Web-building spiders form a major component of the generalist predator fauna in arable fields. They have been purported to contribute to the biological control of pests such as aphids and leafhoppers. However, their successful contribution to pest suppression is contingent upon their ability to adapt to highly-disturbed agroecosystems. We examined the population dynamics of these important natural enemies to compare phenological patterns in relation to crop cycles among species in an alfalfa agroecosystem using quadrat-based sampling and time-series analysis. Three common species of web-building spiders had generation times similar to the duration of a crop cycle (31 to 44 days), with peak abundances of adult spiders occurring at 15–18 days after harvest. The timing of these peaks corresponds with the critical early phase of the pest population cycle, during which natural enemies can have the maximum impact on pest populations, suggesting that these spiders are capable of contributing to pest suppression as part of an assemblage of natural enemies.