Brachinus, Weber, 1801

Fikáček, Martin, Skuhrovec, Jiří & Šípek, Petr, 2007, Abstracts of the Immature Beetles Meeting 2007, Acta Entomologica Musei Nationalis Pragae 47, pp. 287-306 : 300-302

Biology of Brachinus larvae ( Coleoptera : Carabidae ), coleopteran parasitoids

Pavel SASKA & Alois HONĚK

Department of Entomology, Crop Research Institute, Drnovská 507, CZ-161 06 Praha 6 – Ruzyně, Czech Republic; e-mail: [e-mail]

Parasitoid larvae develop at the expense of a single host, which is killed by the parasitoid. Parasitoid mode of life occurs mainly in Hymenoptera and Diptera, but they can be found also in other orders of insects. In Coleoptera , this life strategy is rare. In Carabidae , parasitoid species are known from three tribes: Brachinini , Peleciini and Lebiini (ERWIN 1979) .

North American wetland species of Brachinus parasitize the pupae of water beetles ( Dytiscidae , Gyrinidae and Hydrophilidae ), and develop through five instars (ERWIN 1979). So far the life history has been described for two European dry land species only: Brachinus explodens Duftschmid and B. crepitans (Linnaeus) . Both species were reared on pupae of other carabids of the genus Amara (SASKA & HONĚK 2004, 2005). Up to now, four species of Amara have been successfully used as host for Brachinus larvae: A. aenea (DeGeer) , A. familiaris (Duftschmid) , A. similata (Gyllenhal) and A. littorea (Schiødte) (SASKA & HONĚK 2004, 2005; Saska, unpubl. data).

Females of Brachinus usually lay eggs singly on the substrate surface, stuck to the soil particles. Freshly laid eggs are narrowly oval and soft. Several hours before eclosion, the mandibles of the embryo became dark and visible through the chorion. Larval stage has three instars, but the larval development goes through three distinct behavioural phases different from the instars: the searching, feeding and resting phase. The newly hatched first instar larva is typically caraboid. After hatching it immediately starts searching for a host. On finding a suitable pupa, a larva starts feeding, usually on the host’s appendages. The larva pierces the pupal cuticle and lick up the haemolymph that exuded from the wound. Larvae moult to the second instar without moving away from the host pupa. The second instar larva is erucoid, with a white soft body. The mode of feeding is as in the first instar, but the preferred feeding locations are at the intersegmental membrane connecting head and prothorax, prothoracic pleura, pronotal sternum or abdominal pleura. During feeding, the second instar larva is always attached to the host by its ventral side. The third instar larva is erucoid, with a deeply folded cuticle, which allows a great increase in volume during the short period of intensive feeding. After moulting, the larva adopts a typical feeding position, attached by its dorsal surface to the host’s body and with its head and thorax bent backwards. In contrast to the first and second instars, larvae of the third instar chew the tissues of their hosts with their mandibles. The total feeding period (from the attachment of the first instar larva until the host is consumed by the third instar larva) lasts 6-7 days at 25°C. When the feeding is finished, the larva turns onto its ventral side with the remains of the host remaining on the dorsum and rest for another 3-5 days and thereafter pupate. During this period the shape of the larva changes from obovate to pear-shaped, reflecting the body shape of the adult. The fresh pupa is white. Two days before adult emergence the mandibles darken, and 12 h before emergence the head, pronotum and legs become pale orange. The pupal stage of both species lasts 8-10 days.

Since duration of all development phases except the searching phase varied with temperature, the thermal requirements for development of eggs, larvae and pupae of both species were established using three constant temperatures between 17.7-27.4°C. The LDT, the lower development threshold for eggs, is 9.4°C for B. explodens and 7.2°C for B. crepitans , respectively; the SET, the sums of effective temperatures, are 154.4 and 180.7 day degrees, respectively. LDT for the total postembryonic development (except the searching phase) is 12.3°C in B. explodens and 10.5°C in B. crepitans , respectively, and SET are 209.2 and 289.5 day degrees, respectively.

The conversion of ingested food during the pre-imaginal development is very high in both Brachinus species , as it ranges from 38.5 to 85.5% of the fresh mass for larval, and 33.5- 81.5% of the fresh mass for total pre-adult development. The efficiency of conversion does not change with temperature. Growth rate, consumption rate, relative growth rate and relative consumption rate are high for larval and moderate to high for total pre-adult development, and significantly increase with temperature in both species.

Acknowledgements. Supported by grant no. 0027006-03.

ERWIN T. L. 1979: A review of the natural history and evolution of ectoparasitoid relationships in carabid beetles. Pp. 479-484. In: ERWIN T. L., BALL G. E., WHITEHEAD D. R. & HALPERN A. L. (eds): Carabid Beetles: Their Evolution, Natural History, and Classification. Dr. W. Junk bv Publishers, The Hague, 635 pp.

SASKA P. & HONĚK A. 2004: Development of the beetle parasitoids, Brachinus explodens and B. crepitans ( Coleoptera : Carabidae ). Journal of Zoology (London) 262: 29-36.

SASKA P. & HONĚK A. 2005: Development of the ground-beetle parasitoids, Brachinus explodens and B. crepitans ( Coleoptera : Carabidae ): effect of temperature. Pp. 267-274. In: LÖVEI G. L. & TOFT S. (eds): European Carabidology 2003. Proceedings of the 11th European Carabidologists’ Meeting. DIAS Report 114: 3-401.