With oak moths the question is whether to spray or not to spray
The California oak moth is a creature of extremes. Most of the time these small white insects fly completely under the radar. But every once in a while, they go on a breeding frenzy that is impossible to ignore.
Swarms of moths lay their eggs in oak trees and the newly hatched caterpillars, known as oak worms, strip oak trees of leaves and litter sidewalks and driveways with their waste.
The bare trees and the caterpillar waste-covered sidewalks can be an eyesore and a nuisance for those experiencing an oak moth breeding frenzy. But not everyone is convinced that getting rid of them is the right solution.
As the name suggests, California oak moths are a native species, and their offspring like to feed on native coast live oak trees, or any other oak tree that happens to be in their path. Despite being discovered more than a century ago, much about these insect’s behavior and life history is still a mystery, says California State Monterey Bay environmental scientist Fred Watson. For instance, scientists know that the moths go through two reproductive cycles in a year – one in the spring and one in the fall – and that once every couple of years they go through a population boom, which is when their effect on the oak trees is the most apparent. However, they don’t know how and why these booms occur.
“For something that has such a profound effect on the ecosystem, it’s weird that we don’t have some of the most basic questions answered,” Watson said.
Because so little is understood about this species, no one knows when and where they will strike next. Although the booms are supposed to happen every 3-5 years, some think they might have become more frequent in recent years.
Wayne Belville, the pest department manager at local pest control service Casner Exterminating, said his team treated hundreds of trees last year, and that they are treating hundreds again this year, with outbreaks currently occurring in Pebble Beach and Carmel.
Peter Oboyski, an entomologist at the Essig Museum of Entomology at the University of California, Berkeley, recently made an announcement stating he moth’s numbers are likely to be high in Berkeley again this year, despite also having been hit with the moths two years in a row in 2019 and 2020.
“The reasons for outbreaks, the where, the when, seem to be random,” he said. “Nobody’s really able to get a handle on why.”
The damage these oak worms cause is undeniable. “It only takes one season to completely defoliate the tree,” said Belville. At UC Berkeley, where Oboyski works, the caterpillar’s unsightly strike occurred just before dignitaries were supposed to arrive on campus. “It’s usually a little too much for people to take,” Oboyski said.
Although the damage can look devastating, Watson says that it’s not usually as bad as it might seem. Although the caterpillars devour the oak’s leaves, they leave behind any leaf buds, which allows the oak trees to completely regrow their leaves. And since the outbreaks usually only happen once every few years, trees should have the time to recover.
“There’s probably some kind of balance to keep both the moths and the oak trees healthy,” Watson said.
Moreover, although losing their leaves can be tough for the trees, Oboyski said that the oak moths can actually provide benefits to the tree in the form of soil fertilization. The bodies and feces of the caterpillars are full of nitrogen, an important nutrient that isn’t always readily available. “I’m sure if the oak trees could speak for themselves, they’d say ‘I’d rather keep my leaves, thank you,’” Oboyski said. “But at least the caterpillars are replenishing the soil.”
Both Oboyski and Watson argue that since oak moths are a native species, which we still have much to learn about, the best thing one can do when experiencing an oak moth outbreak is to let them be. Since oaks can regrow their leaves after an infestation, the worm’s effects are only temporary, they argue. Moreover, Oboyski says that treating a tree for oak worms could have unintended consequences, potentially harming other wildlife occupying the tree. “Spraying these trees would be killing all those other native things that the lizards and the birds and everyone else depend on.”
However, Belville argues that it’s not that simple. Even though oak trees can regrow their leaves after an oak moth infestation, he says that the process is a very strenuous one. If the moths come back again the next year, which he says they have been doing, the stress on the tree’s defenses can make it vulnerable to infestations from other pests, which could lead to its death. These deaths are not only damaging for property values, they can also be difficult emotionally.
“I’ve seen people devastated when their oak tree dies,” Belville said. Some people have had these trees on their property for decades, sometimes centuries, he said, and have many memories associated with them. “It’s just heartbreaking.”
Property owners who want to prevent oakworm infestations should make sure to treat their trees as soon as possible, Belville says, ideally as soon as they see the little white moths flying around their trees. Injections to the tree during the egg-laying process, or when the caterpillars are still quite small, prevent most of the damage to the tree’s leaves. But whether or not people choose to get rid of their oak moths depends in the end on what their priorities are, says Watson.
“It’s all a matter of perspective,” he said. “If you don’t like squishy caterpillars on your sidewalk, then you could think of them as pests. But if you value natural processes with native organisms, this is that, and we celebrate that.”
