Tree borer.

susieq9, Feb 25, 9:15am
Our kowhai, lemon and lime tree have borer. They are also riddled with ants. Have put a paste of neem granuules in the holes, but the ants are just plentiful. Anyone have any suggestion for the ants. The lime tree leaves are really black and not sure if it is the borer or the ants.

devine-spark, Feb 25, 1:50pm
I had to take down 5 robinia mop tops at the end of last year because of borer. most upsetting to me.

wheelz, Feb 25, 6:53pm
I would imagine you have more than just borer, for the ants to be there. They go after honeydew excreted by insects, like brown scale, white fly,mealy bug or aphid. Get rid of the other insects and the ants will go too.
The black is called sooty mould, which is caused by the honeydew.

http://m.yates.co.nz/problem-solver/problems/sooty-mould/

susieq9, Mar 4, 4:04am
Thank you for your reply. Have sprayed with conqueror oil. Will now see what happens.

mark_g, Mar 4, 7:25am
Often it takes 2-3 sprays, a few days or a week apart.

maclad, Mar 4, 4:51pm
Even when dead the sooty can hang on for ages. After 3 or 4 weeks blast it with high pressure water.

fresiaa, Mar 4, 9:44pm
For soot, do you see white flies?
If so, just hose the tree once or twice a day for a few days with a strong stream to get rid of the flies.

For the black leaves: you can put a bit of cooking oil, tiny bit of dishwasher liquid and lots of water in a spray bottle. Spray the leaves.
It loosens the soot, though it can take a few years before it is all washed off. It easily rubbs off with your fingers, but honestly, thousands of leaves by hand. :) LOL

susieq9, May 4, 7:49am
Have just been down to water the feijoa and kowhai tree. Under the kowhai I still noticed fresh dust from the borer holes. I let the water spray into one of the larger holes and out comes what looks like slaters. Could they be slaters doing this, or are they just going into the borer holes. Squashed them any way.