Reduced geomagnetic field may affect positive phototaxis and flight capacity of a migratory rice planthopper

The geomagnetic field (GMF) is not constant but changes over space and time. In addition to the current slow decay of GMF, sudden GMF anomalies, mainly characterized by reduced field intensity, and variation in GMF strength between the migration origin and destination areas of migratory animals have attracted growing attention from researchers. We investigated magnetic field effects (MFEs) triggered by the near-zero magnetic field (NZMF), to some extent a proxy for the reduced GMF, on the positive phototaxis and flight capacity of the adult migratory white-backed planthopper, Sogatella furcifera. For the first time, we report the significant enhancement of adult positive phototaxis by the NZMF compared with the GMF. Compared with the GMF, NZMF also affected the flight capacity of adults (with significantly lower body weights) in a sexually dimorphic way. We further found that expression patterns of the multifunctional cryptochromes (genes CRY1 and CRY2) as circadian photoreceptor and magnetoreceptor, along with the genes AKH and AKHR in the adipokinetic hormone (AKH)/AKH receptor (AKHR) signalling system which is related to energy metabolism and antioxidative stress reactions, correlated with phenotypic changes in positive phototaxis and flight endurance of adults. Therefore, potentially acting through an antioxidative stress-related CRYs-circadian clock-AKH/AKHR signalling pathway, we suggest that a reduced field intensity, and even just variation in GMF, may influence positive phototaxis and flight (migratory) behaviour of insects. Our study presents a feasible link between the probable mechanisms underlying MFEs, including cryptochrome-mediated light-dependent magnetosensitivity, cryptochrome-mediated light-dependent magnetosensitivity of the circadian clock and magnetic field-triggered oxidative stress, and provides new insights into the complexity of potential MFEs induced by variation in GMF. (C) 2016 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.