Conopid Fly (Diptera: Conopidae) Attacking Large Orchid Bees (Hymenoptera: Apidae: Eulaema)

Abstract

Conopidae is a large and widespread family of parasitic flies. The conopid genus Physocephala is often associated with the social bumble bees (Hymenoptera: Apidae: Bombus). Smith (1966) lists 13 named Physocephala species and their hosts, of which 11 were known to attack Bombus in the Nearctic and Palaearctic regions. In Europe, their parasitic association with bumble bees has been the subject of significant research. In a recent review, Schmid-Hempel (2001) concluded that conopid parasitization is common in bumble bee foragers, and that the flies have a preference for foragers of larger species. The fly deposits an egg on the adult host, which is killed by an endoparasitic larva within 10-12 days after oviposition. After parasitization, the host changes behavior, visiting different plants from those of non-parasitized relatives. Ultimately, the host digs itself into the ground and dies (Schmid-Hempel, 2001). Studies of parasitization of large orchid bees (Apidae: Eulaema) have revealed numerous nest parasites (Cameron and Ramirez, 2001, and references therein). However, to our knowledge, other than a speculation by Bennett (1966) that conopids should parasitize some of the larger Euglossini, no euglossine bee or other neotropical bee has been reported as a host for conopid flies. On 19 September 2001, at 0945 hr, a Physocephala rufithorax Kr?ber (Diptera: Conopidae) was observed repeatedly attacking certain of the male orchid bees (Apidae: Euglossini) attracted to chemical baits. The chemical attractants were placed in the buffer zone of the Tambopata Reserve and the Bahuaja-Sonene National Park, Madre de Dios, Peru, about 700 m into the forest from the riverbank opposite the Tambopata Jungle Lodge (225 m a.s.l., 12?49.456'S, 69?24.163'W). For details about the area, refer to Erwin (1985). The orchid bee species arriving at the site were attracted to the five most common baits recommended for sampling euglossine bees in that region (Pearson and Dressier, 1985). The temperature at the time of observation was 24.8?C and there was no rain during the day. Situated 12 m from the baiting site was an active surface-nesting colony of the Amazonian bumble bee, Bombus transversalis (Olivier). While making a general survey of male orchid bees, a single female of P. rufithorax was observed attacking orchid bees of the large Eulaema (18-31 mm), including Eulaema cf. meriana (Olivier) (refer to discussion of species mimicry in Dressier, 1979) and E. cingulata (Fabricius), even though the smaller Euglossa spp. (9-19 mm) were more abundant on the baits at the time of observation. The conopid fly alighted on the upper surface of the leaves of a nearby tree, approximately 1.2 m above ground (the same height as the baits) and about 1 m from the primary chemical attractant (methyl salicylate). From there the fly attacked the Eulaema at high speed, striking it to the ground. Once on the ground, the conopid flew off, while the male Eulaema lay stunned, unable to fly for several seconds. This behavior of in-flight attack in the vicinity of chemical baits is analogous to observations of conopid attacks on European bumble bees foraging at flowers (Schmid-Hempel, 2001; P. Schmid-Hempel, pers. comm.). The fly returned to its vantage point on the tree to groom before making another attack. Grooming involved rubbing the hind tarsi together for several minutes and, intermittently, rubbing the exposed cercus and syntergites 8 + 9 (terminology according to McAlpine et al., 1987) backwards and away from the body with the hind legs (approx. 6 times lasting 3-A seconds). The fly was observed to attack no fewer than five males of Eulaema over ten minutes, whereupon it was collected for identification and documentation. None of the attacked bees were used for further study in the laboratory. The observation of a euglossine-conopid association is worthy of note in the context of host-parasite evolution. Two of the eusocial apid tribes, Bombini and Apini, are well-known conopid hosts (Smith, 1966; Schmid Hempel, 2001), and the parasites may show a generalized preference for social, or rudimentarily social species such as Eulaema. They seem to prefer relatively large hosts, as appeared to be the case in Physocephala's preference for the larger Eulaema over the smaller Euglossa species in this observation.