Rearing Thyridanthrax fenestratus prepupae

Thyridanthrax fenestratus (Fallén, 1814) is a bombyliid with poorly understood biology. It was recently shown to locally but frequently parasitize Pemphredon fabricii (M. Müller, 1911) (Hymenoptera: Crabronidae), a crabronid wasp that abundantly nests in old Lipara-induced galls on the common reed Phragmites australis (Cav.) Trin. ex Steud., 1840. The parasitism modes in Bombyliidae and Thyridanthrax spp. are not uniform. Here, we report that Th. fenestratus switches facultatively between killing the host almost immediately (idiobiont strategy) and killing the host at a later developmental stage (koinobiont strategy). We document the koinobiont parasitoid strategy for a series of Th. fenestratus larvae parasitizing P. fabricii. We found that a significant portion of Th. fenestratus larvae spend winter as young larvae and start feeding on fully developed and defecated prepupae of P. fabricii only after the end of cold-induced winter diapause. The time needed for the development of Th. fenestratus larvae exceeds several times the time needed for pupation of P. fabricii prepupae; the parasitized prepupae, therefore, remain paralyzed until the parasitic larva completes feeding. Fungicides, which alter the pupation of the host larva, seem to have negligible effects on Th. fenestratus larvae. The ability to switch between the two parasitism strategies has already been reported for several Anthrax spp., though the ability to block the host in the defecated prepupa stage and prevent its pupation following cold-induced diapause is herein reported for the first time.