Adult parasitoids of honeydew-producing insects prefer honeydew sugars to cover their energetic needs

To meet their carbohydrate requirements, adult parasitoids exploit a broad range of sugar resources, including floral and extrafloral nectar and honeydew. Although honeydew might be the predominant sugar source, especially in agricultural systems, it often is nutritionally inferior to sugar sources like nectar. Given its broad availability, it may be expected that sugar-feeding insects have evolved specialized adaptations to deal with this typically inferior sugar source. This would apply especially to organisms that have a close association with honeydew producers. Here, we hypothesized that parasitoids of honeydew-producing insects should show a pronounced response to sugars, such as fructose, sucrose, melezitose, and trehalose, and to a lesser extent glucose. To test this hypothesis, we investigated sugar consumption, feeding behavior and survival of the aphid parasitoid Aphidius ervi on several sugars (equiweight solutions). Our results show that A. ervi adults consumed typical honeydew sugars (sucrose, fructose, trehalose, and melezitose) the most, while consuming considerably less glucose or melibiose. Rhamnose, which does not occur in aphid honeydew, was not, or was only marginally, consumed. When different sugars were provided at the same time, A. ervi adults preferred sucrose or fructose over glucose or melezitose. Furthermore, pre-exposure to sucrose or fructose significantly reduced subsequent intake of glucose, suggesting an acquired distaste for glucose after being previously exposed to highly preferred sugars such as sucrose and fructose. Altogether, this study shows that A. ervi adults prefer sugars (fructose, melezitose, trehalose, and sucrose) that are overrepresented in aphid honeydew and show a lower preference to one (glucose) that is underrepresented in honeydew

Sweet Scents: Nectar Specialist Yeasts Enhance Nectar Attraction of a Generalist Aphid Parasitoid Without Affecting Survival

Floral nectar is commonly inhabited by microorganisms, mostly yeasts and bacteria, which can have a strong impact on nectar chemistry and scent. Yet, little is known about the effects of nectar microbes on the behavior and survival of insects belonging to the third trophic level such as parasitoids. Here, we used five nectar-inhabiting yeast species to test the hypothesis that yeast species that almost solely occur in nectar, and therefore substantially rely on floral visitors for dispersal, produce volatile compounds that enhance insect attraction without compromising insect life history parameters, such as survival. Experiments were performed using two nectar specialist yeasts (Metschnikowia gruessii and M. reukaufii) and three generalist species (Aureobasidium pullulans, Hanseniaspora uvarum, and Sporobolomyces roseus). Saccharomyces cerevisiae was included as a reference yeast. We compared olfactory responses of the generalist aphid parasitoid Aphidius ervi (Haliday) (Hymenoptera: Braconidae) when exposed to these microorganisms inoculated in synthetic nectar. Nectar-inhabiting yeasts had a significant impact on nectar chemistry and produced distinct volatile blends, some of which were attractive, while others were neutral or repellent. Among the different yeast species tested, the nectar specialists M. gruessii and M. reukaufii were the only species that produced a highly attractive nectar to parasitoid females, which simultaneously had no adverse effects on longevity and survival of adults. By contrast, parasitoids that fed on nectars fermented with the reference strain, A. pullulans, H. uvarum or S. roseus showed shortest longevity and lowest survival. Additionally, nectars fermented by A. pullulans or S. roseus were consumed significantly less, suggesting a lack of important nutrients or undesirable changes in the nectar chemical profiles. Altogether our results indicate that nectar-inhabiting yeasts play an important, but so far largely overlooked, role in plant-insect interactions by modulating the chemical composition of nectar, and may have important ecological consequences for plant pollination and biological control of herbivorous insects

Potential of insect-microbe chemical interactions to improve biological control of insect pests

