Ladybugs are Wolves in Sheep’s Clothing
Wolf in sheep’s clothing: a predatory ladybird larva (Cryptolaemus montrouzieri) is disguised and protected by its woolly coat of wax filaments.
Courtesy of David Cappaert, Michigan State University
CSIRO research has revealed that the tremendous diversity of ladybug beetle species is linked to their ability to produce larvae which, with impunity, poach members of ‘herds’ of tiny, soft-bodied scale insects from under the noses of the aggressive ants that tend them. Reconstructing the evolutionary history of ladybug beetles (family Coccinellidae), the researchers found that the ladybugs’ first major evolutionary shift was from feeding on hard-bodied (“armoured”) scale insects to soft-bodied scale insects.
“Soft-bodied scales are easier to eat, but present a whole new challenge,” says Ainsley Seago, a researcher with the CSIRO’s Australian National Insect Collection. “These soft-bodied sap-feeding insects are tended by ants, which guard the defenseless scales and collect a ‘reward’ of sugary honeydew. The ant tenders aggressively defend their scale insect ‘livestock’ and are always ready to attack any predator that threatens their herd.”
Therein lay the evolutionary problem confronting ladybugs, whose larvae were highly vulnerable to ant attacks. To avoid being killed as they poach the ant’s scales, ladybug larvae evolved to produce two anti-ant defenses: an impregnable woolly coat of wax filaments, and glands which produce defensive chemicals. Most of the ladybird family’s 6,000 species are found in lineages with one or both of these defenses.
“We found that most of ladybird [ladybug] species’ richness is concentrated in groups with these special larval defenses,” Seago said. “These groups are more successful than any other lineage of ladybird beetle. Furthermore, these defenses have been ‘lost’ in the few species that have abandoned soft-scale poaching in favor of eating pollen or plant leaves… This is an unusual way for diversity to arise in an insect group… In most previous research, insect species richness has been linked to co-evolution or adaptive ‘arms races’ with plants.”
This research helps to place Australia’s ladybugs in the evolutionary tree of life for insects, and helps us to understand the complex system of mechanisms by which beetle diversity has arisen.