We learned this tip from a fellow student in Seed School back in the day.
As many gardeners know all too well, the cabbage butterfly (latin name Pieris rapae) is a small white butterfly with black spots. Often called a cabbage moth, the female lays her eggs on plants in the cabbage/mustard ("Brassica") family. These include cabbages, Brussels sprouts, broccoli, kales, radish, cauliflower, collards, pak choi, etc. The larvae - greenish caterpillars, aka "cabbage worms" - that hatch live on the leaf undersides and eat the plant leaves for sustenance, causing those holes that seem to appear overnight. Unchecked, the caterpillars can seriously impact the quality of the developing plant. Usually, by the time you start to see the butterflies flitting about your brassicas, the eggs have been laid and damage is already being done.
Lots of remedies are touted for protecting the brassicas. Among them:
- companion planting plants near or among brassicas and which repel the butterfly. These include aromatic herbs (lemon balm, sage, oregano, borage, hyssop, dill, rosemary) and/or high blossom flowers (tall marigold, calendula)
- companion planting plants at a distance and which attract the moths. Nasturtiums are great for this.
- spraying repellant solutions on the plant. DIY solutions generally include some dishwashing liquid and optionally, one or more of: neem oil (or equivalent), garlic and cayenne. Lots of recipes are offered online.
- hand cleaning - turning each leaf by hand and removing the cabbage worms. This can be hard to do once the worms get into a developing brussels sprout or cabbage head.
New idea we learned
It turns out the butterfly is apparently territorial. So, if you provide a "look-alike" decoy among your brassicas, the real insects will stay away. There are all sorts of decoys suggested on the internet. Everything from exact replicas, to white X's with black spots on them. People also have made them out of all kinds of materials: paper, white milk jug plastic, fabric, and full 3D printed plastic. Folks also have planted the decoys on stakes or tied them to something so they flit about a bit. All variations seem to work.
It seems that the two key features needed for success are:
- include the black marking of the female (two spots/upper wing, vs the male with only 1spot/upper wing), and
- making the decoy slightly bigger than the real thing. The wingspan of the adult is about 1.5-2 inches, so making the decoy about 2 inches is good.
Here's an easy to construct decoy with a simple installation that seems to work well. We like it because it's easy to make and the tape both protects the decoy against the elements and provides a surface that moves well in a breeze. Let us know if it works for you!
We first read about this particular decoy here: (http://www.reddit.com/r/gardening/comments/2yz6e0/i_have_a_terrible_problem_with_white_cabbage/)
Step 1. Cut out paper decoy representations of the butterfly. Here's a single page template you can download
Step 2. Cut lengths of string or thread and space them out on the sticky side of a clear piece of packing tape, spaced out so there is one thread for every cut out you are going to use. We cut our thread to different lengths so they'd be at different heights in the garden.
Step 3. Place your cut outs on top of the thread on the tape.
Step 4. Add a second piece of tape over the first, so that the cut outs and thread are sandwiched between the tape. Now cut the tape into squares with a butterfly cutout in each square. The tape provides enough stiffness so the decoy will flit about in the breeze.
Step 5. Tie your decoys to stakes, plant them in your garden, and let your decoys do the rest!