Page 10 - 2019 Sonoma County Gardeners Resource Guide

Coast Live Oak & Birds
By Lisa Hug
If you happen to have a large Coast Live Oak tree in your yard, consider
yourself lucky. Coast Live Oaks are the sustaining feature of our local ecosystem. The whole Santa Rosa Plain was once a composite of rich wetlands and majestic oak woodlands before we altered the landscape to build our houses and businesses that we need.
The Coast Live Oak can live up to 250 years and attain a height of 80 feet. But, the spread of the branches is the more impressive characteristic of this locally native oak, reaching a spread of 130 feet. Sometimes the horizontal branches grow long and heavy and reach the ground. The twists and turns of their trunks and branches give each Live Oak tree an individualized profile worth treasuring.
Why am I writing about trees? What do they have to do with birds?
Well, everything, in fact. They provide shelter in inclement weather, nesting sites during the breeding season and an endless supply of food. The food may be acorns, or it may be the insects that the trees harbor.
It’s true that mosquitoes bite
and carry diseases. But most
insects are harmless. One insect that really gets misrepresented in our culture is the “oak worm.” Pest control companies would have us believe that these “worms” will kill our Oak Trees if we don’t apply pesticide to get rid of them.
“Oak Worms” are not worms at all. They are native caterpillars.
10 - www.sonomacountygazette.com - 3/19
The list includes many of our favorite garden birds such as Western Bluebirds, American Robins, Bullock’s Orioles, and Yellow-rumped Warblers. So, let’s do our best to try and keep this food source healthy for these beautiful birds that give us so much joy in our lives.
Insects often get a bad rap.
They are very small caterpillars that metamorphose into small brown or
tan diurnal oak moths. As caterpillars, they eat oak leaves voraciously. These oak moth caterpillars are always around, but usually we don’t notice them. However, about every 10 years or so, we have a large outbreak of oak moth caterpillars and these outbreaks usually last for two years. It is not known why these outbreaks occur. During these outbreak events, there may be so many tiny caterpillars eating Coast Live Oak leaves that a tree may temporarily be completed defoliated. But the leaves grow back and the number of Oak Moth caterpillars declines, as their natural enemies, the beneficial insects get their numbers back under control.
There are several natural predators of the Oak Moth caterpillars, including parasitic wasps, soldier bugs and even parasitic flies. “Ewe” you say “They sound ‘nasty.’” They are nasty, but only to Oak moth caterpillars.
However, if the tree is sprayed with a general insecticide, all insects (including the beneficial ones) are affected and these poisons travel up the food chain to birds and other wildlife.
But, if the tree is healthy to start with, it can recover from this event on its own. Botanists theorize that the Coast Live Oak tree and the Oak Moth have a beneficial relationship with each other. The tree provides food for the caterpillars and the caterpillars exude frass that accumulates at the base of the tree and provides natural fertilizer. After the caterpillars have formed into pupae (inside cocoons), they emerge as tiny brown, day-flying moths. These moths don’t even have mouth parts because they don’t need them. They don’t eat. They only live long enough to mate and lay eggs, about one day.
The best way to take care of a Coast Live Oak tree is to protect it from bacterial and fungal diseases. This is done by protecting the tree’s root zone from damage. The root zone is the ground area encompassing the circumference of the crown of the tree plus a buffer zone. This should keep your tree healthy and capable of handling Oak Moth caterpillar infestations.
Our native birds can be seen feasting on these slow-flying moths.
Oak moths and their caterpillars are food to many native birds.