Imported Cabbage Worms: now feasting in a garden near you!

Imported Cabbage Worm feeding damage. They've been feeding for more than a few days.

Since mid-June I’ve been finding Imported Cabbage Worms in my vegetable gardens, munching on the leaves of cabbage and kohlrabi.  From the looks of my cabbage they’ve been feasting since early June.

Signs of Imported Cabbage Worms:

* leaves that look like Swiss cheese.

* Worm feces – fresh droppings are moist dark green blobs.

When you see these signs look closely to find the culprit. The imported cabbage worm is a slender, velvety green caterpillar  up to 1.25” long which feeds on many plants in the cabbage family.

Imported Cabbage Worm - larger than life

The adult is a white butterfly with black tipped fore-wings and one or two black dots on each wing. You may see several of these butterflies fluttering over a garden planted with collards, kale, or cabbage. The female lays her off-white eggs singly on leaves of host plants.  The newly hatched larvae are tiny and pale green.

Imported Cabbage Worm butterfly

Imported Cabbage Worms feeding. Note droppings.

Imported cabbages worms can be controlled by hand picking from crops such as cabbage and kohlrabi.  The caterpillars often align themselves with the edges, veins, or ribs of a leaf  and they feed on both sides of the leaf. Close inspection is necessary. I’m brutal: whenever I find a cabbage worm, I crush it on a rock.

On broccoli, cabbage worms crawl up the stalks and feed on the underside of the head where they are almost impossible to find. My first experience cooking broccoli with cabbage worms is still vivid.  I knew they were there so I had inspected and cleaned carefully. I even soaked the broccoli in salted water before cooking which caused a few more to float out. Finally I boiled the broccoli and discovered the large number that still remained.  The broccoli went to the compost pile.

Broccoli plants protected by a floating row cover.

A floating row cover, applied immediately after transplanting, can control cabbage worms by preventing the butterfly from laying her eggs.  A floating row cover is extremely light weight non-woven fabric which blocks insects but lets almost all sunlight through.  Drape it loosely over the plants with the edges held against the ground by rocks, wood, or soil. There must be slack in the fabric to allow the plants to push it up as they grow. A floating row cover also protects against cabbage maggot.

There are also insecticides which can be used against cabbage worms. The only one I use is Bacillus thuringiensis (Bt) which can be used in organic gardens. It is available in garden centers under a couple different product names.  Bt works by infecting the caterpillar’s gut, causing it to stop eating and eventually die. Bt works fastest when the caterpillar is tiny. I generally use Bt when I’ll be away from my garden for several days, and unable to hand-pick the cabbage worms. I also use Bt on broccoli plants after I’ve harvested the main head, and the plant is too large to fit under my row covers.

Imported Cabbage Worm pupa and catterpiller on a harvested cabbage.

My favorite method of eliminating imported cabbage worms is to have someone else do it for me.  For many years my garden has contained a pair of bird houses favored by house wrens.  Wrens are voracious insect eaters, gleaning meals as they hop around the garden and bushes.  Their nestlings  enjoy many delicious meals of cabbage worms.

Parsnips in Bloom

I’m enjoying the bright yellow splash of parsnip flowers in my vegetable garden. Parsnips are biennials; they bloom and set seed in their second year.

Parsnip 'Hollow Crown' flowers and buds, 5/29/2010

Last fall I left my parsnips in the garden to harvest as needed for cooking. The soil never freezes under 6″ of shredded leaves, so I harvested parsnips all winter.  When the weather warmed, the remaining parsnips resumed growing.  The texture of the parsnip changes during the new growth, so I stop harvesting.

One year old parsnips growing new leaves 4/4.

Year old parsnips grow rapidly. By the end of May the flower stalks were 5′ tall and in full bloom. The flowers resemble Queen Anne’s Lace flowers but in yellow.

Parsnips in full bloom 5/29.

Now, in mid-June, the stalks are top-heavy with blossoms and developing seed. Many stalks have toppled. The seeds currently look like flat, green disks. I’ll be watching as it matures, in the hope of collecting seed to plant next spring.

Parsnip flowers and seed 6/12

All the pictures are of the same planting of parsnip ‘Hollow Crown’.  The plants in the background are snow peas.