It's no secret that global temperatures are getting hotter and scientists around the world are trying to predict how animals will cope with the warmer weather. But scientists make predictions about the survival of many types of animals based on studies of the adults. This is problematic for insects such as the spongy moth (Lymantria dispar dispar), which starts out as a very vulnerable caterpillar that is expending lots of energy on growing bigger. During their time as caterpillars, spongy moths grow from 0.003 to 0.114 g before they morph into moths. ‘To give you some perspective, for a caterpillar hatched the size of an adult human, that would be like growing to the size of a small elephant over a period of about 7 weeks’, says Sean Powers of Virginia Commonwealth University, USA. Because of this drastic change, it seemed likely that the caterpillars might also be responding to temperature differently as they age, and differently from the adults. To address this gap in knowledge, Powers teamed up with Salvatore Agosta, also of Virginia Commonwealth University, Kristine Grayson of the University of Richmond, USA, and Eloy Martinez of Nova Southeastern University, USA, to see how the metabolism of spongy moth caterpillars might respond when the temperature changes.
After the caterpillars hatched, the team waited until the insects reached a specific age (2, 7, 11, 15 or 21 days old) before measuring their metabolism at different temperatures (13, 18, 23, 28, 30, 32, 34 and 36°C). Unsurprisingly, the caterpillars’ metabolism got faster as the temperature got warmer, no matter how old they were. However, when the researchers looked at how quickly the metabolism of each age group started rising as the temperatures heated up, they noticed something interesting. While 2, 7 and 15 day old caterpillars all increased their metabolism to the same extent when the temperature rose, the 11 day old caterpillars bumped up their metabolism even more from one temperature to the next. Interestingly, this stage seemed to be the most sensitive to changes in temperature and this is also right in the middle of their development.
When the scientists looked at the 21 day old and largest stage, they took much longer to ramp up their metabolism when the temperature got warmer, and this was especially true when the temperatures got over 28°C. ‘After the third developmental stage [∼11 days old], caterpillars get so big that they do not process energy as well as smaller caterpillars, and they are unable to increase their metabolism as quickly when temperatures get warmer’, explains Powers. This suggests that these caterpillars respond to temperature differently as they age.
In the case of the spongy moth, the caterpillars react to temperature differently as they age; their metabolism gets a bigger boost as the temperature rises, peaking at 11 days before becoming more sluggish at 21 days of age. As the world warms, understanding how all stages of an insect's life cycle react to warmer temperatures will be important for predicting whether or not a species survives. Without this knowledge, scientists risk projecting that an insect can handle warmer temperatures when, in reality, they could succumb to the changing climate.
