I have watched video on youtube where guy pours molten aluminium into fire ant colony to make casing. In the comments below there's huge discussion on is that a right thing to do. I am on the side that one should not exterminate colony of living things to get aluminium shape of questionable artistic value.
I believe every living organism (even single cell ones) experience pain somehow, because it is important to stay alive. The scientific answer depends on how you define pain...
Evidence suggests that some, if not all, invertebrates have the potential to suffer through current practices that do not take into consideration that invertebrates may experience something like pain and stress and have the capacity for advanced and unexpected cognitive abilities.
Genetic analysis of nociceptive behaviors in the nematode Caenorhabditis elegans and the fruit fly Drosophila melanogaster has led to the discovery of conserved sensory transduction channels and signaling molecules. These are embedded in neurons and circuits that generate responses to noxious signals. This article reviews the neurons and molecular mechanisms that underlie invertebrate nociception. We begin with the neurobiology of invertebrate nociception, and then focus on molecules with conserved functions in vertebrate nociception and sensory biology.
Just to mention fire ants you possibly meet are invasive species, so you should not protect them.
The red imported fire ant, Solenopsis invicta Buren, is an invasive pest that has become widespread in the southern United States and Caribbean after accidental introduction from South America in the 1930's. This species, which has diverse detrimental impacts on recipient communities, was recently discovered in Australia and New Zealand and has the potential to colonize numerous other regions.
- Figure 1 - fire ants worldwide - source
In short, scientists usually conclude that pain is unlikely to be found in insects in the way it is defined in humans, but this is difficult or impossible to test directly.
In humans, pain is often described as a "subjective experience involving a class of sensations with which is associated a characteristic 'negative affect and aversive drive'". So to simplify, pain is defined as a sensation that causes negative emotions and reduces the behaviors that an individual was performing before or during the stimulus. Note that with this definition, it is hard to differentiate pain from fear or response to an unpleasant but non-painful stimulus.
Eisemann, C. H., Jorgensen, W. K., Merritt, D. J., Rice, M. J., Cribb, B. W., Webb, P. D., & Zalucki, M. P. (1984). Do insects feel pain?—A biological view. Experientia, 40(2), 164-167.
To put this more clearly, pain in humans is defined as resulting in a "a protective stimulus-avoidance response".
Fiorito, G. (1986). Is there “pain” in invertebrates?. Behavioural processes, 12(4), 383-388.
Fiorito (1986) notes "Venom used for incapacitation of prey probably does not inflict pain, as it may be disavantageous because pain may increase struggling on the part of the prey: it is indeed when venoms are painful that they can function in defense.". This is probably the argument most commonly employed to support the idea that invertebrates don't feel pain: insects can't do much about injuries or intoxications so feeling pain is unlikely to be adaptive because it is unclear how it could elicit a response that would increase their survival. Many invertebrates are autotomic which means they can voluntarily detach legs or antennae (like a lizard's tail) to avoid being captured by a predator, in this case it is hard to imagine an insect experiencing a pain that would be comparable to the level of pain of a human loosing a leg, because it would incapacitate the insect and reduce its chances of survival. Insects could instead have a direct stimulus to behavior response without any conscious emotions or feelings, comparable to a very simple computer program: "if pressure on this leg; drop the leg".
However, there is at least one case known in ants of wound treatment that would suggest this argument is not valid at least for this species of ants:
Frank, E. T., Wehrhahn, M., & Linsenmair, K. E. (2018). Wound treatment and selective help in a termite-hunting ant. Proceedings of the Royal Society B: Biological Sciences, 285(1872), 20172457.