Insects of one type or another feed on almost every part of a tree, from the leaves to the roots. A typical mid-elevation mixed species forest in the Northeast has a variety of hardwood tree species such as maples, birches, beech, and oaks as well as a few conifers like spruce, fir, or hemlock. There are live and dead trees of all ages standing, as well as old fallen logs on the ground. The deciduous trees contribute to a deep layer of leaf litter on the forest floor. The structural complexities of this forest, as well as the diversity of its trees, provide food and habitat to many different species of insects, and many feeding habits are represented.
While evidence of insect feeding is often quite obvious, the insects themselves may be more difficult to see. Because of their life histories and behavioral traits, particular species may be noticed only when their numbers are very high. They may be present on trees for only a short time. Some insects restrict feeding to evening hours. Others make quick escapes or are so cryptic in their appearance or behavior that they are seldom seen.
The results of insect feeding are often available for observation long after the insect is gone. Sometimes a forester can tell the actual species of an insect just by looking at the feeding damage, as with the unique gallery designs created by particular species of bark beetles. General symptoms, like chewed leaves, allow the observer to determine a broader range of possible agents, but the information collected is still valuable.
No matter what the feeding style, for many purposes it is convenient to group insects by the part of the tree upon which they feed. Not all damage will fit neatly into just one feeding category. Some insects feed as miners when they are tiny, eating the leaf tissue between the upper and lower leaf surfaces, but change to free-feeders, feeding on all parts of the leaf, as they grow. Others may fold or roll a leaf and then feed on the soft tissue between the veins of the leaf in a process called skeletonizing. These combinations of food and shelter on the same leaf are part of what makes insect study so fascinating.
Vermont Entomological Society: http://www.vermontinsects.org/