Field studies were conducted in 2001 through 2004 to assess the compatibility of two herbicides, 2,4-D (2,4-dichlorophenoxyacetic acid) and clopyralid (3,6-dichloropicolinic acid), with two root insects, Cyphocleonus achates (Fahraeus), and Agapeta zoegana L., introduced for biological control of spotted knapweed, Centaurea stoebe Lamarck subsp. micranthos (formerly C. maculosa) in Montana. Both herbicides were applied at the fall and spring rosette stage. In 2003, both herbicides reduced knapweed canopy cover by ≈98% compared with ≈85% in 2004. The number of live larvae of both insect species was significantly lower in treated plots than in controls in 2003. In 2004, the number of live C. achates larvae was significantly lower in treated plots than in controls at the fall application, but larval numbers were not different at the spring application. Larval numbers of C. achates were not different between application times in 2003 but were significantly lower in fall-treated plots than in spring-treated plots in 2004. Numbers of A. zoegana larvae were not different between treated plots and controls in 2004. Larval numbers of A. zoegana were significantly lower in fall-treated plots than in spring-treated plots in 2003, but there was no difference between application times in 2004. Larval numbers of each insect species were similar between herbicides in both years. We conclude that fall applications of both herbicides are not compatible with the two insects. Spring applications of the two herbicides may be compatible with both insect species if delayed until late spring.