|dc.description.abstract||The toxicity of hexythiazox (tradename Nissorun) to adult female twospotted spider mites (Tetranychus urticae Koch) and their eggs was investigated in laboratory studies.
Hexythiazox was found to be relatively non-toxic to adult female T. urticae. The calculated LC₅₀ value was 59 times that of the suggested field application rate (2.5 x 10⁻³ % a.i.). Exposure of adult female T. urticae to direct sprays and resuidues on leaf surfaces both caused chemosterilisation of the mites. Eggs produced following treatment failed to hatch. However, this effect was temporary and following removal of the adult female mites from hexythiazox residues, percentage hatch rose to levels equivalent to that of the controls.
A leaf disc method was used to examine the ovicidal activity of hexythiazox. Baseline toxicity data for hexythiazox on T. urticae eggs were determined. LC₅₀’s of 1.6 x 10⁻⁴ % a.i. for direct spray and 3.2 x 10⁻⁴ % a.i. for residue exposure were calculated.
The effect of post-treatment temperature on toxicity of hexythiazox to eggs was investigated. An inverse relationship was found to occur over the temperature range of 15 to 30°C. Eggs maintained at 15°C were 8.7 times more susceptible than eggs maintained at 30°C.
The effect of egg age, at the time of treatment, on toxicity of hexythiazox to eggs was investigated. An inverse relationship was observed. Eggs treated at 48 – 72 hours were four times more tolerant than eggs treated at 0 – 6 hours. However, a greater than 100 fold increase in tolerance in eggs treated at 72 – 96 hours over the previous age group (49 – 72 hours) occurred.
The effect of leaf type on the toxicity of hexythiazox to T. urticae eggs was examined. Five leaf types were used: broad bean (Vicia faba, cv. ‘Exhibition Long Pod’), strawberry ( Fragaria X ananassa, cv. ‘Red Gauntlet’), raspberry (Rubus idaeus, cv. ‘Glen Prosen’) and two apple cultivars ( Malus sp. Cv. ‘Red Delicious’ and ‘Granny Smith’). Significant differences in toxicity between the leaf types occurred. Hexythiazox was most toxic to eggs laid on broad bean leaves, least toxic to eggs laid on the apple cultivars, and of intermediate toxicity to eggs laid on strawberry and raspberry.
Microscopic examination of eggs treated with a lethal dose of hexythiazox, showed that embryos reach an advanced stage of development before dying. Killed embryos were indistinguishable from newly emerged larvae.||en