Why are there so many caterpillars this year — and can you keep them at bay?

They're creepy and they're crawly, and some caterpillars have been wreaking havoc in people's gardens over the past few months.

Protracted wet weather and high temperatures throughout much of the east coast have created abundant food sources and ideal breeding conditions for caterpillars.

One species in particular — the larvae of the poinciana looper moth — is emerging in plague-like proportions in some areas.

But what, if anything, can be done to prevent the caterpillars from making a meal of our gardens?

What it's like to live with a caterpillar infestation

Don Hewitt has lived in Redland City, on Quandamooka Country, east of Brisbane, for more than two decades.

Caterpillars had never been an issue at his property until an infestation in a large tree in his front yard last month.

"I've never had them before and that tree has been there for 25-plus years," Mr Hewitt says.

The poinciana looper (Pericyma cruegeri) caterpillars have a long, thin body and grow to about 7 centimetres in length. Mr Hewitt says the interlopers left a large volume of droppings, which stained concrete and his car.

"It was hard to get the marks out of the car paint and the concrete; it turns a brown colour and the stain gets into the exposed aggregate concrete," he says.

"I actually chopped the [infested] tree back, because one of our cars sits out on the driveway and it was just getting covered in this black stuff.

"There were literally thousands of [the caterpillars] on our poinciana tree — I could break a branch off and get a handful of them and feed them to our chooks … the chooks loved them."

Not all his pets were as thrilled by their arrival, though.

"The pond was directly underneath the poinciana tree. I didn't realise the droppings had gone in there … fish I'd had for 20 years are all gone, unfortunately."

Why caterpillars are thriving

Nigel Andrew, a professor of entomology at Southern Cross University in Lismore, on Bundjalung Country, says the poinciana looper caterpillars are likely "always around" in their preferred habitats, which also include acacias (wattles) and leopard tees.

"It's just that a really high percentage of the eggs might not usually successfully go through their change from the egg to the larvae, and then go through their growth profile," Professor Andrew says.

"So, they're not necessarily only coming in this year, it's just that you have particular years where the climatic conditions and weather conditions are good, and you get outbreaks."

In Mr Hewitt's case, Professor Andrew also suggests the poinciana trees may have reached the ideal size range for the moths to attack them, and says females can lay up to 200,000 eggs if conditions are right.

"They might have been in low numbers in the area and just by virtue of the tree being a perfect size for them — and for the moths it's the perfect food source — and the adults just get attracted to it."

Is there anything you can do to manage outbreaks?

Professor Andrew warns against using pesticides in an effort to control outbreaks, saying the environmental costs of using these sprays far outweighs any intended benefits.

"It kills off so many other species that are in your backyard or in your environment. They're also bad for your health … because a lot of the toxins do stay in the environment and can cause all sorts of problems."

Tree banding, which involves covering sections of a tree trunk with adhesive tape or purpose-made straps, is often mooted as an effective management tool. But Professor Andrew says this is more suitable for preventing pest species such as armyworms from moving between trees.

"You have to also remember the adults can also easily fly from tree to tree," he adds.

Then there are decoys, faux butterflies often made of plastic that purport to ward off some species of butterflies and moths in gardens by fooling real ones into thinking you already have a resident population.

"They're a great way to give your money away, I guess," Professor Andrew says, pointing out that a lot of the visual cues adults react to are based on the ultraviolet spectrum, and they also rely on chemical communication.

Instead, Professor Andrew says one of the keys to preventing infestations of pest species such as poinciana looper caterpillars is to enhance biodiversity by growing a variety of locally native trees and shrubs.

"If you have lots of diversity, that's usually good, because diversity enhances the opportunities for predators and parasitoids, which will prey on pest species."

Why the caterpillar boom could end soon

The looper caterpillar completes its life cycle from egg to moth in about 40 days, and exists in what Professor Andrew calls "a boom-and-bust cycle".

"They are outbreak specialists and they're great food sources for all the other animals that feed on them, but they can do that because their biology allows them to explode with their numbers [when conditions are favourable]," he says.

Brisbane City Council says infestations are thought to be associated with wetter than average seasons and it does not typically treat infestations, which it describes as "an ecological process that does not cause long-term damage to the trees".

Professor Andrew says the caterpillars are fast approaching the "bust" end of their cycle and "as you get towards the end of April and into May, it's going down — it was probably at the peak of its season a couple of weeks ago".

"Basically, when they come out, they often do come out in big numbers, and they can defoliate trees … but it's one of those things where they might have a big outbreak and then sort of go away."