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After settlers started moving into forested areas, forest fires were threatening private property and the school of thought was to extinguish those fires as quickly as possible. The number of trees in the forest grew exponentially. Today, efforts are being made to thin the number of trees in the forest, clean up fuel on the forest floor with control burns and when a fire occurs, control the fire to protect life and private property, but let fire do what it has been doing naturally for thousands of years — reducing the number of trees to create a healthier forest.

Bark beetle populations rise and fall depending on the weather and food supply, a common phenomenon with most insect populations. DeGomez and a team of researchers track bark beetle populations by capturing the insects in special traps set up on an elevational gradient from the peaks of Flagstaff to the woodlands of Young. He is testing to see if a higher number of trapped insects corresponds with a higher rate of tree mortality.

“Forests in northern Arizona have become ripe for a ‘perfect storm,’ ripe for bark beetle outbreaks,” DeGomez said. “If you look back at fall and winter moisture levels compared to bark beetle outbreaks, 2001-02 was 7.6 inches below normal, which led to the high mortality rate of trees. In 2002-03, precipitation was 2.9 inches below normal, which led to moderately lower levels of mortality in ponderosa pine, but extremely higher levels of mortality in piñon pine.

“Heavy snowfall in 2004-05 reduced the mortality rate even further. This year, (2005-06) precipitation also was lower than normal and only time will tell if any late winter or spring precipitation will make a difference.”

Pine bark beetles in Arizona are from the genus Ips and Dendroctonus. Both bore into stressed, weakened trees. Trees attacked by Ips generally die from the top down. Trees attacked by Dendroctonus generally die from the ground up, DeGomez said.

“If trees do not have sufficient water, they cannot create pitch,” DeGomez said. “Trees that cannot produce pitch cannot expel bark beetles. Trees that have been damaged or weakened by lightning, disease, lack of water, have been growing in dry sites and rocky soil usually are not able to withstand a bark beetle attack as well as healthy trees.

Stressed trees produce one or more signals that bark beetles sense and know when a particular tree is ripe for invasion. Beetles become active in April and early May. Adults that have wintered fly out to find a new suitable host. They bore into a tree, and lay eggs. The eggs hatch after one week, and the larva start feeding on the inner bark for six to eight weeks before they mature. Then these new adults bore out of the tree and look for a new host. When an infested tree will not sustain any new arrivals, bark beetles produce an anti-aggregation pheromone that tells new arrivals that there is no more room and to find another tree. Bark beetles usually regenerate two or three times annually. In times of warm, dry winters, bark beetles may have additional generations, DeGomez said.

According to the Arizona Forest Health Web site, Pine bark beetles kill trees primarily by feeding on the inner bark. This has the same effect as peeling off the bark of the tree. Damage caused by their feeding acts as an internal tourniquet cutting off the flow of nutrients from the leaves to the other parts of the tree. As the damage progresses, sugars and other complex compounds cannot be translocated downward from the leaves to non-photosynthetic areas of the tree. The beetle can also introduce a blue stain fungus that grows into the wood. This fungus prevents water from being transported upward to the leaves. Both of these factors contribute to the decline and death of colonized trees.