Convenient synthesis of mosquito oviposition pheromone and a highly fluorinated analog retaining biological activity

Chemosensory Cues for Mosquito Oviposition Site Selection

Gravid mosquitoes use chemosensory (olfactory, gustatory, or both) cues to select oviposition sites suitable for their offspring. In nature, these cues originate from plant infusions, microbes, mosquito immature stages, and predators. While attractants and stimulants are cues that could show the availability of food (plant infusions and microbes) and suitable conditions (the presence of conspecifics), repellents and deterrents show the risk of predation, infection with pathogens, or strong competition. Many studies have addressed the question of which substances can act as positive or negative cues in different mosquito species, with sometimes apparently contradicting results. These studies often differ in species, substance concentration, and other experimental details, making it difficult to compare the results. In this review, we compiled the available information for a wide range of species and substances, with particular attention to cues originating from larval food, immature stages, predators, and to synthetic compounds. We note that the effect of many substances differs between species, and that many substances have been tested in few species only, revealing that the information is scattered across species, substances, and experimental conditions.

Novel olfactory ligands via terpene synthases

A synthetic biology approach to the rational design of novel olfactory ligands is described.

Diastereodivergent total synthesis of mosquito oviposition pheromone

The unnatural threo-6-acetoxy-5-hexadecanolide and the natural mosquito oviposition pheromone erythro-6-acetoxy-5-hexadecanolide were synthesized in a diastereodivergent fashion in 44% and 33% overall yield respectively from 5-bromovaleric acid and undecanal. The key step utilized a chemoenzymatic epoxidation-lactonization of a naturally available fatty acid to form the 6-hydroxy-5-hexadecanolide core.

Reverse and conventional chemical ecology approaches for the development of oviposition attractants for Culex mosquitoes

Synthetic mosquito oviposition attractants are sorely needed for surveillance and control programs for Culex species, which are major vectors of pathogens causing various human diseases, including filariasis, encephalitis, and West Nile encephalomyelitis. We employed novel and conventional chemical ecology approaches to identify potential attractants, which were demonstrated in field tests to be effective for monitoring populations of Cx. p. quinquefasciatus in human dwellings. Immunohistochemistry studies showed that an odorant-binding protein from this species, CquiOBP1, is expressed in trichoid sensilla on the antennae, including short, sharp-tipped trichoid sensilla type, which house an olfactory receptor neuron sensitive to a previously identified mosquito oviposition pheromone (MOP), 6-acetoxy-5-hexadecanolide. CquiOBP1 exists in monomeric and dimeric forms. Monomeric CquiOBP1 bound MOP in a pH-dependent manner, with a change in secondary structure apparently related to the loss of binding at low pH. The pheromone antipode showed higher affinity than the natural stereoisomer. By using both CquiOBP1 as a molecular target in binding assays and gas chromatography-electroantennographic detection (GC-EAD), we identified nonanal, trimethylamine (TMA), and skatole as test compounds. Extensive field evaluations in Recife, Brazil, a region with high populations of Cx. p. quinquefasciatus, showed that a combination of TMA (0.9 µg/l) and nonanal (0.15 ng/µl) is equivalent in attraction to the currently used infusion-based lure, and superior in that the offensive smell of infusions was eliminated in the newly developed synthetic mixture.

The role of mosquito olfaction in oviposition site location and in the avoidance of unsuitable hosts

