Cleistocactus strausii
As a houseplant, the Silver Torch cactus is an easy to take care of so long as over-watering is avoided. To naturally simulate the porous effects of the sand in its native habitat, it's important to ensure that excess water will not pool in the soil, causing the roots to become water logged.
This plant’s native habitat does not have winter, but when temperatures drop to the low twenties, it can exhibit signs of tip frost. 
The Silver Torch cactus is most susceptible to 
mealybugs 
and spider mite.
Bundala National Park
The seeds of the Opuntia cactuses (called Kathu Potak in Sinhala) are spread by macaque monkeys, and perhaps other animals and birds, that eat the fruits. It is also spread by people cutting down the cactus but leaving the cuttings, which then re-sprout. No biological control measures have been carried out with the moth "Cactoblastis cactorum" so far. Manual removal of the cactuses will be impossible as the area where they have spread over is vast.