Impact of Water Hardness on the Toxicity of Acequinocyl Against Tetranychus urticae (Acari: Tetranychidae)

Abstract

The twospotted spider mite, Tetranychus urticae Koch, is one of the most damaging agricultural pests due to its broad host range and its capacity to rapidly develop resistance to acaricides. Although chemical control remains a main management tool, factors affecting acaricide performance are often underestimated, particularly the quality of the water used to prepare sprays. Under laboratory conditions, we evaluated the effect of water hardness on the toxicity of the acaricide acequinocyl to adult females of T. urticae. Bioassays were conducted using a susceptible laboratory colony and a leaf disc immersion method. A concentration–response analysis showed that acequinocyl caused mortality ranging from 0 to 100% within a concentration range of 0.5 to 12.3 mg/L, with estimated lethal concentrations (LC) of 3.52 mg/L for the LC50 and 7.93 mg/L for the LC90. The LC90 was subsequently used to assess the effect of water hardness. When acequinocyl was prepared with potable water, mortality remained consistently high, ranging from 78.6 to 81.1%. In contrast, preparing acaricide with hard water reduced mortality to intermediate levels between 34.9 and 45.0%, representing a reduction of up to 40% in efficacy. Mortality in the untreated control remained low, ranging from 1.4 to 4.1%. These patterns were consistent across all evaluation times. Overall, the results demonstrate that water hardness substantially reduces the biological efficacy of acequinocyl against T. urticae, highlighting the importance of considering spray water quality to optimize acaricide performance within integrated pest management programs.