Land, Water and People: Insect populations on the decline


The 2016 forest health survey results were released a month ago and showed a decline in activity of the three forest insects that have been most noticeably impacting the Rio Grande National Forest: The spruce beetle, spruce budworm and western tent caterpillar.

The spruce beetle infestation spread another 22,000 acres in 2016, which was down from 34,000 acres in 2015 and 78,000 acres in 2014. In total, the spruce beetle has infested about 610,000 acres of spruce-fir and mixed conifer forest on the Rio Grande National Forest. That is one third of the Forest. The rice-sized beetles and their larva were active on 93,000 acres on the Forest in 2016 compared to 137,000 acres in 2015.  

The decline in the spread is a result of the spruce beetles running out of food. The primary food source for the spruce beetles in this area is our high elevation Engelmann spruce and there aren’t many uninfested stands left. The beetles will infest blue spruce too, and this has occurred in some places, but mostly they prefer Engelmann spruce.

When spruce beetles move into an area, they don’t infest and kill all the trees at once. The active acreage identified in the aerial survey is an indication of the areas where the beetles haven’t killed all the spruce and are still present and attacking trees. In many areas, the beetles have killed all the spruce trees five inches in diameter and larger. Sometimes they will leave an area before killing all the large spruce. This has happened in areas on the Forest only to have the beetles return a few years later. Similar to the reduction in the spread of the spruce beetle, the drop in the active acres indicates there are fewer areas that still have live mature spruce trees.

There was never anything the Rio Grande National Forest could do to stop the spruce beetle epidemic, but in response, the Forest has sold more than 50 timber sales in the last decade to salvage dead and dying trees for commercial sawtimber. In that same time period, the volume of timber put up for sale on an annual basis has tripled to 30 million board feet, or enough timber to build almost 1,900 average-sized single family homes.

Some people are concerned about the increase in cutting timber, while others think the Forest isn’t doing enough. The primary purpose of the increased harvest is to use the dead timber while it still has value as sawtimber. The annual amount the Forest puts up for sale is a function of available funding.

Logging beetle-killed spruce only occurs in areas identified in the forest plan as suitable for timber harvesting and that have existing road systems. About nine percent of the Forest falls into this category.

Spruce beetles aren’t the only insects affecting Rio Grande National Forest trees. Spruce budworms eat the new needles on coniferous trees such as Douglas fir, white fir, subalpine fir and spruce. They typically don’t kill trees unless they stay active in an area for several years. According to the aerial surveys, the area affected by spruce budworm declined from 46,000 acres in 2015 to 25,000 acres in 2016.

Many people have called the Forest about “tent worms” eating all the aspen leaves. “Tent worms” are more accurately called tent caterpillars and, indeed, we had several years in a row where large areas of aspen forest were defoliated by them. The number of acres of aspen impacted by tent caterpillars declined sharply last year from 14,000 acres in 2015 to 3,500 acres in 2016. Tent caterpillars don’t typically kill aspen, as the aspen will put out new leaves later in the summer after being defoliated. The loss of leaves can weaken the aspen, though, opening the door for other insects and diseases to attack the trees.

All three of these insect species are native to Colorado. Often, they will have temporary localized impacts to the forest, while other times, such as the spruce beetle epidemic, their affect will be noticeable for many decades.

Mike Blakeman is the public affairs officer for the Rio Grande National Forest. He spends much of his free time scrambling around the mountains with a camera in his hand.

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