We investigated the response of two species of lady beetles, Coccinella 7—punctata and Hippodamia variegata, to spatial variation in the density of aphids on fireweed, Epilobium angustifolium. Our approach involved experimental manipulations of aphid densities and field observations of lady beetle foraging at three different scales: (1) Individual lady beetles on individual fireweed stems (2) populations of lady beetles on individual fireweed stems, and (3) populations of lady beetles in populations of fireweed stems. At the finest scale, we found that individual lady beetles remained on single fireweed stems longer in the presence of larger aphid colonies. However, this behavioral response to aphid density was weak, explaining only 11 and 4% of the total variance in the length of time C. 7—punctata and H. variegata remained on the stems. To examine directly the population—level effects of this individual lady beetle behavior, we conducted a second series of experiments that focused on the response of populations of lady beetles; we released large numbers of lady beetles into field plots in which we had manipulated the number and size of aphid colonies. By sampling the distribution of lady beetles, we showed that the number of lady beetles found on fireweed stems depended significantly on the presence and size of aphid colonies. The average number of both C. 7—punctata and H. variegata found on a stem was 10—20 times greater when the stem contained a large aphid colony that when the stem contained no aphids. Finally, we examined the consequence of these patterns at the scale of populations of lady beetles and populations of fireweed in 25—m2 field plots. We found that the number and size of aphid colonies within the plots explained 50 and 90% of the variation among plots in the average length of time that C. 7—punctata and H. variegata remained. Taken together, these results demonstrate that although individual lady beetle response to aphid density is extremely weak, the cumulative effect of many individuals can produce strong population—level aggregation of lady beetles in areas of high aphid density.