**2. Biology**

The colony of *V. v. nigrithorax* is started by a single inseminated queen that builds, using fibrous substances of plants origin and saliva, a primary nest after overwintering, typically in April, thus producing the first workers. During the warm season, they enlarge the primary nest (which has an approximate size between 4 cm and 15 cm) directly or build a secondary nest normally on treetops [18]. Nests have normally a circular shape and can grow up to 100 cm in diameter, containing several thousands of hornets. Rome et al. [34] report up to 13,300 adults and 563 new queens from a single nest. At the end of the summer, reproductive individuals emerge and mate; the colonies generally collapse in late autumn or winter, while newly-mated queens search for a place where they can overwinter and, the following year, they start a new cycle [8,27,34,35], Figure 2.

**Figure 2.** Life cycle of *V. v. nigrithorax*.

Hornets use olfactory stimuli to search for long-distance food sources, especially with regard to the localization of honey bee colonies, but the nature of these stimuli is not ye<sup>t</sup> entirely clear [36–38].

The components of the hive, which attract the most attention of *V. v. nigrithorax*, have been the subject of study for some years. Hornets are strongly attracted by the odour of some hive products, especially pollen and honey [37]. A laboratory study showed that *V. v. nigrithorax* workers can use both visual and olfactory cues to locate honey bees [39].

Honey bees attract the attention of the hornets thanks to the production of geraniol, component of the aggregation pheromone of the colony. Less effective than pollen and honey, but still attractive is the royal jelly, thanks to the presence of homovanillyl alcohol (HVA) and methyl-4hydrobenzoate (HOB), substances that are part of the pheromone produced by the honey bee queens, but also present in the royal jelly. Betaocimene emitted by larvae also produces olfactory stimuli that are very attractive to hornets [37].

The pheromones produced by the hornet colony's components are being studied for their possible use in biological control techniques [40]. Couto et al. [41], in a neurobiological works on *V. v. nigrithorax*, showed the presence of several microstructures in the antennal lobe of the males, which are probably linked to sex pheromones. Recently Wen et al. [42] announced the isolation of the sex pheromones from *V. velutina* queens. Cheng et al. [43] proved that *V. velutina* uses sting venom volatiles as an alarm pheromone.
