EVALUATION OF 3 LARVAL INSTARS OF THE MIDGE CHIRONOMUS-PETIOLATUS AS BIOASSAY TOOLS USING A COMPUTATIONALLY INTENSIVE STATISTICAL ALGORITHM

Sensitivity to toxicants is a major criterion for selecting organisms for bioassay testing. If a sensitive species is also abundant and occu pies a role as prey for many other species within a community, then th e species become a valuable tool in environmental monitoring. These fe atures apply to larval midge Chironomus petiolatus in freshwater envir onments of central Chile. The youngest larval instar is the most sensi tive and presents the additional feature of lower survival within cont rol arenas, making it more difficult to discern toxicant-related morta lity from background mortality. In this work, we perform acute bioassa ys with the three larval stages of C. petiolatus and K2Cr2O7 as refere nce toxicant, with the goal of selecting a particular instar as the be st bioassay tool using two criteria: sensitivity and background mortal ity. Sensitivity is evaluated through Monte Carlo estimation of LC50 a nd background mortality through bootstrap resampling, and a final Bioa ssay Performance Index as the product of LC50 and background mortality . For this task we developed a new computationally intensive statistic al algorithm. Results show that the best bioassay tool is not the youn gest and most sensitive instar but an intermediate one.