The boll weevil infestation of 1892-1922 had a clear and lasting impact on the US South's economy. In this paper, we show that it also affected the region’s demography. When the boll weevil hit the cotton South, it encountered a region populated by families of tenant farmers. Tenant farming created both economic opportunities and economic incentives for prospective tenants to marry at young ages. The boll weevil infestation undermined this family-based organization of agricultural labor. Using data from historical US Department of Agriculture maps, complete-count Census of Population data from 1900-1930, and Census of Agriculture data from 1889-1929, we show that the boll weevil’s arrival reduced both the share of farms worked by tenants and the share of African Americans who married at young ages. We also document that increases in tenancy over time increased the prevalence of marriage among young people, particularly young African Americans. Our results provide new evidence about the effect of economic institutions on demographic transformations.