Biological control using natural enemies such as arthropod predators and parasitoids has become an important alternative way of pest management. However, the efficacy of biological pest control can be seriously hampered when naturally occurring enemies are not sufficiently abundant or effective. Therefore, naturally occurring beneficial insects are often complemented with the release of commercially-reared natural enemies. Despite these efforts, a major challenge in biological pest control remains to attract and retain the beneficial insects in the crop so that they reach high population densities in the crop and control the pest insects whenever needed. While the increasingly applied provisioning of supplemental food sources and attractants to lure and augment natural enemy populations appears to be a promising approach to increase biocontrol efficacy, these sugar sources are often not tailored to selectively support the natural enemies and may also benefit harmful insects like herbivores and hyperparasitoids. The latter constitute an important fourth trophic level of organisms that parasitize the primary parasitoids and therefore can disrupt biological pest control, ultimately leading to pest outbreaks. The behaviour of natural enemies is largely determined by chemical cues released in the environment by insects or plants, so-called "semiochemicals". While most research in this field has focused on cues derived from plants, there is mounting evidence that microorganisms emit volatile compounds that also play a role in insect behaviour. However, so far little is known about how microbial volatiles affect the foraging behaviour of natural enemies, and whether they can be applied to improve biological control of insect pests. The overall aim of this PhD study was to investigate the potential of tailored sugar mixtures and microbial volatile organic compounds (mVOCs) to improve the biological control of insect pests. To this end, we used the aphid parasitoids Aphidius colemani Vierick and Aphidius matricariae Haliday (Hymenoptera: Braconidae) and one of their hyperparasitoids, Dendrocerus aphidum Rodani (Hymenoptera: Megaspilidae), as study organisms. Both Aphidius species are solitary generalist endoparasitoids that attack many aphid species, including numerous species of economic importance. In a first part of this PhD study (Chapter 2), we investigated the feeding behaviour and longevity of both parasitoid species and their hyperparasitoid when provided with one of eight plant- and/or insect-derived sugars (fructose, galactose, glucose, melibiose, melezitose, rhamnose, sucrose and trehalose). We first evaluated sugar consumption over a 9-h period of time by using a capillary feeder (CAFE) assay. Next, we studied survival of the parasitoids when fed with the different sugars. Results showed that the studied insect species consumed the largest amounts of sugars that are most commonly found in honeydew (sucrose, fructose, glucose and melezitose) and also survived best when feeding on these sugars. Both Aphidius spp. survived well on melibiose, whereas D. aphidum performed poorly on this sugar. When melibiose was offered in a mixture with glucose, a significant reduction in longevity was observed for D. aphidum when compared to glucose only, while this was less pronounced for Aphidius, suggesting that this mixture can be used to predominantly support Aphidius parasitoids. In Chapter 3, we used Y-tube olfactometer experiments to assess how volatile compounds emitted by bacteria affected the olfactory response of A. colemani and D. aphidum. Olfactory responses were evaluated for volatile blends emitted by bacteria that were isolated from diverse sources from the parasitoid's habitat, including aphids, aphid mummies and honeydew, and from the parasitoids themselves. Results revealed that A. colemani showed wide variation in response to bacterial volatiles, ranging from significant attraction over no response to significant repellence. Interestingly, the olfactory response of A. colemani to bacterial volatile emissions was significantly different from that of D. aphidum. Gas chromatography-mass spectrometry (GC-MS) analyses revealed that the volatile blends repellent to A. colemani contained significantly higher amounts of esters, organic acids, aromatics and cycloalkanes than attractive blends. Bacterial volatile blends repellent to D. aphidum contained significantly higher amounts of alcohols and ketones, whereas the volatile blends attractive to D. aphidum contained higher amounts of the monoterpenes limonene, linalool and geraniol than the repellent blends. The results further showed that closely related species of the genus Bacillus elicited a similar olfactory response (attraction) in A. colemani, suggesting that volatile composition and, as a result, parasitoid attraction, are phylogenetically conserved traits. In Chapter 4, we tested in more detail the hypothesis that phylogenetic relationships among microorganisms predict microbial volatile composition and the olfactory response of insects. Results revealed that phylogenetically closely related Bacillus strains emitted similar volatile blends and elicited a comparable olfactory response of A. colemani in Y-tube olfactometer bioassays, varying between attraction and repellence. Analysis of the chemical composition of the mVOC blends revealed that all Bacillus strains produced the same set of volatiles, but in different concentrations and ratios. Benzaldehyde was produced in relatively higher concentrations by strains that repel A. colemani compared to strains that are attractive, while attractive mVOC blends contained relatively higher amounts of acetoin, 2,3-butanediol, 2,3-butanedione, eucalyptol and isoamylamine. Overall, these results support our hypothesis that bacterial phylogeny predicts mVOC composition and the olfactory responses of A. colemani. Despite an increased understanding of the role of microbial volatile emissions as insect semiochemicals, at present it is not well known which microbial volatiles or blends of microbial volatiles define the insects' response. Therefore, in Chapter 5 we aimed at identifying specific compounds in bacterial volatile blends that attract A. colemani by using a combination of gas chromatography-electroantennography (GC-EAG), gas chromatography-mass spectrometry (GC-MS), and Y-tube olfactometer bioassays using synthetic volatile compounds. Next, the most promising mixture of putatively attractive synthetic compounds was evaluated in two-choice cage experiments to investigate whether A. colemani parasitoids responded to the volatile blend under greenhouse conditions. Results revealed a number of compounds that were significantly attractive or repellent. In particular, a mixture consisting of 100 ng/µL styrene and 1 ng/µL benzaldehyde was most attractive for A. colemani, both in laboratory and greenhouse experiments. Overall, these results indicate that a limited number of volatiles released under particular concentrations can have an important impact on insect olfactory responses and therefore open new opportunities to attract or retain natural enemies of pest species in the crop and possibly to enhance biological pest control. Altogether, this PhD study has provided a better understanding of volatile-mediated interactions between microorganisms, parasitoids and hyperparasitoids. This knowledge combined with a selectively supportive food source for natural enemies may be exploited to develop novel tools that attract, retain and sustain natural enemies of pest species, and potentially lead to improved biological control efficacy and consequently more sustainable agricultural practices.status: publishe

