Cool Bug Facts: The Pinyon Pine and the Pinyon Engraver Beetle

First published in The Midden*, Great Basin National Park's newsletter, Vol. 21, No. 2, Winter 2021. By David Greene, biological science technician.*

The pinyon engraver (Ips confusus) is a bark beetle, in the subfamily Scolytinae. It is native, and plays a useful part in pinyon-juniper forests by killing weak or damaged pinyon pines. That can increase habitat diversity, open gaps in the canopy where sun-loving plants in the seed bank can sprout, provide standing dead trees (snags) for wildlife, and add organic matter to the soil — and the beetles are food for other animals.

How the beetle kills a tree

The beetles bore into the living cambium just under the bark and feed on the xylem and phloem, cutting off the movement and storage of water, nutrients and sugars. Healthy pinyons are normally not at risk — but beetle population booms, drought, fungal infections, or a combination can put even healthy trees in danger.

Figure 1. Globs of sap from pitch tubes sometimes trap invading beetles. NPS photo by G. Baker.

The tree's defence. A pinyon answers an attack with a pitch tube — a flood of sap at the beetle's entry hole that pushes the beetle out and seals the wound.

What breaks it. Drought weakens the pitch-tube defence, making the beetle's job easier. Slash left from logging or thinning gives beetle populations extra habitat to boom — and booms can overwhelm even healthy trees and kill pinyons across wide areas.

Calling in the swarm. Once a beetle is in, the sap turns from defence to lure: the beetle converts terpenes in the sap into pheromones that signal a suitable host, drawing flying beetles in large numbers. The more beetles arrive, the stronger the signal. Depending on conditions and signal strength, some Ips species can detect it from more than eleven miles away.

The life cycle

Figure 2. A pinyon engraver nuptial chamber (a), egg galleries (b) and the niches where eggs are laid (c). NPS photo by David Greene.

1. Usually the males colonise a tree, first boring a nuptial chamber, where a male mates with two to six females.
2. Each female bores an egg gallery out from the chamber along the grain of the wood, and cuts about 20 to 30 small niches, laying one egg in each, packed in with debris.
3. Eggs hatch after about seven days; larvae pupate after three to six weeks and emerge as adults, which follow the pheromone trail to the next host tree.
4. The year's first generation usually emerges in April or May, depending on temperature, and there are commonly three to four generations a year. Adults overwinter in winding feeding galleries in the tree.

Pinyon-juniper woodlands

These woodlands span five states of the Southwest, covering about 15 percent of the land within that range, and play an important ecological role in the Great Basin. At Great Basin National Park they cover about 7,000 acres, some 10 percent of the park. With more drought and higher temperatures predicted as the climate changes, the park will probably see more pinyon engraver activity and more impact on its pinyon woodlands.

Sources

• National Park Service, Great Basin National Park, "Cool Bug Facts: The Pinyon and The Engraver," The Midden, Vol. 21, No. 2, Winter 2021. https://www.nps.gov/articles/000/cool-bug-facts-the-pinyon-and-the-engraver.htm
• Rewritten in hubnx's own words.