Brisbane poincianas have been suffering from a plague of caterpillars. The caterpillars are defoliating the poincianas, descending from trees and infesting local residents properties.

The caterpillar plague is a phenomenon that occurs around once every seven years in Brisbane. Unfortunately, past caterpillar events have shown that poincianas badly attacked by the caterpillars (or inchworms) often die the following year.

Poinciana caterpillars (Pericyma cruegeri) occur in most native Asian habitats of the Caesalpiniceae family (of which poinciana is the only one common locally). They have also become naturalised in Queensland and northern New South Wales.

The bad news is there’s still some time left in the caterpillar invasion, at the time of writing this article. They are currently green caterpillars of around 3-4cm in length. They grow to around 7cm, changing colour to a mottled brown. The good news is that treatment is available.

We have a multi-prong approach to dealing with poinciana caterpillars. We start by raking debris (mulch, leaf litter and sticks) away from the base of the tree. Then we surround the poincianas trunk with diatomaceous earth. Diatomaceous earth is a fine sand consisting of very sharp grains. It discourages caterpillars from crossing to the tree.

We then apply a tree barrier around 45 cm above the root crown. This is to protect the tree from the horticultural glue we apply. Horticultural glue stays sticky even when it dries, and caterpillars are unable to reach the leaves of the tree.

This causes the death of the inchworms in two ways. First, they are unable to reach the foliage to feed. They will either move on or starve. Secondly, they mate in trees. Being unable to reach the upper limbs ends the life-cycle of the caterpillar.

To immediately kill caterpillars on the poinciana and around it, we also spray them with Bacillus thuringiensis or Bt. The strain of Bacillus thuringiensis we use is so targeted that it only kills organisms that feed on leaves. Other Bt. strains are used on insects in cropping.

Some Brisbane residents have been calling in pest controllers. Unfortunately, this pest really can’t be controlled with chemicals. Most insecticides are systemic, meaning they are sprayed on trees or plants and the insect dies when it sucks on the sap of the plant. While caterpillars are insects, even systemic chemicals targeted to them won’t kill them as they don’t ingest enough sap from eating poinciana leaves.

Further, Carbaryl, the chemical usually used against caterpillars, only kills caterpillars on contact. Every caterpillar has to be sprayed. It’s just as effective to crush caterpillars manually.

In fact, killing them by hand is helpful, and much less dangerous than using Carbaryl. Carbaryl is dangerous to humans and isn’t particularly selective. It also kills ladybirds and other beneficial insects that kill the caterpillars.

Our service costs from $189, more for large trees and for heavy infestations. It includes

1. diatomaceous earth
2. installation of tree band with horticultural glue
3. spraying with Bacillus thuringiensis.

We also follow up by respraying Bt. if there is heavy rain within three days of the original spraying. While this treatment should kill all of your caterpillars, if a few do remain they won’t be able to damage your poinciana further due to the barriers in place.

David’s field notes

Interestingly, of four poincianas I examined today that were infested with caterpillars, three of them have suffered from poor pruning techniques over the last year. Trees have natural defenses against organisms attacking them, bio-chemicals in sap. If they are suffering from external stress their defenses are lowered leaving them more vulnerable to attack.

The moral of the story is to ensure the health of your trees and use only an arborist to avoid further tree problems later down the track.