Weighty matters:: Body size, diet and specialization in aphidophagous ladybird beetles (Coleoptera: Coccinellidae)

Aphidophagous ladybirds exhibit a broad range of body sizes. Until now this has been thought to be a function of the different prey densities that they feed at, with smaller ladybirds feeding at lower prey densities. The size of the prey species they feed on has been considered to have no relationship with ladybird body size. However, these arguments possess a limited capacity to explain observed data from the field. I here demonstrate a more realistic, complex approach incorporating both prey density and the size of prey species. Small ladybirds call feed on small aphids at both low and high densities. However when the aphid species is large the), cannot catch the older, bigger, more energy-rich aphid instars Clue to their small size. They are thus unable to feed oil large aphid prey at low densities, although at higher densities numbers of the smaller instars may be Sufficient to sustain them. By contrast large ladybirds can feed on large aphids at both low and high densities due to their superior ability to catch the bigger, more energy-rich older aphids; however they cannot be Sustained by low densities of small aphids due to food limitation consequent oil their large size. This more complex association between ladybird size, prey size and prey density possesses a better explanatory power for earlier field data. Because of this relationship, ladybird body size also provides all important trade-off determining dietary breadth and specialization in the aphidophagous Coccinellidae. Dietary specialists more closely match the size of their limited prey species, have higher overall capture efficiencies and can thus Continue to reproduce at lower aphid densities for longer. By contrast dietary generalists adopt a one-size-fits-all strategy, are medium-sized and have lower capture efficiencies of individual prey Species, thus requiring higher aphid densities. The role of body-sizc in dietary specialization is supported by data from the British fauna. Rather than trade-offs related to prey chemistry, which have hitherto been the centre of attention, body size trade-offs are the likely most important universal factor underlying dietary specialization in aphidophagous coccinellids.