R. Varsano and B. Rubin (1991) Phytoparasitica 19(3):225-236 INCREASED HERBICIDAL ACTIVITY OF TRIAZINE HERBICIDES BY PIPERONYL BUTOXIDE RINA VARSANO and B. RUBIN 1 Piperonyl butoxide (PB) is a known synergist which enhances the activity of insecticides by inhibiting their biotransformation to less active products. We have evaluated the possible use of PB as a herbicide synergist using triazine herbicides in sensitive, tolerant, and resistant plants. The effects of PB, triazine herbicides, and their combinations were examined in whole plants as well as in chloroplasts isolated from triazine-sensitive (S) and -resistant (R) weed biotypes. PB itself, applied postemergence (0.1-0.5%, v/v), was slightly toxic to the plants tested. However, foliar application of PB combined with atrazine, terbutryn or prometryn to maize seedlings significantly increased the phytotoxicity of the herbicides. Low rates of atrazine, prometryn, and terbutryn in a tank-mixture with PB, effectively controlled Solanum nigrum L. and Abutilon theophrasti Medik. PB enhanced atrazine efficacy in both S and R biotypes of Lolium rigidum Gaud. The synergistic effect of PB was evident also in vitro when atrazine and methabenzthiazuron were used to inhibit photosystem II electron transport in chloroplasts isolated from resistant weeds. These data demonstrate the potential of PB as a herbicide synergist and its possible utilization as an aid for improving the activity of triazine herbicides in sensitive, tolerant and resistant plants. KEY WORDS: Piperonyl butoxide; herbicide synergist; atrazine; terbutryn; prometryn; methabenzthiazuron; triazine resistance; Zea mays, Solanum nigrum; Abutilon theophrasti; Lolium rigidum; Alopecurus rnyosuroides. INTRODUCTION Application of herbicides simultaneously or in sequence with other agrochemicals may result in an addition to, synergism or antagonism of the biological activity of the mixture (11). Adjuvants (13), fertilizers (22), pesticides (3,16,17) and growth regulators Received May 2, 1991; received in final form July 15, 1991. 1 Dept. of Field and Vegetable Crops, Faculty of Agriculture, The ttebrew University of Jerusalem, Rehovot 76100, Israel. Phytoparasitica 19:3, 1991 225