Caterpillars bring insatiable appetites to the vegetable garden.

Along with colorful butterflies, smaller and more ordinary looking moths flit around the vegetable garden this time of year, laying eggs that hatch into caterpillars whose sole purpose is to eat and excrete. If you are growing brassicas your plants have probably already been under attack by these very hungry caterpillars.

Cross-striped caterpillar (left) and cabbage looper (right).

The main culprits here in Texas are the cabbage looper (mottled brown moth with a small white marking on each wing), cross-striped caterpillar (brownish-gray moth with darker brown spots) and cabbageworm (whitish-yellow moth with a single black dot on each wing).

What looks like a small yellow blob (less than 1/8 inch across) on the back of a broccoli leaf is actually a mass of cross-striped caterpillar eggs waiting to hatch.

These creatures are stealth; I seldom notice the moths that lay the eggs (they are more active at night) and the freshly hatched caterpillars are so miniscule and blend so well into the foliage that they can do severe damage before they are detected. Inspect your plants often, especially on the underside of leaves; if you miss them you may be surprised to find your plants decimated the next morning.

As the caterpillar grows so does its appetite.

Though these tiny munchers are good at camouflage there is one sure way to affirm their presence before too much damage is done: miniature dark green balls of excrement. Yep, what goes in must come out and caterpillars are prolific poopers.

Dark green droppings mean caterpillars are feasting nearby.

And the bigger they get the more they eat and the more they eat the more they poop. I have had more than one novice gardener tell me that they thought those little green balls were eggs, but if you look above or near the excrement you will almost assuredly find a caterpillar or two or three chewing away.

If you only have a few plants a good defense is to hand pick and destroy caterpillars or infested leaves every time you inspect your plants, but if you have many plants an insecticide will be more effective and a more efficient use of your time. Bacillus thuringiensis (Bt for short) is the recommended control for caterpillars, especially in the earliest stages of growth. It is an organic insecticide made from a naturally occurring bacteria found in soil; when caterpillars ingest the Bt-coated leaves it destroys their gut, causing them to stop feeding and die within a couple of days.

Note the tiny egg mass inside the black circle (left); so tiny yet so destructive (right).

Bt is only effective against caterpillars; it will not harm humans, cats, dogs or beneficial insects but it will kill any caterpillars that ingest it, including butterfly larvae, so apply it only to edibles that are being damaged. Be sure to follow label instructions for application rate and frequency as follow-up applications may be necessary for control. Bt is sold under different trade names, including Dipel and Thuricide, and can be applied as a spray or a dust. Liquid Bt seems to roll off some of the thick, waxy leaves of cabbage, kale and collards, so I have found it beneficial to apply Bt as a dust.

The Dustin Mizer is a good tool for applying Bt as a dust.

Several years ago my brother gave me a tool called the Dustin Mizer that I use quite often for this purpose. When cranked it emits a fine dusting of powder over and under the plant.  It is especially important to direct Bt to the underside of the leaves as that is where the caterpillars are usually feeding.

It’s always a good idea to vary insect control methods in the vegetable garden so consider alternating Bt with a product containing the active ingredient spinosad (also derived from a soil bacterium and also organic). One other option is to use lightweight floating row cover to protect cabbage crops; cover plants as soon as they go in the ground so the moth never has access to the plants to lay her eggs.

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