Trish Currall, of Hamilton, asks :-

I have seen wasps attack monarch butterflies and their caterpillars. Where did this nasty species originate from, as only a few years ago, there was no problem hatching these larvae, but now we have to cover the swan plants with netting to allow any of the grubs to hatch.

John Marris, an entomologist in charge of the research collection at Lincoln University, responded.

Wasps are regularly reported to feed on monarch butterfly caterpillars. Four predatory wasp species are known assailants – the German and common wasps and two species of paper wasps, the Australian paper wasp and the Chinese paper wasp. The German and common wasps are the familiar and widespread, yellow and black striped wasps that build large nests, predominantly underground. Paper wasps are longer and more slender species and are found mainly in the North Island. They build smaller, aerial nests that often hang from trees and shrubs. All four wasp species were accidentally introduced to New Zealand and each are capable of delivering a nasty sting to humans.

Monarch caterpillars feed almost exclusively on milkweed plants, which contains toxic compounds. Rather than harming the caterpillars, they store these toxins and use them for chemical defence against predators. The chemicals are transferred through to the adults to give the same protection to the butterfly stage. Not content with simply being poisonous, both the caterpillar and butterfly advertise their distastefulness through their bright warning colouration.

Despite these chemical deterrents, the four wasp species are still able to feed successfully on monarch caterpillars. It is unclear, however, as to how these wasps cope with the defensive toxins. One overseas study showed wasps had a slight preference for caterpillars with lower toxin levels. Other studies suggest that wasps feed on parts of caterpillars that have lower toxin concentration.

Another insect predator of monarch caterpillars is the brown soldier bug, which stabs its prey with its needle-like mouthparts and sucks out the contents of the unfortunate larva.

The best account of monarchs in New Zealand, including the impacts of wasps and other predators, is in George Gibbs’ excellent book, The Monarch Butterfly in New Zealand, published in 2013 by the Entomological Society of New Zealand.