Savage Garden and an attack by killer wasps.

Chemical warfare in a tiny Rotorua garden.

On a sunny afternoon relaxing in suburban Rotorua, above the lake not far from the CBD, bad things happen in tiny gardens.

Early summer and dark chemical processes are at work in the bright sunlight. Swan plants are producing and stockpiling deadly molecules in their sap and leaves. A chemical defence system designed to kill insects foraging for food. The main components of this molecular arsenal are cardenolide glycosides. They are steroids with a sugar molecule attached. These toxins can induce cardiac arrest.

Monarch caterpillars and butterflies are immune to these poisons. Caterpillars voraciously devour swan plant leaves. They hijack the plant’s chemical defence system to deter hungry predators. Birds associate becoming sick with a diet of brightly coloured caterpillars or butterflies and drop them from their diet.

Asian Paper Wasps.

Asian paper wasps are very determined predators of monarch caterpillars and are unaffected by the cardenolides. Wasps need protein for nest construction and to feed developing larvae in spring and early summer. Caterpillars are a source of protein at this stage.

I watched wasps creep through dense foliage in relentless pursuit of their prey.

Monarch caterpillars appear to understand the danger and seek refuge amongst the spines of the swan plant seed pods. The aerial bombardment is relentless, and a plant can be completely stripped of the caterpillar population in a day.

Large mature caterpillars are a favourite target. I have seen them stand up and head-but wasps in attempts to beat off the merciless attackers.

The relationship between steroids and cardenolide glycosides.

Oleandrin is one of the cardenolide glycosides found in swan plants. The structure on the bottom left of the steroid skeleton is a sugar unit hence the glycoside in the name.

Swan plants are on steroids.