# Bacterial Semichemicals and Transkingdom Interactions with Insets and Plants
## 1 Introduction
### Bacteria and Presocial Insect Communication  
The bacterial contribution to chemical crosstalk was first investigated in simple phenomena such as attraction/cohesion processes or sexual behavior. For example, [PERSON] and colleagues [21; 22] demonstrated that in the desert locust _Schistocerca gregaria_ (Orthoptera, Acrididae), the aggregation pheromone guaiacol was produced in vitro by _Pantoa agglomerans_ isolated from the insect gut using vanillic acid introduced by diet as a substrate (Figure 1). Figure 1: Bacteria and insect communication. In _S. gregaria_ (**A**), the bacterium _P. agglomerans_ produces guaiacol, which acts as an aggregation signal. In _D. valens_ (**B**), bacterial community convert confer monoterpenoids in verbenone, a multifunctional pheromone. In _B. germanica_ (**C**), the aggregation is stimulated by succinic acid and other acids produced by gut bacteria. In _A. gambiae_ (**D**), pyrazines, carboxylic acids, and alcohols stimulate oviposition. In _P. regina_ (**E**), bacterial volatile organic compounds (BVOCs) stimulate attraction between individuals. In eusocial insects (**F**) (Hymenoptera), social and behavioral skills are managed by pyrazines,ticular hydrocarbons, and other semiochemicals. Microbial biotransformations are, indeed, rather common in insects. In the bark beetle _Dendroctomus valens_ (Coleoptera, Curculionidae), the most important mortality agent in the coniferous forest, the gut microbiota transforms toxic monoterpencoids of conifers into the multifunctional pheromone verbenone [23; 24]. In _D. valens_, the verbenone works as an aggregation pheromone at a low concentration, while it is an alarm (anti-aggregation) pheromone at a high concentration. Thus, it has been proposed for coniferous forest management. Gut bacteria transform _cis_-verbenol, a toxic precursor for bacteria, to verbenone. It subsequently benefits from this conversion due to the reduced toxicity of verbenone. At the same time, by producing and modulating the bark beetle ambivalent pheromone, they play a key role in controlling _D. valens_ population density (Figure 1). Specific groups of terpene-metabolizing bacteria that are associated with monoterpene tolerance were described in different _Dendroctous_ species. In _D. valens_, the terpene-metabolizing bacteria were almost equally represented by both Gram-positive (Actinobacteria, Bacilli) and Gram-negative (Gamma-Proteobacteria) species, while in _D. ponderosae_, the Gram-positive species were predominant. Compared to the bacteria isolated from _D. valens_, the bacteria from _D. ponderosae_ were not able to grow using \(\alpha\)-pinene and 3-carene as carbon substrate. They were therefore inhibited by different monoterpenes [24], suggesting that a reduction in Gammaproteobacteria may affect their tolerance to monoterpenes. A different study reported that several _Serratia_ strains from _D. ponderosae_ were able to degrade monoterpenes but not \(\alpha\)-pinene that was, instead, degraded by _Rahnella_. On the other hand, _Serratia_ and _Brevundimonas_ were able to reduce diterpenes [25]. Compounds and strains involved in these systems are reported in Table S1 of Supplementary Materials. The upmost importance of bacteria in insect aggregation was also demonstrated in the German cockroach _Blattella germanica_ (Blattodea, Blattellidae). This cockroach is a major pest of the built environment, where it can acquire and transmit pathogens and produce allergens that cause disease in humans. [PERSON] and coworkers [26] demonstrated that the feces of normal cockroaches emitted volatile carboxylic acids (VCAs) that elicited insect aggregation, whereas bacteria-free feces contained few VCAs and were relatively unattractive. Among bacteria, _Enterococcus avium_ and _Weissella cibaria_ were shown to produce aggregation BVOCs 3-isovaleric acid and valeric, succinic acid, benzoic acid, and phenylacetic acid [26] (Figure 1).