Some Aspects of Host Selection in Eucarcelia Rutilla Vill. (Diptera: Tachinidae)

Abstract

Host finding, acceptance and suitability in Eucarcelia rutilla Vill. were studied in the field and in the laboratory. The literature on nomenclature, morphology of adult and larva, life history, hosts, synchronisation and geographical distribution is surveyed. The species is univoltine, well-synchronized with its two main hosts, Bupalus piniarius and Semiothisa liturata. It deposits macro-type eggs in which the maggots are ready to hatch. Descriptions and relevant aspects of the biology of pine dwelling caterpillars and Diprion larvae are also given. In the field gravid Eucarcelia females coincide mainly with Bupalus larvae in their second and third instar. Eggs were also found upon specimens of Semiothisa liturata, Ellopia prosapiaria, and Thera obeliscata. Superparasitism is not avoided under natural conditions. Host selection was studied experimentally. Attention was paid to detectability and acceptability. A preliminary study of factors involved in host finding reveals that visual stimuli play an important role. In host acceptance stimuli originating from the host's surface are involved. Some evidence was found that these stimuli are mediated through receptors on the tarsi and ovipositor. In experiments there was no discrimination between parasitized and non-parasitized hosts. Preimaginal conditioning in oviposition preference was not noticeable. The bigger the caterpillars, the greater the chance of detection and the greater the risk of parasitization. The resting position of the caterpillar upon the needle has a strong influence on its chance of being found by the parasite. Specimens resting at the base are almost exempt from attacks. The detectability and acceptability of nine species of Lepidoptera and six species of Hymenoptera were tested. Only 4 differed significantly in detectability from Bupalus, but 10 were wholly unacceptable for oviposition. The reaction of the hosts towards attacking females and their eggs differed markedly among species. Upon Bupalus piniarius, Thera obeliscata, T. firmata, and Hyloicus pinastri the eggs are deposited preferably on the thorax. In Semiothisa liturata, Ellopia prosapiaria, Panolis flammea, and Diprionidae other distributions were obtained. As regards the side of the host chosen, the wider its girth the less often the eggs are attached ventrally. The risk of destruction of eggs upon the host's integument varies widely among species: Panolis flammea and Eupithecia indigata remove almost all the eggs, whereas in Ellopia prosapiaria no eggs were dislodged, and in Hyloicus pinastri almost none. Bupalus piniarius, Semiothisa liturata, Thera obeliscata, and T. firmata demonstrate a moderate reaction towards eggs placed upon the abdomen. In Bupalus the instars II and III have a stronger reaction than the other instars. Eggs laid dorsally on abdominal segments as a rule run a greater risk than those placed ventrally. Eucarcelia can develop only in Bupalus, Semiothisa, and Thera obeliscata. In about 20% of the cases Thera firmata, too, can give rise to full-grown parasite larvae. Host suitability also depends on site of penetration. Posterior penetration into Bupalus lead to greater losses than anterior penetration. The frequencies of settlement of maggots in wall of the foregut (intima), mandibular muscles, subcuticular sites, segmental muscles, and haemocoel were recorded for the various instars of Bupalus. Maggots which have settled in intima or in subcuticular sites must migrate at each moult of the host. This migration takes the maggot outside the host's body which it re-enters by piercing the integument at a sharply defined spot mid-dorsally in the membrane between head-capsule and first thoracic segment. Its main function certainly is to prevent removal with the shed intima or cuticle. In addition it is conjectured that migration may help the maggot to avoid exposure to the host's hormones in concentrations which might cause it to moult prematurely. Mortality of maggots can be brought about by encapsulation by haemocytes or removal from the host. Larval mortality is higher in cases where more than one maggot has entered the host than when only one is present. Site of settlement exerts a considerable influence upon parasite mortality. Settlement in haemocoel or wall of the mid/hindgut is unfavourable. It is argued that the relatively high mortality of maggots penetrated into posterior segments of younger instars is due to lower suitability of the sites of settlement attained by these maggots.