Getting to the Heart of Stress Reactivity in Honeybees

Abstract

Honey bee colony collapse disorder is a concerning phenomenon. It appears that the key to unlocking the factors driving CCD lies in understanding the honey bee stress response. Increased heart rate is a hallmark of acute stress, as it aids the transport of blood and oxygen to the brain, heart, and muscles, which enhances an individual’s survival. Recent studies have suggested that allatostatins (AT’s) may function as stress hormones in insects. In this study, the effects of honey bee allatostatins, ASTC, ASTCC and ASTA on honey bee forager heart rate in vitro were investigated. Concentrations ranging from 10-8 M to 10-4 M were exogenously applied to semi-isolated bee hearts. The effects of ASTs on heart rate and arrhythmic activity were measured over ten minutes. The results showed that ASTC and ASTCC have biphasic effects on heart rate; 10-8 M ASTC and ASTCC significantly increased heart rate, whereas at a concentration of 10-5 M, ASTC and ASTCC significantly reduced heart rate, relative to controls treated with the vehicle alone. In contrast, all ASTA concentrations tested significantly increased heart rate in bee preparations compared to controls. Low concentrations of ASTC and ASTCC, and ASTA at all concentrations tested also reduced the occurrence of heart arrhythmias. Arrhythmias that did occur in heart preparations exposed to these allatostatins occurred later compared to controls. The effects of Varroa (a key honey bee stressor) on heart rate in stage nine pupal bees were also assessed. Heart rate in forager bees was compared to heart rate in stage 9 pupal bees (P9’s) immediately prior to their emergence as adults. Two groups of P9’s were examined; bees in cells highly infested with Varroa, and P9’s taken from brood cells that were not infested with Varroa. The results show that in foragers heart rate decreased significantly over ten minutes, to a rate that was similar to the heart rates observed in P9’s. Interestingly, P9’s exhibited stable heart rates over the ten minutes of recording. Furthermore, P9’s highly infested with Varroa did not display significantly different heart rates from P9’s not infested with Varroa. The absence of a Varroa effect on heart rate in P9’s adds further weight to the hypothesis that P9’s may not exhibit a stress response. Taken together, these results lend support to the hypothesis that allatostatins function as stress hormones in bees.