Puriri moth caterpillars (larvae) ring barked
branches for the first time in 2011/2012. The moth is a New
Zealand native, and attacks native trees, mainly puriri and
putaputaweta, and a range of introduced trees. The first hint
was a wilting branch....
The point of entry was just above a small branch that had died
back following a failed graft attempt...
The larva chews a channel right around the branch, effectively
ringbarking it. And when an avocado tree exudes sap, the
unique avocado sugars crystallise into a dusty pure white powder
around the damaged area.
The groove is covered with webbing, and originates from a hiding
hole the grub has chewed into the trunk.The Puriri moth
caterpillar chews out the bark around the entrance to its hole,
making a depression where the bark has been removed.
The tree tries to heal the damage with callus tissue, and it is
the callus tissue that the Puriri moth caterpillar feeds on. It
covers the depressed area with webbing, and sticks bits of bark
onto the webbing effectively camouflaging its hiding place.
The give-away is the area of moist poo ('frass') at the bottom
of the webbing.
The Puriri caterpillar chews the hole more or less vertically
down the branch - but unlike the lemon tree borer, it does not
keep going deeper and deeper, and there are no little frass
ejection holes at intervals down the branch....
The whitey - colored larvae can seriously weaken smaller limbs.
Their body shape and color is very similar to lemon tree borer
larvae, but lemon tree borer is a beetle, and this pest is a
moth caterpillar. The caterpillar has 6 true legs just behind
the brown head, and several stumpy 'prolegs' at the rear.
Apparently the first stage of the Puriri caterpillars life is
spent eating fungus growing on dead wood on the forest floor. I
try to keep bits of rotting wood under my little avocado trees
as part of the battle against Phytopthora
root rot fungus, so the caterpillars have an ideal food
So far, I have spotted 4 Puriri caterpillar holes in my young
trees. The caterpillar seems to start its entry attempts at the
point where a youngish branch joins either a main limb or the
tree trunk, usually on the upper side of the join.
When the caterpillar starts a fair way out on a limb, all it
will do is kill the branch above its hole. But where it drills
into the young tree trunk and ring barks it, the entire top the
the tree trunk will either die or snap off in a wind.
This is probably not critical if you grow avocados in the
conventional way as a multi truck tree, but if you are growing
them as a single trunk, you may end up with the entire top half
of your tree snapped off.
Oak (bellota Oak, Evergreen Oak etc)
We have a few Quercus ilex ssp bellota seedlings planted in the
hope of one day harvesting the edible acorns. Most of the half
dozen or so trees have been unwitting hosts to larvae of the
Puriri moth. I only discovered the damage recently when pruning
the trees, as the moth leaves a thin 'skin' of bark over its
workings, and the frass resembles the bark somewhat. I found two
trees with empty pupal cases just protruding from the holes.
Given the 'tree stage' of these caterpillars live for an average
of about 2 years before they pupate and turn into moths, the
larvae have had plenty of time to do damage.
Frass obscuring the
Pupal case and the damage done - the thin bark cover pulled
pupal case showing the size of the caterpillar...!