Smooth bark surfaces can defend trees against insect attack: resurrecting a 'slippery' hypothesis

1. Smooth bark on trees and shrubs was historically hypothesized to be an anatomical defence against epiphytic vegetation and phytophagous insects. This hypothesis has fallen from favour, yet no clear tests of bark texture as a defence against insects have been published. 2. We tested the smooth bark defence hypothesis using bark beetles specialized in attacking pine trees as model insects, and Pinus flexilis (limber pine) - a widespread tree that can have both smooth and rough bark surfaces on the same stem - as the model tree. We investigated the effects of bark texture on the locations of bark beetle attacks on trees with a combination of field surveys and experiments in the Colorado Rocky Mountains, USA. 3. Bark beetle attacks were overwhelmingly located on rough bark surfaces and virtually absent from smooth bark. Increasing proportional coverage by smooth bark was negatively related to bark beetle attacks per square metre of bark surface. Experimental tests of bark beetles' ability to grip smooth versus rough bark revealed that bark beetles have difficulty gripping and quickly fell from smooth bark but not from rough bark. 4. Smooth bark was negatively related to increasing tree size, but our models indicated that even partial coverage by smooth bark on a tree's trunk can significantly reduce total bark beetle attacks - this reduction likely improves tree fitness as bark beetles must aggregate to overcome tree defences. 5. Synthesis. Our results indicate that smooth bark on trees can act as an anatomical defence against insects by reducing their ability to grip a tree's surface - even for insects specialized in attacking tree stems. Similar to other forms of anti-insect defence (i.e. secondary chemistry, leaf toughness), smooth bark appears to be influenced by plant ontogeny whereby younger trees have greater defences than older trees. Understanding the adaptive significance of bark texture will require continued field and genetic study. Nevertheless, our results revealed that smooth bark texture increases tree resistance to phytophagous insects calling for the resurrection and vetting of the smooth bark defence hypothesis.