Cabbage white butterflies have had a lovely time with my broccoli this year. There are several conditions that make brassicas very attractive to these pests and the brown cabbage moth. See Cabbage white butterfly.
My problem was that I inadvertently added some very alkaline compost to that area of the garden, (See recent post on Compost pH) and it is taking a while for the soil to get back to a neutral pH.
After spending a week or so removing eggs, squashing tiny caterpillars, or feeding larger ones to the chooks, I remembered a tip someone gave me long ago to deter these pests but have not needed to use before. The tip was to slip the plastic clips that seal loaves of sliced bread onto the edge of some of the Brassica leaves. White seals to deter the C W butterfly and beige ones for cabbage moth. The theory being that the adult butterflies and moths will “think” that eggs are already being laid on these plants and they look for another food source for their larvae.
My broccoli plants are looking healthier already and I have not found any more eggs under the leaves, but I don’t know if the seals are working or the pests are no longer present in our area. Has anyone else tried this tip?
If you come across a cluster of small, yellow cocoons on leaves of Brassica vegetables – don’t spray them or feed them to the chooks. They do not belong to garden pests. (See photos below.)
These cocoons are, in fact, the pupation stage of a very small, black wasp. This wasp belongs to the Braconidae family. Braconids are parasitic wasps and very beneficial insects to have in your garden.
The female adult wasps, which are barely 5 mm long, lay their eggs in caterpillars of the Cabbage White Butterfly, which feeds on the leaves of stressed broccoli, Brussels sprouts, cabbage, cauliflower, horseradish, kale, radish, rocket, swedes and turnips.
The wasp larvae then feed on the caterpillars from the inside until they are ready to pupate. (Sounds gruesome, doesn’t it.)
They each then spin a small cocoon on the remains of the caterpillar, and hatch out two or three weeks later to repeat the cycle.
Adult wasps feed on nectar from flowers. It is worthwhile growing some nectar-producing plants to encourage these useful pest predators.
These flirtatious little butterflies can be destructive to stressed Brassica plants. If your plants have been attacked, squash any green caterpillars or feed them to the chooks. They often hide along the mid ribs of leaves making them difficult to see. Check daily for newly laid CWB eggs (bright yellow dots – usually on the backs of leaves), and brush them off. Also check for newly hatched larvae – these appear as fine green threads hiding under the leaves, and can look like leaf veins.
White cabbage moth attack is a sign that either:
1) Your plants could do with more water. Brassicas need thorough, regular watering – not a daily sprinkle. Mulching the bed reduces water loss and encourages horizontal movement of water through soil.
2) You have been a bit mean with the complete fertiliser when preparing the bed. Brassicas prefer a humus rich soil to provide a good supply of fertiliser. If they are not making steady growth, a side dressing of compost (under mulch) or applications of complete fertiliser, applied as a tea, can correct the problem. Or, they are missing some essential trace elements that you can supply with a drink or two of good quality seaweed extract tea (such as Acadian, Natrakelp or Seasol).
3) You have added enough fertiliser but the soil is too acid or alkaline for the plants to absorb what they need for pest resistance. If it is a case of too acid – and this can be remedied with an application of dolomite or agricultural lime. If you suspect acidity, apply a handful per square metre of bed and water it in. Avoid using hydrated lime on beds that contain plants as it can burn plant roots. If your soil is too alkaline, the addition of elemental sulphur will reduce alkalinity.
However, if they are making a total mess of your plants, apply Dipel while waiting for soil conditions to improve. Dipel will kill the caterpillars without killing good insects.
This family, called the Brassicas, tend to be more susceptible to attack from the Cabbage Moth and the Cabbage White Butterfly when conditions are too warm for them or when the soil they are growing in is too acid for their liking, especially while the plants are young.
The cabbage family includes Brussels sprouts, Chinese cabbage, cauliflower, horseradish, kale, kohlrabi, mizuna, mustard greens, swede, tatsoi, turnip, watercress and stock, and is also related to radish. These plants need both boron and molybdenum for healthy growth and these are only available to plants when the soil pH is close to neutral. If you know that your soil is acid, and your Brassica plants are being attacked, give the bed a drink of dolomite or agricultural lime. Dissolve a generous handful in a full watering can and apply this to each square metre of the bed. Repeat the application if pests are still hanging around in two weeks.
In the meantime, remove all pest eggs from under leaves and leave crushed caterpillars on the leaves. This helps to deter further egg laying.