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September 10, 2025

Toxicity Profiling of Dimethoate and Diflubenzuron in Daphnia magna Integrating Heart Rate Alteration and Survival Analysis

Key Points

Dimethoate and diflubenzuron exhibit differing levels of acute toxicity in daphnia magna, affecting survival rates significantly.
At 0.0075 ppm, dimethoate led to a survival rate of only 20% at 120 hours, contrasting with 60% for diflubenzuron.
Heart rate analysis demonstrated acute excitotoxic responses to dimethoate, while diflubenzuron caused chronic physiological stress.
Integrating survival and physiological data enhances the predictive value of toxicity assessments in aquatic environments.

Abstract

This study investigates the acute and sublethal effects of two widely applied agricultural pesticides dimethoate, an organophosphate, and diflubenzuron, an insect growth regulator on Daphnia magna, a sentinel organism in aquatic ecotoxicology. Adult Daphnia were exposed to three concentrations (0.001, 0.005, and 0.0075 ppm) of each pesticide under standardized laboratory conditions. Survival was monitored at 0, 24, 48, 72, 96, and 120 hours. No mortality was recorded within the initial 72 hours; however, time-dependent mortality was observed at higher concentrations thereafter. Dimethoate at 0.0075 ppm resulted in a survival rate of 40% at 96 hours and 20% at 120 hours, whereas diflubenzuron exhibited comparatively lower acute toxicity, with 60% survival at 120 hours. Given the delayed mortality patterns, sublethal toxicological endpoints were assessed via cardiac monitoring. Heart rate was quantified at 0, 6, and 24 hours post-exposure using low-light digital microscopy and open-source video analysis software (Tracker). Significant alterations in cardiac activity were demonstrated in response to both pesticides. Dimethoate triggered a rapid increase in heart rate up to 133.41 beats per 10 seconds at 6 hours followed by a decline to 100.98 beats at 24 hours, indicating an acute excitotoxic response followed by physiological depression. In contrast, diflubenzuron induced a more gradual yet sustained elevation in heart rate (from 71.11 to 104.98 beats per 10 seconds at 0.001 ppm), suggesting chronic physiological stress due to growth regulatory disruption. These findings validate the sensitivity of Daphnia magna as a responsive biomonitor for detecting both acute and sublethal pesticide toxicity. Integrating survival data with physiological biomarkers such as heart rate enhances the predictive value of ecotoxicological risk assessments. This study affirms the importance of using sublethal endpoints in environmental monitoring and supports evidence-based pesticide regulation to safeguard freshwater.

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