was 1.3-fold lower than in the controls. Moreover, in a greenhouse insect-proof cage, there were 1.5 fewer adult whiteflies on tomato intercropped with wild oregano from southern Martinique than on tomato alone after 96 h exposure. Our study generated further insight into the potential of P. amboinicus for B. tabaci biocontrol on tomato crops. Wild oregano extracts were repellent to B. tabaci and could be used as a companion plant to prevent whitefly infestations on tomato crops. However, the B. tabaci behavior depends on the plant origin.

Olfactory dysfunction in aging and neurodegenerative diseases

Alterations in olfactory functions are proposed to be early biomarkers for neurodegeneration. Many neurodegenerative diseases are age-related, including two of the most common, Parkinson's disease (PD) and Alzheimer's disease (AD). The establishment of biomarkers that promote early risk identification is critical for the implementation of early treatment to postpone or avert pathological development. Olfactory dysfunction (OD) is seen in 90% of early-stage PD patients and 85% of patients with early-stage AD, which makes it an attractive biomarker for early diagnosis of these diseases. Here, we systematically review widely applied smelling tests available for humans as well as olfaction assessments performed in some animal models and the relationships between OD and normal aging, PD, AD, and other conditions. The utility of OD as a biomarker for neurodegenerative disease diagnosis and future research directions are also discussed.

Olfactory Learning Supports an Adaptive Sugar-Aversion Gustatory Phenotype in the German Cockroach

Simple Summary

Toxic baits that contain an insecticide and phagosimulatory sugars, including glucose, are most effective in German cockroach control. However, cockroaches have evolved behavioral resistance, where they perceive glucose as a deterrent and avoid eating the bait (glucose-aversion, GA), resulting in failure to control infestations. We hypothesized that the GA phenotype may be extended by associative learning of specific odors with glucose. We demonstrated that GA cockroaches associated attractive food odors, such as vanilla and chocolate, with glucose (deterrent) and learned to avoid these odors. In contrast, wild type (WT) cockroaches that associated these odors with glucose (phagostimulant) increased their preference for the odors. The aversive and appetitive memories were retained for at least three days. Generally, when toxic baits are deployed, GA cockroaches are first attracted to the bait, and they repeatedly experience its aversive taste as they reject eating the deterrent bait. The recurring non-rewarding foraging experience may contribute to the formation of an aversive olfactory memory. Even if the baits are later reformulated without aversive tastants, GA cockroaches may avoid the new bait because they associate it with aversive olfactory stimuli. Our findings will guide the rational development of baits that consider the olfactory learning abilities of cockroaches.

Abstract

An association of food sources with odors prominently guides foraging behavior in animals. To understand the interaction of olfactory memory and food preferences, we used glucose-averse (GA) German cockroaches. Multiple populations of cockroaches evolved a gustatory polymorphism where glucose is perceived as a deterrent and enables GA cockroaches to avoid eating glucose-containing toxic baits. Comparative behavioral analysis using an operant conditioning paradigm revealed that learning and memory guide foraging decisions. Cockroaches learned to associate specific food odors with fructose (phagostimulant, reward) within only a 1 h conditioning session, and with caffeine (deterrent, punishment) after only three 1 h conditioning sessions. Glucose acted as reward in wild type (WT) cockroaches, but GA cockroaches learned to avoid an innately attractive odor that was associated with glucose. Olfactory memory was retained for at least 3 days after three 1 h conditioning sessions. Our results reveal that specific tastants can serve as potent reward or punishment in olfactory associative learning, which reinforces gustatory food preferences. Olfactory learning, therefore, reinforces behavioral resistance of GA cockroaches to sugar-containing toxic baits. Cockroaches may also generalize their olfactory learning to baits that contain the same or similar attractive odors even if they do not contain glucose.

Identification and application of bacterial volatiles to attract a generalist aphid parasitoid: from laboratory to greenhouse assays

BACKGROUND

Recent studies have shown that microorganisms emit volatile compounds that affect insect behaviour. However, it remains largely unclear whether microbes can be exploited as a source of attractants to improve biological control of insect pests. In this study, we used a combination of coupled gas chromatography-electroantennography (GC-EAG) and Y-tube olfactometer bioassays to identify attractive compounds in the volatile extracts of three bacterial strains that are associated with the habitat of the generalist aphid parasitoid Aphidius colemani, and to create mixtures of synthetic compounds to find attractive blends for A. colemani. Subsequently, the most attractive blend was evaluated in two-choice cage experiments under greenhouse conditions.

RESULTS

GC-EAG analysis revealed 20 compounds that were linked to behaviourally attractive bacterial strains. A mixture of two EAG-active compounds, styrene and benzaldehyde applied at a respective dose of 1 μg and 10 ng, was more attractive than the single compounds or the culture medium of the bacteria in Y-tube olfactometer bioassays. Application of this synthetic mixture under greenhouse conditions resulted in significant attraction of the parasitoids, and outperformed application of the bacterial culture medium.

CONCLUSION

Compounds isolated from bacterial blends were capable of attracting parasitoids both in laboratory and greenhouse assays, indicating that microbial cultures are an effective source of insect attractants. This opens new opportunities to attract and retain natural enemies of pest species and to enhance biological pest control.