Effect of some chemical substances on the fertility of Tetranychus urticae Koch

The results of a search for chemicals affecting the reproduction of the two-spotted spider mite, Tetranychus urticae Koch are recorded. The arrhenotokous type of reproduction of the spider mites has meant that male sterilization does not hold any great advantage over female sterilization. The present investigation was therefore concentrated on chemicals suppressing reproduction in the females. Contiguous to this study is a review of the literature with particular reference to the range of chemicals which can act as chemosterilants, the processes of chemical induction of sterility, and the biology and habits of T. urticae. An understanding of the latter is of fundamental importance before any application of chemosterilants can be contemplated. Preliminary tests with candidate chemosterilants were carried out and of those chemicals showing activity, the trialkyl phosphate group was selected for further study owing to the relatively low toxicity, ease of synthesis and sterilizing properties of some chemicals in this group. Trimethyl phosphate, triethyl phosphate, dimethyl methylphosphonate and diethyl cyanomethylphosphonate reduced egg lay and egg hatch while trimethyl thiophosphate reduced egg lay alone. Experiments indicated that trimethyl phosphate was a more active reproduction suppressant than trimethyl phosphate but showed a degree of chemical instability. Triethyl phosphate was therefore used for comparison with diethyl cyanomethylphosphonate for residual effects and the permanency of sterility induced by the chemicals. Diethyl cyanomethylphosphonate had greater residual properties and the sterility was more permanent suggesting that diethyl cyanomethylphosphonate would have greater effects than triethyl phosphate on the mite populations if these chemicals were used for population suppression.