Antibacterial activity of 4-oxo-(E)-2-hexenal from adults and nymphs of the heteropteran, Dolycoris baccarum (Heteroptera: Pentatomidae)

Effect of Bed Bug (Hemiptera: Cimicidae) Aldehydes on Efficacy of Fungal Biopesticides

The use of the entomopathogenic fungus Beauveria bassiana (Bals. - Criv.) Vuill. (Hypocreales: Cordycipitaceae) has been recently incorporated in the management of bed bugs, Cimex lectularius L. (Hemiptera: Cimicidae). Bed bugs produce a set of aldehydes that are known to affect the growth of some fungi. Considering that bed bugs or their exuviae release these aldehydes, it was suspected that the bed bugs' aggregation sites would contain an increased level of the bed bug aldehydes. The current study examined if elevated levels of the bed bug aldehydes in the microhabitats would impact the efficacy of B. bassiana. Following a brief exposure to the residues of commercial products containing B. bassiana, the treated bed bugs were kept in a vial with or without a natural or artificial blend of bed bug aldehydes (i.e., exuviae or synthetic compounds). For a B. bassiana product that is not currently registered for bed bugs control, the presence of aldehydes significantly reduced 15-d mortality (61-62%) compared to the no aldehydes control (97.7%). However, when tested with a B. bassiana formulation designed for bed bug control, the aldehydes only caused delayed mortality for the treated bed bugs. When tested in culture, the growth rate of B. bassiana on a medium was significantly reduced when the bed bug aldehydes were provided in the headspace. Implications on practical bed bug management using fungal biopesticides are discussed.

Identification of saturated and unsaturated 1-methoxyalkanes from the Thai millipede Orthomorpha communis as potential "Raincoat Compounds"

Mixtures of saturated and unsaturated 1-methoxyalkanes (alkyl methyl ethers, representing more than 45.4% of the millipede hexane extracts) were newly identified from the Thai polydesmid millipede, Orthomorpha communis, in addition to well-known polydesmid defense allomones (benzaldehyde, benzoyl cyanide, benzoic acid, mandelonitrile, and mandelonitrile benzoate) and phenolics (phenol, o- and p-cresol, 2-methoxyphenol, 2-methoxy-5-methylphenol and 3-methoxy-4-methylphenol). The major compound was 1-methoxy-n-hexadecane (32.9%), and the mixture might function as “raincoat compounds” for the species to keep off water penetration and also to prevent desiccation.

Aroma volatile analyses and 2AP characterization at various developmental stages in Basmati and Non-Basmati scented rice (Oryza sativa L.) cultivars

BACKGROUND

Rice plant growth is comprised of distinct phases, such as vegetative, reproductive, grain filling and maturity phases. In these phases synthesis and availability of primary and secondary metabolites including volatile organic compounds (VOC's) is highly variable. In scented rice, aroma volatiles are synthesized in aerial plant parts and deposited in mature grains. There are more than 100 VOCs reported to be responsible for flavor in basmati rice. It will be interesting to keep track of aroma volatiles across the developmental stages in scented rice. Therefore, the aroma volatiles contributing in aroma with special reference to the major compound 2 acetyl-1-pyrroline (2AP) were screened at seven developmental stages in scented rice cultivars Basmati-370 and Ambemohar-157 along with non-scented rice cultivar IR-64 as a control following HS-SPME-GC-MS method. In addition, the expression levels of key genes and precursor levels involved in 2AP biosynthesis were studied.

RESULTS

The study indicated that volatilome of scented rice cultivars is more complex than non-scented rice cultivar. N-heterocyclic class was the major distinguishing class between scented from non-scented rice. A total of 14 compounds including, 2AP were detected specifically in scented rice cultivars. Maximum number of compounds were synthesized at seedling stage and decreased gradually at reproductive and maturity. The seedling stage is an active phase of development where maximum number green leaf volatiles were synthesized which are known to act as defense molecules for protection of young plant parts. Among the 14 odor active compounds (OACs), 10 OACs were accumulated at higher concentrations significantly in scented rice cultivars and contribute in the aroma. 2AP content was highest in mature grains followed by at booting stage. Gene expression analysis revealed that reduced expression of betaine aldehyde dehydrogenase 2 (badh2) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and elevated level of triose phosphate isomerase (TPI) and Δ1-Pyrolline-5-carboxylic acid synthetase (P5CS) transcript enhances 2AP accumulation.

CONCLUSIONS

Most diverse compounds were synthesized at seedling stage and OACs were accumulated more at flowering followed by seedling stage. Distinct accumulation pattern exists for 2AP and other aroma volatiles at various developmental stages. The study revealed the mechanism of 2AP accumulation such that 2AP in mature grains might be transported from leaves and stem sheath and accumulation takes place in grains.

