Whether and how to fill in a tree hollow

We have a tree with an enormous hollow cavern in it. I wouldn't worry about it, except it is full of brackish water and is clearly a mosquito breeding ground. Is it ok to fill in a tree hollow, and if so, with what? My step-son suggested concrete -- but I'm sure that won't do. We could just put a mosquito dunk in, but I'm worried that it might poison the tree.

The mosquito dunks won't poison the tree. They contain Bt (Bacillus thuringiensis), it's a type of bacteria that will take care of the mosquito larvae. But since it's not a chemical pesticide it won't have any impact on the tree. I'll let someone else comment on whether you should fill in the cavity--my gut instinct would be not to fill it in but leave it alone but since I have no real info to back that opinion up with I'll leave that for someone else to advise you. One thing you should consider though is the location and structural integrity of the tree--if the cavity is big enough and has weakened the trunk considerably and if it's in a location where it could fall on your house, then you might want to consider taking the tree out.

Definitely don't fill the cavity!

Resin

Thanks! The tree isn't too big and isn't near any structures of concern. It is right on a fence line. The worst that would happen if it falls is that it'll fall on the fence, but I don't think it will fall in that direction.

Ok, I won't fill it, and I'll put in dunks. Thanks!

Would anybody suggest drilling a hole towards the bottom to allow for drainage? I would think it wouldn't do any damage to the tree. It would be like using a tree borer or tapping a maple for sap.

But won't the sitting water hasten the decay? The hollow is probably a foot deep.

Not a lot, as the tree lays down chemical barriers to decay in the wood; drilling a hole breaches that barrier.

Resin

Resin: Thank you very very much for the advice. I really want to do the right thing by this old tree.

Resin how is drilling a hole any different than taking a core sample to get the age or drilling a hole to collect sap?

It isn't any different to taking a core sample - that can be equally damaging to the tree in the long term. There is sometimes a difference in that trees growing in very dry areas (e.g. bristlecone pines), everything is so dry that decay fungi can't survive and the tree is OK, but as a general rule, taking core samples is not good practice.

Drilling a hole to collect sap is different, as that only cuts into the inner bark or the very outermost layers of sapwood, which is living and able to occlude the wound.

Resin

Resin, my curiosity has finally gotten the best of me. You are an acknowledged expert on on birds and and all sorts of flora and fauna. How is it that you are such an expert? I know that if anyone has a question they will get a definitive answer from you. Please explain and I'm not trying to be a "pain" or smart aleck. I stand in awe of your knowledge. Thanks!!

Ann

I'm curious too plantladyhou. Resin always has the best advice--he has a LOT of knowledge in his head!