Konrad Dettner

Gut bacteria may be involved in interactions between plants, herbivores and their predators: Microbial biosynthesis of N- acylglutamine surfactants as elicitors of plant volatiles

Abstract N-Acylamino acids are dominant and widespread constituents of insect oral secretions (regurgitants), serving the insect as biosurfactants in the digestive process. During feeding the conjugates may be introduced into damaged leaves and contribute there to the elicitation of plant defenses such as the induction of volatile biosynthesis. From gut segments of Spodoptera exigua, Mamestra brassicae and Agrotis segetum 23 bacterial strains were isolated, ten of which were able to synthesise typical lepidopteran N-acylamino acids from externally added precursors. Four strains, Providencia rettgeri, Ochrobactrum spec., Myroides odoratus and Acinetobacter sp. genospecies 11 were identified on the basis of their 16 S rDNA. The organisms displayed a very broad substrate tolerance, since fatty acids of different chain length and different degree of saturation were converted into N-acylamino acids. Moreover, most of the proteinogenic amino acids, but not glutamic and aspartic acid, were used as substrates. The dominant occurrence of fatty acids conjugated with glutamine may result from a preferred transport of glutamine from the hemolymph into the gut of the insects. The involvement of bacteria in the biosynthesis of compounds which play a pivotal role in the interaction of plants, herbivores and their predators adds a new trophic level to this complex network of interactions. Due to their short generation cycle and the ease of adaptation endosymbiontic bacteria may have an outstanding importance for the coevolution of plant-insect interactions.

Role of cuticular hydrocarbons of aphid parasitoids in their relationship to aphid-attending ants

Lysiphlebus cardui, the dominant aphidiid parasitoid of the black bean aphid,Aphis fabae cirsiiacanthoidis (Afc), on creeping thistle, is able to forage in ant-attended aphid colonies without being attacked by ants. Several behavioral observations and experimental studies led to the hypothesis thatL. cardui mimics the cuticular hydrocarbon profile of its host aphid. Chemical analysis of the cuticular extracts revealed that bothL. cardui and Afc exclusively possess saturated hydrocarbons:n-alkanes, monomethyl (MMA), dimethyl (DMA), and trimethyl alkanes (TMA). Comparison of the hydrocarbon profiles of parasitoid and aphid showed great qualitative resemblance between parasitoid and host:L. cardui possesses almost all host-specific compounds in addition to species-specific hydrocarbons of mainly higher molecular weight (>C30). However, there is a lesser quantitative correspondence between parasitoid and host aphid. Furthermore, we analyzed the cuticular hydrocarbon profile of another parasitoid of Afc,Trioxys angelicae. This aphidiid species is vigorously attacked and finally killed by honeydewcollecting ants when encountered in aphid colonies. Its cuticular hydrocarbon profile is characterized by the presence of large amounts of (Z)-11-alkenes of chain lenghts C27, C29, C31, and C33, in addition to alkanes and presumably trienes. The role of the unsaturated hydrocarbons onT. angelicae as recognition cues for aphid-attending ants is discussed.

On the cytotoxity of oxidized phytosterols isolated from photoautotrophic cell cultures of Chenopodium rubrum tested on meal-worms Tenebrio molitor

Abstract Cell injury—as evoked by oxidative stress, insect attack or ageing—causes lipid peroxidation of polyunsaturated acids, e.g. linoleic acid. Hydroperoxides (LOOH) so produced transform fatty acids, terpenes and sterols to epoxides. We describe a bioassay to determine the cytotoxity of sterol oxidation products by measuring the mortality of meal-worms ( Tenebrio molitor ) after injection of diluted test compounds. LOOH was tested in concentrations from 10 −5 M up to 10 −2 M and developed recognizable mortality (25%, 10 −2 M, 23 hr). A direct relation between activity and concentration was observed. 5,6-Epoxides of phytosterols and derived 3,5,6-trihydroxysteranes have been isolated from Chenopodium rubrum cell cultures. Using 5,6-epoxycholesterol and 3,5,6-trihydroxycholestane as standards of known bioactivity, the toxicity of epoxides of sitosterol and stigmasterol, as well as the trihydroxy compounds thereof, was checked. The phytosterol derivatives are by a factor of five less active than the corresponding cholesterol oxidation products. 5,6-Epoxides and 5,6-chlorohydrins showed half of the activity of the corresponding 3,5,6-trihydroxyphytosterols. These developed highest cytotoxity (40% mortality in 5 × 10 −3 M solution after 23 hr). The activity of 5,6α-and 5,6β-epoxides was found to be equal. Metabolism and proposed biological function of oxidized phytosterols is discussed.

