Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Plant, Environmental Management and Soil Sciences

First Advisor

James L. Griffin


Field studies were conducted at two locations in Northeast Louisiana, to investigate interactions associated with the herbicide pyrithiobac and insecticide combinations in respect to weed control and insecticide efficacy and cotton response. In addition, controlled environment experiments were conducted to evaluate the effect of temperature regimes and simulated thrips damage on pyrithiobac plus insecticide phytotoxicity to cotton. Pyrithiobac and insecticide combinations when compared with pyrithiobac alone did not reduce cotton leaf area, height, main stem node number, main stem nodes to first square, days to first square or flower, main stem nodes above white flower, or seedcotton yield. Acephate insecticide alone in one experiment and oxamyl insecticide in two experiments reduced thrips larvae more than when in combination with pyrithiobac. Weed control was equivalent when pyrithiobac was applied alone or in combination with the insecticides acephate, dicrotophos, fipronil, imidacloprid, lambda-cyhalothrin, oxamyl, carbofuran, or dimethoate. Presence of thrips on cotton did not affect cotton tolerance to pyrithiobac. However, differential cotton response to rate of pyrithiobac was observed. Cotton growth not reduced by pyrithiobac applied at four times the labeled rate of 0.07 kg ai/ha compared with nontreated cotton. Leaf area reductions as high as 58% were observed with eight and sixteen times the labeled rate. Significant reductions in cotton height were observed, but varied among years and locations. At Winnsboro, LA, pyrithiobac at 1.22 kg/ha increased total main stem nodes at square initiation by 1.2 nodes. Although significant reductions were observed in leaf area, cotton yield was reduced only in 1998 by pyrithiobac at 1.22 kg/ha (19%). In an additional study, the presence of thrips or simulated thrips damage did not change cotton tolerance to pyrithiobac regardless of application timing. In the controlled environment study, cotton response to pyrithiobac; was not effected when cotton was stressed under a cool temperature regime (22/10 C day/night for 48 hours prior to application) or terminal removal to simulate thrips damage. Some injury to cotton was observed 7 days after treatment when pyrithiobac was applied with the insecticide malathion or dimethoate but injury was transient and less than 5% 21 days later.