a chewed Blue Gum leaf

If you have spent more time outdoors the last two years exploring new trails in the area, you may have noticed the notched leaves on the large Blue Gum eucalyptus trees that line certain trails and cover large swaths of real estate in Tilden Regional Park, and wondered “What kind of bug is chewing these semi-circular notches on the leaves? It must be a big bug!”
Not a big bug but…
The notches you are seeing are produced by the Eucalyptus Tortoise beetle which is only 1/4 to 3/8 inches long. About the same size and shape as a large ladybug only brown not red. However, like most wildlife, they are difficult to see as they tend to hide under loose bark during the day and feed primarily at night. And they tend to hang out high up in the tree where it’s hard to reach them.  
How bad is the damage they inflict on trees?
Adult beetles and larvae chew semicircular holes or irregular notches along edges of eucalyptus leaves. The beetles can remove most of a leaf’s surface, leaving only the midvein. Unsightly, tattered leaves are usually just an annoyance that does not appear to threaten eucalyptus survival. Well-established and properly maintained eucalyptus appear to tolerate extensive leaf feeding. Thus, no tortoise beetle control is needed in most landscape situations.
During heavy infestations, trees can lose most of their leaves, which increases tree stress. Although these beetles alone are not known to kill trees, their feeding adds to that of more than a dozen other new eucalyptus pests introduced into California during the last three decades. Combined stress from multiple pests, especially if growing conditions or tree care practices are not optimal, could eventually lead to tree death.

Eucalyptus Tortoise Beetle

Wait! They’ve only been here 30 years? How is that?
You may or may not be aware that eucalyptus is not native to California. They were first introduced in the 1850s brought by Australian gold miners (The history of eucalyptus trees in California is fascinating and a topic for another newsletter). The eucalyptus trees lived a charmed life for 150 years because when they were brought here they left all the pests and diseases that afflicted the trees back home in Australia. However, that all changed in 1998 when biologists discovered several different bugs, including the Eucalyptus Tortoise beetle, had hitch-hiked a ride to southern California. Scientists are not sure how they made this journey but they suspect they hid out in packing material on a container ship and were released when the container was unloaded. Exotic pests from other countries are a huge concern for wildlife managers as they can wreak havoc on the native populations of trees and plants. 

Eucalyptus Blue Gum

Eucalyptus Tortoise beetles are not a big problem in Australia
What’s interesting is that in Australasia the Eucalyptus Tortoise beetle barely moves the needle on the panic alarm of devastating eucalyptus pests. That’s because the natural enemies of the beetle keep their populations in check. However, those insects did not make the journey to California and therefore the Eucalyptus Tortoise beetles can proliferate without any natural constraint. 
I have a Silver Dollar eucalyptus in my yard, should I look for beetles on it?
Right now, the only species of eucalyptus that is most affected by the Tortoise beetle are the Blue Gums (Eucalyptus globulus) the giant-sized eucalyptus species which tend to grow in wide-open spaces, like public parks. They are rarely found as landscape trees in backyards because of their overwhelming size. But if you are one of those lucky people with a Blue Gum in your yard, you can help minimize damage from the Tortoise beetle with good cultural care. Most important is to make sure they are well hydrated. Even though eucalyptus are drought tolerant they can benefit from infrequent irrigation (possibly once a month during drought periods) but with sufficient amounts so the water penetrates deeply into the soil (1 foot or more below the surface). However, avoid prolonged waterlogging, especially around the root crown, because eucalyptus trees are susceptible to pathogens that cause Armillaria root rot and Phytophthora root rot, which are favored by wet soils. 
Dazzle family and friends with your entomology knowledge 
During the pandemic, we were encouraged to take walks in nature to help relieve stress. It was discovered that you could improve stress relief by being observant of your surroundings―noticing trees, birds, wildflowers, etc during your walks. Now when you go hiking and you pass by a big Blue Gum stop and look and see if you can find the distinctive notched leaves, either on the ground or on leaves in the tree. You can impress your friends and family by proclaiming “I know what caused those notches. The Eucalyptus Tortoise beetle!”