Developments in the exploitation of mosquito olfaction are traced, in collaborative studies with various groups, from the first identification of a mosquito pheromone through to a discussion of non-host avoidance. The characterization of the oviposition pheromone for mosquitoes in the genus Culex, e.g. Culex quinquefasciatus, as a novel chiral lactone ester provided the impetus for a number of sophisticated asymmetric syntheses and economical large-scale routes to racemic products. The latter have provided material for successful field trials in three continents. During the course of this field work, we obtained evidence that semiochemicals originating directly from the oviposition site are essential for activity of the oviposition pheromone. Recent studies are elucidating the nature of these agents and their geographical variability. Initially, we used synthetic oviposition pheromone to attract mosquitoes to sites treated with a biorational larvicide. However, recyclable biological control agents offer better prospects for resource-poor regions. A biotechnological approach to pheromone production has been devised involving the generation of inexpensive starting materials by the cultivation of a higher plant. New studies on dipterous pests feeding on farm animals indicate a semiochemically based mechanism by which unsuitable individuals within the host species are avoided. There appears to be an analogous process in which mosquitoes avoid certain potential human hosts, thereby raising prospects for the development of novel, rationally identified repellents once the semiochemical/olfactory interactions have been fully elucidated.

Semiochemicals for biting fly control: their identification and exploitation

Insects that feed on the blood of vertebrates are difficult to control, and many previous efforts have been unsuccessful. This is becoming an ever increasing issue, not only in developing countries, but also in developed countries, as exemplified by the recent spread of West Nile virus by mosquitoes across the USA and recent dengue outbreaks in Singapore and Australia. Investigating the ways in which biting insects interact with each other, their environment and their hosts is providing valuable knowledge that will lead to the development of improved control technologies. For instance, recent advances in chemical ecology research have led to the identification of new semiochemicals that show great potential as control agents against biting insects. Exciting new chemical ecology tools and control technologies for the future are discussed.

Oviposition Responses of Gravid Female Culex quinquefasciatus to Egg Rafts and Low Doses of Oviposition Pheromone Under Semifield Conditions

Semifield experiments were conducted to study the oviposition of gravid Culex quinquefasciatus females in response to one or 10 egg rafts, or 3.0 microg of synthetic oviposition pheromone, (-)-(5R, 6S)-6-acetoxy-5-hexadecanolide, a dose equivalent to 10 egg rafts. These treatments were added to the small bowls filled with either hay infusion or water in a small (0.3-m spacing) or a large-square design (3.4-m spacing). Oviposition choice was more pronounced in the "small square" assays. Mean number of egg rafts laid in response to a single egg raft in an oviposition jar filled with hay infusion was significantly greater than with all other treatments. When the oviposition pheromone dose was increased from one to 10 rafts or when 3.0 microg synthetic oviposition pheromone were dispensed on a floating receptacle, synergistic effects were observed between the oviposition pheromone and the hay infusion at both distances. This study is the first demonstration that the amount in a single raft induces oviposition of gravid Cx. quinquefasciatus females under semi-natural conditions.

Laboratory and field responses of the mosquito, Culex quinquefasciatus, to plant-derived Culex spp. oviposition pheromone and the oviposition cue skatole

Laboratory and field studies were conducted on the oviposition behavior of the pathogen-vectoring mosquito, Culex quinquefasciatus, in response to the oviposition pheromone 6-acetoxy-5-hexadecanolide, produced from a renewable plant resource, Kochia scoparia (Chenopodiaceae) (plant-derived pheromone, PDP), and via an established synthetic route (synthetic oviposition pheromone, SOP). Responses to the oviposition cue skatole (3-methylindole), presented individually and in combination with the plant-derived and synthetic oviposition pheromone, were also studied. Both laboratory and field assays showed that PDP and SOP were equally attractive. Synergistic effects were observed with one combination of PDP and skatole combinations in laboratory assays. Synergy was also observed under field conditions. SOP and skatole combinations showed additive effects in laboratory assays, but were not tested in field bioassays. Although synergism has been previously demonstrated with combinations of SOP and polluted waters, the work presented here is the first example of synergy between a specific oviposition attractant and the oviposition pheromone. Furthermore, the efficacy of mosquito pheromone produced from a cheap, renewable botanical source has been demonstrated.

