Notes on the biology of ambrosia beetles of the genus Trachyostus Schedl (Coleoptera: Platypodidae) in West Africa
Authors: Roberts Hywell 
Published: July 2009

The biology of three West African ambrosia beetles, Trachyostus aterrimus (Schauf.), T. ghanaensis Schedi and T. schaufussi schaufussi (Strohm.) is described on the basis of studies made in China and Nigeria, and notes are given on the six other species and subspecies of Trachyostus that have been taken in West Africa.

All species of Trachyostus are confined to forest areas in West Africa. some of the species extend into montane and swamp forest, but the genus is most abundant in lowland rain forest. It is suggested that susceptibility to desiccation of the young larvae and of the ambrosia fungus is the principal factor in limiting Trachyostus species to forest areas. Two species, T. ghanaensis and T. interstitialis Schedl, are confined to the forests between Sierra Leone and Ghana, but the remaining species are found throughout the lowland forest of west Africa.

Host selection by species of Trachyostus depends mainly on the taxonomic status of the host, the condition of the tree, and the time of year. The genus as a whole is considered very selective. T. ghanaesis attacks living trees of only one host species, selecting individual trees and favouring the most vigorous. All other species of Trachyostus normally attack felled trees of a wide range of host species, but show preferences for those of a small number of plant genera. Attack on living trees by species other than T, ghanaensis occurs in the dry season. It is suggested that T. ghanaensis is sensitive to subtle changes in tree condition, while the remaining species require a more drastic change before attacking such trees.

The process of nest initiation and establishment is described for ome species. Species of Trachyostus differ from other tropical Platypodids in that the length of time elapsing between nest initiation and the emergence of the first young adults is exceptionally long. This is thought to be mainly due to the ambrosia fungus requiring a long time to establish itself on the gallery walls of the nest.

All species of Trachyostus are active in daylight hours. Emergence of young adults from the nest occurs only in daylight and is dependent on a drying atmosphere. Falling light intensities probably contribute towards ending emergence in the late afternoon. The thresholds of humidity and illumination for emergence of T. schaufussi schaufussi appear to be higher than those of T. aterrimus.

Nests of T. aterrrimus and T. schaufussi schaufussi remain functional for about nine months, but those of T. ghanaensis can still be active after four years. The protracted nest duration of T. ghanaensis is apparently a consequence of the habit of this species of attacking living host trees in which the rate of development seems to be slowed down. Only a small proportion of the nests initiated by species of Trachyostus are successful. In T, ghanaensis this high failure rate is attributable to failure of the ambbrosia fungus, but in the other species insect nest-associates and predators are apparently the main causes.

The habits and life-history of T. ghanacnsis can only be compared with those of one other Platypodid, Dendroplatypus impar (Schedl), a Malaysian species. Both species attack only living trees, and show a high degree of host selection. The intensity of attack on the host tree by these species is very low, and the life-cycle of each seems to take a long time for completion.