Prey nutrient content creates omnivores out of predators

Abstract

The proximate forces that create omnivores out of herbivores and predators have long fascinated ecologists, but the causal reasons for a shift to omnivory are poorly understood. Determining what factors influence changes in trophic position are essential as omnivory plays a central role in theoretical and applied ecology. We used sevenspotted lady beetles (Coccinella septempunctata) to test how prey nutrient content affects beetles’ propensity to engage in herbivory. We show that beetles consuming an all‐prey diet demonstrate normal growth and development, but suffer a complete loss of fitness (spermatogenic failure) that is restored via herbivory and supplementation with phytosterols and cholesterol. Furthermore, we show that lady beetles possess a state‐dependent sterol‐specific appetite and redressed their sterol deficit by feeding on foliage. These results demonstrate that predators balance their nutrient intake via herbivory when prey quality is low, and reveal a selective force (sterol nutrition) that drives predatory taxa to omnivory.

Plant Secondary Metabolites as Defenses, Regulators, and Primary Metabolites: The Blurred Functional Trichotomy

“…Box 1 cited articles: Glauser et al, 2011;Maag et al, 2016;Robert et al, 2012;Robert et al, 2017. Box 2: Multi-functionality of plant secondary metabolites in tritrophic interactions. Box 2 cited articles: Hunter, 2003;Sarfraz et al, 2009;Ugine et al, 2019;Aartsma et al, 2017;Erb, 2018, Sloggett andDavis, 2010;Robert et al, 2017;Sun et al, 2019a;Fink and Brower, 1981;Rafter et al, 2017;Zhang et al, 2019. (Glauser et al, 2011;Maag et al, 2016), the regulation of callose deposition as a defense against aphids (Ahmad et al, 2011;Meihls et al, 2013), and iron uptake through their capacity to chelate iron in the rhizosphere (Hu et al, 2018).…”

“…Beyond plants and herbivores, the multifunctionality of plant secondary metabolites may also extend to the third trophic level. The current view on this topic is that i) plant primary metabolites influence tritrophic interactions by directly or indirectly feeding natural enemies of herbivores (Hunter, 2003;Sarfraz et al, 2009;Ugine et al, 2019), ii) volatile secondary metabolites can serve as foraging cues (Aartsma et al, 2017;Turlings and Erb, 2018), and iii) nonvolatile secondary metabolites can reduce herbivore host quality and thereby negatively affect natural enemies of herbivores, in particular when they are sequestered by specialized herbivores (Nishida, 2002;Opitz and Müller, 2009). To what extent natural enemies of herbivores may use plant secondary metabolites as primary metabolites for their own nutrition or as hormone-like regulators is currently unknown.…”

Identifying plant DNA in the faeces of a generalist insect pest to inform trap cropping strategy

“…MOTUs at the genus level and gives no information about predation on other invertebrates. This last point is important because prey nutrient content can modify omnivorous insect's propensity to engage in herbivory (Ugine et al 2019). Secondly, the RRA may not be an accurate quantitative measure of actual amount of each species consumed (Deagle 2019).…”

Plant defense resistance in natural enemies of a specialist insect herbivore

“…Herbivore natural enemies are regularly exposed to plant defense metabolites, either by coming into contact with plants directly, or by preying on herbivores that contain plant defenses (33-36). Many herbivore natural enemies have been found to avoid plant defenses by rejecting herbivores that accumulate or sequester toxins (37-39).…”

“…Whether evolved resistance to plant toxins is common in herbivore natural enemies remains to be determined. Given that exposure of natural enemies to plant toxins is frequent in nature (33-36) and that different natural enemies have been reported to resist and accumulate plant toxins (18-21), we expect evolved behavioral avoidance, metabolic resistance and tolerance strategies to be widespread among members of the third trophic level. Host diversity and diet breadth will likely determine the prevalence and biochemical architecture of these traits in herbivore natural enemies (20).…”