Adult Parasitoids of Honeydew-Producing Insects Prefer Honeydew Sugars to Cover their Energetic Needs

To meet their carbohydrate requirements, adult parasitoids exploit a broad range of sugar resources, including floral and extrafloral nectar and honeydew. Although honeydew might be the predominant sugar source, especially in agricultural systems, it often is nutritionally inferior to sugar sources like nectar. Given its broad availability, it may be expected that sugar-feeding insects have evolved specialized adaptations to deal with this typically inferior sugar source. This would apply especially to organisms that have a close association with honeydew producers. Here, we hypothesized that parasitoids of honeydew-producing insects should show a pronounced response to sugars, such as fructose, sucrose, melezitose, and trehalose, and to a lesser extent glucose. To test this hypothesis, we investigated sugar consumption, feeding behavior and survival of the aphid parasitoid Aphidius ervi on several sugars (equiweight solutions). Our results show that A. ervi adults consumed typical honeydew sugars (sucrose, fructose, trehalose, and melezitose) the most, while consuming considerably less glucose or melibiose. Rhamnose, which does not occur in aphid honeydew, was not, or was only marginally, consumed. When different sugars were provided at the same time, A. ervi adults preferred sucrose or fructose over glucose or melezitose. Furthermore, pre-exposure to sucrose or fructose significantly reduced subsequent intake of glucose, suggesting an acquired distaste for glucose after being previously exposed to highly preferred sugars such as sucrose and fructose. Altogether, this study shows that A. ervi adults prefer sugars (fructose, melezitose, trehalose, and sucrose) that are overrepresented in aphid honeydew and show a lower preference to one (glucose) that is underrepresented in honeydew.status: publishe

Associative learning and memory retention of nectar yeast volatiles in a generalist parasitoid

status: publishe

Gustatory response and longevity in Aphidius parasitoids and their hyperparasitoid Dendrocerus aphidum

Aphid parasitoids are commonly used in the biological control of aphids. However, their success in biological control largely depends on the availability of carbohydrate-rich food as an energy source for maintenance and reproduction. Therefore, as these resources have become rare in modern agricultural systems, external sugar sources like flowering plants or artificial sugar solutions are more and more used to provide the biocontrol agents with the necessary sugars. When developing such artificial food sources it is essential to carefully select the sugars that best support the target parasitoids without benefiting non-target insects, such as pest insects or hyperparasitoids. Here we investigated the gustatory response and longevity of two commonly used aphid parasitoids (Aphidius colemani and Aphidius matricariae) and their hyperparasitoid Dendrocerus aphidum when provided with one of eight plant- and/or insect-derived sugars (fructose, galactose, glucose, melibiose, melezitose, rhamnose, sucrose and trehalose). Our results showed that the studied insect species consumed the largest amounts of sugars that are most commonly found in honeydew (sucrose, fructose, glucose and melezitose) and also survived best when feeding on these sugars. Both Aphidius spp. survived well on melibiose, whereas D. aphidum performed poorly on this sugar. When melibiose was offered in a mixture with glucose, a significant reduction in longevity was observed for D. aphidum when compared to glucose only, while this was less pronounced for Aphidius spp. This knowledge can be exploited in tailoring food sources to selectively support Aphidius parasitoids, enhancing the biological control of aphids.status: publishe

Effect of citalopram on esophageal motility in healthy subjects-Implications for reflux episodes, dysphagia, and globus

