Using Sclerodermus brevicornis to understand sociality in parasitoids

Sociality is common among the aculeate Hymenoptera, and full range of levels of sociality is exhibited, from solitary to eusocial. Most attention has been paid to eusocial and semisocial species mainly in bees, ants, and paper wasps. There has been far less consideration of sociality in parasitoid species, chiefly because most of them are socially solitary. However, in some taxa of parasitoids, mainly belonging to the aculeate family Bethylidae, a range of degrees of sociality is present. Sub-sociality behaviour is the most primitive level of sociality and is evidenced by species that show single mother maternal care of the brood (from passive egg guarding to an array of complex grooming, feeding, protection, and nesting behaviour). This kind of sociality is typical of species in the genus Goniozus Förster in which a single mother paralyses and oviposits eggs in group on a victim and takes care of the offspring from hatching until subsequent emergence. A more complex degree of sociality (quasi-sociality) is represented by a group of females (foundresses) collaborating in the paralysis of a large host that they share as an oviposition site, and they provide mutual care for each other offspring. This is a typical behaviour in the bethylids species of the genus Sclerodermus Latreille. Recently, the interest toward these species has increased as they can become active part in the control of xylophagous pest beetles. Adult females are highly active in searching for hosts in the pre-existing galleries bored in the wood, helped by their morphological adaptations such as small size and flattened body. The species Sclerodermus brevicornis (Kieffer) has been found in association with the exotic longhorn beetle Psacothea hilaris hilaris (Pascoe) in Italy and, as it can be reared in laboratory, it can be used as a model species to study many different characteristics related to quasi-social behaviour. Different behavioural strategies of this species, from host sharing including host paralysis, to egg oviposition, brood guarding until and beyond offspring pupation are described. Also the capability to aggregate and segregate in relation to host availability and dimension has been documented. While these parasitoids exhibit cooperative social behaviour, cooperation is not carried out without reference to the relatedness between social partners. Selected tests are reported, always including a greater degree of complexity and observations on cooperative associative strategies between females, coming from the same brood or different broods, to evidence if females are most willing to take risks attacking hosts when the beneficiaries will include their sisters. Understanding the evolution of quasi-sociality and associated behaviours in S. brevicornis will assist in biological control programmes because it allows the design of mass rearing and the optimisation of the mass release technique.