The effect of prey type and abundance on the reproductive success and pre-ovipositional activity of the ground beetle Calosoma sycophanta L. was investigated to better understand their relationship with the gypsy moth, Lymantria dispar (L.). We obtained the numbers of ovipositing females, eggs per oviposition, and viable eggs for beetle pairs fed differing numbers of L. dispar and Galleria mellonella L. larvae for 6 wk after simulated emergence. Beetle pairs fed 10 L. dispar larvae at each feeding produced significantly higher numbers of eggs than those of any other treatment. Beetles fed G. mellonella were generally not reproductively successful, except when the diet also included adequate numbers of L. dispar . We monitored the post-emergence activity of beetles in similar feeding groups with a video camera recording 1 s of every 30 s for 2 wk. By the 2nd wk, male beetles were significantly more active than females in all groups except those receiving high numbers of L. dispar . These results indicate that abundant prey is critical and that L. dispar is important to the diet of C. sycophanta if successful reproduction is to occur. In particular, female beetle activity is related to prey type and availability.