Physiology of crapemyrtle bark scale, Acanthococcus lagerstroemiae (Kuwana), associated with seasonally altered cold tolerance

Temperature is one of the most important abiotic factors influencing the adaptation and diversification of insects. Diverse and complex physiological mechanisms have evolved to help insects adapt to seasonal changes in temperature and prevent cold injury. Although the mechanisms of seasonal adaptation to low temperatures have been studied for insects in different taxa, none of these mechanisms have been investigated in scale insects in the superfamily Coccoidea. The crapemyrtle bark scale, Acanthococcus lagerstroemiae (Kuwana) (Hemiptera: Eriococcidae), is a newly introduced scale pest of crapemyrtles, Lagerstroemia spp. (Myrtales: Lythraceae). Our previous study concerning the cold tolerance of this pest suggested that, from summer to winter, A. lagerstroemiae seasonally adapted to lower temperature with a 5 °C reduction of supercooling points. In addition, time required to achieve the same levels of mortality at lower temperatures also increased. In this study, we used A. lagerstroemiae as a model system to investigate the physiological mechanisms correlated with changes in cold tolerance in scale insects, by measuring water content, lipid content and fatty acid composition, and cryoprotective polyols and sugars every other month. Results suggested that water content was lower in winter and early spring than in summer and early fall (40.8% vs. 63.3%). The proportions of the fatty acids in PL were similar over seasons, but in TAG, shorter chain fatty acids (from C6:0 to C10:0) increased in winter as longer chain fatty acids (from C14:0 to C18:0) decreased. Among all measured polyols and sugars, including glycerol, D-mannitol, myo-inositol, and D-trehalose, the levels of D-mannitol were the highest in January 2016, which were 19-times of those in March 2016 and 4.5-times of those in September 2016. Results from this study provide a better understanding on how A. lagerstroemiae overwinters, which may give insights into the overwintering strategies of other scale insects.