8 The Use of Unisexual Wasps in

Biological Control

R. Stouthamer*

Laboratory of Entomology, Wageningen University, PO Box 8031, 6700 EH

Wageningen, The Netherlands

Introduction

Biological control workers have long been
fascinated by the phenomenon of unisexual
reproduction. Timberlake and Clausen (1924)
explained the possible advantage of a uni-
sexual parasitoid over a form reproducing
sexually by calculating the population
increase of the sexual form compared with
the unisexual form. The difference in popula-
tion growth rate in such calculations can be
astonishing (Fig. 8.1). In such calculations we
assume that the unisexual wasps produce
equal numbers of offspring to the sexual

forms and therefore over time the unisexual

population should outcompete the sexual

form since all the offspring of the unisexual

form consist of females only.

The concept of choosing a unisexual mode

of reproduction for the wasps to be used in

biological control is interesting; however, up

until now, this can only be applied in those

cases where two modes of reproduction are

present in a species. In the not too distant

future, it may be possible to render sexual

forms unisexual by infection with parthe-

nogenesis-inducing (PI) microorganisms.

Natural infection with these bacteria is the

*Present address: Department of Entomology, University of California at Riverside, Riverside,
CA 92521, USA.

© CAB International 2003. Quality Control and Production of Biological Control Agents:
Theory and Testing Procedures (ed. J.C. van Lenteren) 93

Abstract

Unisexual reproduction has long been seen as a clear advantage for wasps to be applied in biological
control projects. The discovery that the mode of reproduction in parasitoid wasps may be manipulated
from sexual to unisexual and vice versa will allow biocontrol workers to test the advantage of either
mode of reproduction for biological pest control. Here a review is presented of the cases of unisexual
reproduction found in parasitoid wasps. Unisexual reproduction is not rare among parasitoids; at least
150 cases of unisexual reproduction have been reported. The literature is reviewed for cases where
both unisexual and sexual forms are used in the same control project to determine if the theoretical
advantage of unisexual reproduction indeed materializes. Few cases can be used to test the presumed
advantage of unisexuals. Some evidence is found for two advantages of unisexual reproduction: uni-
sexuals are cheaper to produce in mass rearing than sexuals, and in classical biocontrol projects they
are more easily established.