Scaling up: Linking thermal ecology to species interactions in a sessile herbivore

Austin R. Cruz and Judith L. Bronstein, Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ, Martha S. Hunter and Goggy Davidowitz, Department of Entomology, University of Arizona, Tucson, AZ

Background/Question/Methods
All organisms must maintain optimal biochemical conditions in order to survive and reproduce. One mechanism is thermoregulation, the ability to regulate body temperature within specific boundaries. For ectotherms such as insects, which have a high surface area-to-volume ratio and whose body temperatures fluctuate with ambient temperature, physiological traits and/or a combination of behavioral mechanisms allow them to temporarily avoid sub-optimal or extreme temperatures such as those found in hot, arid regions. However, sessile organisms are faced with the challenge of being unable to easily avoid sub-optimal environmental temperatures. We explored how a sessile scale insect, cochineal (Dactylopius spp. [Hemiptera: Dactylopiidae]), avoids extreme heat in the Sonoran Desert through the synthesis and placement of white, cottony wax over its body. Using artificial warming assays accompanied by wax removal treatments, infrared imaging of cochineal on cactus (Opuntia spp. [Cactaceae]) pads, and measurements of temperature and wax surface areas, we assessed the potential thermal function of the wax. We tested two hypotheses: first, that the wax buffers cochineal from high temperatures relative to the cactus pad, and second, that the surface area of the wax affects the temperature that the cochineal experience relative to the pad.

Results/Conclusions
Evidence was consistent with both hypotheses: cochineal covered by wax were significantly cooler than cochineal without wax under the warming treatment, and the greater the surface area of the wax produced by an individual or cluster of individuals, the cooler the cochineal were relative to an adjacent area of the cactus pad. Our results illuminate a physiological mechanism that allows these sessile insects to regulate body temperature in extreme heat. Moreover, these results suggest a novel hypothesis in this system, the “thermal Allee effect:” there is a positive association between wax surface area and per capita population growth. This hypothesis connects organismal physiology and fitness with larger, conspecific spatial distribution patterns and species interactions across the insect’s host plant. As global warming accelerates across diverse ecological systems, organisms will increasingly be exposed to thermal stress. We offer our results as an example in which an understanding of thermal responses will enhance our ability to predict the conditions under which organisms might perish or persist.