The Influence of Fluorinated Molecules (Semiochemicals and Enzyme Substrate Analogues) on the Insect Communication System

Can the introduction of fluorine atoms affect the bioactivity of natural semiochemicals? Can fluorine contribute in the creation of specific enzyme inhibitors to interrupt or disrupt the insect communication system? The first step for the bioactivity of a molecule is interaction with the biological sensor. Hydrogen and fluorine are almost bioisosteric and the receptor site of the enzyme can still recognize and accept the fluoro analogue of its natural substrate. However, the peculiar electronegativity of the fluorine atom can affect the binding, absorption, and transport of the molecule. The differences in the molecule's electronic properties can lead to differences in the chemical interactions between the receptor and the fluorinated substrate. Fluorine introduction can modify the metabolic stability and pathway of the semiochemicals in many different ways. Fluorinated analogues can show synergism, inhibition, or hyperagonism effects on insect behaviors, that is, the activity of the nonfluorinated parent compounds can be mimicked, lost, or increased. In any case, the fluorinated molecules can interact with the bioreceptors in a new and disrupting way. The semiochemicals are olfactory substances: fluorine can affect their volatility or smell. Production of semiochemicals from exogenous substances, perception at antennal receptors, and processing of biological responses are the main steps of communication among insects. In the production step, the fluorinated molecules can interact with enzymes that catalyze the biosynthesis of the natural pheromones. In the perception step, fluorinated semiochemicals can interact with the olfactory receptor cells; this often leads to totally unpredictable behaviors. Fluorinated molecules have been developed as probes to elucidate the complex chemorecognition processes of insects. Many of these molecules have been tested to find highly effective behavior-modifying chemicals. New analogues have been synthesized to investigate the metabolic pathway of a pheromone molecule and many of them are promising disrupting agents. Despite such titanic research efforts, the results have often been random, rational trends in the induced behaviors have sometimes been impossible to find, and practical applications of the fluorinated semiochemicals are still uncertain.

Sampling gravid Culex quinquefasciatus (Diptera: Culicidae) in Tanzania with traps baited with synthetic oviposition pheromone and grass infusions

The effectiveness of traps baited with (5R,6S)-6-acetoxy-5-hexadecanolide (the synthetic oviposition pheromone) and grass infusions in sampling a population of gravid Culex quinquefasciatus Say was conducted in Muheza, Northeast Tanzania. A counterflow geometry (CFG) trap baited with pheromone and set outdoors, adjacent to a pit latrine building, collected more gravid Cx. quinquefasciatus than a CDC trap baited with pheromone and operated without light. Inside pit latrine buildings, significantly more gravid Cx. quinquefasciatus were collected in a CFG trap-baited with pheromone or grass infusion than in traps baited with tap water. CFG traps baited with either grass infusion or pheromone and set outdoors, away from known breeding sites, caught significantly more gravid Cx. quinquefasciatus than traps baited with tap water. CFG traps baited with pheromone + grass infusion caught significantly more gravid Cx. quinquefasciatus than CFG traps baited with either grass infusion or pheromone. In both cases, the proportion of gravid mosquitoes increased as traps were moved away from a natural emergence site. More gravid Cx. quinquefasciatus were collected in a pheromone-baited CFG trap than were egg rafts deposited in a jar with pheromone-treated water. It is concluded that CFG traps baited with oviposition attractants can be used effectively to sample gravid Cx. quinquefasciatus.

Insect parapheromones in olfaction research and semiochemical-based pest control strategies

The possibility of disrupting the chemical communication of insect pests has initiated the development of new semiochemicals, parapheromones, which are anthropogenic compounds structurally related to natural pheromone components. Modification at the chain and/or at the polar group, isosteric replacements, halogenation or introduction of labeled atoms have been the most common modifications of the pheromone structure. Parapheromones have shown a large variety of effects, and accordingly have been called agonists, pheromone mimics, synergists and hyperagonists, or else pheromone antagonists, antipheromones and inhibitors. Pheromone analogues have been used in quantitative structure-activity relationship studies of insect olfaction, and from a practical point of view they can replace pheromones when these are costly to prepare or unstable under field conditions.