Selection of an appropriate life-cycle stage of Chironomus riparius meigen for use in chronic sediment toxicity testing

The first larval instar was identified as the most suitable life-cycle stage of the insect Chironomus riparius Meigen with which to initiate chronic sediment toxicity studies utilising the emergence of adults as the test end-point. The emergence of adults from formulated reference sediment was recorded in three test systems seeded with either individual eggs, egg-rope sections or first instar larvae. Analysis of total adult numbers emerging revealed a significant difference between the three systems. The system seeded with individual first instar larvae resulted in 100% emergence of adults compared to 90% and 94% in the systems seeded with egg-rope sections and individual eggs respectively. In all three systems, the median emergence time (EmT50) for males was significantly less than that of the females in the corresponding system. Analysis of combined male and female EmT50 values demonstrated that adults arising from the system seeded with individual eggs emerged significantly earlier than their counterparts in the remaining two systems. The relevance of these results in relation to the standardisation of existing test procedures amongst various operating agencies is discussed.