Rapid Cold Hardening Capacity and Its Impact on Performance of Russian Wheat Aphid (Hemiptera: Aphididae)

Thermal tolerance variations and physiological adjustments in a winter active and a summer active aphid species

The Russian wheat aphid Diuraphis noxia (Kurdjumov) and melon aphid Aphis gossypii Glover are known as winter and summer active species, respectively. It is hypothesized that differences in the aphids' seasonal activities might be related to their response to temperature extremes and different physiological mechanisms. To study the aphids' thermal tolerance variations and mode of their physiological basis, they were cold acclimated at 20, 15, 10, 5, and 0 °C for 168 h (7 days) and heat acclimated at 20, 25, 30 °C for 168 h and 35 °C for 48 h. At the end of each thermal regime, survival at low and high temperatures was determined, and changes in sugars and polyols and heat shock protein 70 (Hsp70) were investigated. D. noxia was more cold-tolerant, while A. gossypii was a more heat-tolerant insect. The type and pattern of sugars and polyols were similar in both species under cold acclimation (ACC) and heat acclimation (HCC). In both species, glucose and mannitol were the major identified compounds involved in cold and heat tolerance. However, they showed different patterns of Hsp70 level, with D. noxia having a higher level of Hsp70 under ACC and A. gossypii having a higher level of Hsp70 under HCC. These results demonstrated that their differences in thermal tolerance might explain the seasonal activities of the aphids.

Cold Tolerance of the Tribolium castaneum (Coleoptera: Tenebrionidae), Under Different Thermal Regimes: Impact of Cold Acclimation

The red flour beetle, Tribolium castaneum (Herbst), is a serious pest of stored product worldwide. Cold tolerance or cold hardiness is an important ecophysiological trait related directly to survival, fitness, and distribution of insects. In this study, the effects of four thermal regimes, i.e., control (C), cold acclimation (CA), rapid cold hardening (RCH), and fluctuating-acclimation (FA), were examined for their effects on cold tolerance, supercooling point (SCP), lower lethal temperature (LLT), and chill-coma recovery time (CCRT) of the red flour beetle. In addition, changes in cryoprotectant (trehalose, sorbitol, and myo-inositol) levels were investigated under each thermal treatment. The results documented a substantial enhancement in the SCP, cold hardiness, and cryoprotectant levels of the adults of T. castaneum under CA regimes. The lowest SCP, highest trehalose and myo-inositol contents, and, subsequently, the greatest survival rate were observed in cold-acclimated beetles. In addition, coordination between cryoprotectant level, SCP, and cold tolerance of the pest was observed. The highest and lowest CCRT were observed at control and CA, respectively. In RCH regime with the highest impact, LLT reached the lowest level of -22°C. As most of the mortality of T. castaneum occurred at a temperature above the SCP, so this pest could be considered as a chill-susceptible insect.

Water-induced strong protection against acute exposure to low subzero temperature of adult Aedes albopictus

As an important vector of dengue and Zika, Aedes albopictus has been the fastest spreading invasive mosquitoes in the world over the last 3–4 decades. Cold tolerance is important for survival and expansion of insects. Ae. albopictus adults are generally considered to be cold-intolerant that cannot survive at subzero temperature. However, we found that Ae. albopictus could survive for several hours’ exposure to -9 to -19 ^oC so long as it was exposed with water. Median lethal time (LT₅₀) of Ae. albopictus exposed to -15 and -19 ^oC with water increased by more than 100 times compared to those exposed to the same subzero temperature without water. This phenomenon also existed in adult Aedes aegypti and Culex quinquefasciatus. Ae. albopictus female adults which exposed to low subzero temperature at -9 ^oC with water had similar longevity and reproductive capacity to those of females without cold exposure. Cold exposure after a blood meal also have no detrimental impact on survival capacity of female adult Ae. albopictus compared with those cold exposed without a blood meal. Moreover, our results showed that rapid cold hardening (RCH) was induced in Ae. albopictus during exposing to low subzero temperature with water. Both the RCH and the relative high subzero temperature of water immediate after cold exposure might provide this strong protection against low subzero temperature. The molecular basis of water-induced protection for Ae. albopictus might refer to the increased glycerol during cold exposure, as well as the increased glucose and hsp70 during recovery from cold exposure. Our results suggested that the water-induced strong protection against acute decrease of air temperature for adult mosquitoes might be important for the survival and rapid expansion of Ae. albopictus.

Aedes albopictus is one of two most important vectors for dengue and zika. During the last 3–4 decades, this mosquito has spread from native Asian area to all continents except Antarctica, becoming the most invasive mosquitoes which imposed extensive public health threat to human beings throughout the world. Cold tolerance is important for distribution and survival of insects. During the expansion of Ae. albopictus, especially a spatial expansion to cooler climate areas, it needs to cope with cold temperatures. Moreover, because of such widespread distribution adult Ae. albopictus will certainly often encounter sudden drops in air temperature even below subzero that often happens in early spring and winter, and late autumn. Thus far, adult Ae. albopictus are generally considered to be cold-intolerant that can not survive at subzero temperature. In this study, we found that water can provide strong protection against low subzero temperature even below -10 ^oC. Cold exposure of adult female Ae. albopictus to low subzero temperature with water either before or after a blood meal have no detrimental impact on fitness costs of these adult mosquitoes. Considering water is common in nature, our results indicated that during the expansion of Ae. albopictus especially when adult mosquitoes encounter a sudden drop in air temperature water could be a good shelter for cope with such cold temperature below subzero.

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.