Most people think of weevils as little maggoty grubs that infest stored grain products. Which is true, but the reality is that the weevil family is the most diverse family of organisms on the planet, with more than 50,000 species. Weevils are beetles, and adults are characterised by having a long snout and antennae bent at right-angles. They range in size from less than 2 mm to about 50 mm long. Weevils all start their lives as eggs from which larvae (grubs) hatch, and it is the larvae of the maize weevil or rice weevil (both in the genus Sitophilus) that you might find in your old opened packet of muesli.
New Zealand has a diverse weevil fauna that includes several large flightless species at risk of extinction. Many weevil species feed exclusively on a single plant species, or group of related species. One example in New Zealand is the flax weevil (Anagotus fairburni) – a flightless species about 20 mm long. Flax weevils occur only on New Zealand flax – both harakeke (Phormium tenax) and the smaller wharariki (Phormium cookianum). The larvae live in the soil and feed on the flax roots, while the adults emerge at night and gnaw on the edges of the leaves.
Predation by rats and mice has confined flax weevils to a few small rodent-free islands plus some alpine areas where few rodents live. They formerly occurred throughout New Zealand, as evident from their relictual distribution from Northland (the Poor Knights Islands) to Fiordland (islands in Breaksea Sound), as well as on a few islands in the Marlborough Sounds.
As part of the ecological restoration of Mana Island, 80 adult flax weevils were translocated from Maud Island (Marlborough Sounds) in 2004. The weevils thrived at their release site on Mana Island, where mice had been eradicated in 1989. Within a few years their characteristic ragged feeding sign was evident on flax leaves, and adult weevils were readily found at night.
But the weevil population kept growing and growing. By 2013 the flaxes at the release site had collapsed and died, and adult weevils swarmed over unopened flower stalks on nearby plants, consuming them before they could grow and open. The weevil population is in the tens of thousands, in contrast to other islands, where usually only a few adult weevils occur on each plant, and cause only cosmetic damage to the leaves. So why have flax weevils become so abundant on Mana Island?
There are several theories, but no answers as yet. The two main theories are:
- That the flaxes on Mana Island are less resistant to weevil browsing than those on islands where weevils still occur. The flaxes at the release site are a hybrid swarm between native wharariki from the nearby cliffs and a harakeke cultivar planted as windbreaks when the island was farmed in the 1970s. Maybe the artificially selected cultivar has lost some level of immunity to weevil browse.
- That the weevils had benefited from some form of predator or parasitoid absence when they were translocated. Many beetle species are attacked by parasitoid wasps that lay eggs in their larvae or pupae, consuming them alive before they can mature into adults. Maybe moving only adult weevils broke the life-cycle of an as-yet unrecognised parasitoid that infests flax weevils, allowing the new population to grow unchecked.
The answer to the riddle is likely to require detailed field studies and possibly planting trials of different flaxes – and would be an ideal thesis topic.
There are conservation values at risk from too many flax weevils on Mana Island, and restoration of the island could benefit from an understanding of what usually limits their population. Mana Island holds the largest population of the rare goldstripe gecko, a lizard species that on the island lives almost exclusively within flax bushes. In addition, tui, bellbirds and two species of geckos (common geckos and goldstripe geckos) feed on flax nectar.
Flax weevils are neat critters, but healthy flaxes are an important part of the Mana Island ecosystem too.