INFLUENCE OF DEVELOPMENTAL CONDITIONS ON COLD-HARDINESS OF ADULT DALLUS LEAFHOPPERS - IMPLICATIONS FOR OVERWINTERING

Little is known about how adults of the com leafhopper, Dalbulus maidi s (DeLong & Wolcott), and its congeners survive subfreezing temperatur es at high elevations during the dry winter in Mexico. In the laborato ry, duration of survival at -5-degrees-C 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 America n species, Baldulus tripsaci Kramer & Whitcomb. Adult leafhoppers rear ed under environmental conditions that simulated the beginning of the dry winter season during October in Mexico ('October-reared') were at least twice as tolerant of -5-degrees-C than adults reared under envir onmental conditions that simulated the beginning of the wet summer sea son during June ('June-reared'). Dalbulus species found primarily at h igh elevations, such as D. elimatus, were seven times more tolerant of -5-degrees-C than D. quinquenotatus, a species which overwinters at l ow to mid elevations on its Tripsacum hosts. October-reared D. maidis adults survived relatively short periods at -5-degrees-C (LT50 = 8.9 h ) compared to October-reared D. elimatus adults (LT50 = 42.3 h). This suggests that in Mexico, D. maidis either overwinters in protected hab itats 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-degrees-C (LT50 = 2.6 h). A conditioning perio d for 1 h at +5-degrees-C before and after exposure to -5-degrees-C si gnificantly improved survival for D. maidis. Super-cooling points (SCP s) were between -23 and -20-degrees-C, indicating that mortality of th ese leafhoppers at -5-degrees-C was due to cold shock injury rather th an internal ice formation.