Endophenazines A-D, New Phenazine Antibiotics from the Arthropod Associated Endosymbiont Streptomyces anulatus

Four new members of the phenazine family, endophenazines A-D, and the already known phenazine-1-carboxylic acid (tubermycin B) were detected in the culture broth of various endosymbiotic Streptomyces anulatus strains by chemical screening in a combination of TLC-staining reagents and HPLC-diode array analysis. The endosymbiotic strains were isolated from four different arthropod hosts at various sites. The new phenazine compounds showed antimicrobial activities against Gram-positive bacteria and some filamentous fungi, and herbicidal activity against Lemna minor (duckweed).

Recognition of aphid parasitoids by honeydew-collecting ants: The role of cuticular lipids in a chemical mimicry system

The aphidiid waspLysiphlebus cardui parasitizes in ant-attendedAphis fabae cirsiiacanthoidis colonies without causing aggressive behavior in the antLasius niger. By contrast,Trioxys angelicae, another aphidiid parasitoid of aphids, is rapidly recognized and vigorously attacked by the ants.L. niger workers also responded differently to dead individuals ofL. cardui andT. angelicae. DeadL. cardui parasitoids were often ignored when encountered byL. niger, whereas deadT. angelicae individuals were immediately grasped by ants that discovered them. However, hexane-washed parasitoids caused a similar reaction pattern in the ants, in that both aphidiid species were tolerated in the aphid colony. Lure experiments demonstrated that chemical stimuli on the cuticle are major cues for the ants to distinguish between the parasitoids. The hexane extract ofL. cardui transferred to washed individuals ofT. angelicae resulted in ant responses characteristic towardsL. cardui, andL. niger workers displayed the typical removal pattern they normally showed towardsT. angelicae whenT. angelicae extract was applied toL. cardui individuals. Both parasitoid species treated with the hexane extract ofA. fabae cirsiiacanthoidis were similarily treated by the ants as were aphid control individuals. The suggestion that the aphidiid waspL. cardui uses chemical mimicry is discussed.

Defensive secretions and exocrine glands in free-living staphylinid bettles—their bearing on phylogeny (Coleoptera: Staphylinidae)

Abstract Due to their reduced elytra many adults of free living rove beetle taxa (Coleoptera: Staphylinidae) bear exocrine defensive glands where an astonishing variety of defensive compounds, antimicrobics, fungicides, adhesives or spreading active compounds are produced. Chemically these exocrines cover hydrocarbons, aliphatic alcohols, aldehydes, ketones, acids, esters, sprioacetales, iridoids, aromatics, quinones or various nitrogen-containing compounds. Factors such as presence, position, structure and variation of these exocrine glands are used more and more in order to establish a natural system of Staphylinidae. This paper surveys the chemical defensive systems of staphylinid beetles and considers the biological significance and multifunctional role of these exocrines. Within the omaliine group, Oxytelinae/Pseudopsinae, Aleocharinae, Paederinae, Staphylininae/Xantholininae, Steninae and Silphidae where a common homologous gland system is present in each case, character patterns and evolutionary trends, especially with respect to secretion chemistry are presented. In the subfamily Oxytelinae these glandular and micromolecular patterns are used especially in order to construct a phylogeny for this subfamily which is based on phylogenetic systematics. It was shown that identification of character stage transformations of these micromolecules in advanced and primitive species is necessary for estimating apomorphic and plesiomorphic character states. Optimizing of a defensive secretion may on the one hand be achieved by gradual intervention of new secretion types (from one precursor) with increasing topical irritancy from primitive towards advanced taxa. A further improvement of the secretion may be achieved by changing and adjusting secretion formulation in order to optimize these mixtures by synergisms from primitive towards advanced species. Finally the necessity of biogenetic data is stressed in order to discuss the gradual sequence of evolution of gland constituents.

The male pheromone of the old house borerHylotrupes bajulus (L.) (Coleoptera: Cerambycidae): Identification and female response

