Abstract:
Obligate hyperparasitoids are widely considered an important ecological disturbance
to biological control of insect pests, as they develop at the expense of primary parasitoids.
However, supporting evidence is largely derived from direct trophic interactions in simple food
webs. Yet, a multitude of insect pest populations simultaneously support development of several
primary parasitoid species in horticultural and natural systems. Since primary parasitoid species
in a community can differ in vulnerability to obligate hyperparasitoids, it is desirable to establish
if the invulnerable primary parasitoids can take advantage of reduced competition from affected
species by increasing their contribution to total primary parasitism levels thereby mitigating
effects of hyperparasitism on biological control. To investigate this question, populations of the
diamondback moth, Plutella xylostella (Linnaeus) (Plutellidae), its primary parasitoids and
hyperparasitoids were monitored on unsprayed cabbage plots at weekly intervals over six
consecutive years. Cotesia vestalis (Haliday) (Braconidae), a dominant primary parasitoid in this
system, was a secondary host to three obligate hyperparasitoids: Mesochorus sp.
(Ichneumonidae), Eurytoma sp. (Eurytomidae) and Pteromalus sp. (Pteromalidae). The higher
efficiency of C. vestalis in utilizing younger host larvae at lower hyperparasitism levels limited
host availability to other major primary parasitoids. But, as hyperparasitism levels increased and
its populations declined, populations of Oomyzus sokolowskii (Kurdjumov) (Eulophidae) and
Diadromus collaris (Gravenhorst) (Ichneumonidae) increased significantly as they parasitized a
greater proportion of available hosts. As a consequence, the impact of hyperparasitoids did not
result in trophic cascades, as their impact on total primary parasitism levels and infestation levels
was insignificant. This study shows that primary parasitoid species that are invulnerable to
hyper-parasitism can take over the function of vulnerable ones in communities where
interspecific interactions among species are strong. Thus, an approach that considers both direct
and indirect effects of hyperparasitoids in primary parasitoid communities improves our
understanding of the net impact of hyperparasitism on biological control of insect pests.
