Parasitoids and predators of herbivores have evolved and function within a multitrophic context. Consequently, their physiology and behavior are in­ fluenced by elements from other trophic levels such as their herbivore victim (second trophic level) and its plant food (first trophic level) (126) . Natural enemies base their foraging decisions on information from these different trophic levels, and chemical information plays an important role. This review is restricted to the ecology of chemical information from the first and second trophic levels. The importance of so-called infochemicals, a subcategory of semiochemicals, in foraging by parasitoids and predators has been well documented (e.g. reviewed in 31, 78, Il l , 183, 185) , and we do not intend to repeat the details. But because of a lack of testable hypotheses, all this research is conducted rather haphazardly: the total puzzle of infochemical use has not been solved for any natural enemy species. Here we approach the use of infochemicals by natural enemies from an evolutionary and ecological standpoint. Our basic concept is that information from the first and second trophic levels differs in availability and in reliability, a difference that shapes the way infochemicals are used by a species. We generate hypotheses on (a)

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Ecology of infochemical use by natural enemies in a tritrophic context.

Part 1 The importance of blood-sucking insects. Part 2 The evolution of the blood-sucking habit: prolonged close association with vertebrates morphological pre-adaptation for piercing. Part 3 Feeding preferences of blood-sucking insects: host choice host choice and species complexes. Part 4 Location of the host: the behavioural framework of host location appetitive searching activation and orientation attraction movement between hosts. Part 5 Ingestion of the blood meal: vertebrate haemostasis insect anti-haemostatic factors probing stimulants phagostimulants mouthparts blood intake. Part 6 Managing the blood meal: midgut anatomy the blood meal gonotrophic concordance nutrition host hormones in the blood meal partitioning of resources from the blood meal autogeny. Part 7 Host - insect interactions: insect distribution on the surface of the host morphological specializations for life on the host host immune responses to insect salivary secretions behavioural defences of the host density dependent effects on feeding success. Part 8 Transmission of parasites by blood-sucking insects: transmission routes specificity in vector-parasite relationships origin of vector parasite relationships.

Biology of Blood-Sucking Insects

Predictive theory on how plant diversity promotes herbivore suppression through movement patterns, host associations, and predation promises a potential alternative to pesticide-intensive monoculture crop production We used meta-analysis on 552 experiments in 45 articles published over the last 10 years to test if plant diversification schemes reduce herbivores and/or increase the natural enemies of herbivores as predicted by associational resistance hypotheses, the enemies hypothesis, and attraction and repellency model applications in agriculture We found extensive support for these models with intercropping schemes, inclusion of flowering plants, and use of plants that repel herbivores or attract them away from the crop Overall, herbivore suppression, enemy enhancement, and crop damage suppression effects were significantly stronger on diversified crops than on crops with none or fewer associated plant species However, a relatively small, but significantly negative, mean effect size for crop yield indicated that pest-suppressive diversification schemes interfered with production, in part because of reducing densities of the main crop by replacing it with intercrops or non-crop plants This first use of meta-analysis to evaluate the effects of diversification schemes, a potentially more powerful tool than tallies of significant positive and negative outcomes (vote-counting), revealed stronger overall effects on all parameters measured compared to previous reviews Our analysis of the same articles used in a recent review facilitates comparisons of vote-counting and meta-analysis, and shows that pronounced results of the meta-analysis are not well explained by a reduction in articles that met its stricter criteria Rather, compared to outcome counts, effect sizes were rarely neutral (equal to zero), and a mean effect size value for mixed outcomes could be calculated Problematic statistical properties of vote-counting were avoided with meta-analysis, thus providing a more precise test of the hypotheses The unambiguous and encouraging results from this meta-analysis of previous research should motivate ecologists to conduct more mechanistic experiments to improve the odds of designing effective crop diversification schemes for improved pest regulation and enhanced crop yield

Does plant diversity benefit agroecosystems? A synthetic review

Behavioral adaptations to pathogens and parasites: Five strategies

Host‐feeding strategies in hymenopteran parasitoids

. 

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 Plutella xylostella (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 do 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.

Aggregation in field parasitoid populations: foraging time allocation by a population of Diadegma (Hymenoptera, Ichneumonidae)

. 1. Theoretical models by Hamilton and others predict that the optimal sex ratio (proportion males) 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.

Sib-mating and sex ratio strategies in scelionid wasps

. The locomotory response of walking parasitoids to a contact chemical from their host is reviewed, and in particular the response of the ichneumonid, Nemeritis canescens Grav., to the mandibular gland secretion of its host, Plodia interpunctella Hubn., is investigated. In response to the presentation of the host chemical on a surface, a walking Nemeritis exhibits a complex ortho-kinetic 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 opposition while on the patch.

Arrestment responses of the parasitoid, Nemeritis canescens, to a contact chemical produced by its host, Plodia interpunctella

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 Phanomen in Parasitoiden von Insekten und es kann unter bestimmten Umstanden vorteilhaft sein. Diese Arbeit macht Unterschied zwischen zwei Formen von Superparasitismus, jeder mit seiner eigenen adaptiven Konsequenz fur 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 Gesamtverhaltnis von mannlichen 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 wahrend Selbst-Superparasitismus und Konspezifisch-Superparasitismus. Diese Ergebnisse werden diskutiert im Rahmen der Theorien fur die Belegung des Wirtes in bezug auf die Anzahl und das Geschlecht der Eier.

Self and conspecific superparasitism by the egg parasitoid Trichogramma evanescens

The evolution of close vertebrate associations has occurred in seven orders of insects, resulting in a great diversity of interactions which range from commensalism to true parasitism. The evolution of each taxon of vertebrate associates is discussed in turn, some new ideas on the development of certain groups are presented and, on a broader scale, a general model for the evolution of ectoparasitic insects is proposed. It argues that all vertebrate associates have evolved along one of two macroevolutionary pathways which differ only in the sequencing of adaptations facilitating host association and host feeding. These pathways lead to parasite types which differ greatly in their life history and intimacy of host association.



Some microevolutionary processes influencing the diversification of ectoparasites are discussed, in particular the process of insect/vertebrate coevolution and the forms this may take. Host specificity, one consequence of coevolution, is recognised as an important factor influencing the structure of ectoparasite communities, and a hypothesis is presented that competition between ectoparasite species, mediated by host defensive responses, is also important in determining community structure.

Jeffrey K. Waage

Papers

Aggregation in field parasitoid populations: foraging time allocation by a population of Diadegma (Hymenoptera, Ichneumonidae)

Sib-mating and sex ratio strategies in scelionid wasps

Arrestment responses of the parasitoid, Nemeritis canescens, to a contact chemical produced by its host, Plodia interpunctella

Self and conspecific superparasitism by the egg parasitoid Trichogramma evanescens

The evolution of insect/vertebrate associations