Host selection is not expressed for underground hosts in a pupal parasitoid

Abstract

Host-parasitoid dynamics is critically influenced by variability in parasitism risk among hosts, and the host preference of the parasitoid is one of the factors causing such variability. This study examined the host preferences of the pupal parasitoid Dirhinus giffardii (Silvestri) (Hymenoptera: Chalcididae) on pupae of the oriental fruit fly Bactrocera dorsalis (Hendel) (Diptera: Tephritidae) in the laboratory. Dirhinus giffardii prefers large hosts and avoids hosts nearing eclosion when hosts are exposed to the parasitoid. This study reexamined the preferences when host pupae are in the soil (simulated by peat moss) because tephritid hosts, including B. dorsalis, pupate in the soil. Results showed that D. giffardii preferentially parasitized large hosts when hosts were exposed, but host sizedependent parasitism was absent when hosts were in the soil. Preference on host age was absent even when hosts were exposed, contradicting the results of previous studies. The discrepancy was examined by performing a follow-up experiment to test the hypothesis that D. giffardii cannot distinguish host age, but eggs that are laid on older hosts cannot develop successfully. The parasitoid successfully hatched regardless of host age, and the hypothesis was rejected. Potential explanations for the discrepancy are discussed.

Digging and burying of hosts by a pupal parasitoid

“…The present study describes a host-handling behaviour of D. giffardii in which it excavates underground hosts to the ground surface and buries them back in the ground (see a movie in Video S1). This behaviour, which can be observed for hosts located at relatively shallow depths (<1 cm), may reflect an adaptation to evaluate host quality, considering that D. giffardii is capable of evaluating hosts out of the soil (Okuyama, 2021). The pupation depths of tephritid flies (common hosts of D. giffardii) vary greatly from 0 to several centimetres, while large proportions of them pupate in <1-cm depth (Hodgson et al, 1998;Jackson et al, 1998;Dimou et al, 2003;Okuyama, 2019).…”

“…Pupae used in the experiments were at least 3 days old since pupation. The parasitoid did not exhibit host selection based on host age (Okuyama, 2021), but the body of the host in the puparium was not fully developed in young hosts, which made dissection (as described below) of the young hosts difficult.…”

“…The hypothesis for excavation was tested by examining whether the host excavation behaviour of D. giffardii was influenced by host quality, that is, large, small, and empty hosts. Dirhinus giffardii preferentially parasitised large hosts when the hosts were on the ground (Okuyama, 2021); thus, if the parasitoid could not evaluate the underground hosts, the excavation rate was not influenced by the host type. Empty hosts (i.e., puparia left after eclosion) were included as clearly unsuitable hosts, and the excavation of empty puparia would support the hypothesis that D. giffardii cannot evaluate the underground hosts.…”

“…The pupal parasitoid Dirhinus giffardii (Silvestri) (Hymenoptera: Chalcididae) prefers large hosts (Wang & Messing, 2004a;Okuyama, 2021). Although superparasitism is common in D. giffardii, it prefers to parasitise unparasitised hosts over parasitised hosts when the two types of hosts are available simultaneously (Okuyama, 2019).…”

“…However, these preferences have been described in simple experimental set-ups in which the hosts were directly accessible to the parasitoids (i.e., the hosts were located outside the soil). A recent study demonstrated that the host selection based on the host size is expressed only when the hosts are out of the soil, but not expressed when the hosts are in the soil (Okuyama, 2021). Because hosts naturally occur in the soil (Hodgson et al, 1998;Alyokhin et al, 2001), these results may indicate that the host selection behaviour is not exhibited in ecologically realistic context.…”