Ruud Kleinpaste
Insect damage can be unsightly, are they all equally dangerous? Ruud Kleinpaste looks at the evidence.
The pōhutukawa leaf miner.
Over the past goodness-know-show-many decades that I have been writing for this august magazine, the topic of sucking, chewing and rasping damage has come up on a number of occasions. It’s simply a matter of putting bug activity on plants (and the damage they cause) into perspective: how damaging, really, are caterpillars chewing leaves or aphids distributing honeydew willy-nilly?
If you really want to experience how gardeners think about chewing invertebrates, raise the topic of bronze beetles, green looper caterpillars and leafrollers. Alternatively check out the phobias surrounding codling moth caterpillars in apples and the tunnels through branches left behind by lemon tree borers and k¯ anuka longhorn beetles.
There’s a lot of bad feeling out there for the chewing insects that make a living out of creating holes and gaps and tunnels, as well as notches and windows in leaves. In fact, it is often the most important topic of talk-back radio, from what I can determine.
The answer is always the same: our biodiversity is part of a huge food chain that benefits an extraordinary range of predators and insectivores higher up.
In terms of the severity of the “damage” to the plants, chewing is really not a big deal. Yes, you lose some foliage, there may be some branch die-back, and the nibbling in the apple core really doesn’t harm the plants much at all. A wellestablished tree, shrub or plant will regrow the lost parts quite quickly, as if nothing has happened.
My favourite example is the way you prune your roses: 80 per cent gone overnight, simply through a hungry pair of secateurs… and the plant grows it all back next spring.
But every now and then, you come across some serious vandalism that makes a total mess of your trees or shrubs, and this time it was on the grounds of Parliament where some carefully planted p¯ohutukawa had been defaced by the larvae of Neomycta rubida.
You may have never heard of that species in your life. Its common name is p¯ohutukawa leaf miner and if you live in the northern part of New Zealand, you will have seen its characteristic workings from time to time.
The small, but cute, little brown weevils appear in late spring and summer, and find a mate. Females deposit eggs inside the pristine, new leaves at the tip of branches. When the tiny larvae hatch, they tunnel into those leaves and create a messy leaf mine, chewing their way forward undercover of the epidermal layers of those leaves. Out of sight, out of mind. Often, the affected leaves fall off the tree when the miner has barely started. In those cases the leaf and its hidden inhabitant will perish.
But when the leaf miner has enough nutriment to create a pupa, the life cycle can be completed. It is the small brown weevils’ turn to have a go at the young, developing leaves. And do they!
Their chewing mouthparts (mandibles) take pot shots at the foliage, creating tiny, often perfectly round holes, right through the leaf.
When the leaf grows larger, the holes are stretched as well, with the edges of the feeding damage showing some brown necrosis (die-back).
Often, the affected leaves fall off the tree when the miner has barely started. In those cases the leaf and its hidden inhabitant will perish.
Now it really looks ugly – as if someone had taken a shotgun to the tree.
Have a look at your p¯ohutukawa (if you live in an area where they grow) to see if you can find this damage – it has been particularly bad over the past few years.
Interestingly enough, there seems to be a trend: this weevil’s vandalism is more obvious in metrosideros growing in cities or in places away from the sea. There’s even a suggestion that it is worse in areas outside the p¯ohutukawa’s natural range of distribution (North Cape to a line from north Hawkes Bay to Taranaki) – hence the visual devastation in Wellington!
I reckon that when a tree grows outside its ecological “happy place”, it could well be more susceptible to some opportunistic chewers. Who knows?
I realise this weevil is not going to kill its host, but nevertheless, it would be great to find out its distribution and incidence on our summertime taonga. ✤