Bark Beetle Biology
Bark beetles are members of a group of insects in the order Coleoptera, family Scolytidae, and genus Dendroctonus. Four species naturally occur in the forests of Southern California: the Western Pine Beetle (Dendroctonus brevicomis), Mountain Pine Beetle (D. ponderosae), Jeffrey Pine Beetle (D. jeffreyi) and the Red Turpentine Beetle (D. valens). These beetles are always present in the forest at relatively low populations levels. Periodically, when conditions favor high reproductive success, population levels increase to epidemic proportions, leading to mass tree mortality. Each species of bark beetle is host specific, attacking only one or a few tree species. The Jeffrey Pine Beetle is host specific to Jeffrey pines. The Mountain pine beetle primarily attacks Ponderosa pines and Sugar pines and the Western pine beetle invades primarily Coulter pines in Southern California.
Bark beetles spend the majority of their lives beneath the bark of a single host tree. All bark beetles undergo four stages of development: egg, larva, pupa, and adult. An adult female lays eggs in egg galleries bored between the inner bark and outer sapwood of the host tree. The eggs hatch to become larvae, which feed on the phloem and nutritious fluids of the tree until they are ready to pupate. After 4 larval stages (instars) the larva then bores into the inner bark where it forms a hardened outer casing where it becomes a pupae. Within the casing the beetle develops into the adult form. Adults are 3 - 8 mm long and feed on either the inner bark, phloem, or both. The adults eventually emerge from the host tree to colonize a new tree and reproduce when the surrounding air temperature is higher than about 60o F (16o C). Each beetle species has similar patterns of development with varying generation times.
There are many mechanisms of natural population control for bark beetles. The host trees act as a limiting resource when the forest is healthy and the beetle populations are at normally low levels. An invaded tree functions as both the site for reproduction and as a food source for the bark beetles. The few vulnerable trees available for beetle invasion act to keep beetle population levels low. During winter low temperatures (below - 20oF) for extended periods reduce the number of eggs that hatch, further controlling population levels. Natural predators are another mechanism of beetle population control. Woodpeckers and predaceous insects, such as checkered beetles and dolichopodid flies, feed on bark beetle larvae. Parasitic wasps lays their eggs on bark beetle larvae and the larvae of other beetle species compete directly with bark beetle larvae for food, further reducing bark beetle reproductive success. Nuthatches and other insectivorous birds feed on exposed adults. Bark beetles are also subject to a variety of parasites, such as nematodes (internal parasitic worms), which further impact their population numbers. Although predators help to control population levels under normal conditions, they have little impact during an outbreak.