We report here the identification of the long-range, male-produced sex pheromone of the Old house borerHylotrupes bajulus. Chemical analysis of hexane extracts obtained by surface extraction from dissected prothoracic glands and from headspace samples of the two sexes, revealed male-specific compounds: (3R)-3-hydroxy-2-hexanone, 2-hydroxy-3-hexanone, the diastereomeric diols (2R, 3R)-2,3-hexanediol and (2S, 3R)-2,3-hexanediol, 2,3-hexanedione, as well as 1-butanol. In wind tunnel bioassays we tested the influence of these male-specific compounds from the prothoracal glands on the behaviour of unmated and mated females. Specific behavioural sequences of the tested females (activity, running behaviour, searching, cleaning, flying, extension of ovipositor) were recorded. Unmated females were attracted by male beetles, headspace extracts of males, synthetic blends of the major pheromone compounds as well as by the components (3R)-3-hydroxy-2-hexanone, and the diastereomeric diols. Hexane, female beetles and 2,3-hexanedione did not attract unmated females. The reactions of mated females to male beetles and headspace samples did not differ significantly from those of the controls. The results of the bioassays show that the two-stage premating behaviour is initiated by emission of a long-range sex pheromone from the male prothoracal glands, which functions as an activator, attractant, and possibly aphrodisiac for unmated females.

Allocation of pederin during lifetime ofPaederus rove beetles (Coleoptera: Staphylinidae): Evidence for polymorphism of hemolymph toxin

Pederin, a hemolymph toxin found in the rove beetle genusPaederus, is quantified in individual specimens ofP. fuscipes andP. riparius. Whereas males always contain only small quantities of the toxin (0.1–1.5 µg), females are not so restricted (0.2–20.5 µg) and in most cases possess roughly 10-fold that in males. There are, however, females containing as little pederin as the males, and so two clearly separate groups of females can be discerned. These two groups hint at two types of females defined by the eggs they lay. About 85% of the females, i.e., those containing much of the substance, transfer pederin into every egg and thus are denoted (+)-females. The remaining 15%, the (−)-females, contain at best small amounts of the toxin, which is transferred in minute quantities into the first eggs while the subsequent ones lack pederin. With respect to the pederin content of their eggs, there is no overlap between these two types of females. If fed with pederin, (−)-females can transfer it into the eggs like (+)-females. After hatching the larvae store pederin when present. Larvae are not able to biosynthesize the toxin on their own, but storage of that received in the egg is very efficient, and the difference between larvae with and without pederin is preserved until imaginal eclosion. In (+)-females, pederin increases, probably reflecting a biosynthetic capacity, but in males and presumptive (−)-females the amount of pederin never exceeds the quantity transferred by parental (+)-females. Consequently, males and (−)-females probably are unable to biosynthesize pederin. This polymorphism of females may have a genetic basis. Individuals of all stages nevertheless sequester pederin if it is supplied with the diet.

Biosynthesis of iridoid monoterpenes in insects - defensive secretions from larvae of leaf beetles (coleoptera, chrysomelidae)

Abstract Larvae of the leaf beetles Phaedon amoraciae, Phaedon cochleariae, Gastrophysa viridula, and Plagiodera versicolora biosynthesize the two iridoids chrysomelidial 1 and/or plagiodial 3 from geraniol 7 by an oxidative sequence related to that known from plants en route to the iridane skeleton. Following ω-oxidation of geraniol 7 the resulting 8-hydroxygeraniol 8 is oxidized further to 8-oxocitral 9. The mode of the cyclization of 8-oxocitral 9 to chrysomelidial 1 depends on the chrysomelid species. In larvae of Gastrophysa viridula chrysomelidial 1 originates directly from the acyclic precursor 9 while in both Phaedon spp chrysomelidial 1 is produced via plagiodial 3 followed by isomerization of the endocyclic doble bond.

Effect of Bt corn leaf suspension on food consumption by Chilo partellus and life history parameters of its parasitoid Cotesia flavipes under laboratory conditions

Laboratory scale experiments were conducted in order to assess the potential effect of Bacillus thuringiensis‐corn leaf material on the parasitized herbivore Chilo partellus Swinhoe (Lepidoptera: Crambidae) and on its parasitoid Cotesia flavipes (Cameron) (Hymenoptera: Braconidae). Food consumption and relative consumption rate of parasitized hosts exposed to Bt‐corn leaf‐material were strongly reduced compared to the control. The number of hosts allowing parasitoid larvae to complete their development was also reduced in the Bt group. Moreover, the fresh weight of parasitoid pupae and the dry weight of parasitoid adults was lower than in the control. Only in the Bt group, were strong negative correlations found between food intake by the host, and the number of parasitoid cocoons. Strong positive correlations were also only found in the Bt group, between food intake and parasitoid development time. As effects of Bt on the oviposition behaviour of C. flavipes could be excluded, differences between the Bt group and the control could only be due to the effect of Bt toxin on the parasitoid larva developing inside the host. Whenever food consumption can be measured, the methods used in this study are proposed as a model for future risk assessments on different types of insect‐resistant transgenic plants, herbivores, parasitoids, and predators.