Diminutive warty birch caterpillars (Falcaria bilineata), less than 1.5mm long, ardently defend their leaf tip homes from invading caterpillars by scraping and pounding the leaf to warn off potential invaders. But how might the day-old caterpillars defend themselves when voracious ladybeetles (known as ladybugs in the US) are on the prowl?
Jayne Yack and colleagues from Carleton University, Canada, report in Journal of Experimental Biology that tiny warty birch caterpillars can distinguish between the approaching footsteps of deathly ladybeetles and invading caterpillars. The young homeowners are smart enough to keep quiet when a ladybeetle approaches and throw themselves off the leaf for safety when the attacker gets too close. But when an invading caterpillar closes in, the defender makes a lot of noise to try to scare them off.
Curious to find out how young warty birch caterpillars react when under threat, Yack and Emilie Mauduit also from Carleton University, allowed newly hatched caterpillars to establish home on a leaf tip, filming the resident, while recording the vibrations generated by the youngster using a laser beam trained on the leaf. Then the team released an adult ladybeetle (Hippodamia convergens) onto the leaf and watched what happened.
The caterpillar fell almost quiet, became completely silent as the ladybeetle drew nearer and froze. The caterpillars weren't even trying to deter the ladybeetles. And some caterpillars flung themselves off the leaf within 5 s of the ladybeetle's arrival, remaining suspended on a silken thread until the adult ladybeetle departed, because the stakes were high: 43% of the caterpillars did not survive their encounters with adult ladybeetles.
When faced with a lady beetle larva, which crawl around on 6 legs, the residents made some effort to warn off the intruder, beating and scraping the leaf a little more often than when alone on the leaf, before falling silent. They also took up to 40 s before deciding to evacuate.
However, when Sarah Matheson (Carleton University) recorded intruding warty birch caterpillars approaching an occupied leaf tip, this time the caterpillar resident ramped up their beating and scraping to every couple of seconds to send a strong warning message.
'[The caterpillars] are capable of recognising the level of threat and responding in accordance', says Mauduit.
The caterpillars could distinguish between different intruders, but how could they tell the difference? Might the vibrations produced by the thundering footsteps of a 20 mg adult ladybeetle, the pattering of 3 mg ladybeetle larvae and shuffling 0.8 mg caterpillar intruders be sufficiently different for the caterpillars to distinguish them?
The team recorded the vibrations produced by each type of intruder as they charged past the vibration-sensing laser beam, comparing various aspects of the vibrations – including the intensity, pitch and bandwidth – to find out how they differed and realised that the intruders' footsteps were all uniquely distinct.
The behemoth ladybeetles produced the loudest vibrations, which travelled through the leaf providing an early warning for the vulnerable resident caterpillars. However, the vibrations produced by the ladybeetle larva intruders were more similar to those of the shuffling caterpillars, possibly explaining why the resident caterpillars were initially confused, trying to warn off the intruder before realising their mistake and falling silent.
So tiny warty birch caterpillars can distinguish the footsteps of threatening ladybeetles from interlopers that would evict them, and decide to take defensive or evasive action, which is quite extraordinary for such a minute day-old creature.
'That caterpillars 1/20th the size of a grain of rice have such sophisticated sensory capabilities is astounding', says Yack.
