An astonishing number of cabbage hornets and butterflies were “all over the place” when Prof Mark Elgar walked out of the University of Melbourne campus on to Royal Parade last Friday.
“I was really quite struck, I haven’t seen something like that before,” he said. In the bioscientist’s front garden, they were “hovering all around” the citrus plants too.
“Melbourne and Sydney gardeners have got to be on the lookout, otherwise you are going to find a lot of leaf damage,” Elgar said.
Pieris rapae is colloquially referred to as a cabbage moth, but is in fact a butterfly.
It is an invasive species originally from Europe, considered a pest in Australia because it will snack on any plant from the brassica family, which includes lettuce, cabbage, broccoli, cauliflower, kale and brussels sprouts.
The insect goes through distinct stages of growth, starting at hatching from an egg and turning into a larva.
“That is when they will damage your plants,” Elgar said.
The larva feeds on plants until they have expanded beyond their exoskeletons and go through a moulting phase. Then it wraps silk around its body and “goes through this extraordinary metamorphosis where it changes into a butterfly”.
Female adults then “search for somewhere to lay their eggs,” which tend to be the underside of leaves.
Elgar said the cabbage butterfly influx was occurring in Melbourne and Sydney but “probably hidden in places in-between as well”.
“I am sure it is not just an urban city thing.”
Gardeners across several states have been taking to social media with images and anecdotes of a “plague” of larvae “decimating” brassica plants and adult butterflies hovering around their gardens.
“The reason for that would be the climate,” said Dr Thomas White, an ecologist from the University of Sydney.
A mild winter meant more pupate survived than usual. A very warm start to spring, with days as hot as 40C combined with a few inches of rain, created “ideal butterfly and insect conditions” for an abundance of adults to survive and continue laying eggs for the start of a new 30-day reproductive cycle.
How to protect your garden
Part of the reason the insect is considered a pest is because “they are just so hardy and resilient in a broad range of conditions,” White said, recalling seeing adults “floating around in winter”.
“Numbers are steadily higher than previous years,” he said.
The “huge burst” means many adults are “fluttering around, looking to meet with other butterflies to lay their eggs”, White said.
“If you stand there, and you see them floating around your garden, I am putting my money on them having eggs around the place as well.”
Minuscule eggs are often found on the underside of leaves.
“Anything that is green-like and living on a plant is living there because it is being provided with a huge larder,” Elgar said. “They will happily munch their way through the leaves, and often go near the tips and new leaves.”
Reaching for chemical repellants should be a “last resort”, White said, but there are other ways to remove cabbage butterflies.
Other insects such as lady beetles and wasps will target caterpillars, “so you can encourage the natural enemies of these critters” by planting a diverse mix of flowers that attract them, White said.
In Queensland and Northern Territory, green tree ants are sometimes used to control insect pests by eating the larvae before it turns into an adult.
In a small garden, mechanically inspecting each leaf, and rubbing or flicking eggs and larvae off, is a manageable fix. Installing nets over plant bushes will prevent adults laying more eggs.
• The headline, text and picture caption of this article were amended on 16 November 2023. An earlier version referred to the cabbage butterfly as a moth; to clarify this species of butterfly is often called a “cabbage moth” but is in fact a butterfly.
