K.G.A. Hamilton
Leafhoppers and their relatives are jumping insects with powerful hind legs. They disperse largely by running and leaping, but they also disperse by flight even when most of the population are short-winged (“brachypterous”) and flightless. They have been recorded to migrate over thousands of kilometres in one season (Medler 1962; Cheng et al. 1979; Ghauri 1983) when aided by strong winds. Yet many species have very restricted distributions: over 90 species of leafhoppers plus 35 planthoppers, 8 treehoppers, 7 spittlebugs and 4 cicadas are known from only very small areas of Canada (unpublished checklist, 1997). This seeming contradiction apparently reflects the diversity of life styles found in these insects.
Most migratory bugs are light-bodied insects not more than 4 mm long that are easily carried by air currents. They usually show modifications for flight: their wings are usually more than four times as long as wide, and (in many delphacid planthoppers) their eyes are very large compared to the width of the head. Wind-carried insects apparently include most “microleafhoppers” (subfamily Typhlocybinae) and a portion of delphacid planthoppers. Migratory delphacids are mainly tropical species; only a very few migratory species are known from western Canada and one of them is an introduced species. Thus, of the native fauna only Typhlocybinae are excluded from the following biogeographic discussion.
A great number of species of leafhoppers fly, but only few individuals of most species are found in flight intercept traps. The main exception to this rule is the genus Xestocephalus which are believed to be ant-guest insects; apparently adults fly actively near ground level in search of ant nests. Traps more than 1 metre above the ground collect few leafhoppers, mainly long- winged species of Macrosteles (Waloff 1973), at least some of which are known to be migratory (Chiykowski and Chapman 1965). Among other leafhoppers, tree canopy species are more commonly collected in such traps than species from low vegetation, as the usual flight path of leafhoppers is obliquely downwards. The exception seems to be sexually immature individuals (Waloff 1973); possibly these actively disperse over short distances to prevent inbreeding. By the time females become gravid they usually lose the power of flight.
The rate at which such populations spread is best observed in species imported by human activity. These “exotics” expand their ranges at rates between 10 and 100 km/year (Hamilton 1983). These figures may be taken to be upward extremes for leafhoppers as introduced species are often the most aggressive ones. Introduced insects disperse readily by following introduced floras along transportation corridors. A few of these “exotics” are also migrants, arriving annually in southern British Columbia from warmer climes, including the delphacid planthopper Delphacodes propinqua (Fieber) and possibly the leafhopper Xyphon triguttatum (Nottingham), a Californian species that has been found once in the Okanagan.
Native species or ones with fragmented habitats appear to spread at much slower rates. For example, only one arctic species out of 24 has been able to invade islands across major water channels and a third of the arctic Alaskan-Yukon leafhoppers have not crossed the Mackenzie River valley in the 12,000 years since deglaciation (Hamilton 1997). Some native species have been spread unnaturally quickly by human activities; thus, for example, the leafhopper delphacid Liburniella ornata (Stål) and the buffalo treehopper Ceresa alta Walker (formerly Stictocephala bubalus auctt. nec Fabricius: Simões de Andrade 1997) have established themselves in British Columbia although their native ranges were confined to areas east of the Rocky Mountains (Van Duzee 1917). Another such insect, the leafhopper Agallia quadripunctata (Provancher) has been in British Columbia for a much longer time, yet still retains a disjunct distribution from its naturally-occuring eastern populations.
