Eradication programs have eliminated boll weevil as an economic pest from 4.5 million acres in 8 states (Cunningham and Greffenstette 1998). Eradication program protocols have relied heavily on the use of malathion to eliminate boll weevil. Catolaccus grandis (Burks) has been extensively evaluated for suppression of boll weevil in cotton and may be used in eradication programs where synthetic insecticide applications are problematic (King et al. 1995, Morales-Ramos 1995, Summy et al. 1995). However, if used in conjunction with malathion applications, more understanding of residual activity of malathion against Catolaccus grandis is needed. Malathion was the most toxic insecticide to an in-vivo strain of C. grandis of 10 insecticides tested in laboratory bioassays (Elzen et al. 1999). The objective of this study was to evaluate residual activity against C. grandis in the field, using standard and microencapsulated formulations of malathion.