Top-down suppression of arthropod herbivory in intercropped maize and organic farms evidenced from δ13C and δ15N stable isotope analyses

Abstract

Maize is a globally important cereal crop and a staple in sub-Saharan Africa, where it is predominantly grown by small-scale farmers. Its production, however, is undermined by numerous herbivorous arthropods, and agrochemicals used for controlling such pests are increasingly unaffordable. Farmers therefore require cheaper, effective and environmentally sustainable pest management alternatives. This study explored the hypothesis that boosting habitat heterogeneity through organic farming and intercropping enhances arthropod predator-herbivore feeding linkages that naturally suppress herbivory across non-Bt maize fields. To test this, δ 13 C and δ 15 N stable isotope analyses were conducted to establish feeding pathways from maize, legume intercrops, and marginal vegetation, through arthropod herbivores and predators across 15 small-scale maize fields in western Kenya. Farming and cropping systems' roles in trophic linkages were also assessed. Feeding connections and plant food source contributions to arthropod consumer diets were subsequently determined using Bayesian mixing models, and predator relative efficiencies also evaluated. The results showed significantly stronger predator-herbivore trophic linkages within intercropped than monoculture fields, while farming system showed no effect. Herbivores showed stronger connections to crops than to noncrops, suggesting higher vulnerability for crops. For predators, earwigs derived most basal carbon from maize; wasps and predatory beetles, from legumes; ants, from both maize and legumes; and spiders, from both crops and noncrops. Ants and earwigs are important in maize herbivore regulation, particularly for intercropping; wasps and predatory beetles for regulating legume herbivores; and spiders for whole-field herbivore regulation. Most studies have focused on single species at singletrophic levels, but here we demonstrate, for the first time in sub-Saharan Africa, application of stable isotope analyses to characterize multitrophic feeding interactions that indicate effective agronomic practices for fostering top-down arthropod herbivore suppression in non-Bt maize fields. The results are useful in prescribing field practices with low-impact habitat management for sustainable small-scale agriculture rather than pesticide-based arthropod herbivore control.

Push-pull cropping system soil legacy alter maize metabolism and fall armyworm, Spodoptera frugiperda (Lepidoptera: Noctuidae) resistance through tritrophic interactions”

“…This implies the possibility that plants in push-pull soil may constitutively attract natural enemies of herbivores into the crop plant population and so mediate constitutive indirect resistance. Previous studies have shown that intercropping systems are associated with higher natural enemy numbers of herbivores and play a role in reducing herbivore numbers in these systems through predation and increased parasitism ( Otieno et al, 2019;Khan et al, 1997). In our study, we suggest intercrop-induced VOC emission as a new mechanism for increased pest control in intercropping systems in general and push-pull in particular.…”

Tropical insectivorous birds’ predation patterns that promote forest–farmland trophic connectivity for integrated top–down pest biocontrol

“…Except from the last two and a half decades when official forest protection enforcement began to improve, the trends in forest's destruction, degradation, and fragmentation had traditionally been tied to the fact that it is surrounded by farmland and a dense human population practicing mainly subsistence agriculture and small-scale livestock husbandry but with considerable dependence on the forest for wood, fiber, and other products, though this is now regulated by forest management authorities (Farwig et al, 2008). The main crop on the eastern side of the forest includes mainly non-Bt maize, which is commonly intercropped with legume crops (Otieno et al, 2019a). Farmers also typically maintain some stand of woodlots and fruit orchards, and also plant trees along property boundaries, and occasionally establish fast-growing agroforestry trees alongside row-crops (Diwani et al, 2013).…”

“…From the perspective of spatial and temporal fluxes in resource abundance such as the cycles of crop planting seasons followed by after-harvest transitions, such taxon-level food resource diversification and flexibility are important for the herbivores' survival and persistence (Driscoll et al, 2013). However, the herbivores 'apparent overall preference of maize over the other two plant food sources likely owed to the position of maize as the predominant and most abundant subsistence row-crop, as well as being the most common across these particular farming landscapes (Laube et al, 2008;Otieno et al, 2019a) For this reason. therefore, of the three plant food options assessed here, maize was the most readily available to most herbivorous species of the three arthropod taxa.…”

“…However, these discrepancies may also arise from the intraguild predation processes (Sanders et al, 2011), which were, however, not focal to the present study. For instance, cisticollid warblers could also derive maize carbon through consuming arthropod natural enemies of maize pests such as ants and spiders (Sanders et al, 2011;Otieno et al, 2019a).…”

Aromatic border plants in early season berries do not increase parasitism of spotted wing drosophila, Drosophila suzukii

“…By providing resources such as nectar, pollen, and habitat, intercropped flowers may increase the diversity and abundance of the natural enemies of various pest species. 8,9 This has been shown to be an effective, ecology-based method of both attracting the natural enemies of pest species and repelling pest insects in some cropping systems. [10][11][12] An intercrop of particular interest in the control of D. suzukii is sweet alyssum (Lobularia maritima).…”