Effect of biological ice nucleators on insect supercooling capacity varies with anatomic site of application

Topical application of ice nucleating active (INA) bacteria or fungi decreases the cold tolerance of freeze-intolerant insects by raising their supercooling points (SCPs). However, the route by which INA agents come in contact with insect body water is unknown. To determine their effect on the SCP, we topically applied a suspension of INA Pseudomonas syringae to four anatomic sites of the freeze-intolerant lady beetle, Hippodamia convergens. Aqueous suspensions of either cultured or lyophilized, ultraviolet irradiated (UVI) P. syringae produced significantly higher mean SCPs than control treatments when applied to the thoracic spiracle of the insect, − 7.7 and − 5.6 °C, respectively, compared with the control treatment's mean SCP of − 14.9 °C. Application of an aqueous suspension of UVI P. syringae to three other anatomic sites on the beetle produced less dramatic and more varied increases in mean SCP. Application of the INA fungus Fusarium avenaceum to the thoracic spiracle significantly elevated the mean SCP to approx. − 10 °C. Application of powdered UVI P. syringae to the thoracic spiracle resulted in a SCP increase from − 14.9 to − 4.6 °C, the most dramatic increase in this study. These results indicate that the efficacy of INA microorganisms in elevating the SCP varies with the microorganism and its site of application.