On the origin of the desert locustSchistocerca gregaria(Forskål) (Orthoptera: Acrididae: Cyrtacanthacridinae)

Abstract

The locust genus Schistocerca (Stål) has a transatlantic disjunction, which has been controversial for more than a century. Among 50 species within the genus, only one species, the desert locust (S. gregaria Forskål), occurs in the Old World, and the rest occur in the New World. Earlier taxonomists suggested that the desert locust is a migrant from America, but this view was strongly challenged when a large swarm of the desert locust successfully crossed the Atlantic Ocean from West Africa to the West Indies in 1988. The currently accepted view, supported by this incident, is that the New World species are descendants of a gregaria-like ancestor, and the desert locust would be ancestral to the rest of the genus. However, there is surprisingly little evidence to support this view other than the 1988 swarm. I present the most comprehensive phylogenetic study that suggests that the desert locust originated from the New World, contrary to the accepted view. I also present a hypothesis about how the ancestral Schistocerca might have colonized the New World in the first place in light of phylogenetic relationships with other cyrtacanthacridine genera.

Ancient trans-Atlantic flight explains locust biogeography: molecular phylogenetics ofSchistocerca

“…Thus, its phylogenetic position has considerable relevance for biogeographical scenarios. Chapman et al (2000) suggested that Halmenus and Schistocerca were not closely related, while Song (2004) found the two genera to be sister taxa.…”

“…In this view, the African Schistocerca lineage evolved first, and trans-Atlantic invasions of the New World were facilitated by the ability of these locusts to undertake extreme long-distance flights in very large swarms. This scenario has been called the 'Old World Origin' hypothesis for Schistocerca (Song 2004). …”

“…Similarly, Grunshaw et al (1990) suggested that cuticular hydrocarbon profiles, showing a close relationship between S. gregaria and the South American Schistocerca cancellata, provided evidence for a recent dispersal from South America to Africa. Song (2004) presented the first comprehensive cladistic analysis of Schistocerca based on morphology, and found S. gregaria to be nested within a clade of 'mobile' New World Schistocerca species that included both S. americana and S. cancellata. Based on this result, Song supported the New World Origin hypothesis:…”

“…(Song 2004(Song , p. 1644 The most important test of the alternative Schistocerca origin biogeographical scenarios is the phylogenetic position of S. gregaria relative to other Schistocerca species (figure 2; see also Song 2004). Single or multiple west to east crossings (New World Origin hypothesis) predict that S. gregaria will be embedded within the New World clade and sister to the New World species or lineages that traversed the Atlantic (figure 2a).…”

“…Known as the 'New World Origin' hypothesis (Song 2004), this scenario was based on the apparent close relationship between S. gregaria and a subset of American locusts. For example, Dirsh (1974) argued that the desert locust was best considered a subspecies of the American bird locust, Schistocerca americana, which therefore indicated a recent dispersal from the New to the Old World.…”