The cabbage moth is the main pest of concern in the vegetable garden.
Some of the most powerful chemicals on Earth occur naturally in plants. Many plants and animals manipulate these chemicals to survive and none more so than the caterpillar of the Monarch butterfly which exclusively eats the toxic leaves of the poisonous milkweed plant. The toxins are sequestered by the caterpillar thus keeping it safe as the toxins make it unpalatable to predators.
In the same way we can spray garden plants with botanical oils that repel chewing and stinging insects, there are many plants that contain chemicals that will either, off-gas a repellent odour, contain chemicals in their roots which are released into the soil, or contain chemicals in their foliage which is toxic to chewing insects. The gardener can use plants containing these natural chemicals to protect susceptible plants.
The cabbage moth is the main pest of concern in the vegetable garden, as its name suggests its target species is the brassica and beet family which include most of the healthy green vegetables we like to grow in FNQ at this time of the year.
This pesky moth is around 10mm long, with pale grey wings and the adult can lay a cluster of over 100 eggs on the underside leaf of its host plant every night.
Within a week the eggs hatch into caterpillars that can completely eat and destroy a large vegetable plant in one day.
The caterpillars munch away for a couple of weeks before they encase themselves in a thin, silky cocoon, about 9mm long, this pupa will descend 2 to 10mm into the soil and within the week the adult moth emerges ready to mate immediately and repeat another life cycle.
The cabbage moth caterpillar is the single pest that causes most vegetable gardeners to give up, but this pest can be outsmarted.
Firstly reduce the numbers of moths laying eggs in your garden, this can be achieved by interplanting cabbages and beets with plants that off-gas chemicals that mask the smell of these host plants. This way the moth cannot identify the plants she is seeking.
To fool the cabbage moth interplant all members of the cabbage and beet family with either onions, shallots, or leeks.
In addition plant Thai basil and citronella grass borders to the vegetable garden. Thai basil and citronella grass are perennial plants that will grow to 1.5 metres. Prunings from these plants are good to use as vegetable garden mulch which will double their repellent capabilities.
Make a “tea” from the leaves of plants containing repellent volatile oils and use to spray on to cabbage leaves or other host plants to make them unpalatable to caterpillars. Soak overnight the leaves of either marigolds, citronella grass, nasturtiums, Thai basil, garlic, mint or chilli fruits in a bucket of warm water and add a tablespoon of liquid soap.
The next day pour the solution through a sieve and use to spray on to the susceptible plants, do so regularly.
The cabbage moth is territorial and spreading crushed white egg shells around the garden will confuse her into thinking there are other moths already around the plants. Traps used in combination with a Pherodis dispenser can be made to trap and monitor moth numbers.
A wide range of botanical chemicals are commercially available as insecticide sprays for the gardener. Be mindful, they will also kill beneficial insects.
These insects include natural caterpillar predators that should be encouraged to create a balanced ecosystem in the vegetable garden. Encourage birds, mantis, spiders and wasps to take up residence in or near the garden and they will help control caterpillars and other pests.
Hand removal of moth eggs and caterpillars is effective but plants need to be inspected daily. The moths lay their eggs at night and can sometimes be found still doing so in the early morning.
Another method of control is the use of fine nets to exclude the pests. However this will also exclude pollinators and other beneficial insects.
The netting can be a problem in weather events where it might collapse on to the plants and it’s also a bother when weeding, fertilising and mulching the garden.
Identify and get to know the pest as that makes the tasks of control so much easier.