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By  Ian Dewar Andrea Wild 16 January 2024 7 min read

Key points

  • Weevils are beetles with long snouts (also called a rostrum).
  • Weevils can be very picky eaters and make perfect biocontrol agents for the weeds they feed on.
  • Some weevils are invasive pests that need to be managed or eradicated.

Australian weevils include more than 4110 species and that’s just the described species! They represent only a fraction of the total number of weevil species here.

As pollinators, some weevils use their powers for good. As pests attacking crops, pastures, flowers, trees and timber, some use their powers for weevil.

Many weevils eat only very specific plants. Known as "high host specificity" this makes them ideal biocontrol agents. It's why they’ve been widely used to fight weeds on land and water.

Let’s take a look at some of our fave resident weevils and one evil weevil.

Water hyacinth weevils

The Mottled water hyacinth weevil Neochetina eichhorniae. ©  Ricardo Arredondo T. CC BY-NC 4.0 (Int)

Water hyacinth is one of the world's worst aquatic weeds. It takes over waterways and damages aquatic ecology and fisheries.

We've seen it in Lake Victoria, Africa and the Sepik River in Papua New Guinea. In these places, it was choking waterways and stopping transport and fishing, affecting whole communities.

Two biocontrol weevils came to the rescue: the Mottled water hyacinth weevil (Neochetina eichhorniae) and the Chevroned water hyacinth weevil (Neochetina bruchi).

Biocontrol uses a weed’s natural enemies against it. All biocontrol agents are rigorously studied by our scientists before they are approved for release. This ensures they do not pose a risk to native plants or the environment.

Our scientists researched and released the weevils in Australia. Then they helped with weevil programs in Africa and Papua New Guinea.

The adult weevils eat the leaves and the larvae tunnel into the stems and crown. This reduces the growth, flowering and seed production of the plant. It can also cause the plants to rot and sink.

The weevils significantly reduced the spread of this weed. Local communities benefitted as vital waterways opened up again for transport and fishing.

Darwin’s weevil

A weevil collected by Charles Darwin in 1836. ©  Gordon Gullock

One of the oldest specimens in our Australian National Insect Collection is a weevil. It was collected by Charles Darwin near present-day Albany, Western Australia in 1836.

Darwin visited Australia during his long voyage on board HMS Beagle. Observing and comparing the world’s plants and animals during this voyage led Darwin to write On the Origin of Species, which introduced the scientific theory of evolution.

This tiny weevil is not much bigger than the head of the pin securing it.

Paterson's curse root weevil

An adult of the Paterson's curse Root weevil, Mogulones geographicus, on a Paterson's curse flower. The weevil is a biological control agent of Paterson's curse.

Paterson's curse (Echium plantagineum) is Australia's worst broadleaf temperate pasture weed. It can produce thousands of seeds per square metre. The seeds can last more than seven years and can germinate at any time of year, given the right conditions. This makes Paterson’s curse a persistent weed.

Two weevil species have been tested and released to tackle this weed. The Crown weevil (Mogulones larvatus) is the most damaging, often killing the weed outright on a large scale.

The Root weevil (Mogulones geographicus) feeds on leaf stalks at the base of the plant. The larvae tunnel into the tap root. They pupate in the soil and emerge in spring. In summer adults burrow into the soil and become dormant until after autumn rains.

An evil weevil

The Polyphagous shot hole borer, Euwallacea fornicatus. ©  Karl Magnacca CC BY-NC 4.0 (Int)

The Polyphagous shot-hole borer (Euwallacea fornicatus) is an invasive introduced species detected in Western Australia in 2021. It is just 2 millimetres long.

This evil weevil belongs to the subfamily Scolytinae. It is part of a group of weevils called ambrosia beetles.

Named for the food of the Greek Gods, ambrosia beetles tunnel into trees and farm fungi inside their tunnels. The farming lifestyle has evolved independently at least 16 times among these weevil species.

Ambrosia beetles have lost the long rostrums characteristic of weevils, probably because these would get in the way of tunnelling through wood.

Most species tunnel into dead wood. But when they invade new places, ambrosia beetles tend to attack living trees. Damage caused by tunnelling or the fungi they farm can kill the trees. The Polyphagous shot-hole borer can attack more than 400 different species of trees.

Dr James Bickerstaff is a CERC postdoctoral researcher at our Australia National Insect Collection. He is part of a team using genomics to help eradicate this weevil from Australia.

“We’re sequencing the genome of this species as the starting point for genetics research and future genetic-based management techniques,” James said.

“Female Polyphagous shot-hole borers are the key. Males are small, flightless, blind and have only one copy of each chromosome. Females, like most other animals, have two copies.”

“Individuals of this species usually mate with their siblings. The resulting inbred offspring tend to be healthier than outbred offspring. It’s pretty weird but it explains why they’re successful invaders. A single pregnant female can give rise to a large population.”

Platypus weevils

Platypus norfolkensis, an ambrosia beetle ©  Debbie Jennings, CSIRO

James also works on another group of ambrosia weevils that farm fungi: the subfamily Platypodinae.

They include the original Platypus, a name given to these ambrosia beetles in 1793. The name platypus, meaning flat foot, was also given to the aquatic mammal, a few years later.

The group includes Austroplatypus, the world’s only eusocial beetle. Eusociality represents one of the most complex and organised social structures in the animal kingdom. Key characteristics include cooperative care of young, multiple generations living together, division of labour, and reproductive specialisation, where only certain individuals reproduce.

Austroplatypus queens and their daughters live together in the fungus-lined tunnels they create, like bees in hives. Much like queen bees, only one female Austroplatypus can reproduce. Her daughters assist in fungal farming and caring for larvae.

“Ambrosia beetles have specialised organs, called mycangia. They use these to transport their fungus from the tunnels where they are born to the new homes they create,” James said.

“The morphology of mycangia vary widely. Some are sack-like organs inside their head capsule; others are pits in their exoskeleton.”

Scotch thistle seed-head weevil

Close up of an adult Seed-head weevil Larinus latus on a spiky Scotch thistle head.
An adult Seed-head weevil, Larinus latus, on a Scotch thistle.

Thistles are an introduced weed across south-eastern Australia. They’re drought resistant and fast spreading. Thistles can form dense tall thorny thickets. And they’re prolific seed producers – up to 20,000 seeds a year, some of which can remain viable for more than 20 years.

After extensive research, two biocontrol weevils were released on Scotch thistle (Onopordum acanthium): the Seed-head weevil (Larinus latus) and the Stem-boring weevil (Lixus cardui).

They’re now widespread and together have reduced seed rain at several sites in NSW by more than 90 per cent. It’s a great example of biocontrol agents as a long-term, sustainable, and environmentally sensitive means of controlling weeds at a landscape scale.

Adult Seed-head weevils are large, up to 20 mm long and 10 mm wide. The young adults are covered with a yellow waxy coating which gradually wears away so that older weevils become increasingly black.

The larvae tunnel into the flowerheads and eat the developing seeds.

A weevil frozen in time

A weevil preserved in amber around 100 million years old. ©  Gordon Gullock

Amber is fossilised tree resin. It sometimes preserves insects so perfectly they look like they just fell into a drop of honey a few minutes ago.

This piece of amber and the insect preserved inside it are 100 million years old. It contains a tiny weevil that lived during the Cretaceous period.

Fossils like this help us construct evolutionary trees, name species from the past and understand how insects and flowering plants evolved together.

Our necessary weevils

So, there you have it – a tale of our necessary weevils.

Snout out to all our weevil geniuses. They’re ready to rock you.

A tray of wonderful weevils at our Australian National Insect Collection.


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