Natural Predators And Oakworm Defense

By Jutta Thoerner   UCCE Master Gardener

Click, Click, Click. The noise is relentless, insistent, and amplified once I step outside under my Coast Live Oak. I look up and peer into the foliage: half eaten leaves and skeletonized leaf ribs are staring back at me. I pull down a lower branch, examining the leaf. I find the culprits: yellowish-green caterpillars with dark stripes. The oakworm. Now I see hundreds more, many suspended in air on fine silk threads. A few days later I find them crawling up the side of my house, with many underfoot on the sidewalks. My Coastal Live Oak is naked, stripped of its green foliage. 

This happened a couple years back in April and May. Looking out my window today, that same oak tree is beautiful, green and healthy.  The oakworm, the larval stage of the oak moth, seems to descend on our oak trees in a cyclic fashion; coming every 2-3 years, devouring the oak leaves in spring (April-June) and fall (Sep-Oct).  Oak trees have other caterpillars that eat their leaves, most of which are “hairy” like the tent caterpillars. These moths are less damaging as they have one caterpillar stage a year and infestations are over by mid-summer.  The problem with the oak moth is that they can have up to 3 generations of caterpillars in one year, causing extensive defoliation of our oak forests.

It is unnerving to stand by and watch a beautiful oak tree being stripped of all its leaves, but stand by you should! Studies show that unless the oak tree is already stressed from summer overwatering, root cutting or severe drought, they will withstand these haircuts without lasting consequences. But we are doers, we are gardeners; we want to do something!  Okay, here is what you can do: help out the natural predators that eat the oakworm: spiders, big eyed bugs, lacewing larvae, ground beetles, damsel bugs, assign bugs, spined soldier bugs, two species of parasitic wasps and countless birds.  Provide an insecticide free environment for the predatory insects and water and shelter for the birds. Then watch how nature takes care of its own.