Mortality of Sugarcane Aphid, Melanaphis sacchari (Zehntner, Hemiptera: Aphididae), at Low Temperatures.

We conditioned colonies of Melanaphis sacchari (Zehntner, Hemiptera: Aphididae) to three different temperature regimes over a period of 2 mo: 30°C, 25°C, and a cycle of 18/8°C (L:D), all under a diurnal photoperiod of 12:12 (L:D). Plants of susceptible sorghum, 85Y40, were grown to the five-leaf stage in 4-liter pots, 2 per pot, and then infested with mixtures of second-fourth instar M. sacchari nymphs, either 50 (18/8°C treatment) or 100 (25 and 30°C treatments) aphids per plant. In each run of the experiment, a series of two to four plants of each treatment group were lowered to the desired cold temperature (4.0, 2.0, 0.0, -2.0, and -4.0°C) in a climate-controlled chamber and then evaluated for aphid survival after 12 h in complete darkness. Results revealed substantial survival of aphids from the 18/8 and 25°C conditioned treatments, even at -4.0°C, a temperature sufficient to kill the plants, and >90 and 60% survival, respectively, at 0.0°C. In contrast, aphids conditioned to 30°C experienced about 50% mortality at 4.0°C, and complete mortality at 0.0°C. Our results indicate that M. sacchari is able to survive periods of sub-freezing temperatures quite easily and will not suffer 'cold-shock' mortality unless subjected to a rapid drop in temperature from very warm conditions (e.g., 30°C or higher). Therefore, only temperatures that are cold enough to kill sorghum plants are likely to kill all apterous virginoparae of M. sacchari.