TOXICITY OF THE HERBICIDE GLUFOSINATE-AMMONIUM TO TETRANYCHUS-URTICAE(ACARI, TETRANYCHIDAE) UNDER LABORATORY AND FIELD CONDITIONS

The toxicity of herbicides widely used in apple orchards to the twospo tted spider mite (Tetranychus urticae) was evaluated in laboratory and held studies. In a laboratory study with susceptible T. urticae, gluf osinate-ammonium was highly effective against larvae, protonymphs and adults, but non-toxic to eggs. Its efficacy was much greater than that of the commonly used acaricide azocyclotin. The immatures died within 24 h after treatment, suggesting that the nymphicidal action may be a ttributable to a direct effect rather than an inhibitory action of chi tin synthesis. Glufosinate-ammonium showed a positive temperature coef ficient of toxicity against T. urticae adults at six temperatures from 10 to 32 degrees C, being more toxic at higher temperatures. Very low levels of resistance to the herbicide were observed in the seven held -collected T, urticae populations resistant to various acaricides. Tre atment with glufosinate-ammonium did not cause a repellent response fr om either adults or immature stages of T. urticae. Paraquat dichloride and glyphosate were ineffective against all stages of T. urticae. In a held study of a population of T. urticae, glufosinate-ammonium when sprayed to weeds caused significant decrease in T. urticae population densities in apple trees for nine weeks after treatment, as compared w ith the control. Thereafter, a single application of standard acaricid es to apple foliage greatly reduced population densities, although the re was no difference in the densities between the glufosinate-ammonium -treated and control plots. Based upon laboratory and field data, two single treatments with glufosinate-ammonium to weeds in May and a sele ctive acaricide to apple trees in July may be used to prevent damage b y T. urticae.