The Harlequin Bug, Dindymus versicolor, is a native Australian insect that can become a pest in gardens and agriculture.
You can buy poison to kill it; I wouldn’t put poison in my garden if I can avoid it.
For ornamental plants, the commercial product Confidor is available; its active ingredient is Imidacloprid, a neonicotinoid – a class of chemicals now banned in some countries because of their suspected role in colony collapse disorder of bees.
Heidi, the blogger at A Year In A Gippsland Garden, relays the recipe from Gardening Australia’s Peter Cundall: a strong mixture of cheap (hopefully, biodegradable) detergent in water. Spray it on the bugs, or knock them into a bucket of it.
How to get it out of your garden by other means? I looked for scientific papers and there isn’t much, but here’s what I found.
Control of mallow/hibiscus species (like the weeds that commonly grow in my lawn here) seems sensible, based on a paper from Latrobe University, published in the Australian Journal of Ecology in 1981 (see table reproduced below). The study suggests that while the bugs may feed on many plants (and also carrion, and they are cannibals), they start the year principally in Malvaceae (mallow, hibiscus, hollyhock etc). The native hollyhock Malva Preissiana (previously called Lavatera Plebeia) is apparently their main native host plant.
As the summer progresses and the mallows go to seed (and become less tasty to the bugs), the bugs then opportunistically move to other convenient plants. So I’ll hypothesise that if the mallow weeds are eradicated well in early spring , the outbreaks of the bug could be reduced later on.
I’ll test this hypothesis next Spring! It will mean a bit of a dilemma for what to do with the beautiful native hollyhock growing in my front garden – it was the most beautiful floral display this spring just gone!
The other possibility for control is perhaps in their dormant stage. They apparently overwinter under bark or in leaf litter and mulch, so depending on how your garden is set up, you may be able to reduce these refuges. Certainly, getting rid of mulch from your vegie garden in winter can reduce slug and snail populations, and lets the sun warm the soil, so it’s not a bad idea (if it rains a lot, your soil may get compacted though). Removing bark from trees and mulch under shrubbery isn’t so easy!
For those wondering about natural predators, there isn’t much information available. One record I could find was from the CSIRO journal Emu (“Examination of Contents of Stomachs and Crops of Australian Birds,” Vol 11 No 2, 1911 – may require paid access), which in a survey of the contents of birds’ guts/gizzards, found a D. versicolor in the gut of a fan-tailed cuckoo (Cacomantis flabelliformis). This native bird is usually found in closed forests from cape york south, and across SE Australia, according to my Birds of Australia guide. Whether you can get it into your garden, and whether it will eat many of the bugs, I don’t know yet!