[box]Since I wrote this, I have discovered the culprit is cabbage maggots, which you can read about in Ants, Cabbage Maggots, and Broccoli. I may still experiment with vaseline to smoother eggs or deter egg laying.[/box]
One of my cabbages has had the life sucked out of it. It is just one in a series of crimes perpetuated over the last few years. The cabbage or broccoli will sprout, grow to a well established juvenile stage and then, steadily, the leaves change to a sick purple until the plant falls over, limp and shriveled. Scrutinizing the evidence, I determined there were not any bugs on the plants. No holes in the leaves, no eggs. The leaves didn’t curl or have spots. In fact, in spite of the gradual change to purple, the leaves were thick and healthy until the final demise. Two of the victims are pictured below:
The only slightly suspicious thing I saw was ants in the raised bed, but no book or internet site I found listed ants as destructive to cole crops. Time and time again, however, there would be ants circling the base of the cabbage or broccoli that was dying, but none around the adjacent healthy ones. The stem they were closely circling, and sometimes climbing part way up, always got thinner and thinner. A closer look revealed that it was uniformly cut away, always below a crisply defined border.
See the healthy stem below for comparison:
I spread diatomaceous earth, which did not have any noticeable effect, but the plants may have already been too far gone; I also probably needed to wait more than a day… Then I looked up “ants” in one of my favorite garden remedy books, The Truth About Garden Remedies by Jeff Gillman. It recommended insecticidal soap. For the few plants I was going to spray, I just used a regular spray bottle, but after spraying the whole section my hand was cramping and fatigued and I wished I had started with a pump pressure sprayer. The ants disappeared for a while. Still, I wasn’t convinced it was the ants. I needed to investigate further.
(See “my” ants below next to a maple tree seed. These are small ants.)
Rodale’s Garden Problem Solver has been the backbone of my garden detective work for years, but it doesn’t say anything about ants bothering cabbage or broccoli. The descriptions within are clear and precise and in many other cases have helped me. This time, I have looked up cutworms, though I never find them at the scene of the crime. Today, I dug up the dirt at the base of three terminal plants, and still didn’t find any cutworms. I did find nearly non-existent root systems and a millipede. Now, I think the ants are just opportunistic scavengers with a taste for freshly peeled cabbage or broccoli; the millipede is an innocent bystander. The cutworm has the means and the motive. I am indicting the cutworm.
Edit 6-4-2015: After dealing with this for 3 years, it does really look like it is the ants chewing around the base of the stalks. If I am very diligent with a good layer of diatomaceous earth it helps keep them at bay. Other than that, Bonide Chemical QT Eight Vegetable Fruit and Flower Spraykills them and also keeps them away for a couple of days.