Host finding by Rhyssa persuasoria (L.), an ichneumonid parasite of siricid woodwasps

Abstract

1. 1. Exploratory behaviour over the bark of infested timber comprises a survey of the surface with the antennae, sustained antennal activity (palpating) in areas of special interest, and ovipositor probing to a depth of 1 to 3 mm into the wood. 2. 2. Drilling to a depth of more than 5 mm is elicited by stimuli emanating from within the timber, with surface features such as cracks in bark and wood, influencing the point of ovipositor insertion. Siricid oviposition punctures do not attract the parasite or stimulate drilling. 3. 3. Antagonistic behaviour between females results in the acquisition of loosely defined territories. 4. 4. The majority of drills are made in response to siricid tunnels and are not random, but the parasite does not apparently follow concentration gradients along individual tunnels. 5. 5. Females are able to detect and parasitize dead hosts in logs, demonstrating that sound is not a necessary stimulus for drilling, and is not essential for host detection. 6. 6. The majority of drills are elicited by siricid frass; washed siricid larvae alone stimulate no host-searching response. The frass nearest the host is most attractive, and the attraction of the frass decreases with increase in distance from the host. Wet frass elicits more drilling than dry frass. 7. 7. Cultured fungal symbionts 3 to 4 months old are the most attractive, coinciding with maturation of host larvae in timber under similar conditions. There is no evidence that the species of tree influences attraction, or of pre-imaginal conditioning of the parasite to its host's symbiont. Comparison of the symbiont with several common woodland fungi demonstrated the specific attractiveness of the symbiont. 8. 8. Comparison of named Stereum and Amylostereum species with the symbionts demonstrated a marked preference for the S. juvencus symbiont which is probably closely related to Amylostereum species. 9. 9. Aqueous and ethanolic extracts of frass and fungus stimulate drilling. 10. 10. When siricid larvae are maintained on frass or fungus medium, or when siricid larvalgut extract, and chitinase are incorporated into the medium, the attractiveness of the substrate is increased.