Submitted to: Comparative Biochemistry and Physiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/4/2011
Publication Date: 12/1/2011
Citation: Yocum, G.D., Greenlee, K.J., Rinehart, J.P., Bennett, M.E., Kemp, W.P. 2011. Cyclic CO2 emissions during the high temperature pulse of fluctuating thermal regime in eye-pigmented pupae of Megachile rotundata. Comparative Biochemistry and Physiology. 160(4):480-485. doi:10.1016/j.cbpa.2011.08.004. Interpretive Summary: The alfalfa leafcutting bee, Megachile rotundata (Hymenoptera: Megachilidae) is the primary pollinator of alfalfa, Medicago sativa L., in North America. US growers typically use 70,000 to 90,000 bees per hectare for the pollination needed for alfalfa seed production. During the 2007 growing season, 44,839 hectares of alfalfa seed were harvested, yielding approximately 31,000 tons of clean seed with an estimated value of $88.5 million. In late spring each year, post-diapausing prepupae are transferred to 29 °C to allow completion of development and adult emergence. The timing of this transfer is critical for synchronizing bee nesting activity with the alfalfa bloom. If meteorological conditions changes such that the bloom will be delayed, bee managers, to insure optimal synchronization of bees and bloom, slow development of the bees by lowering the incubation temperature. Extended periods of low temperature exposure can be detrimental to insects. M. rotundata can only be stored at 6 °C for 7 days before survival significantly decreases. Treating developing M. rotundata stored at 6 °C with a daily one hour pulse of 20 °C (fluctuating thermal regime, FTR) permitted storage to four weeks without a significant decrease in survival. As the first step in investigating the physiological mechanism M. rotundata response to FTR, respirometry experiments were conducted. The objectives of this investigation were two-fold: 1) To characterize respiratory patterns of M. rotundata during the low temperature phase and high temperature pulse of a FTR. 2) Determine the respiratory response of bees exposed to different FTRs varying in peak temperature and the duration of the high temperature pulse. The results of this investigation lays the foundation for determining the mechanisms enabling the increase survival of M. rotundata during low-temperature storage and to accelerate the development FTR protocols for other pollinators and other agricultural important insects.
Technical Abstract: Megachile rotundata, the primary pollinator used in alfalfa seed production, may need to be exposed to low-temperature storage to slow the bees’ development to better match spring emergence with the alfalfa bloom. It has been demonstrated that using a fluctuating thermal regime (FTR) improves the bees’ tolerance to low temperatures. CO2 emission rates were compared between four different FTRs, all with a base temperature of 6 °C and a daily high-temperature pulse. Four different high-temperature pulses were examined, 15 or 25 °C for two hours and 20 °C for one or two hours. Pupae at the base temperature of 6 °C exhibited continuous gas exchange and, once ramped to 20 or 25 °C, shifted to cyclic gas exchange. As temperatures were ramped down from the high-temperature pulse to 6 °C, the pupae reverted to continuous gas exchange. The following conclusions about the effect of FTR on the CO2 emissions of M. rotundata pupae exposed to low-temperature storage during the spring incubation were reached: 1) The high temperature component of the FTR was the best predictor of respiratory pattern; 2) Neither pupal body mass nor days in FTR significantly affected which respiratory pattern was expressed during FTRs; 3) Cyclic gas exchange was induced only in pupae exposed to temperatures greater than 15 °C during the FTR high temperature pulse; 4) A two hour pulse at 25 °C doubled the number of CO2 peaks observed during the FTR pulse as compared to a two hour pulse at 20 °C.