Antifungal actinomycetes associated with the pine bark beetle, Orthotomicus erosus, in South Africa

Abstract

Actinomycete bacteria are often associated with insects that have a mutualistic association with fungi. These bacteria are believed to be important to this insect–fungus association as they produce antibiotics that exclude other saprophytic fungi from the immediate environment. The aim of this study was to investigate the presence of potentially protective actinomycetes associated with Orthotomicus erosus, an alien invasive pine bark beetle, in South Africa. This bark beetle and its relatives have an association with Ophiostomatales species which are often the only fungi found in the bark beetle galleries. We hypothesised that antibiotic-producing actinomycetes could be responsible for the paucity of other fungi in the galleries by producing compounds to which the Ophiostoma spp. are tolerant. Several actinomycetes in the genus Streptomyces and one Gordonia sp. were isolated from the beetle. Interestingly, most isolates were from the same species as actinomycetes associated with other pine-infesting insects from other parts of the world, including bark beetles and the woodwasp Sirex noctilio. Most actinomycetes isolated had strong antifungal properties against the selected test fungi, including Ophiostoma ips, which is the most common fungal symbiont of Orthotomicus erosus. Although the actinomycetes did not benefit Ophiostoma ips and the hypothesis was not supported, their sporadic association with Orthotomicus erosus suggests that they could have some impact on the composition of the fungal communities present in the bark beetle galleries, which is at present poorly understood.

Outbreak of Orthotomicus erosus (Coleoptera, Curculionidae) on Aleppo Pine in the Mediterranean Region in Croatia

“…Furthermore, O. erosus is associated with several ophiostomatoid species [13,59]. Some ophiostomatoid fungal species in association with bark beetles show increased virulence and it is assumed that they help their vectors, bark beetles, to break the defensive mechanisms of the host plant.…”

“…It has been introduced to Fiji, South Africa, Swaziland [10] and the USA [11]. It can occur in association with other bark beetle species (Pityogenes calcaratus, Tomicus destruens [12]) and in symbiotic relationship with blue stain fungi (Ophiostomatidae) [13]. It is considered as a secondary pest found on recently dead or felled trees, but can also attack weakened living trees (e.g.…”

Cycloheximide-ProducingStreptomycesAssociated withXyleborinus saxeseniiandXyleborus affinisFungus-Farming Ambrosia Beetles

“…Interestingly, species in the fungal order Ophiostomatotales (including Raffaelea spp. ), are known to largely be resistant to cycloheximide [28, 78]. Although not determined here, X. affinis also cultivates a cycloheximide-insensitive Raffaelea sp.…”

“…Its ubiquity suggests that XylebKG-1 may be a defensive mutualist of these ambrosia beetles that inhibits the growth of all but a few fungi, including its mutualistic fungal food source. Future studies should include natural nests collected from a wider range of species and geographies to establish the breadth and prevalence of XylebKG-1 in bark and ambrosia beetle nests [28]. These studies should also include greater phylogenetic power as 16S r RNA gene analyses is not sufficient to resolve species within the S. griseus clade [79].…”

“…To help defend the cultivar from this parasite the ants use actinobacterial symbionts that produce antibiotics [23–26]. A similar type of defensive symbiosis has been shown in the fungus-associated bark beetle Dendroctonus frontalis [27], and has been further suggested in the Mediterranean Pine Engraver bark beetle, Orthotomicus erosus [28], as well as fungus-growing termites [29]. Beyond defending fungal mutualists in agricultural associations, Actinobacteria are well adapted for insect dispersal (e.g.…”