The generalist predator Geocoris pallens Stäl is a common natural enemy in many agro-ecosystems in California. Although G. pallens is regarded as a biocontrol agent of two main pests in cotton, Lygus hesperus Knight and the two-spotted spider mites (Tetranychus urticae Koch, TSM), its efficacy in doing so remains unclear. Various factors may affect the capacity of biocontrol exerted by G. pallens, including intraguild predation (IGP) by Zelus renardii Kolenati. A season-long manipulative field experiment was conducted in cotton to evaluate the biocontrol capacity of G. pallens on L. hesperus and TSM and the effects of IGP. Results show that G. pallens could not control TSM. G. pallens provided biocontrol of L. hesperus and the addition of IGP strengthened this biocontrol. Cotton plant weight did not differ among treatments, but G. pallens and IGP treatments improved cotton square retention. This study suggests that IGP does not necessarily suppress biocontrol. More work is needed to evaluate the biocontrol capacity of G. pallens and its ecology.