Changes in the numbers of adults of Coccinella californica Mannerheim and C. trifasciata Mulsant in small plots of alfalfa and oats were monitored over the summers of 1974 and 1975 by mark–recapture methods. Aphid population dynamics in the plots were studied concurrently. In the second summer, movements of the beetles were also monitored by means of individual marking and sticky net traps. Approximately 1000 beetles of each species were marked in the first summer; in the second, roughly 5000 and 3000 respectively. The capture–recapture method was tested with a caged population, and was found to be feasible for estimation of the coccinellid numbers. Catch per unit effort, using visual sampling that was not dependent on marking, provided an acceptable index of numbers which could give population estimates when calibrated against mark–recapture estimates.

The number of beetles trapped on the sticky nets was primarily a function of numbers in the plots and temperature, with the two species having different relationships between temperature and flight. But for C. californica, the numbers trapped were also a function of aphid density in the plots; the data suggested that for C. trifasciata the same is probably true, but the range of densities of pea aphids was too narrow to show it. In both seasons, C. trifasciata was predominant in the alfalfa and C. californica in the oats. Plot to plot movements by overwintered beetles reflected the same apparent preference. Changes in the numbers of coccinellids in the experimental plots, and their reproductive success, are discussed in the light of these findings.