BACKGROUND: Drugs such as citalopram, "targeting" the serotonin pathway, can alter esophageal mechano-chemical sensitivity and gastrointestinal motility. The aim of this study was to clarify the effect of citalopram on esophageal motility and sphincter function, transient lower esophageal sphincter relaxations (TLESRs), and reflux events. METHODS: Sixteen healthy volunteers (HV) receiving 20 mg citalopram or placebo intravenously, in a randomized cross-over fashion, underwent two high-resolution impedance manometry studies involving liquid swallows and a high-fat, high-caloric meal. Manometric, reflux, and symptom-related parameters were studied. KEY RESULTS: A lower distal contractile integral was recorded under citalopram, compared with placebo (P = 0.026). Upper esophageal sphincter (UES) resting pressure was significantly higher after citalopram administration throughout the study (P < 0.05, all periods). Similarly, the UES postswallow mean and maximum pressures were higher in the citalopram condition (P < 0.0001, in both cases) and this was also the case for the 0.2 s integrated relaxation pressure (P = 0.04). Esophagogastric junction resting pressures in the citalopram visit were significantly higher during swallow protocol, preprandial period, and the first postprandial hour (P < 0.05, in all cases). TLESRs and total reflux events were both reduced after citalopram infusion (P = 0.01, in both cases). During treatment with citalopram, five participants complained about globus sensation (P = 0.06). This citalopram-induced globus was associated with higher UES postswallow mean and maximum pressure values (P = 0.01 and P = 0.04, respectively). CONCLUSIONS AND INFERENCES: Administration of citalopram exerts a diversified response on esophageal motility and sphincter function, linked to clinically relevant phenomena: a reduction in postprandial TLESRs and the induction of drug-induced globus.status: publishe

Floral nectar yeasts enhance parasitoid foraging and maintenance

Floral nectar is a sugar-rich, nutritional reward offered by many plant species to recruit flower-visiting insects for their ecological services such as pollination and plant protection. Owing to its high concentration of sugars, floral nectar is a typical habitat for many nectarivorous microbes, most often nectar-inhabiting yeasts (NIYs) and bacteria. NIYs alter nectar chemistry and change its odor by releasing a specific blend of microbial volatile organic compounds (MVOCs). These changes by NIYs may impact the insect attraction and the fitness of nectar-consuming insects. In this study, we tested the hypothesis that different NIYs affect insect attraction, nectar intake and adult life span disparately, by altering the chemistry of nectar and/or producing distinct aroma-active compounds. We used the generalist aphid parasitoid Aphidius ervi (Haliday) as a study organism. Adult parasitoids were provided with a synthetic nectar solution that was fermented with one of five different NIYs that commonly occur in floral nectar. Overall, NIYs significantly impacted nectar chemistry by changing its acidity, sugar and amino acid profiles. Moreover, the NIYs produced a species-specific blend of MVOCs that was substantially different from yeast-free (mock) nectar. Olfactometer bioassays revealed that A. ervi females showed a strong preference for nectar fermented with Metschnikowia reukauffii, followed by Metschnikowia gruessii and Aureobasidium pullulans, but did not distinguish between mock nectar and nectar fermented with Hanseniaspora uvarum. Nectar fermented with Sporobolomyces roseus was clearly repellent to female parasitoids. Parasitoids showed a significant difference in intake among the various fermented nectars. Furthermore, adults of A. ervi survived longest when fed with nectar fermented with M. reukauffii and M. gruessii. The outcomes of our study provide further insight into the ecological implications of NIYs in tritrophic interactions

Data from: Nectar bacteria affect life history of a generalist aphid parasitoid by altering nectar chemistry

1. Nectar is a crucial energy resource that strongly mediates the interactions between plants and animal pollinators or plant defenders. Previous research has shown that nectar is commonly colonized by microorganisms, most commonly bacteria and yeasts, which can have a strong impact on nectar chemistry. However, at present little is known about the effects of microorganisms on the fitness of animals feeding on nectar. 2. We used three nectar bacteria representing different metabolic groups (Asaia sp., Lactococcus sp. and Rosenbergiella sp.) and the common generalist aphid parasitoid Aphidius ervi (Haliday) (Hymenoptera: Braconidae) to test the hypothesis that different nectar-dwelling bacteria affect nectar consumption and insect longevity differently by altering the chemistry of nectar. 3. Bacteria significantly affected nectar chemistry by altering its acidity, sugar and amino acids composition/concentration and by adding compounds synthesized by the microbes. Although inoculation with bacteria did not affect nectar consumption, a significant difference in insect longevity was observed. The impact on longevity was species specific, with Lactococcus being beneficial and Asaia having a detrimental effect. 4. Bacteria have a strong impact on nectar chemistry and changes in nectar chemistry may not only influence the foraging behavior of flower-visiting animals and impact on plant fitness, but also influence the fitness of nectar-consuming organisms. As effects were species dependent, changes in nectar chemistry induced by different bacteria may have contrasting effects on the interactions between plants and insects. It is therefore essential to know how different microbes alter nectar chemistry to understand the relationships between plants, nectar-inhabiting microbes and nectar-consuming animals