What would happen if you fed termites a drug that killed trichonympha?

Answer: they won't be able to digest there food the way they use to

Termite will not perform its function which is digesting cellulose. Hence termite will unable to decay woods.

Explanation:

Termites and trichonympha (a protist) are in symbiotic relation. Trichonympha lives in the hind gut of a termite. Termites give a home to trichonympha while trichonympha produce an enzyme which is used by termites to digest cellulose. So without trichonympha, there will be no enzyme and no digesting activity by termites.