Poincianas and Caterpillars

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

poinciana defoliated by caterpillarThe 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.

diatomaceous earth caterpillar barrierWe 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.

horticultural glue caterpillar barrier on a poincianaTo 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.

killing a caterpillar with bacillus thurengiensisSome 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, usually in chemicals in sap. If they are suffering from external stress their defenses are lowered leaving them more vulnerable to attack.

a badly pruned poinciana

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

 

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Comments

  1. Anne Jarvis says

    Hi David,
    Would you please advise me of what causes a poinciana tree to bleed sap.
    Regards
    Anne Jarvis

    • David Taylor says

      Hi Ann. There are several possible causes, so I’ll mention each to assist you in deducing which is the most likely. Firstly, injury. I list it first as it’s probably easy to rule out if you aren’t aware of any vehicles or yard tools (think whipper snippers) that may have caused it.

      We then move on to biotic factors: pests and diseases. Both can cause bleeding and can be indicative of major health problems in this case. Longicorn borers are most likely if they are also present. Examine some dead wood and look for small holes, exit wounds. If a poinciana is stressed these can move into live wood and go unnoticed until they have done irrepairable damage to the tree. Unfortunately, the only defense is maintaining the health of the tree so it doesn’t get stressed to that point.

      Finally, it’s possibly caused by a fungal infection. We can use a Phytopthera test kit to check, though we may be able to clinically diagnose it also.

  2. Anne Back says

    I have a horrible feeling our 2 45 year poincianas are about to be hit by another plague of these pest. We had a serious problem last year but the trees fought back and have reasonable foliage but little flowers in the summer.
    We live in Yabba St which is usually splendid in summer. HELP
    I need your services

  3. Genevieve Wills says

    Hi David, Thanks for the great blog. Your site is the only place I have found this problem mentioned. Do you know what stage of the lifecycle the caterpillar is at now? As you said it is a 7 yearly problem usually, are you expecting another big infestation this summer or not?
    Thanks, Gen

    • David Taylor says

      Hi Gen, it was great to catch up with you last Sunday at the community meeting about poinciana caterpillars. While the rule of thumb is 7 years, the proliferation of caterpillars already this year has me suspecting there may be some shift in an underlying environmental factor affecting the caterpillars.

  4. louise white says

    Hi David, Thank you, I found this post very helpful and another feed you have been replying on a similar topic on your website. I wish I had read this a couple of months ago in order to have saved my tree. Now that the catepillar’s and leaves have gone, is there anything I can do to nurture my tree back to health? It is >50years old big beautiful old girl. Thank you

    • David Taylor says

      Hi Louise. A study into poplars demonstrated increased morbidity in defoliated trees that had previously been fertilized, so I suggest not adding nutrients to the soil. I can only suggest treating any remaining leaves with Bt if the tree is still suffering loss, and applying a tree band to block access to the tree from further infestation.

      Your tree’s age may actually be an advantage as the same study I quoted above showed a positive correlation between diametre at breast height (DBH) and survival.

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