Trees as sources of food
The feeding habits of herbivorous (plant-feeding) insects and other herbivorous animals cause damage to trees. The most nutritious tree tissues are found in actively growing parts of a tree, such as the foliage (leaves or needles), the cambium (a thin layer of growing cells immediately under the bark), and to a lesser extent the vascular systems from the roots to the branches. Herbivores that feed on buds and leaves, such as spruce budworm and hemlock looper, are called defoliators. Those that feed under the bark on the cambium and vascular tissues, such as mountain pine beetle and white pine weevil, are called wood-boring or xylophagous insects.
In Canada, most insects feed on only one or a few closely related tree species. The name of an insect’s preferred tree host is often part of its common name, for example, native elm bark beetle or Douglas-fir tussock moth. Within the tree species chosen as a host, different age classes of trees may be preferred because of the match between the tree resource and the insect’s life cycle. For example, large, vigorous, mature trees commonly host open-feeding defoliators (e.g., forest tent caterpillar) and sap-sucking adelgids (e.g., pine bark adelgid). Wood-boring bark beetles are closely associated with weakened, older trees (e.g., spruce beetle), while others congregate on immature, rapidly growing juvenile trees (e.g., white pine weevil). During irruptive outbreaks, like those of mountain pine beetle or spruce budworm, all trees in a stand may become susceptible.
Larger animals such as mammals and birds also use trees as a food source, grazing on foliage or eating bark, shoots, fruit, and seed. In the winter, ungulates such as deer, elk, and moose often rely on shoots, twigs, and bark as valuable food resources.
Fungal pathogens, unlike insects and other animals that eat trees, grow in or on their hosts, secreting enzymes to break down plant tissues into nutrients, which are then absorbed through their cell walls. This can result in wood decay, such as heart rot (i.e., brown trunk rot), or other symptoms, such as cankers (hypoxylon canker) and leaf spots (septoria leaf spot of poplars). In some cases, fungal pathogens also excrete toxins that can result in symptoms such as wilting or dieback, or reduced growth. Some fungi are saprophytic, only growing on dead tissue, which is most often heartwood in standing or fallen trees, while others are parasitic, living on live plant tissues. The rust fungi are obligate parasites, meaning that they cannot grow or complete their life cycle unless they are growing on a living host. Many parasitic fungi, especially rusts, are host-specific, growing on a single host plant family or genus (e.g., white pine blister rust).
Plant pathogenic bacteria, which also obtain nutrients by absorption, cause disease by producing toxins, clogging water-conducting tissues with their growth, or altering tree growth by producing tumors.
Viruses trick their host tree into using its own energy to replicate more virus particles. First, they inject their genetic material into their hosts’ cells. Then, the introduced viral genetic material programs the host cells into making new virus particles, diverting energy from the tree’s normal functions.
Although plant-feeding nematodes are animals (roundworms), they are usually treated as plant pathogens because they are microscopic and because the damage from their feeding causes disease symptoms such as wilt. Nematodes (e.g., pinewood nematode) feed by piercing plant cells with a microscopic needle-like oral stylet.