Cabbage White Butterfly
One is often encouraged to believe that a garden full of healthy insect life is a good omen. Bees and butterflies pollinate plants and flowers, making them important for a gardens survival. It is often easy to be filled with pride upon the sight of fluttering butterflies and buzzing worker bees; signs of a thriving garden, a bustling eco-system full of life.
However, there is always a downside to every up. And for any aspiring vegetable grower, there are few creatures more annoying than the cabbage white butterfly. Sure, it may be a wondrous event to behold a horde of flapping white majesties drifting around your garden, but two weeks later the ugly truth will be revealed. The eggs hatch and suddenly what was once a lovely spectacle is now turning into a chaotic situation as crops are slowly eradicated.
Deceptively alluring, the species scientifically named Pieris rapae, has been the bane of many vegetable farmers globally. Originally populating Europe, North Africa and Asia, it was accidentally introduced to North America, Australia and New Zealand, where the species proceeded to spread rapidly causing extensive damage to crops.
The Butterfly Effect
Owing to geographical boundaries, such as oceans or mountains, certain ecologies are never exposed to a wide variety of alien species. The white cabbage butterfly was mistakenly introduced into North America in the mid 1800’s. It was first spotted in Canada in 1860, after which it proceeded to spread through the United States, reaching Hawaii in 1893.
New Zealand was the next country to be invaded, with first sightings being recorded in Napier in March, 1930. It later reached Australia’s shores and was so favored by the environment that it spread across the whole country in less than four years. The damage was so overwhelming that natural enemies such as parasitic wasps were introduced.
Know your Enemy
Attacking cruciferous crops such as cabbages, swedes, turnips and other brassicas, the cabbage white butterfly is an insect to be wary of. The adult butterfly, however, is not directly the problem, but rather the larvae. With powerful mandibles for cutting through leaves, this caterpillar can be devastating to crops.
The white cabbage butterfly is often the first butterfly to appear at the beginning of summer. Often affectionately called the summer snowflake, the adults can be identified by their white wings with one or two dark spots.
The adults mate and the female lays her eggs on the inside of the crucifers or the undersides of the leaves. The eggs are laid singly, and are the shape of a bullet, of less than 1mm in length, with a distinctive yellow colour, making them difficult to spot.
The eggs then hatch into green larvae anywhere from five to fourteen days after they were laid. These little monsters are the vegetable gardeners’ biggest problem. They devour the leaves, leaving gaping holes and more often than not they dig deeper to the heart of the plant, leaving a rotting shell of a vegetable in its place. When they burrow through the leaves they leave a green or brown deposit behind them.
The larvae then pupate and emerge as the second brood of butterflies, ready to start the whole cycle again.
Prevention is better than cure
Thankfully, there are a variety of ways in which you can save your vegetables from this pest. Depending on where you live and what your personal preferences are towards insecticides, there are plenty of non-invasive methods to choose from.
As the government in mid-19th century Australia realised, introducing parasites, such as native parasitic wasps, is a noninvasive method of killing off the white cabbage butterfly. The wasp Apanteles sp. parasitizes the cabbage white caterpillars, laying its eggs inside the caterpillars’ body. The eggs hatch and the wasps’ larvae devour the caterpillar from the inside out. While this method is more suited to the commercial growing of crucifers, it should be considered as it results in little to no damage to the ecology of your garden.
Other parasites to attract include: braconid and the tachonid fly. Both of these are attracted by plants such as dill and parsley, so think about introducing these and other herbs into your garden. Also, reduce the amount of insecticides used which may kill not only pests but also beneficial insects.
If you find dead, parasite-infested caterpillars don’t throw them away! Take advantage of the situation and lay them in problem areas of your patch to help spread the parasite and eliminate the problem.
The white cabbage butterfly may not look appealing to the seasoned gardener, but to various garden predators it looks absolutely delicious. One great way of preventing the development and spread of the pest is to introduce one or even better yet, more of its many natural predators.
Ladybird beetles are a good sign that natural biological control is occurring. Attract and maintain levels of ladybird beetles by planting flowering plants of different species to give the beetles a wide selection of food, otherwise they end up eating each other! Not only will this help increase your vegetable yield, but you will also be creating a beautiful garden to boot.
Other natural predators include lacewings and the weaver ant. Weaver ants require a lot of effort to attract to a garden, and if you’re looking to go down this route speak to a horticultural expert at your local nursery. Lacewings can be attracted by dill, sunflowers and carrots – flowering plants are a good choice.
Moving to predators on a larger scale, it is recommended that you encourage small, insect-eating birds to populate your garden. You can do this by using bird feeders and having a constant supply of water set up. Constructing places for them to nest will also encourage birds to stay.
Use special plant covering, stockings or just mosquito netting, to swathe your plants and prevent the adults from laying their eggs.
If you feel like doing something a little more hands on, walk around and inspect the underside of the leaves, of all your brassicas. Pick off the caterpillars and give the fish a tasty treat or look out for the signature yellow eggs and be sure to crush them by rubbing them between your fingers. The adult females generally lay them around the sharper edges where there is more grip, so this should be the first place to check.
The best time to do this is after you see large numbers of cabbage whites hovering around your plants, as their eggs have almost certainly been produced.
A butterfly net is another useful tool and can be used to catch the adults, although this requires a little more practice and effort than squashing the butterflies eggs before they hatch; a much loved activity for young children.
One purported deterrent is the use of white stones and eggshells, placed near and around the crucifers. These totems are supposed to resemble the white wings of the butterfly, and are meant to trick the adults into thinking that the plant is already overpopulated. This can also be achieved by tying strips of white fabric to canes or trellises above or near crops, resembling butterflies with wings that flap in the breeze.
Unfortunately, if the larvae have hatched and established it is hard to save your crop, especially if they have eaten through the heart. However there are a few methods for killing off and surviving caterpillars.
One effective way of treating the caterpillars is by introducing a naturally occurring bacteria Bacillus thuringiensis, which kills the caterpillars and leaves the predatory insects unharmed. The caterpillar eats the treated leaf, gets an upset stomach stops eating and will die within four days, the bacteria should be re-applied weekly for the duration of the infestation. It should be applied in the late afternoon, and should not be sprayed when wet weather is imminent, as it will wash away and become ineffective. In Australia this product has gained organic certification, is safe for use in conjunction with fish, bees, ladybirds, mammals and pets, and is commonly sold by Yates under the name Dipel.
This is a great alternative to chemical insecticides, which not only require vegetables to be thoroughly washed before eating, but also kill beneficial insect life. Other substitutes to commercial chemical sprays include using garlic and chilli spray or diluted worm leachate, which can also act as a repellant.
Always be sure to carefully follow the manufacturers’ instructions when mixing products. Note the safety warnings and wear appropriate protective equipment.
Just because your cruciferous crops are suffering does not necessarily mean that the white cabbage butterfly is to blame. If you notice slime trails encircling the plants, then your culprit is the slug or snail. This may seem a small detail, but if details are overlooked, plenty of time and money can be invested to solve a problem that does not even exist.
Keep your eyes open and your mind aware and your garden will be fine. Next time you see a fluttering of white across your crops, admire it for a minute, take a picture and grab your butterfly net.