Influence of developmental conditions on cold-hardiness of adult <Emphasis Type="Italic">Dalbulus </Emphasis> leafhoppers: Implications for overwintering

Little is known about how adults of the corn leafhopper, Dalbulus maidis (DeLong & Wolcott), and its congeners survive subfreezing temperatures at high elevations during the dry winter in Mexico. In the laboratory, duration of survival at -5 ~ was measured for four Mexican Dalbulus species: D. maidis, D. elimatus (Ball), D. gelbus DeLong and D. quinquenotatus DeLong & Nault; and a closely related North American species, Baldulus tripsaci Kramer & Whitcomb. Adult leafhoppers reared under environmental conditions that simulated the beginning of the dry winter season during October in Mexico ('Octoberreared') were at least twice as tolerant of -5 ~ than adults reared under environmental conditions that simulated the beginning of the wet summer season during June ('June-reared'). Dalbulus species found primarily at high elevations, such as D. elimatus, were seven times more tolerant of -5 ~ than D. quinquenotatus, a species which overwinters at low to mid elevations on its Tripsacum hosts. October-reared D. maidis adults survived relatively short periods at -5 ~ (LTs0 = 8.9 h) compared to October-reared D. elimatus adults (LTso = 42.3 h). This suggests that in Mexico, D. maidis either overwinters in protected habitats at higher elevations or it migrates to lower, frost-free regions. October-reared B. tripsaci adults, which overwinter in the egg stage, were intolerant of -5 ~ (LTs0 = 2.6 h). A conditioning period for 1 h at + 5 ~ before and after exposure to -5 ~ significantly improved survival for D. maidis. Supercooling points (SCPs) were between -23 and -20 ~ indicating that mortality of these leafhoppers at -5 ~ was due to cold shock injury rather than internal ice formation.