The toxicity of cyromazine to Chironomus zealandicus (chironomidae) and Deleatidium sp. (leptophlebiidae)

Abstract

AbstractThe toxicity of cyromazine and a commercial formulation, ‘Vetrazin’®, to Chironomus zealandicus (thummi) Hudson and Deleatidium sp. was investigated. Under acute test conditions, the LC50 values for each species were quite comparable. For C. zealandicus, the value varied according to instar, 100–400 mg litre−1 for second‐ and third‐instar to 1000–10000 mg litre−1 for older fourth‐instars. For the one size class of Deleatidium tested (c.10 mm long), the value was 300–400 mg litre−1. High control mortalities of C. zealandicus limit that species' usefulness as an acute bioassay candidate. Under chronic test conditions, cyromazine showed a high toxicity to eggs or early‐instar larvae of C. zealandicus. The maximum acceptable toxicant concentration for cyromazine against C. zealandicus was approximately 17.5 μg litre−1. The possibility of water contamination at this level is discussed. Whole‐of‐life chronic tests with C. zealandicus indicated that the most susceptible stage was in the egg or soon after larval emergence. These results highlight the dangers of using short‐term acute toxicity results to formulate environmental exposure limits for modern pesticides that do not have dysfunction of the nervous system as their mode of action.