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Why are there so many of them and how can you protect your crops?
Jul
Cabbage whites – Pieris outside – are among the most common garden visitors in southern and eastern Australia. The butterfly looks elegant in white with black dots on its wings: females have a pair of black spots, while males have a single spot on each forewing. But their velvety green caterpillars are voracious beasts on brassicas – a family of plants that includes popular vegetable crops such as cabbage, cauliflower, broccoli, kale and pak choi.
The species was accidentally introduced to Melbourne in 1929 from Europe. Since then, cabbage whites have spread throughout Australia, finally reaching Perth in 1943.
Because of the caterpillars’ dependence on cabbage loopers, it is one of the most widespread crop pests in the world. Recent conditions have been favorable, resulting in enormous numbers of white cabbage loopers.
A single female can lay up to 800 eggs. After hatching, the caterpillars prefer densely populated hosts in saturated, toasty habitats. biomass could double every daymaking them one of the fastest growing cabbage caterpillars.
Anna N. Chapman/Wikimedia Commons
Read more: Why Red Fire Ants and Yellow Crazy Ants Gave The Green Airy To Invade Australia
Why are there so many of them this season?
One reason white-tailed beetles are so common these days is their ability to take advantage of last winter’s soggy, toasty conditions.
Scott Nelson/Flickr
They can also cope with the frosty. The pupa – the stage where the caterpillar develops in a cocoon into an adult butterfly – can survive temperatures as low as -20°C. Their cells can produce antifreeze proteins that lower their freezing point if the weather turns frosty.
Milder winters mean that overwintering pupae emerge and reproduce early. Adult females soon begin laying eggs on planted cabbages.
The next generation of caterpillars can begin feeding without being preyed upon, because their natural enemies take longer to emerge after milder weather. The numbers of cabbage whites then raise extremely rapidly.
What is attractive about cabbage plants?
The caterpillars are attracted to and dependent on chemicals found in cabbage leaves. These include mustard oil glucosides (glucosinolates) – particularly sinigrin, which initiates caterpillar feeding.
Adult females are attracted to brassicas by another glucosinolate, glucobrassine, which prompts them to lay eggs on the leaves. Females can “smell” these chemicals via hairs on their front legs.
Females also prefer green plants, which they find in well-watered and fertilized vegetable gardens, to lay their eggs.
Echinacea/Flickr, CC BY-NC
How to live with them
The key to controlling caterpillars is to prevent them from getting to the crop in the first place. Once the leaves start to unfurl, cover the crop with insect-proof netting. You can operate garden hoops or bamboo as a frame to hold the netting.
If you are going to remove the netting to water or remove weeds, do it in the early morning or behind schedule afternoon when the adults are not flying.
White butterfly lures usually hang on sticks they do not act to stop females from laying eggsThere is no evidence that female cabbage whiteflies are territorial.
It may be useful to provide a “sacrificial” plant. Leave them in the open to attract an adult female to lay eggs.
As the caterpillars grow in numbers, they will begin to attract beneficial predators and parasitic insects that lay eggs on the caterpillars. Parasitoids are primarily wasps and can be very effective biological agents. Their larvae feed on the body fluids or internal organs of the host caterpillar, ultimately killing it.
These beneficial insects need a source of nectar to stay dynamic, and they will be attracted to gardens that are a bit “messy” with a variety of habitats and flowers.
Remember that some green caterpillar-like animals they are good guys. So if you like crushing caterpillars, make sure they are the ones eating the leaves, not the voracious predators, especially aphids, that eat the herbivores.
Read more: Secret agents protecting our crops and gardens
To make things more engaging, the caterpillars that feed on them are generally around the age of 100 times more likely fall victim to predators and parasites than caterpillars that hide. Longer periods of caterpillar feeding usually occur on plants with lower nitrogen levels – so if you have a sacrificial plant, do not fertilize it.
It can also aid to not plant all your cabbages together. Mix your greens and herbs. This will give your precious kale companion plants and make it harder for caterpillars to move from one plant to another.
Planting companion plants allows beneficial insects to find hiding places closer to the caterpillars, while making it harder for female butterflies to find the brassicas.
Read more: These 3 tips will aid you create a prospering, pollinator-friendly garden this winter
Photos
Avoid pesticides as much as possible
Don’t spray your garden plants with pesticides unless you desperately need to feed a enormous family or are a serial fruit and vegetable exhibitor at your local show. The cost is enormous relative to the benefits the chemicals bring you. In most cases, you will kill many of the beneficial creatures in your garden.
Read more: Fighting insects: time to end chemical warfare
If you want, the least harmful spray for humans and other natural enemies of cabbage whiteflies is Dipel. It is an insecticide containing toxins from bacteria, Bacillus thuringiensis kurtaki (Btk), which occurs naturally in soil and on plants. However, it can be toxic to other butterflies and moths that pollinate your vegetables, so be careful where and when you spray.
Netting your plot instead of spraying and providing space and food for natural enemies are great ways to maintain diversity in your garden. Allowing a little damage to your prized yard crops allows for engaging biological interactions to occur in areas that may have been missing for decades.
