By Tim Clancy.
One of the great pleasures of working with trees is the opportunity to observe the relationships of creatures with trees. Of course, there are many "pests" which make their livings in and on trees. Many of these so-called pests do little significant damage to trees. If you are interested in knowing the potential damage a pest can do, a little research on the University of California Statewide Integrated Pest Management Program website will answer your questions.
The pepper tree psyllid is one good example of a "pest" that we don't need to rush to eradicate. It has been around for almost thirty years, and I have seen very few pepper trees without psyllid in the last few years. The pest can be found in abundance, yet the negative effect on a tree is minimal. In this case, while there are pesticides that will knock out the pest (for a single season), the treatment isn't really necessary. Aphids, too, can attack trees, but again, the impact is typically negligible. I managed about thirty-five jacaranda trees for fifteen years, and while they were hosts for aphids year after year, I never once treated for aphids. I allowed mother nature to take its course and the trees are thriving.
In addition to small insects like psyllids and aphids, larger animals use trees for shelter and food. One of my favorite animals is the tree kangaroo, a marsupial that spends almost its entire life in the forest trees of Papua New Guinea, Indonesia, and Australia. And, of course, we are all familiar with birds that nest and find and store food in trees.
Recently, I was asked to inspect a tree alongside a man-made lake in Thousand Oaks. It is a very large Fremont cottonwood (Populus fremontii) that was declining and an opinion was requested about pruning or removal. Upon approaching, I could hear the drumming of an acorn woodpecker. It was working in the tree on a dead branch. This tree was a family granary and the woodpecker was storing food for winter consumption. Upon closer examination, it was clear the woodpecker family had been very busy because there were hundreds, if not thousands, of holes drilled in dead tree parts and filled with acorns. One acorn per hole. This behavior in and of itself is fascinating, but what makes it even more interesting is that the granary is also maintained by the woodpeckers. As the acorns lose moisture, they shrink in size. This makes it easier for other birds like blue jays to steal the food from the woodpecker. However, acorn woodpeckers have a strategy. As the acorns shrink, the birds will move them to smaller holes to protect the stockpile.
Sadly, there is no hope for the tree's recovery. It is large and old and has reached the end of its service life as a living plant. It is now serving a different purpose as a bona fide habitat tree. So my recommendation in this case is to do some pruning to remove some dangerous limbs and then leave the tree in place for use by wildlife. This one's for the birds.