Cabbage worms

As soon as I see little white butterflies with greyish-brown dots on its wings visit my garden, I know it is time to protect my vegetable garden from cabbage worms.

Cabbage Worm (length = approximately 1")

These butterflies lay hundreds of tiny yellow eggs on the underside of the leaves. The eggs hatch and release velvety green larvae (cabbage worms) that love the taste of broccoli, cabbage, collard greens, and other members of the brassica genus. The larvae feed on the foliage of these vegetables, creating numerous holes. About three weeks after hatching, the cabbage worm form a cocoon-like pupal case. A few days later, a new butterfly emerges and the cycle starts again...and again, throughout the summer. In the fall, these pests overwinter and emerge again in the spring.

Cabbage worms can be very destructive during their short lifespan. Leaves become riddled with small holes, altering the appearance of the plant, reducing yield, and eventually impacting the plant's ability to undergo photosynthesis.

Some gardeners cover their crops with a light floating row cover to prevent the butterflies from laying eggs. This sounds great, but I practice intensive gardening. It is too difficult to cover just the brassica crops. AND, the floating row cover should be applied at transplanting to be the most effective. 

Alternatively, cabbage worms can be hand-picked off the plants. Yeah, this sounds like a lot of fun. Not really! This is a labor-intensive endevour that I know I will not win, so I don't even attempt to fight this battle.

I prefer to use an "organic" control that relies on the bacterium, bacillus thuringiensis (also called "Bt"). Bt can be found at most garden centers under the brand names "Dipel" or "Thuricide".

Although harmless to humans, Bt is toxic to a number of destructive worms upon digestion, including: cabbage worms, tomato hornworms, and cutworms. However, it must be applied when the cabbage worms are actively feeding on the leaves. Once the cabbage worm ingests the Bt, it will stop feeding and die of starvation within 3-5 days.

Since Bt rapidly degrades in the sun, I sprinkle it sparingly over the leaves surface just after the sun goes down in the evening. A bonus: cabbage worms mainly feed at night so the Bt will be most effective at this time as well. Unfortunately, Bt must be reapplied after it rains.

It is possible to still get harvestable ("sellable") produce despite a cabbage worm infestation, provided that it is caught and treated early in the season.