Experiments were conducted from 1994 through 1996 to determine the effect timing of malathion applications have on cotton response relative to postemergence applications of pyrithiobac. Treatments were initiated approximately 1 mo after planting (four to six leaves) and included pyrithiobac at 70 g ae/ha applied alone or in combination or with malathion applied at 1 or 3 d before and at 1 or 3 d after pyrithiobac. At 7 d after treatment (DAT), pyrithiobac plus malathion applied in combination caused 34, 28, and 21% cotton injury in 1994, 1995, and 1996, respectively. Malathion applied 1 d before pyrithiobac resulted in 18% injury in 1994, 11% in 1995, and 15% injury in 1996, whereas malathion applied 1 d after pyrithiobac injured cotton 15 and 11% in 1994 and 1996, respectively. Injury of cotton subjected to malathion applied 3 d before pyrithiobac was not different from pyrithiobac applied alone. When malathion was applied 3 d after pyrithiobac, injury occurred only in 1995. At 14 and 21 DAT, the combination of pyrithiobac plus malathion caused the most cotton injury. Seed cotton yield was not adversely affected by any treatment when compared with pyrithiobac applied alone.

Nomenclature: Malathion, O,O-dimethyl phosphorodithioate of diethyl mercaptosuccinate; pyrithiobac; cotton, Gossypium hirsutum L. ‘DES 119’, ‘DPL 50’.

Additional index words: Crop injury, insecticide, interaction, pesticide, timing.

Abbreviations: DAT, days after treatment; POST, postemergence; PRE, preemergence.