Colonization of weakened trees by mass-attacking bark beetles: no penalty for pioneers, scattered initial distributions and final regular patterns

Abstract

Bark beetles use aggregation pheromones to promote group foraging, thus increasing the chances of an individual to find a host and, when relevant, to overwhelm the defences of healthy trees. When a male beetle finds a suitable host, it releases pheromones that attract potential mates as well as other ‘spying’ males, which result in aggregations on the new host. To date, most studies have been concerned with the use of aggregation pheromones by bark beetles to overcome the defences of living, well-protected trees. How insects behave when facing undefended or poorly defended hosts remains largely unknown. The spatio-temporal pattern of resource colonization by the European eight-toothed spruce bark beetle, Ips typographus, was quantified when weakly defended hosts (fallen trees) were attacked. In many of the replicates, colonization began with the insects rapidly scattering over the available surface and then randomly filling the gaps until a regular distribution was established, which resulted in a constant decrease in nearest-neighbour distances to a minimum below which attacks were not initiated. The scattered distribution of the first attacks suggested that the trees were only weakly defended. A minimal theoretical distance of 2.5 cm to the earlier settlers (corresponding to a density of 3.13 attacks dm−2) was calculated, but the attack density always remained lower, between 0.4 and 1.2 holes dm−2, according to our observations.

Attraction of Adults of Cyclocephala lunulata and Cyclocephala barrerai (Coleoptera: Scarabaeoidea: Melolonthidae) towards Bacteria Volatiles Isolated from Their Genital Chambers

“…The attraction of both male and female individuals of C. lunulata towards several of the VOCsB indicates that these are aggregation attractants [ 62 , 63 ]. Aggregation has been cited for species from Dynastinae; although the production of these chemical substances has been primarily associated with males [ 37 , 39 , 64 , 65 , 66 , 67 , 68 , 69 ], there are reports of female beetles emitting this type of pheromone as well [ 70 , 71 ]. In this study, the females of Cyclocephala produced the compounds, possibly with the assistance of bacteria associated with their genital chamber, and this coincides with other studies reporting that VOCsB act as aggregation attractants for their insect hosts [ 35 , 36 ].…”

Bark Beetle Population Dynamics in the Anthropocene: Challenges and Solutions

“…The beetle is associated with a diverse and dynamic community of bacterial and fungal symbionts suggested to contribute to the exhaustion of tree defenses [43,53], the detoxification of tree defenses [36,37,54], and nutrient provisioning [40]. Intraspecific competition is probably one of the major drivers of ESBB population dynamics [42]. However, such competition is relaxed in the population build-up phase due to large numbers of dead or weakened trees.…”

Priming of inducible defenses protects Norway spruce against tree‐killing bark beetles

“…Some of these inducible defenses, such as the formation of traumatic resin ducts, may take more than 2 weeks to form (Nagy, Franceschi, Solheim, Krekling, & Christiansen, 2000). By this time, the critical phase in the interaction between beetle attack and tree defenses may already be over (Toffin, Gabriel, Louis, Deneubourg, & Grégoire, 2018) and the beetles can successfully colonize and reproduce in many of the trees, with hightree mortality as a result. For these trees, the allocation of resources to defense comes too late and the trees suffer the ultimate fitness cost: death ( Figure 6).…”