Plant Doctor Archive

Pohutukawa

My pohutukawa tree has bubbles on its new leaves. What causes this?

This is very common on pohutukawa and rata, and it's caused by an absolutely fascinating native insect called a psyllid. It also occurs on the karo — or the pittosporum. On pittosporum, the pits are made by a different species of psyllid, namely the pittosporum psyllid.

This one is totally host-specific, as is the one on the pohutukawa. It only goes for metrosideros.

The adults, which you can see on the new foliage, look a bit like aphids, and they only lay eggs on the new leaves. Out of the eggs come little nymphs, and they put their stylets into the leaf material, and then the leaf forms the bubble.

These psyllids excrete crystalline sugar, a crystalline form of honeydew, which is normally picked up by ants which take the bits of sugar back to their nest. At the same time, these ants will protect the psyllids that live on the leaf, and they also protect the leaf on which the psyllids live. That means no chewers will come and take bites out of the leaf.

As for the acmena hedge, a few years ago a foreign species of psyllid got into NZ, and it's hammering the new growth of its host. It doesn't make any dimples, but it's free-living. It can get very messy with honeydew and sooty mould, but this psyllid does not seem to have any ecological relationship with ants. So whilst the damage looks spectacular, it does not seem to harm the fast-growing acmena that much.

If you have major problems with psyllids, try using a spray like pyrethrum or a simple oil spray. In my situation, I'm not worried because the psyllids will only attack the juvenile leaves. When the leaves become adult — with white undersides — the hair tickles them and they won't infest it.

Advice by Dr Dan Blanchon from Unitec's Diploma in Sustainable Horticulture and Bachelor of Resource Management.

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