Life history and larval morphology of Monacon (Hymenoptera: Perilampidae), parasitoids of ambrosia beetles (Coleoptera: Platypodidae)

Abstract

Species of Monacon are distributed from Africa to New Guinea and parasitize ambrosia beetles of economic significance to tropical forestry. This study, based on fieldwork in Papua New Guinea and in Seram, Indonesia, provides the first detailed examination of the interaction between these parasitoids and their hosts; Monacon robertsi is a solitary, monophagous ectoparasitoid of the pupa of Crossotarsus barbatus. The phenology of the parasitoid and its host are summarized with life-history diagrams, and information is presented on oviposition, distribution of parasitoids within the gallery system constructed by the host, behaviour of adults both within and outside the gallery system, and sex ratios. A sex pheromone produced by male C. barbatus during the initiation of a gallery system both reinforces host specificity and synchronizes oviposition by Monacon roberstsi with mating of the host. This results in the parasitoid attacking only the first few of many host egg batches, which reduces the overall parasitism rate. Monacon roberstsi eggs are strongly sculptured, suggesting plastron respiration, and are laid on the bark in the vicinity of the entrance to ambrosia beetle galleries. Development is hypermetamorphic; the first instar is a heavily sclerotized planidium that locates and attaches to the host, and subsequent larval instars are grublike and hymenopteriform. The planidia experience high mortality rates while transferring to successive host instars and only a single planidium remains when metamorphosis and feeding begin, which coincides with host pupation. The immature stages of M. robertsi are described and illustrated using both light and scanning electron microscopy and are compared with those of other genera of Perilampidae and with Eucharitidae. The potential of Monacon species as agents of biological control of economically important species of ambrosia beetles is discussed and judged to be poor.