(1) The locomotory responses which influence patch time allocation were investigated for the parasitoid, Nemeritis canescens (Grav.). Arrestment in patches was brought about by orthokinetic and klinotactic responses to a host-produced chemical and, indirectly, by oviposition. (2) A series of ovipositions increased patch time in a manner dependent on their rate rather than their absolute number. (3) A behavioural model is developed for the duration of a patch visit at different host densities. The aggregative response predicted is compared with experimental findings. (4) The effect of experience on patch time is investigated. Nemeritis avoids already visited patches. More long-term effects of conditioning on patch time are also suggested. (5) The manner in which Nemeritis forages is compared to that predicted by foraging models, and the nature of the parasitoid foraging process is discussed.

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Superparasitism is a widespread phenomenon in insect parasitoids and may be advantageous in certain circumstances. In this paper, we distinguish two kinds of superparasitism with different adaptive consequences for an ovipositing parasitoid: self superparasitism, where a parasitoid oviposits in a host in which she has previously oviposited, and conspecific superparasitism, where she oviposits in a host containing egg(s) from a previous oviposition by a conspecific. Our studies on Trichogramma evanescens Westwood show that females lay smaller clutches on parasitized than on healthy hosts but make the same overall allocation of males and females, although the sequence of sexes during oviposition differs. No difference was found between allocation in self and conspecific superparasitism. These results are discussed in the light of theories for progeny and sex allocation. Zusammenfassung Selbst‐ und Konspezifisch‐Superparasitismus durch die Schlupfwespe Trichogramma evanescens Superparasitismus ist ein weitverbreitetes Phänomen in Parasitoiden von Insekten und es kann unter bestimmten Umständen vorteilhaft sein. Diese Arbeit macht Unterschied zwischen zwei Formen von Superparasitismus, jeder mit seiner eigenen adaptiven Konsequenz für die parasitierende Schlupfwespe: Selbst‐Superparasitismus, wenn eine Schlupfwespe einen Wirt parasitiert die sie vorher selbst parasitiert hat, und Konspezifisch‐Superparasitismus, wenn sie einen Wirt parasitiert die vorher von einem Artgenossen parasitiert worden ist. Unsere Untersuchungen an Trichogramma evanescens zeigen dass die Weibchen auf den belegten Wirten kleinere Gelege haben als auf den gesunden Wirten, aber das Gesamtverhältnis von männlichen und weiblichen Brut ist dasselbe, obschon die Reihenfolge in den die Eier der beiden Geschlechte abgelegt werden sich underscheiden. Keinen Unterschied wurde gefunden zwischen die Ablegung während Selbst‐Superparasitismus und Konspezifisch‐Superparasitismus. Diese Ergebnisse werden diskutiert im Rahmen der Theorien für die Belegung des Wirtes in bezug auf die Anzahl und das Geschlecht der Eier.

The locomotory response of walking parasitoids to a contact chemical from their host is reviewed, and in particular the response of the ichneumonid, Nemeritis cunescens Grav., to the mandibular gland secretion of its host, Plodia interpunctellu Hubn., is investigated. In response to the presentation of the host chemical on a surface, a walking Nemeritis exhibits a complex orthokinetic response involving stopping, walking at a reduced speed and probing with the ovipositor. In response to the removal of the chemical following presentation, as would occur when the insect left the edge of the chemical patch, the wasp exhibits a klinotactic response which directs it back to the patch at an average angle of 157" relative to the orientation at the moment of stimulation. This turning-back response at the patch edge greatly increases the time spent on a patch of contact chemical. The waning of this response determines when a host patch is abandoned. This waning is retarded by increasing the concentration of contact chemical on the patch and by oviposition while on the patch. w ch c, % 2 9 B k T References Ahmad, T. (1936) The influence of ecological factors o n the Mediterranean Flour Moth, Ephestiu kuhniella and its parasite, Nemeritis canescens. Journal of Animal Ecology, 5 , 61-93.

1. Theoretical models by Hamilton and others predict that the optimal sex ratio (proportionmdes) for a species should decrease with increasing levels of sib-mating. A test of this hypothesis is made for members of the parasitoid family Scelionidae.2. Scelionid wasps differ in levels of sib-mating as a consequence of differences in host egg mass size, male-male aggression and female-female aggression.3. Sex ratios for thirty-one species vary with inferred changes in sib-mating as predicted by the models. 4. Interspecific differences in sex ratio may represent different fvted strategies or different manifestations of a single variable strategy (sex ratio game) in response to parasitoid-produced stimuli. 5. Males and females are assigned non-randomly to egg masses in a manner which ensures mixed broods in the proper proportions for a particular species.

1. A field study was made of foraging time allocation by a population of parasitic wasps, Diadegma spp. (Ichneumonidae), to plants containing different densities of their hosts, the caterpillars of Pkutella xylostelka (L.). 2.The parasitoid population exhibited a clear aggregative response, spending more total time on higher density patches, which probably resulted from wasps making more and longer visits to these densities.3. Despite this aggregation, positive density dependent parasitism was not found. The functional response of the Diadegma population exhibited an upper asymptote at high host densities, probably due to an increase in the proportion of time spent handling hosts, which countered the effect of aggregation.4. While Diadegma may select and forage preferentially on plants with higher host density, they d o not exhibit the tendency, predicted by some optional foraging models, to exploit progressively less profitable plants during a foraging bout. Some factors affecting patterns of parasitoid foraging in the field are discussed.

SUMMARYThis review begins with a description of the parasitoid life-style and the ecological and evolutionary factors which generate the remarkable diversity of insect parasitoids. We then describe the various ways that parasitoids have been used in the biological control of insect pests, and survey their success to date. The use of parasitoids remains largely an art, aided by past experience of success and failure. A more fundamental approach, involving basic research and theory, has not as yet contributed significantly to practical biological control. We explore the potential for such a science of parasitoid use and review basic research on parasitoid ecology and evolution which is of particular relevance to biological control. Mathematical models are used to identify and examine those parasitoid and host attributes which lead to successful biological control. Factors such as parasitoid foraging behaviour, fecundity, larval survival and sex ratio are shown to be important in influencing the depression of host populations and/or the stability of host–parasitoid interactions after depression. Multiple release is discussed and a model for inundative release of parasitoids is explored.