Are they termites or ants (or both)? Contrary to what some people think, termites aren't ants, could be described as "social cockroaches". They're more closely related to cockroaches than to ants, they eat vegetable matter. Most termites are grass eaters, those won't eat woody plants. But there's still plenty of termites that will. Ants will often bring in aphids, mealybugs or other sap suckers. They can 'honeycomb' the soil under small plants and cause a drying out of the root system. For ants you can flood the area around the plants and the ants might pack up and go. But remember, Veitchias need good drainage. Otherwise you might need to find a good bait that the workers will take back into the nest to feed the whole colony. Killing individual ants won't work, the colony will keep producing more. There's a bait here called Amdro put out by BASF which works quite well. There's also baits for termites. But if you go down the road of poisons, remember, they often don't just stop around the plants but get washed further along. And a bait shouldn't kill the workers straight away. They must be able to carry it into the nest to feed to the others. Good luck with it.
Thanks for the input. We used large doses of freshly boiled garlic and cinnamon in water and doused the trees for about 5 days and soon saw no trace of yes, ants, rather than termites as initially thought. So far the trees are flourishing, but I am keeping my eyes open!