The aim of this study was to estimate the mortality and fading effects of dye markers Neutral red and Nile blue commonly used in the estimation of termite colony population, in order to determine the decrease rate from the initial number of marked and released termites. This rate can be considered in the application of the different indexes used to calculate the number of termites of the colony. In the laboratory, Reticulitermes grassei Clément, 1977 were forced-fed during four days with Neutral red 0.5% (wt/wt) and Nile blue 0.5% (wt/wt); later the percentage of mortality and fading was recorded after 30, 40, 60, 80 and 137 days post treatment. These allowed us to obtain a fitted linear regression line that can be used for estimating the number of termites that remain marked into the colony at any day. The percentage of mortality observed increased from 3.17% at 30 days to 18.07% at 137 days for the Neutral red dye and from 8.62% at 30 days to 21.89% at 137 days for Nile blue. No fading was observed at 30 and 40 days for Neutral red and until 60 days for Nile blue, but increased from 10.59% at 60 days to 34.68% at 137 days for Neutral red and from 5.94% at 80 days to 12.49% at 137 days for Nile blue. A field estimation of the colony population was made in two colonies from Palenciana town (Córdoba, Spain) using the Lincoln index. The colony population was estimated twice, first considering the total number of marked released termites and later taking into account their decrease (by mortality and fading) in relation with the days elapsed since their release into the colony, estimated with the linear regressions obtained.

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