Agricultural crops are investments that can be exploited by others. Farmer clones of the social amoeba Dictyostelium discoideum carry bacteria to seed out new food populations but they also carry other non-food bacteria such as Burkholderia spp. Here we demonstrate that these farmer-carried Burkholderia inhibit the growth of non-farmer D. discoideum clones that could exploit the farmers' crops. Using supernatants, we show that inhibition is due to molecules secreted by Burkholderia. When farmer and non-farmer amoebae are mixed together at various frequencies and allowed to complete the social stage, the ability of non-farmers to produce spores falls off rapidly with an increase in the percentage of farmers and their defensive symbionts. Conversely, farmer spore production is unaffected by the frequency of non-farmers. Our results suggest that successful farming is a complex evolutionary adaptation because it requires additional strategies, such as recruiting third parties, to effectively defend and privatize crops.