There are a number of bugs that eat pine trees either because of their sap, needles, or wood and bark. Several insect species will infest and then vigorously feed off of them with great enthusiasm. These include borer-type insects such as mountain pine beetles and needle-feeding insects like the European sawfly. Pine aphids can be a problem; pine adelgids love to feed on sap in a pine tree's bark.
The mountain pine beetle is attracted to several types of pine tree, including Scottish and ponderosa varieties. It's about the size of a rice grain and has a hard black exterior shell, or exoskeleton. In initial stages of infestation, these beetles usually attack trees that have been damaged in some way. Later, as their numbers grow, they move on to the healthier pines in a forest or group. Infested pine trees often display sickly yellowish-red foliage and bark-free spotting.
The European sawfly loves to lay its eggs on a pine tree, including Scottish and mugho types. When they hatch, the larvae will often feed in large groups on the older needles. Sawfly larvae are greyish-green in colour and have two light stripes as well as one dark stripe on each side of their bodies. Additionally, their heads and legs are black and shiny. Fully grown, these larvae measure approximately 25 mm (1 inch) in length.
Pine aphids are easy to spot on a tree because they have two small tails that extend back from their abdomens. They're soft-bodied, like to attack white pines and will feed in large numbers, removing sap from a pine's bark. This can interrupt sap movement to pine needles and stunt branch growth on larger trees. In smaller ones, these insects can often kill them off. A sooty mould is also often present, making the tree look dirty.
Pine bark adelgids feed on the sap present in the smooth bark of pine trees. They're small and dark and are covered with a waxy secretion that resembles cotton. When they heavily infest a pine tree's bark, they can make it look like it has been whitewashed. Females will lay their eggs on the tree over the winter, and there can be five generations or more in a year. Fortunately, healthy trees usually don't suffer permanent damage from them.