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.
A brief inspection of the garden this week reveals
holes and tattered margins on the leaves of many
plants. Sometimes 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. A quick
lesson on the life history of moths will help
explain why it seems like caterpillars are
overwhelming our gardens in Orange County both
coastal and inland right now.
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 p.m. that night it was 71 degrees
Fahrenheit. Even at the coldest moment, at precisely
1:30 in the morning, the thermometer was still
sitting at 65, 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 backyard. It was a garden
party of epic proportions, and the attendees were
coming from far and wide.
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, 30 days later, the caterpillars have grown
to become leaf-eating machines.
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 "caterpillar" will work nearly as well.
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 it's
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.
