Morphological response of the red palm weevil, Rhynchophorus ferrugineus, to a transient low temperature analyzed by computer tomography and holographic microscopy.

The red palm weevil (RPW), Rhynchophorus ferrugineus, is one of the worst palm pests worldwide. Our study aims to assess its internal and external morphological response to a sudden but transient decrease in the environmental temperature. Wild pre-pupae were subjected for 7 days to either low (5.0 ± 0.5 °C) or ambient temperature (23 ± 1 °C). Such conditions mimic a thermal anomaly happening in the larval stage most exposed to environmental factors. We quantified the changes undergone at: 1) the internal morphology, by X-Ray Computer Tomography (CT); 2) the 3-D integument' architecture, by Digital Holographic Microscopy (DHM); and 3) the glucose in hemolymph as a potential endogenous cryoprotectant. From X-ray CT we found that both pre-pupae subjected to cold and those remaining at ambient temperature follow a development where their fat body content decreases while a thick and dense cuticle is formed. There was no difference between both groups in the rate of change of fat body/dense tissues. Nevertheless, the cold group presents a slight developmental delay at the level of hemolymph content. Through DHM we again obtained that pre-pupae subjected to cold have not experienced a stop in their development. However, a more obvious developmental delay is now observed in this group at the level of the integumental roughness. Finally, regarding glucose, we found similar levels in control and ambient temperature larvae, while it was clearly increased in 51,7% of those subjected to cold. Our whole results provide morphological and biochemical evidence showing that the larval-pupal transition of the RPW continues almost undisturbed even during the quiescent state induced by a sudden and severe cold event. Nevertheless, a certain developmental delay is observed in both internal and external morphology. Additionally, the increased glucose level only found in the cold group suggests that glucose is part of the RPW cold tolerance strategy.

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