Gardeners in Orange County, either coastal or inland, are noticing an unusual number of holes and chewed leaves on many of their plants. A brief inspection of the garden this week reveals holes and tattered margins on the leaves of many plants. Small holes on some plants, large holes on others. Sometimes the little chewers avoid the veins of the leaves; sometimes they don’t. Some leaves look like they were hit with a shotgun. In the worse cases, entire leaves are gone; chewed to a stub.
The culprits are caterpillars. Caterpillars are the larval stage of moths and butterflies. However, it is moths that cause the majority of the damage in our gardens. Butterfly larvae are seldom pests.
Sometimes gardeners refer to these pests as “worms”, but a “worm” could apply to many slithering, crawling creatures; like earthworms and flatworms, beetle grubs, nematodes, sawfly larvae and more. “Worm” is a bit confusing. What we’re really dealing with are “moth larvae”, but since that will never catch on; the term “caterpillars” will work nearly as well.
Does it seem like there is more caterpillar damage in your garden than usual, especially in the last couple of weeks? In fact, there is, and there’s a good explanation.
A quick lesson on the life history of moths will help explain why it seems like caterpillars are overwhelming our gardens.
Moths fly at night, primarily when the temperatures are warm. Take an evening walk with a flashlight through your garden in winter or spring and you may have a hard time finding a moth. Turn your porch light on in March or even April and see how many moths are attracted. Not many.
Adult moths are active when night temperatures are high. For a moth, the warmer the better. Also, moths navigate by the light of the moon. A night with no moon means fewer moths than a night with a full moon.
Warm nights and full moons result in lots of moths. Now, jump back in time to about a month ago, specifically to the night of July 11. It was a Tuesday and we were in the midst of our first big heat wave. On July 11 the mercury soared in central Costa Mesa and Newport Beach. If you were out in your garden at 10:00 pm that night it was 71° f. Even at the coldest moment, at precisely 1:30 in the morning, the thermometer was still sitting at 65° f, three degrees warmer than the historical average for the day.
Just as important, the moon was full on the night of July 11. A warm and balmy July night; and a full moon. Party time for a moth, and the party was happening in your own back yard. Your garden was abuzz with moths that night. It was a garden party of epic proportions and the attendees were coming from far and wide. There were more moths in the air on July 11 than any other night of the year.
Of course, the moths in your garden that night had a purpose. The reason for their big party was to mate and lay eggs. Moths are incredible egg laying factories. An adult moth can deposit hundreds of eggs on your plants in a night. About 10 days later these eggs hatched and out came hungry larvae . . . or caterpillars. For the first week or two these little guys were so small a gardener either doesn’t notice the damage or doesn’t give it much attention.
But now, thirty days later, the caterpillars have gown to become virtual leaf eating machines. There are lots of holes in the leaves of your plants and it may appear to be getting worse every day. In fact, it probably is.
Birds and diseases claim a few of the caterpillar bounty. Braconid and other wasps, Tachinid flies, and some other beneficial insects will cull a few more. For the rest of the caterpillars, a gardener has a few options. If you’ve one of the lucky ones you may be able to do nothing. A few chewed leaves, in the larger scheme of your garden, may not be a big deal. You can live with it.
If the infestation isn’t too heavy or is isolated to only a few plants you may be able to hunt for them and dispose of them manually. They can be difficult to see, so a keen eye is an asset.
Bacillus thuringiensis var. kurstaki, “Bt” for short, is the best pesticide control. Bt is a naturally occurring microbial pathogen that is specific only to the larval stages of moths and butterflies. It has no effect upon other insects, including other chewing pests like sawfly larvae, grasshoppers, snails and slugs. Likewise, it has no effect on reptiles, amphibians, birds, natural predators, pets or mammals. It can be used on vegetables, herbs and fruits with no contamination issues.
Bt mixed with water and applied as a spray is odorless, tasteless and invisible. Once a young caterpillar eats a small amount of foliage and ingests Bt it stops feeding. In about 24 to 48 hours the caterpillar dies.
Unbeknownst to you, a month ago there was quite a party in your garden. You weren’t invited, but now, a month later, you’ll have to do the cleanup.
Ron Vanderhoff is the Nursery Manager at Roger’s Gardens, Corona del Mar