Enhancing biological control: conservation of alternative hosts of natural enemies

Background: There is a consensus that the conservation of natural enemies is the most important biological control strategy, but it has also been the least attended. The reason is simple: there is a strong contradiction between modern agriculture and biological control. Various strategies have been proposed for the conservation of natural enemies, among others, protection against pesticides and establishment of nectar plants as alternative food. Less attention has been paid to the conservation of alternative hosts for natural enemies. Main body: Natural enemies are not exclusive to pests, on the contrary, numerous species require alternative prey and hosts for their prevalence. Their conservation is a strategy widely referenced in scientific literature; however, this idea is not consistent with the studies developed. From 1973 to 2021, only 21 studies that emphasized the use of alternative hosts were recorded. Most focused on single phytophagous–natural enemy species, and little attention was given to the plant–phytophagous relationship. For example, Asclepias curassavica (Apocynaceae) hosts more than nine species of specialist phytophagous; and they, in turn, attracted 24 species of natural enemies. Although different studies demonstrate the potential of alternative hosts, some presume an adverse or doubtful effect on pest control; for example, additional vegetation in agroecosystems could act as a source, but also as a sink for natural enemies. This analysis tries to fit biological control to the modern agricultural paradigm, and not the other way around, as suggested by ecological theory. We support the idea that conservation biological control should be directed toward the conservation of multiple species, with the aim of controlling not only pest, but also the self-regulation of the agroecosystem. The path that should be followed by the study and application of alternative hosts as a strategy of biological control by conservation is discussed. Conclusion: The wide diversity of non-pest insect species suggests an enormous potential for their exploitation as alternative hosts. The evaluation of host plants of specialized phytophagous species could improve biological control and reduce the risk of pests for crops. This strategy would partly fill the huge gap that exists in modern agroecosystems in terms of biological diversity.