Strategies Involved in the Location of Hosts by the ParasitoidAphidius erviHaliday (Hymenoptera: Braconidae: Aphidiinae)

Publisher Summary Ants, plants, and homoptera interact in a wide variety of ways. Each participant in these interactions can affect the other two either directly or indirectly by affecting the third participant. The outcome of a particular interaction may also be affected by other species interacting with one or more of the participants and by abiotic factors such as fire, soil, and climate. The quantitative significance and the degree of specificity, obligacy, and specialization for each participant vary widely between interactions. There are many interactions between ants and plants that do not involve Homoptera at all, the direct twofold ant-plant components of ant-plant-homopteran interactions are generally negligible unless the plants also possess extrafloral nectaries or ant-attractant food bodies. In direct interactions between Homoptera and plants, the homopterans gain food, habitat, and sometimes allelochemicals from the plants, while the plants suffer tissue damage, loss of metabolites, and most importantly, increased incidence of infection by homopteran-transmitted microbial pathogens: plants therefore employ a range of physical and chemical defenses against Homoptera. The direct interactions between ants and Homoptera fall into two main classes: either the ants prey on the homopterans, or they tend and/or defend them against predators and parasites. Some ants may prey on some species of homopteran and tend others. In either case, the homopterans provide food for the ants, either as prey or by exuding honeydew, which the ants collect. The indirect components of the overall threefold interaction may be as or more significant to the participants than the direct twofold components.

Ant-Plant-Homopteran Interactions

There is increasing evidence that chemical cues play a pivotal role in host selection by the natural enemies of aphids. We use Vinson's (1976) division of the host selection process into habitat location, host location and host acceptance for both parasitoids and predators and review what is known about the role of semiochemicals in aphid selection by natural enemies. For habitat location (i.e. detection of the host plant), volatiles emitted by plants after aphid attack have been described for a number of plant-aphid inter- actions. These synomones indicate not only the presence of an aphid host plant to the predator or parasitoid, but also the presence of aphids. Volatiles emitted from undamaged host plants are often attractive to aphid parasitoids, but less so for predators. Host location by the natural enemy on the food plant is guided by semiochemicals that mostly originate from the aphids, in particular aphid alarm pheromone, honeydew, or the smell of the aphid itself. Host acceptance is guided by contact chemicals for both predators and parasi- toids. In parasitoids, host recognition may be based on visual cues or on contact chemicals on the aphid's cuticle, whereas host acceptance is ultimately based on as yet unknown substances within the aphid's hemolymph. While it appears that many predators and parasitoids are attracted to the same semiochemicals, synergistic and antagonistic interactions among chemical substances have only rarely been investigated. More research into model systems is needed, not only to identify important semiochemicals, but also to determine their range of attraction. Recent progress in the development of analytical techniques has created new opportunities to improve our understanding of the chemical ecology of aphid-natural enemy interactions in the coming years.

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Chemical cues mediating aphid location by natural enemies

Threshold dosage and type of dispersive response by aphids to the alarm pheromone, (E)-s-farnesene, were determined for fourteen species in the subfamilies Aphidinae and Chaitophorinae. Dosage required to disperse 50% of the most and least sensitive species was 0.02 ng and 100 ng, respectively. Some species dispersed at low dosages by walking and at high dosages by falling; others dispersed only by walking regardless of dosage. Sensitivity to alarm pheromone and type of dispersive response are interrelated with aggregation density and whether the aphid is myrmecophilous.



RESUME

COMPARAISON DES REPONSES DE DIVERS APHIDES A LA PHEROMONE D'ALARME (E)-β − FARNESENE (EBF)



Quatorze especes d'Aphides appartenant aux sous-familles des Aphidinae et Chaitophorinae ont ete testees pour leur reponse a la pheromone d'alarme EBF, sur la base de la dose limite efficace et du type de reponse par dispersion des individus. Les doses d'EBF necessaires pour disperser 50% des individus chez l'espece la plus sensible sont de 0,02 ng, contre 100 ng pour l'espece la moins sensible. Certaines especes se dispersaient en marchant pour de faibles doses et en se laissant tomber pour de fortes doses; d'autres se dispersaient seulement en marchant, quelle que soit la dose. La sensibilite et le mode de reponse a la pheromone sont en relation avec la densite des groupements et avec le fait que l'espece est ou non myrmecophile.

COMPARATIVE RESPONSE OF APHIDS TO THE ALARM PHEROMONE, (E)‐ß‐ FARNESENE

The oviposition behaviour of the aphid parasitoid Aphidius ervi Haliday is influenced by both chemical and physical cues. Oviposition attack responses were elicited by paint pigments sealed into the tip of a glass capillary tube. Parasitoids reacted to yellow pigments with repeated oviposition attack responses, but they did not react to green pigments. The spectrum of reflected light from the yellow pigments was very similar to that from the `green' natural host Acyrthosiphon pisum (Harris), with a high proportion of the total radiation energy being emitted in the yellow-orange wavebands (580-660 nm). Pea aphid cornicle secretion also elicited oviposition attack responses, which were not exclusively induced by its pale yellow-green colour. In fact, the oviposition attack response to capillary tips coated with cornicle secretion remained evident under red light conditions, which, in contrast, nearly completely suppressed the response to yellow pigments. Chemical compounds from cornicle secretion do not appear to be involved in parasitoid orientation, even though they stimulate intense oviposition attack responses. Olfactometer experiments showed that the putative kairomone involved acts only at very short range or on contact. Host exuviae, which also elicited strong and persistent oviposition reactions from A. ervi females, appear to be a good alternative source of ovipositional kairomone(s). This work confirms the existence of an aphid cuticular kairomone.

Physical and chemical cues influencing the oviposition behaviour of Aphidius ervi.

Cornicle secretions have been reported to serve multiple functions in aphids. Of particular interest are reports that these sticky secretions serve to defend aphids against attack by predators and parasites. Dixon (1958) observed that the aphid, Microlophium evansi (Theobald) will smear its cornicle secretions on the mouthparts of the coccinellid, Adalia decempunctata (L.) allowing the aphid to escape from the temporarily immobilized predators. Edwards (1966) observed parasitoid wasps ( Aphidius ) trapped in aphid cornic1e secretions in the field.

L. R. Nault

Papers

Aphid Cornicle Secretions Ineffective Against Attack by Parasitoid Wasps