5.6 Wednesday, Jan. 4 A caterpillar grows up: thermal consequences of growing larger on a leaf WOODS, HA; Univ. of Montana email@example.com
Leaf-associated insects live partially or entirely within leaf boundary layers. Because leaves transpire, boundary layers can be cooler and more humid than ambient air. I examined the thermal consequences of living within versus protruding from leaf boundary layers, using larval Manduca sexta on their primary host plant in Arizona, Datura wrightii. A newly-hatched larva projects about 1 mm from the leaf surface. In the subsequent 2 – 3 weeks, the larva increases in mass by 10,000-fold, at which time it projects > 15 mm from the leaf surface. For comparison, leaf boundary layers are generally 1 – 4 mm thick, depending on leaf size and wind speed. These observations indicate that temperatures of eggs and early-instar larvae should be strongly coupled to leaf temperature, whereas those of larger larvae should increasingly match air temperatures, or perhaps exceed air temperatures when larvae are directly illuminated by the sun. I tested this prediction using an infrared video camera to record, during midday, the temperatures of leaves and their associated eggs and larvae (all five instars). Leaf temperatures were usually cooler than ambient air, on average by 4°C and occasionally up to 8°C. Egg and larval temperatures depended strongly on body size. Eggs and first-instar larvae were always within 1°C of leaf temperature, regardless of ambient temperatures. By contrast, fifth-instar larvae were always close to air temperature, regardless of leaf temperature. The middle instars transitioned between these extremes. These results show that the thermal experience of larvae changes dramatically over ontogeny. A consequence is that larger larvae reach maximum temperatures 2 – 6°C higher than those of small larvae, and these natural changes in thermal stress may be mirrored by changes in larval stress physiologies over ontogeny.