Das Ausleseverhalten von Aphelinus semiflavus Howard und die Abwehrreaktionen seiner Wirte (Hymenoptera: Aphelinidae).

Abstract

Aphelinus semiflavus How. is a polyphagous parasite of many species of Aphididae and Callaphididae. The possibility of development in hosts of other aphid families has yet to be proved. The females feed from the body-fluid of the aphids. During the puncture for nourishment, no egg is laid but apparently a substance is secreted to paralyse the aphid. The tip of the ovipositor remains within the body of the victim 3-5 times as long as for oviposition. The duration increases according to the body-size of the prey. Acceptance and refusal of a host for oviposition (host selection) are performed in three phases. The refusal can take place already after external examination of the aphid by the antennae (predominantly without touching) (1st phase), or after internal examination by the ovipositor. In the second case, the host is rejected either by interrupting the puncture prematurely (2nd phase), or by omission of egg deposition during a puncture of normal length (3rd phase). Because the females seldom touch the aphids before oviposition, the internal investigation is of greater importance for the selection of suitable hosts. Hosts containing a living egg are nearly always rejected; hosts otherwise unsuitable for the development of offsprings are partly accepted; but hosts which are obviously suitable nevertheless are often rejected. For nourishment, the aphids are more readily accepted than for oviposition. The selection behaviour of A. semiflavus can be interpreted to be the consequence of the combined action of sign stimuli given by the host and of variable thresholds for the release of reactions by the parasite. Because the duration of puncture for nourishment is dependent on the size of the victim and because the more important part of host selection does not take place till penetration of the ovipositor is performed, the existence of chemoreceptors at this organ can be suspected. Among the defence reaction of the aphids against the parasites, again three phases can be distinguished: 1st, mechanical defence by antennae and legs before the attack, 2nd, intensified mechanical defence by antennae and legs or by oscillating of the whole body during the puncture.. 3rd, physiological reactions destroying eggs or larvae of the parasites by formation of an enclosing capsule. The intensity of the defence movements is depending on age and species of the aphis. Capsules were formed only by Bhopalomyzus ascalonicus (Donc.) (shallot aphid) and Myzotoxoptera tulipaella Theob. (tulip aphid). In the case of the shallot aphid, the selection behaviour of the female parasite was influenced by the formation of the capsules. The differences in the frequency of oviposition into hosts of different species or stages were quantitatively evaluated. These differences are caused predominantly by the different perceptibility of the hosts that increases, independently of the species, according to their body size (slightly more than proportionally), and by the intensity of the defence reactions of the hosts. The defence movements reduce the degree of parasitization and confine the oviposition mainly to young aphids, whereas the physiological reactions reduce the number of suitable host species. Altogether, the experiments indicate that the selection behaviour of the parasite and the defence reactions of the host play an essential part in forming the host-parasite-relationship. Both of them are not perfect: the host selection cannot distinguish perfectly between suitable and unsuitable hosts, and the defence reactions cannot prevent absolutely the parasitization. This incompleteness seems to be to some extent the result of mutual processes of genetical adjustment.