Thursday, June 30, 2011

CA oak moth ~ 06/30/11 ~ Fort Ord

California oak moth on coast live oak
Phryganidia californica on Quercus agrifolia var. agrifolia

The populations of the CA oak moth are known to fluctuate dramatically over the years in an apparent cycle. I tend to forget about them until I start seeing swarms of moths around oak trees on warm June or September/October afternoons. It's really quite an amazing sight. In places of heavy outbreaks, people who normally don't pay attention to such things definitely notice massive numbers of caterpillars swinging on lines of silk from defoliated oak trees and climbing everywhere to pupate, from tree trunks to buildings and cars. I've read the population cycle runs anywhere from 5 to 10 years. It seems to me this would be a difficult thing to monitor, particularly the relative numbers, length of time, and locations. What I recall is that one year it could be heavy out in Carmel Valley and three years later it could be heavy up in Aptos. I've even seen one oak completely defoliated and the oak right next to it not even touched. I wish I had taken notes and photos of my observations through the years.

Many people seem to get alarmed when their precious oak tree in their yard gets defoliated. They call out pest management companies to spray and inject pesticides. I think this is a waste of time and money. I also believe this is harmful to the tree, the fauna that depends on the tree, and surrounding wildlife, including birds that collect the acorns and other moths that don't even eat oaks. Some reputable companies will clearly state oak moth infestations generally only last 2 years at most and they will treat in the second year. Huh? Well, if you wait to the third year without doing anything, the moths mysteriously disappear anyways. Plus, the live oak usually recovers with vigorous new growth, which is not an annual thing for this evergreen oak. It's cost-free pruning with an added benefit of natural fertilizer from the frass. I've seen this first-hand in the oak trees next to our driveway; I actually think they look healthier now than a few years ago before defoliation from the oak moths.

I believe these regular population crashes are density dependent. My theory is that when you intervene in the natural cycle and artificially suppress the peak population size, you prolong the higher than average population numbers. Again just my theory, the reason for this is that parasitoids (tachinids and ichneumonids), viruses, fungi, other diseases, and predators that normally keep the oak moth population in check aren't allowed to do their job as effectively. You end up with a bedraggled oak (think how your hair looks without a hair cut for a long time) and the potential to prolong the stress of repeated partial defoliation, which could in the long-run do more harm to your oak tree.

Okay, I'll get off my soap box. For decent links with better information, check out Elkhorn, University of California Hastings Reserve, Pests of Landscape Trees and Shrubs, and Tree Solutions (yes, a pest management company with well-written information).

ps 10/09/11 - The oak moths have started swirling around our oak tree en masse. Interesting to note, the tree itself does not look defoliated like I've seen other trees around town.


Imperfect and tense said...

Sensible words, Katie.

We seem to be a species that doesn't bat an eyelid at altering vast areas of habitat, but if another species so much as affects one tree, it's time to wheel out the chemical arsenal.

We're a short-sighted, short term bunch, I'm afraid.

Katie (Nature ID) said...

Thanks, Graeme. In a former profession, I worked on a long-term gypsy moth study examining the effects of biological pesticides, Btk (bacteria) and Gypcheck (virus) for potential forest management back east. The official study was 10 years during the time when the gypsy moth was moving into the area and even that wasn't long enough. If I can recall correctly, out of around 1000 species moths, at least 10-15% were negatively impacted by the aerial spraying. Then take into consideration all the other animals that rely on moths for food, the impact magnifies. While gypsy moths and oak moths are different in their roles, the same lessons can be learned about our fiddling with the natural processes. I agree, we are short-sighted and conceited as a species. Somehow taking action is preferable to not doing anything. Maybe because pest control is a highly profitable business?