Diamondback Moth

Diamondback moths are common pests of cole crops.

Cole crops include broccoli, Brussels sprouts, cabbage, cauliflower, and Napa cabbage. That’s where I saw the damage. My Napa cabbage was growing very nicely, enjoying our cool winter weather, but the leaves looked more like Swiss cheese. That’s because diamondback caterpillars prefer older leaves, so the first sign of infestation you will see is a bunch of holes in the larger leaves. They also feed on flower buds and flower stems.

Adult diamondback moth (Olaf Leillinger) CC BY-SA 2.5

Diamondback moth lifecycle

Adult diamondback moths (Plutella xylostella) lay their eggs on the underside of leaves and they are very difficult to see. When the eggs hatch, tiny caterpillars burrow their way into the crown for safety. When a diamondback moth caterpillar is disturbed, it will jump ship, rappelling away from danger using a thin silken thread to swing to safety. After feeding for a couple of weeks, diamondback moth caterpillars spin a loose cocoon on nearby leaves or stems, where they pupate.

Adult diamondback moth (Landry J-F, Hebert PDN) CC BY 3.0

Diamondback moth control

Personally, I have caught a few diamondback moths on good old fashioned fly paper strips. That isn’t a very effective control method however. Maintaining biodiversity in your garden is one way to attract the ichneumonid wasp, a predator of diamondback moth caterpillars. Spraying Bacillus thuringiensis and spinosad are effective organic control methods.

The green diamondback moth larva is alive and well, the reddish one was killed by 'Beauveria bassiana' spores, and the middle one is covered with spores that have erupted from within. (Merle Shepard, et al) CC BY 3.0 US