Agricultural intensification, including the conversion of agroecosystems into monocultures, is a leading cause of global biodiversity loss. Preserving smallholder farms that use traditional mixed cropping systems is one strategy to prevent further declines in arthropod biodiversity and maintaining crucial ecosystem services, such as those provided by natural enemies of pests. We investigated the impact of crop diversity on both the abundance and diversity of herbivores and their natural enemies within the traditional milpa intercropping system, with a particular focus on diversity in response to its widespread decline. We compared arthropod abundance, species richness and evenness as measured by the Shannon diversity index in replicated (n =15) plots (2 m x 2 m) of monocultures, bicultures and tricultures of maize, beans, and squash in an agricultural field in Oaxaca, Mexico. Our findings support the hypothesis that the milpa system fosters higher levels of arthropod diversity compared to monocultures, primarily due to independent effects of each crop species in an additive manner. Notably, our observations also revealed non-additive effects on herbivore diversity within the squash-maize bicultures, through the increase and decrease in abundances of less and more dominant insect groups, respectively. Overall, however, we did not detect an effect of crop diversity on aggregate arthropod abundance. Our results show that polycultures effectively increase biodiversity, but they also emphasize the need of considering plant interactions when selecting specific crop combinations.