Penetration ability of Holepyris sylvanidis into the feeding substrate of its host Tribolium confusum

Abstract

The bethylid wasp Holepyris sylvanidis (Brethes, 1913) is an antagonist of the confused flour beetle Tribolium confusum Jacquelin du Val 1868, a severe pest in the food processing industry and in grain products, primarily in flour mills and bakeries. Females of the larval ectoparasitoid H. sylvanidis have to detect hosts that feed in different depths inside stored products like flour or grist. This study addresses the questions (1) whether successful host finding by H. sylvanidis is dependent on the location of Tribolium larvae in the substrate and (2) whether the type of substrate affects host finding. In laboratory experiments, T. confusum larvae in a Petri dish accessible to the wasps were placed 1, 2, 4 or 8 cm deep in either fine or coarse ground wholemeal grist of wheat (main particle size: < 0.2 mm or 1.4 – 3 mm) in fifteen replicates per substrate and depth. Parasitoids were released onto the surface of the substrate. Tribolium confusum larvae were not able to leave the Petri dish, however they could be pulled outside into the grist by H. sylvanidis . Within the behavioural sequence of parasitisation, pulling away of host larvae is the typical behaviour preceding oviposition. In order to determine host finding success by the parasitoid, the number of missing host larvae was assessed 2 wks after release of the wasps. In fine grist larvae were attacked down to 4 cm depth; however, larvae placed deeper (8 cm) were not found anymore. In contrast, host larvae in coarse grist were still detected at 8 cm depth. The results suggest that host finding by H. sylvanidis is hindered by decrease in particle size of the substrate. Nevertheless, H. sylvanidis may be considered a promising candidate for biological control of T. confusum larvae feeding in coarse grist and in thin layers of fine grist. Keywords : Holepyris sylvanidis, Tribolium confusum , Biological control, Penetration ability, Host finding