Numerical responses of omnivorous arthropods to plant alternative resources suppress prey populations: a meta-analysis

Canola bee pollen is an effective artificial diet additive for improving larval development of predatory coccinellids: a lesson from Harmonia axyridis

BACKGROUND

Pollen is a common plant-derived food source for predatory ladybird beetles under field conditions, yet the potential for pollen to improve the quality of artificial diets remains largely unexplored. In this study, we developed three pollen diets by incorporating varying proportions of canola bee pollen (7.5%, 15.0% and 22.5% with 2.5%, 5.0%, and 7.5% of water, respectively) into a conventional diet. The feeding efficiency of Harmonia axyridis, an omnivorous predator, was evaluated and compared on three pollen diets, a conventional nonpollen diet and pea aphids.

RESULTS

The larvae fed a medium or high pollen diet exhibited significantly higher survival in the 4th instar, pupa and adult stages than those fed a nonpollen diet. These larvae also developed into significantly heavier adults, and their survival rates in adulthood were comparable to those fed pea aphids. Specifically, we revealed the underlying mechanisms through which a high pollen diet enhances pupal development. Consumption of high pollen diet versus nonpollen diet resulted not only in a significant decrease in pupal glycogen content, but also an increase in adult lipid content. Both diet treatments induced similar changes in carbohydrate and glycogen content compared to the aphid diet while exhibiting different alterations in pupal protein content and adult lipid content. Furthermore, the transcriptome analysis revealed that the nutrient metabolism, immune response, and cuticle development pathways were predominantly enriched among the differentially expressed genes (DEGs).

CONCLUSION

Canola bee pollen offers diverse advantages in terms of rearing H. axyridis larvae with an artificial diet, which will advance the development of effective diets for predaceous coccinellids. © 2024 Society of Chemical Industry.

An omnivore vigour hypothesis? Nutrient availability strengthens herbivore suppression by omnivores across 48 field sites

Nutrients regulate herbivore growth from the 'bottom-up' via improved plant vigour and food quality. Nitrogen also affects 'top-down' control of herbivores by moderating attraction of predators and the rates at which they consume herbivorous prey. Tri-trophic consequences of nitrogen availability are more challenging to predict among omnivorous natural enemies who feed on both plants and herbivores, limiting our ability to predict net outcomes of nutrient availability in food webs. In a two-year field survey of insects on zucchini host plants at 48 sites, I predicted that both herbivores and foliar-feeding omnivores would increase with nutrient availability, while predators would not. My results revealed positive relationships between omnivores and foliar nitrogen concentrations, while predators had neutral responses to foliar N. Surprisingly, herbivores declined with increasing foliar N across the field sites. Greenhouse experiments re-enforced these patterns, as herbivore growth inversely correlated with soil N concentrations in communities that included foliar-feeding omnivores. Conversely, herbivore growth was uncorrelated with soil N on plants with predators, nor on predator-free plants. These results suggest that omnivores mount strong and consistent responses to nitrogen in plant tissues in a variety of ecological contexts. In environments where omnivorous arthropods can thrive, their recruitment to nitrogen-rich plants may increase predation and thereby counterbalance and stabilize 'bottom-up' increases in herbivore performance supported by enhanced foliar nutrition.

Numerical responses of omnivorous arthropods to plant alternative resources suppress prey populations: a meta-analysis

Omnivory is ubiquitous in ecological communities. Yet, we lack a consensus of how plant alternative resources impact the ability of omnivores to suppress prey populations. Previous work suggests that plant alternative resources can increase, decrease, or have no effect on the magnitude of omnivore-prey interactions. This discrepancy may arise from 1) the ability of omnivores to numerically respond to plant alternative resources and 2) identity-specific effects of plant alternative resources. We used a meta-analysis to examine how omnivore numerical responses and the identity of plant alternative resources affect 1) omnivore predation rates (mainly reported as per capita predation rate) and 2) omnivore impacts on prey population density. Plant alternative resources reduced omnivore predation rate regardless of identity. The suppression of predation rate by flowers and flowering plants was magnified when pollen alone was tested as the alternative resource. Surprisingly, plant alternative resource availability reduced prey density, suggesting that omnivore predation increased with plant alternative resources. This discrepancy (plant alternative resources decreased omnivore predation rates but also decreased prey density) resulted from experimental differences in the ability of omnivores to respond numerically to plant alternative resources. In the presence of plant alternative resources, allowing omnivore numerical responses decreased prey density, while not allowing numerical responses increased prey density. Because omnivores commonly suppress prey density in the presence of plant alternative resources when numerical responses of omnivores are allowed, the effectiveness of biological control may depend upon the availability of such resources and the facilitation of numerical responses. This article is protected by copyright. All rights reserved.