Omnivory in predatory lady beetles is widespread and driven by an appetite for sterols

1. Animals maintain physiological and behavioural systems that allow them to detect and consume specific macro- and micronutrients to maximize their fitness. One common physiological system is the nutrient-state-dependent or demand-driven appetite. These systems are well described for macronutrient regulation, but not for micronutrients. 2. Sterols are essential micronutrients that all animals need to survive. They are the backbone of many hormones, important in cell signalling and an integral component of cell membranes. 3. Lady beetles are globally distributed predators of insect herbivores. Adult seven-spotted lady beetles maintain a state-dependent sterol appetite and consume plant tissues to obtain sterols, which improves their fitness. Additionally, seven-spotted lady beetles can detect sterols pre-ingestion. 4. We used lady beetle species distributed across the three clades of the Coccinellini to determine (a) whether other beetle species maintain a state-dependent sterol appetite, (b) if sterol structure affects beetles' state-dependent sterol appetite and (c) whether lady beetles consume foliage in a sterol-state dependent manner. Additionally, we determined (a) what sensory organ beetles use to detect sterols, (b) their limit of detection and (c) when during development their appetite manifests. 5. All six beetle species we tested maintained a state-dependent appetite for sterols. Sterol structure affected beetles' propensity to feed on sterol-treated disks, indicating that the number and position of double bonds in sterol molecules affects beetles' ability to detect or desire to feed on them. Only beetles in Glade III fed on plant foliage in response to sterol limitation. Few beetles in any Glade that were supplemented with sterols consumed plant tissue. 6. Beetles' appetite for sterols first appeared during the second larval stadium, and the rate of sterol consumption increased with age. Ablations of sensory organs revealed that beetles use their labial palps to detect sterols, and that they detect them at concentrations as low as 1ppm. 7. These data demonstrate that lady beetles across the Coccinellini maintain a state-dependent appetite for an essential class of micronutrients. They also provide very strong evidence that lady beetles can taste sterols, raising the possibility that they maintain novel as-yet undescribed gustatory receptors for an essential class of lipids.

Automated summary

Animals maintain physiological and behavioral systems to detect and consume specific nutrients, including sterols which are essential for survival. Lady beetles globally have a state-dependent sterol appetite to enhance fitness and can detect sterols pre-ingestion. Beetles across various clades maintain a state-dependent appetite for sterols, with the ability to taste them at concentrations as low as 1ppm.