Subterranean termites are quite efficient at extracting nutrients from lignocellulose. Their ability relies not only on the digestive tract physiology but also on symbiotic relationships established with flagellate protists and bacteria. This work aimed to screen the response of the flagellate protists community of the subterranean termite Reticulitermes grassei Clément to the ingestion of different biocides. The substances chosen were applied at sublethal doses and included antibiotics (amoxicillin), an antiprotozoal (metronidazole), a termite intestine pH alteration agent and respiration inhibitor (boric acid), an essential oil (cloves) and its main constituent (eugenol), together with the solvent (water) and a positive control of pine wood. Termites were captured in three different zones of the same pine forest, sufficiently distant to be considered as different colonies; three replicate samples from each colony were selected for testing. Immediately after termite capture the initial flagellate protists community was evaluated for all samples (initial controls). Groups of termite workers were then fed on diet disks impregnated with the substances and, after the trials the diversity and abundance of the flagellate protist community was evaluated. Twelve morphotypes were present in the controls. The naturally less abundant morphotypes were positively associated with the termites screened before the trials and the ones fed on water treated diet disks or original wood. Metronidazole showed to affect negatively most morphotypes, however, two morphotypes’ abundance increased; these two morphotypes abundances decreased when termites fed on amoxicillin treated diet disks. For eugenol and boric acid significant negative impact was found for one morphotype with parallel increase in abundance of two others. Overall, the results suggest a possible maintenance of hindgut equilibrium or minimum functioning relying both on: changes on abundances of two or three morphotypes; and presence and abundance of the less common morphotypes. Three morphotypes exhibit differentiated response to changes in hindgut conditions, triggered by the addition of substances to the termite diet. This dynamic nutritional symbiosis equilibrium seems to provide a wide range of defences of the termite to exposure to substances potentially harmful and general dietary changes.

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