Biological Observations on some Thynnids of Western Patagonia

Extract

During a survey for whitegrub parasites, six Thynnid species were found in parts of western Patagonia and studied in varying degrees of detail.

Definite host records are established for Elaphroptera scoliaeformis (Hal.), E. (Paralycus) sp. near nigripennis (Smith) and Pseudelaphroptera brevipilosa Durán, which are solitary external parasites of the mature larvae of Aulacopalpus pilicollis (Fairm.) (Rutelinae), Macrosoma glacialis (F.) (Melolonthinae) and Adioristus acuminatus Hust. (Curculionidae), respectively.

No adult feeding was observed during the mating flight of E. scoliaeformis and E. sp. near nigripennis, but pairs of P. brevipilosa and Ornepetes semicincta Turner fed extensively on flower heads of Umbelliferae. Some evidence is presented suggesting that at the conclusion of the mating flight the winged males tend to return the apterous females to the sites where they picked them up.

Hosts are permanently paralysed and malaxated before eggs are laid on them. The eggs of the two Elaphroptera species are placed parallel to the longitudinal axis of the host, on the mid-ventral line of abdominal segments two to seven. In the case of P. brevipilosa, the egg is lightly cemented by its caudal end at an angle of about 45 degrees to the host surface, the attachment being near the spiracular line of the host and between the third thoracic and sixth abdominal segments.

There are five larval instars. The larvae do not normally move over the host surface. In the first four instars feeding is by suction, but the fifth-instar larva tears a hole in the host in the region of the earlier feeding punctures. The film of liquid with which the larva is bathed is produced from its posterior extremity and is not exuded from the feeding puncture. Cocoons of two types may be recognised, corresponding to the sexes, the female type being the shorter and of more rigid construction.

E. scoliaeformis, E. sp. near nigripennis, and P. brevipilosa have a single generation a year. The period from egg deposition to cocoon formation is generally covered in three weeks, and at least nine months are spent in the cocoon as a mature larva or prepupa. Adults probably live up to two months.

The only natural enemies observed were a Bombyliid of the genus Acrophthalmyda and a Carabid predator, Cnemacanthus sp., attacking the cocoon stages of E. sp. near nigripennis. The adult of this Thynnid is also attacked by a mimicking Asilid fly, Lycomya germainii Big.

The function of the mating flight and the economic value of members of the Thynnidae as controlling agents for Scarabaeid pests are briefly discussed.