Case-bearing moths
The "grains of rice" are the cases that the larvae spin for themselves, mostly out of the fibres of the carpet. They pupate inside this case too, and emerge as adult moths
Moths are NOT like bloody salmon! They don't care where they go, as long as there is a dark corner and a food source for their next generation. The adults and larvae are light-shy so tend not to stray very far from where they hatch, which means that an infestation can grow in size very rapidly.
As stated above, the larvae can go unnoticed for a long time if furniture isn't moved around or vacuumed under very often. You'll find the larvae under any furniture that sits really low to the carpet, making it virtually dark. Typically they'll be head down, tail up, munching away on the carpet fibres.
They eat keratin, a protein made up of amino acids - this is actually a very specialised diet, few creatures can break it down to digest it. Keratin is found in hair, fur and feathers (nails too but you don't get many fingernail carpets). Finally we've found a saving grace for polypropylene carpets :
As already stated, the best way of customers
controlling the presence of these pests is by a very thorough vacuuming regime, including the gulleys of the carpet with a crevice tool.
Vacuuming is never going to get rid of all of the larvae, just helps to limit the growth of the population.
ANY insecticide used previously will no longer have an effect after a certain period of time. Insecticides generally work on contact and have little residual effect once dry. Insecticides only kill adult moths and larvae, not pupae or eggs. You can't "put an insecticide down strongly" to prevent insects returning. Glug glug etc...
This is why a growth inhibitor (e.g. Nylar) should be applied to the carpet at the same time (some insecticides might have an inhibitor mixed in with them). Growth inhibitors have a residual effect lasting a good few months. This means that any eggs are prevented from developing, any freshly hatched larvae or adults are prevented from developing to the next stage of the life cycle. This breaks the chain, and destroys the life cycle of the moth, in theory eradicating it from the infested area.
The only problem is that if the next-door neighbour has the moths too, there'll still be a fresh source of adult moths coming in to the house. When the inhibitor has worn off from the treated areas, the infestation may start all over again
The only option would be to treat with inhibitor every 6 months or so and keep an eye on it.