The role of floral resource subsidies in structuring a four tropic-level parasitoid/host system

Abstract

In this study, conservation biological control principles were explored to determine whether the efficacy of Aphidius ervi Haliday (Hymenoptera: Braconidae), a parasitoid of the pea aphid Acyrthosiphon pisum Harris (Homoptera: Aphididae) would be reduced in the presence of the hyperparasitoid Dendrocerus aphidum Rondani (Hymenoptera: Megaspilidae) interacting with floral resource subsidies. Four flowering plant species were selected for laboratory experiments to assess the best plant species to increase the longevity and fecundity of A. ervi and D. aphidum. Buckwheat (F agopyrum esculentum Moench cv. Katowase), coriander (Coriandrum sativum L. cv. Slowbolt), alyssum (Lobularia maritima L. cv. Carpet of Snow) and phacelia (Phacelia tanacetifolia Bentham cv. Balo) significantly increased the longevity and fecundity of A. ervi and the hyperparasitoid D. aphidum. Also, the idiobiont ecto-hyperparasitoid Asaphes vulgaris Walker (Hymenoptera: Pteromalidae), the koinobiont endo-hyperparasitoids Alloxysta victrix Westwood (Hymenoptera: Charipidae), Syrphophagus aphidivorus Mayr (Hymenoptera: Encyrtidae) and Phaenoglyphis villosa Hartig (Hymenoptera: Charipidae) all used buckwheat nectar and this increased their longevity.

Longevity of the parasitoid and the hyperparasitoids was significantly increased by buckwheat compared with other flowering plants.

Buckwheat was deployed in field cages to assess percentage parasitism, percentage hyperparasitism and aphid density. The highest percentage parasitism and hyperparasitism occurred in cages containing buckwheat. The aphid density was lowest in field cages containing buckwheat.

The behaviour of fed and unfed A. ervi and D. aphidum in the laboratory was studied to determine if searching behaviour could be enhanced after nectar feeding. A. ervi and D. Aphidum spent a considerable time displaying characteristic behaviours related to active searching when buckwheat flowers were provided. This suggests that well-fed parasitoids and hyperparasitoids search actively for hosts once they have had nectar access.