Non-crop habitats concurrently drive crop colonization by the millet head miner and regulation by natural enemies

Non-crop habitats, depending on their composition, can enhance the abundance and diversity of natural enemies of crop pests, but also at the same time provide resources to pests, thereby reducing the effect on pest incidence and resulting yield losses. The objective of the present study was to test (1) the effect of semi-natural habitats in the landscape on crop colonization by pests and natural regulation, and (2) the relationship between natural regulation and pest incidence. The pearl millet head miner (MHM) was selected as a case study because it is a key pest of millet cultivated in traditional pesticide-free tree-crop agroforestry systems in which its control mostly relies on the action of natural enemies. A set of 24 millet fields were selected in a 20x20 km area in Senegal, from the analysis of high-resolution satellite images (Pleiades), and hypotheses on the relative abundance of semi-natural habitats (here trees and rangelands) in the agricultural landscape. Millet fields were monitored for pest infestation of panicles and pest natural regulation. We used partial least squares structural equation modelling (PLS-PM) to evaluate the relationships between the abundance and diversity of semi-natural habitats at the landscape scale, crop colonization, natural pest regulation, and pest incidence. Panicle colonization by the MHM was generally high (14-92%) and increased with the abundance of trees and to a lesser extent with the rangeland area at a 1000 m-radius around millet fields. However, regulation provided by natural enemies was amplified by the abundance of trees at a local scale (250 m-radius around millet fields). This was particularly true at early crop colonization of the MHM with parasitism and direct predation on eggs and young larvae. This multi-scale effect of semi-natural habitats on crop colonization and natural regulation could explain why no clear relationship between crop colonization and pest incidence, nor natural regulation and pest incidence, was observed. Future studies on the identification of complex species-specific interactions between trees and natural enemies should provide a better understanding of the ecological processes underlying the performance of natural regulation of MHM populations. (C) 2022 The Authors. Published by Elsevier GmbH on behalf of Gesellschaft fur Okologie.

Automated summary

The study examined the impact of semi-natural habitats on crop colonization by pests and natural regulation, as well as the relationship between natural regulation and pest incidence. Through monitoring millet fields in Senegal, it was found that the abundance and diversity of semi-natural habitats had a certain influence on pest infestation, while the regulation provided by natural enemies was mainly influenced by the abundance of local trees. However, due to the multi-scale effects of semi-natural habitats, there was no clear relationship observed between crop colonization and pest incidence, as well as natural regulation and pest incidence.