Melittobia acasta

Matthews, Robert W. & González, Jorge M., 2008, The acasta conundrum: Polymorphism and taxonomic confusion within the parasitoid genus Melittobia (Hymenoptera: Eulophidae), Zootaxa 1854, pp. 45-54 : 52-53

The acasta conundrum

The puzzle of the acasta species group has several aspects. Paradoxically, the overwhelmingly common MF of the various Melittobia species appear so similar as to be nearly impossible to distinguish in a conventional taxonomic key; yet at the same time, the limits of their variability are unknown, and the BF remain almost entirely unexamined. As noted above, males of the various species are somewhat easier to distinguish, but make up less than 5% of most populations and in most species, further reduce their numbers by dismembering one another; their variability and polymorphisms deserve far more attention than they have received. For both sexes, large population samples are the norm (with 400–600 individuals arising from a single large host cocoon such as T. politum ), but temporal and host-related effects upon individual morphology remain almost entirely unstudied. Recent work on the proximal causes of the BF/MF polymorphism in M. digitata ( Cônsoli & Vinson 2002, 2004) highlights the role of nutrition in determining developmental pathways.

Another part of the conundrum is that although Melittobia are known to be widely polyphagous, most recent samples have been obtained from mud-dauber wasp nests. Interestingly, all of the enigmatic acasta group species were reared from other hosts. The type specimens of M. scapata were from an unidentified Trypoxylon sp. nesting in a sumac twig, and types of M. evansi were from Trypoxylon striatum Provancher from trap nests ( Dahms 1984a); neither of these species has been collected again. The type series of M. megachilis was reared from a cavity-nesting bee, Megachile centuncularis (L.) (Packard 1864). Only the types of M. chalybii were from a mud dauber, Chalybion californicum (Saussure) , in this case a species that reuses old Sceliphron spp. or T. politum nests. Might knowledge of host preferences help to explain perplexing taxonomic variability? An extensive survey of solitary trap-nesting bees and wasps ( Krombein 1967) that might have provided an answer fails to do so, because Melittobia reared from numerous species nests apparently were not kept.