Thrips' responses to thermogenic associated signals in a cycad pollination system: the interplay of temperature, light, humidity and cone volatiles

Abstract

Summary Most functions postulated for thermogenesis in plant reproductive organs are associated with pollination and pollinator activity. During thermogenesis other chemical changes occur, and these also may affect pollinator activity. We address how cone thermogenesis and the associated chemical emissions influence pollinator behaviour in the obligate mutualist Macrozamia lucida and M. macleayi cycad – Cycadothrips chadwicki thrips pollination system. Cones of these dioecious gymnosperms have a diel, mid‐day metabolic burst that is associated with increased cone temperatures, volatile emissions (primarily β‐myrcene), and CO2 and water vapour emissions. Concurrently, thrips leave cones en masse and then return to cones later in the day. We investigated the effects of these cues, individually and in combinations, on their potential to induce thrips to leave cones in the dark and light with a suite of Y‐tube behavioural experiments. The results suggest that ambient light, and high cone temperatures, humidity, and β‐myrcene levels, but not CO2, each induce Cycadothrips to leave cones. At typical overnight temperatures (14 °C) thrips were relatively inactive and negatively phototactic. At typical daytime ambient temperatures, 22–26 °C (lower than typical thermogenic temperatures), thrips were active and positively phototactic. Thrips moved away from or avoided thermogenic temperatures and high concentrations of β‐myrcene, and they preferred dry (<8% RH) to humidified (~88% RH) air. Whereas several variables individually induce thrips to leave a cone's darker interior towards the daytime light, these signals are presented simultaneously during thermogenesis. There thus seems to be enhanced or redundant signalling that enforces thrips departure from pollen cones thus increasing the chances of vectoring pollen to ovulate cones. While high temperature appears necessary to mediate thrips movement, its concurrence with other plant signals that produce similar responses, suggests a dynamic multimodal signalling system that operates as a functional unit.