Chemically Mediated Arrestment of the Bed Bug, Cimex lectularius, by Volatiles Associated with Exuviae of Conspecifics

Extracts of the exuviae (cast skins) of nymphal bed bugs (Cimex lectularius) were analyzed for volatile compounds that might contribute to arrestment of adult bed bugs. Four volatile aldehydes, (E)-2-hexenal, 4-oxo-(E)-2-hexenal, (E)-2-octenal, and 4-oxo-(E)-2-octenal were consistently detected in the headspace of freshly shed exuviae regardless of the developmental stages from which the exuviae were obtained. Quantification of the aldehydes in the solvent extracts of homogenized fresh, 45- or 99-d aged 5th instar exuviae indicated that the aldehydes are present in the exuviae and dissipate over time, through evaporation or degradation. Microscopic observation of the fifth instar exuviae indicated that the dorsal abdominal glands on the exuviae maintained their pocket-like structures with gland reservoirs, within which the aldehydes might be retained. Two-choice olfactometer studies with the volatiles from exuviae or a synthetic blend mimicking the volatiles indicated that adult bed bugs tend to settle close to sources of the aldehydes. Our results imply that the presence and accumulation of bed bug exuviae and the aldehydes volatilizing from the exuviae might mediate bed bugs’ interaction with their microhabitats.

Identification of the Alarm Pheromone of Hygia lativentris and Changes in Composition during Development

Heteropteran insects produce a series of volatile compounds from their scent glands that protect them from predators and parasites. These compounds also play roles in chemical communication that elicit aggregation, dispersal, and mating behaviors. Hygia lativentris (Coreidae) adults frequently aggregate on host plants. When disturbed, they quickly disperse with the release of a sour smell, suggesting that these bugs possess an alarm pheromone in their secretions. This adult secretion-induced dispersal has been examined with a laboratory assay. Hexanal, the predominant component of the adult secretion was identified as a component of the alarm pheromone by evaluation of the adult bug's response time and escape distance from the chemical source. Physicochemical analyses with gas chromatography/mass spectrometry and nuclear magnetic resonance spectroscopy revealed that secretory components differed between nymphs and adults, and also during adult aging. Nymphs produced two unsaturated compounds, (E)-2-hexenal and (E)-4-oxo-2-hexenal, together with hexanal and 1-hexanol, which were found in all developmental stages. In adults, hexyl acetate was the major component of secretions within 3 days of emerging, while the amount of this ester decreased and those of hexanal, hexanoic acid, and hexanal trimer increased with aging. The decomposition of hexyl acetate into hexanal via 1-hexanol was attributed to the presence of esterases and alcohol dehydrogenases specifically found in adult secretory glands. In contrast, the formation of a hexanal trimer may be due to a non-enzymatic reaction under acidic conditions.

Wolbachia-Free Heteropterans Do Not Produce Defensive Chemicals or Alarm Pheromones

The true bugs, or heteropterans, are known for their widespread production of anti-predator chemicals and alarm pheromones in scent glands, a derived trait that constitutes one of the defining characters of the suborder Heteroptera and a potential novel trait that contributed to their diversification. We investigated whether symbiotic bacteria could be involved in the formation of these chemicals using Thasus neocalifornicus, a coreid bug that produces semiochemicals frequently found in other bugs. Using DNA phylogenetic methodology and experiments using antibiotics coupled with molecular techniques, we identified Wolbachia as the microorganism infecting the scent glands of this bug. Decreasing the level of Wobachia infection using antibiotics was correlated with a diminution of heteropteran production of defensive compounds and alarm pheromones, suggesting that this symbiotic bacterium might be implicated in the formation of chemicals.

4-Oxo-(E)-2-hexenal produced by Heteroptera induces permanent locomotive impairment in crickets that correlates with free thiol depletion

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Heteropterans produce 4-keto-2-aldehydes that function as anti-predatory defenses.

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4-Oxo-(E)-2-hexenal (OHE) induces permanent locomotive impairments in crickets.

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Covalent binding of OHE to biologically active thiols is a potential cause affecting insect locomotion.

Heteropterans produce 2-alkenals and 4-keto-2-alkenals that function as defense substances or pheromones. However, in spite of advances in heteropteran chemistry, it is still unclear how these compounds affect insect physiology. We found that exposure to 4-oxo-(E)-2-hexenal (OHE) induced permanent paralysis and death in crickets, an experimental model. The depletion of free thiols in leg tissues of OHE-treated crickets and the in vitro adduct formation of OHE with a thiol compound suggest that covalent binding of OHE to biologically active thiols is a potential cause affecting crickets’ locomotion.

Chemical polymorphism in defense secretions during ontogenetic development of the millipede Niponia nodulosa

A mixture of defense compounds (benzaldehyde, benzoyl cyanide, benzoic acid, mandelonitrile, and mandelonitrile benzoate), found commonly in cyanogenic polydesmid millipedes, was identified in the non-cyanogenic millipede Niponia nodulosa. These compounds were major components in 1st-4th instars, but were absent in older instars and adults. Extracts of older instars and adults contained 1-octen-3-ol, 2-methyl-2-bornene, E-2-octen-1-ol, 2-methyl-isoborneol, and geosmin; these compounds were minor components in 1st-4th instars. This ontogenetic allomone shift may be explained by the high cost of biosynthesis of polydesmid compounds from L-phenylalanine being offset by their potency in protecting the insect during fragile and sensitive growth stages. However, as the cuticle hardens in older juveniles (5th, 6th, 7th instars) and adults, this allows for a switch in defense to using less effective and less costly volatile organic compounds (presumably microbial in origin) that are ubiquitous in the millipede's habitat or are produced by symbiotic microbes and may be readily available through food intake or aspiration.