Friday, June 21, 2013

Flat-headed Borer Beetle

Western Sculptured Pine Borer (c) John Ashley
Adult Western Sculptured Pine Borer
If you live in the sticks, literally, then you have probably met this large, native beetle. The Western Sculptured Pine Borer (Chalcophora angulicollis) is one of the so-called "metallic" or "jeweled" beetles that lives just about everywhere that pine or fir trees grow. This is the only westerner in the group, but four related species chew through the forests of eastern North America.

Western Sculptured Pine Borer (c) John Ashley
The beetle's "sculptured" back
Adult beetles fly and mate in summer, and females lay their eggs on tree bark. The western adults eat leaves while their larvae eat wood, especially Douglas fir and ponderosa pine. Their common name, "flat-headed beetle," comes from these legless white grubs that are t-shaped, sort of like horseshoe nails. After hatching, the larvae chew their way through the tree's protective bark and into the living cambium layer just below, where they tunnel for food. They remain as sawdust-producing larvae for 1-2 years and go through a dormant pupal stage before transforming into the winged adults that we see.

Many of the wood-boring beetles engage trees in a form of complex chemical warfare. The adult beetles key in on scents produced by injured trees, and on the communication pheromones produced by other beetles. Some species attack en masse in order to overcome the tree's defensive measures.

Females are attracted to injured and drought-stricken trees, which emit a defensive compound (myrcene) when stressed. The tree bleeds sticky sap to "pitch out" the first few beetles that attack. But the females make a pre-emptive strike by releasing pheromones to attract additional male and female beetles to the tree.

Male beetles are drawn to the injured tree by the pheromonal message sent out by early-arriving females. Once they locate the tree, males release their own pheromone which attracts even more beetles to the front line. After mating, however, males and females both change their chemical signals. Now they release pheromones that interrupt further attraction by more beetles.

There are entire cohorts of native insects like these that are drawn to forests stressed by drought and scorched by fire. Before we started warming the global climate and putting out forest fires, these insects only affected forests on a small, localized scale. But we have unintentionally tipped the scales to favor these insects, and as a result we are now losing forests on a far larger scale.