Intra- and inter-specific variation in alarm pheromone produced by Solenopsis fire ants

Olfactory Strategies in the Defensive Behaviour of Insects

Most animals must defend themselves in order to survive. Defensive behaviour includes detecting predators or intruders, avoiding them by staying low-key or escaping or deterring them away by means of aggressive behaviour, i.e., attacking them. Responses vary across insect species, ranging from individual responses to coordinated group attacks in group-living species. Among different modalities of sensory perception, insects predominantly use the sense of smell to detect predators, intruders, and other threats. Furthermore, social insects, such as honeybees and ants, communicate about danger by means of alarm pheromones. In this review, we focus on how olfaction is put to use by insects in defensive behaviour. We review the knowledge of how chemical signals such as the alarm pheromone are processed in the insect brain. We further discuss future studies for understanding defensive behaviour and the role of olfaction.

On the Biological Diversity of Ant Alkaloids

Ants have outstanding capacity to mediate inter- and intraspecific interactions by producing structurally diverse metabolites from numerous secretory glands. Since Murray Blum's pioneering studies dating from the 1950s, there has been a growing interest in arthropod toxins as natural products. Over a dozen different alkaloid classes have been reported from approximately 40 ant genera in five subfamilies, with peak diversity within the Myrmicinae tribe Solenopsidini. Most ant alkaloids function as venom, but some derive from other glands with alternative functions. They are used in defense (e.g., alarm, repellants) or offense (e.g., toxins) but also serve as antimicrobials and pheromones. We provide an overview of ant alkaloid diversity and function with an evolutionary perspective. We conclude that more directed integrative research is needed. We suggest that comparative phylogenetics will illuminate compound diversification, while molecular approaches will elucidate genetic origins. Biological context, informed by natural history, remains critical not only for research about focal species, but also to guide applied research.

Comparative Cutaneous Water Loss and Desiccation Tolerance of Four Solenopsis spp. (Hymenoptera: Formicidae) in the Southeastern United States

The high surface area to volume ratio of terrestrial insects makes them highly susceptible to desiccation mainly through the cuticle. Cuticular permeability (CP) is usually the most important factor limiting water loss in terrestrial insects. Water loss rate, percentage of total body water (%TBW) content, CP, and desiccation tolerance were investigated in workers of four Solenopsis species in the southeastern USA. We hypothesized that tropical/subtropical ants (S. invicta and S. geminata) will have lower CP values and tolerate higher levels of desiccation than temperate ants (S. richteri and S. invicta × S. richteri). The %TBW content was similar among species. Solenopsis invicta had a 1.3-fold and 1.1-fold lower CP value than S. invicta × S. richteri and S. richteri, respectively. Solenopsis geminata had a 1.3-fold lower CP value than S. invicta × S. richteri, and a 1.2-fold lower CP value than S. richteri. The LT₅₀ values (lethal time to kill 50% of the population) ranged from 1.5 h (small S. geminata) to 8.5 h (large S. invicta). Desiccation tolerance ranged between 36 and 50 %TBW lost at death and was not related to a species’ location of origin. This study is the first report of water relations of S. invicta × S. richteri. It demonstrates that desiccation stress differentially can affect the survival of different Solenopsis species and implies that environmental stress can affect the distribution of these species in the southeastern USA.

Electrophysiological Responses of Eighteen Species of Insects to Fire Ant Alarm Pheromone

Olfaction plays a dominant role in insect communication. Alarm pheromones, which alert other insects of the same species of impending danger, are a major class of releaser pheromones. The major components of alarm pheromones in red imported fire ants, honeybees and aphids have been identified as 2-ethyl-3,6-dimethylpyrazine (2E-3,6-DP), isopentyl acetate (IPA), and E-β-farnesene (EβF), respectively. In this study, electroantennography (EAG) responses to EDP (a mixture of 2-ethyl-3,6-dimethylpyrazine and 2-ethyl-3,5-dimethylpyrazine), IPA and EβF were investigated in a wide range of insect species. Beside imported fire ants, the EDP (2-ethyl-3,6(5)-dimethylpyrazine) elicited significant EAG response from all other tested insects, including six ant species and one hybrid ant, honeybee, bagrada bug, lady beetle, housefly, small hive beetle, yellow fever mosquito, termite, bedbug, water hyacinth weevil, southern green stink bug and two aphid species. In contrast, IPA elicited significant EAG response only in the honeybee, red imported fire ant, an Aphaenogaster ant, and the water hyacinth weevil. The EβF only elicited EAG responses in two aphids, small hive beetle and housefly. The results clearly indicate that EDP can be detected by widespread insect species that did not coevolve with S. invicta and further suggested alkylpyrazine may activate multiple generally tuned olfactory receptors (ORs) across a wide number of insect species.

HPLC Separation of 2-Ethyl-5(6)-methylpyrazine and Its Electroantennogram and Alarm Activities on Fire Ants (Solenopsis invicta Buren)

2-Ethyl-3,6-dimethylpyrazine (EDMP) was an alarm pheromone component isolated from the mandibular gland of the red imported fire ant, Solenopsis invicta Buren. Several pyrazine analogues have been previously found to elicit significant alarm responses in S. invicta workers. This study aimed to separate the commercially available 2-ethyl-5(6)-methylpyrazine (EMP), i.e., a mixture of 2-ethyl-6-methylpyrazine (2E6MP) and 2-ethyl-5-methylpyrazine (2E5MP), and to examine both electroantennogram (EAG) and behavioral responses of S. invicta workers to EMP and the purified isomers. HPLC separations were achieved using a polysaccharide chiral stationary phase (Chiralpak AD-H) column with both mobile phases: Cyclohexane/isopropanol, and hexane/isopropanol. A ratio of 99:1 was selected for the separation of EMP at semipreparative level. The structures of the isomers obtained through the cyclohexane/isopropanol mobile phase were confirmed by detailed analyses of 2D-HSQC- and -HMBC-NMR data. The two isomers showed differential methine C⁻H correlations evidenced by 2D-HMBC-NMR spectra. The two concentrated fractions obtained through hexane/isopropanol mobile phase were subjected to EAG test and behavioral bioassay on S. invicta workers. The two HPLC−purified isomers, 2E6MP and 2E5MP, and their mixture (1:1) at same dose elicited similar EAG and alarm responses, indicating that these two isomers are equally active. The 2D-NMR−spectroscopic characterization, and electrophysiological and alarm activities of 2E6MP and 2E5MP were reported here for the first time.