Close Encounters

Buckleria paludum, the caterpillar that has a taste for danger

Far from being a victim, the caterpillar of Buckleria paludum feeds on carnivorous plants. But how?

Buckleria paludum is a plume moth found in moorland and peat bogs. Like many moths, it likes to find the right plant for its young, but its choice of plant suggests that these moths have a deep dislike of their children. Buckleria paludum caterpillars feed on Drosera, sundew, plants. This gives B. paludum the common name, the Sundew Plume Moth. This is the reverse of what you might expect because sundews are carnivorous plants. Normally a sundew would welcome a visiting invertebrate as lunch, so how do the caterpillars survive? Haruka Osaki and Kazuki Tagawa have recently published an article in Entomological Science where they take a closer look.

Drosera catch and kill insects through their trichomes, little hairs on the leaf. On the tips of the hairs are little globes of mucilage that sticks to the prey. As the insect attempts to escape it agitates the leaf, and it curls around the prey to digest it. So a caterpillar bumbling among Drosera leaves should have a short life expectancy.

Drosera spatulata. Image: Canva.

Osaki and Tagawa visited Okinawa to see how B. paludum ate D. spatulata. They found that the mucilage wasn’t able to trap the caterpillars because the caterpillars licked the mucilage from the hairs, cleaning them up. Most of the time it left the hairs alone. This means that hairs didn’t bend and the leaves didn’t curl. Sometimes it chewed the bases of the glandular hairs. The authors believe that caterpillars would get more energy in the same amount of time by eating the hairs, rather than licking the mucilage. So they conclude this is a deliberate action by the caterpillar to disarm the traps on the sundew. This is unusual.

“Animals have at least three types of defenses against adhesive leaves: secretion of fluid to lubricate the body surface for movement (Voigt & Gorb 2008; Fleischmann et al . 2016), breaking through mucilage with power (Eisner & Shepherd 1965; Gibson 1991) and avoiding glandular secretions behaviorally by carefully walking with long legs and “tip‐toeing” (Wheeler 2001; Voigt et al . 2007; Krimmel & Pearse 2014). The observed behavior of B. paludum, to lick mucilage, is not covered by any other known defense,” write Osaki and Tagawa in their article.

The advantage of licking the hairs clean is that it reduces the bending response in the hairs and leaves of sundews. Osaki and Tagawa consider that the mucilage might have value for the caterpillar as it has both polysaccharides and protein. However, this doesn’t prevent caterpillars cleaning the sundew leaf to numbs it also being a defensive strategy.

The authors leave highlight some puzzles to solve. There are hundreds of sundew species. How do they all react? Are they all susceptible to having their traps licked, or does B. paludum target specific susceptible species? What are the physiological processes that numb the sundew leaves? What is the mucilage on the leaves doing apart from sticking to insects?

Osaki and Tagawa note this is not the end, but the latest round of an arms race between Drosera and its prey. But for now the Drosera traps have a vulnerability, and the caterpillar has got it licked.