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

1. Animals maintain physiological and behavioral 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 signaling, and an integral
component of cell membranes. 3. Lady beetles are globally distributed
predators of insect herbivores. Adult sevenspotted lady beetles maintain a
state-dependent sterol appetite and consume plant tissues to obtain
sterols, which improves their fitness. Additionally, sevenspotted lady
beetles can detect sterols pre-ingestion. 4. We used lady beetle species
distributed across the three clades of the Coccinellini to determine 1)
whether other beetle species maintain a state-dependent sterol appetite,
2) if sterol structure affects beetles’ state-dependent sterol appetite,
and 3) whether lady beetles consume foliage in a sterol-state dependent
manner. Additionally, we determined 1) what sensory organ beetle’s use to
detect sterols, 2) their limit of detection, and 3) 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
clade three fed on plant foliage in response to sterol-limitation. Few
beetles in any clade 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.