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• Caterpillar-parasitoid food webs on annual vs perennial extensive crops were compared. • Caterpillars were more abundant but equally diverse on alfalfa than on soybean. • Caterpillars on alfalfa supported higher parasitoid richness and parasitism rates. • Food web vulnerability was a key differential parameter between crops. • Parasitism rates did not increase with parasitoid richness or food web vulnerability. Food webs offer a useful approach to the study of complex communities interacting across trophic levels, potentially linking community structure and ecosystem services such as biological control. Herbivore-parasitoid food webs are relevant for agricultural production, in terms of pests and their control. Here, we compare caterpillar-parasitoid food webs in a perennial (alfalfa) and an annual (soybean) crop in Central Argentina, by analyzing their diversity, food web structure and parasitism rates using four years of data. Caterpillar – parasitoid communities in extensive agricultural systems have barely been addressed in a food web context. We found the same eleven defoliating Lepidoptera species in both crops, with similar annual richness values. However, alfalfa sustained larger caterpillar populations and richer parasitoid assemblages than soybean, even when only summer data (when both crops coexisted) were considered. Parasitism rates were also higher on alfalfa (only for yearlong data), suggesting bottom-up effects on the third trophic level, and may reflect the resource continuity offered by this crop. A discriminant analysis separated alfalfa from soybean food webs, mainly on the basis of vulnerability (an indicator of the mean number of parasitoid species per host), which was in turn unrelated to parasitism rates, thus suggesting a breach between two functional approaches, i.e. food webs and ecological processes such as biological control. [ABSTRACT FROM AUTHOR]