Douglas Fir, Spiders and Ants

The Douglas fir makes an attractive holiday tree once it's cleared of ants and spiders.

The spiders and ants that live in Douglas fir trees are an important part of the ecosystem. Douglar fir, Pseudotsuga menziesii, is actually not a fir, pine or spruce. The trees are their own species, named for the two men who first discovered it in North America. Douglas fir trees are also called Oregon pine, British Columbian pine, red fir and Douglastree. Spiders and ants, in some ways natural enemies, live together in Douglas fir trees.

Habitat and Cultivation

Douglas fir trees are prolific throughout the United States; they are the biggest source of softwood in the country. Douglas fir trees grow deep in well-drained, well-aerated soils with a pH range of 5 to 6. Wet, compacted soils are not suitable for Douglas fir. The trees grow very large, with a pyramidal shape, and make attractive foliage trees in garden landscapes. In addition to attracting several different insect pests, Douglas firs are a natural habitat for many species of ants and spiders that aren't at all harmful to the tree.

Enemies Turned Friends

In nature, spiders and ants exist in direct competition with each other. Both prey on other insects and populate the same outdoor spaces, but in the Douglas fir spiders and ants exist in harmony. Scientific studies show that removing ants does not largely impact spiders, nor vice versa. In the Douglas fir tree, ants and spiders live together, but neither depends upon the other. In the trees, ants make up less than 2 percent of the spider diet. Ants do not appear to prey on the spiders at all. Spiders and ants do compete for some of the same food in Douglas fir trees, but studies do not show that either species harms the other.

Predatory Ants

Some of the ants that live in Douglas fir trees are actually beneficial to the tree and to the garden. Carpenter ants do feed on plant sap, but they also feed on grasshoppers, leafhoppers, crickets, aphids, mosquitoes, bees, ants, spiders and flies. The ants also attack Douglas fir moths, spruce budworms and tent caterpillars that are damaging to forest trees like the Douglas fir. Predatory ants and spiders that live in Douglas fir trees reduce pest populations, acting as natural insecticides against undesirable insects.

Pests

Douglas fir trees are susceptible to attack from spider mites, aphids, scales, weevils, webworms, moths and spittlebugs. Spider mites are so named not because they are true spiders but because they create webs like true spiders. Unlike their predatory and beneficial cousins, spider mites are damaging to trees and a wide variety of garden plants. Spider mites feed on the nutrients found in foliage, creating discoloration and wilting damage. Some ants prey on mites, providing natural insect control. When treating Douglas fir trees for pests, use selective insecticides that will not harm beneficial ants and spiders.

References

• Western Wood Products Association; Douglas Fir & Western Larch; January 1996
• USDA Forest Service: Douglas Fir
• JSTOR; Negative Effects of Ant Foraging on Spiders in Douglas-fir Canopies; J. Halaj, et al.; October 1997
• The North Columbia Monthly; Boundaries; Jack Nisbet; May 2007

Resources

• University of California Statewide IPM Program: Douglas Fir - Pseudotsuga Menziesii
• Colorado State University Extension; Spider Mites; W.S. Cranshaw, et al.; November 2006