one minute bugs - Australian garden insect stories by Denis Crawford

Historic weevil

Posted on December 28, 2020 by Denis Crawford — 2 Comments ↓

I first published this story back in April 2015, but decided to revamp it after finding the weevil in the video clip below.

This insect is commonly known as a ‘Botany Bay weevil’ (Chrysolopus spectabilis) or by its other common name ‘diamond weevil’. I see these weevils every year but this is the first time I have been able to capture one taking off – albeit on a windy day and only with a mobile phone camera.

Botany Bay weevils belong to an historic group of insects. This weevil was collected by Sir Joseph Banks on Captain James Cook’s voyage to Australia in 1770. Actually we don’t know that Banks himself collected it – it could have been one of his men, or the Swedish naturalist Daniel Solander who accompanied Banks on the voyage. On his return, Banks gave the task of cataloguing his insect collection to the Danish insect taxonomist Johann Fabricius. The weevil was described in 1775, making it one of the first Australian insects described to western science.

Despite it’s common name the Botany Bay weevil is not only found around Sydney, but from coastal far north Queensland to eastern South Australia. The weevil is certainly common in New South Wales and its common name probably dates back to the early colonial days of Botany Bay.

To this day we don’t actually know where Banks’ specimen was collected. Was the weevil collected at Botany Bay in April 1770, or during July/August 1770 when Cook was repairing the Endeavour near present day Cooktown? The label on the specimen simply says “nova Hollandia”.

Banks was no entomologist. He doesn’t mention our weevil in his journal, but does complain about flies and mosquitoes on various occasions, and notes the sighting of a butterfly. He summarised Australia’s insect fauna as: “Of insects here were but few sorts and among them only the Ants were troublesome to us. Musquetos indeed were in some places tolerably plentyfull but it was our good fortune never to stay any time in such places, and where we did to meet with very few. The ants however made ample amends.”

Botany Bay weevils are associated with about 30 species of Acacia. Female weevils chew holes in the stems of Acacia trees at or below ground level in which they lay their eggs. You can see a female doing just that in the image above.

Upon hatching the larvae bore into the stem and usually down into the roots. Adult weevils emerge in the summer, and what spectacular insects they are with their rich metallic green or blue markings on a black background. The adult weevils also feed on Acacia plants in their characteristic manner of removing the leading shoots several centimetres down the stem.

The Botany Bay weevil has a distinctive defence mechanism. One day I was taking some close-up images of weevils on Acacia provincialis when I got just a bit too close for comfort. The weevil suddenly went stiff, toppled backwards and fell to the ground like an actor in a B-grade Western movie. The trick I learned was to move closer to them slowly, taking photos as I went, so they got used to me and the noise of the camera.

Reference: Waterhouse, D.F. (1971). “Insects and Australia”. J. Aust. Ent. Soc. 10 (3): 145–160.

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The Art of Seeing

Posted on October 27, 2020 by Denis Crawford — 2 Comments ↓

It is a good idea to keep an eye on the plants in your garden, but it’s just as important to understand what you are seeing.

Regular visits to your garden will help you detect any change that may occur, such as an increase of pest insects, or evidence of their damage. On the plus side regular monitoring will also allow you to see the buildup in numbers of beneficial insects such as ladybird beetles or lacewings.

Some insects like leaf miners, borers, and gall-inducing insects spend most of their life cycle hidden from view. You don’t necessarily have to be able to see the pest insect itself to be able to identify it – if you know what the damage they cause looks like. That’s knowledge worth having!

Leaf miner larvae burrow inside leaves where they feed on the leaf tissue between the upper and lower surfaces. The damage you see may be squiggly trails, papery blisters or see-through windows on a leaf, sometimes with a grub visible inside. A number of insects are leaf miners, including the larvae of some moths, flies and sawflies. In many cases you rarely see the adult insects – only the minor damage caused by their larvae. For example, the squiggles caused by the larvae of the cabbage leaf miner fly Liriomyza brassicae pictured below.

Liriomyza brassicae

There are many kinds of wood-boring insects, but the main culprits in gardens are the larvae of longicorn beetles (Cerambycidae) and the larvae of jewel beetles (Buprestidae). Telltale signs that a plant has beetle larvae tunneling away inside include: sawdust on the ground or around the junction of branches, holes with droppings and sawdust in and around them, and flowing sap. Wattles look particularly miserable when infested with wood-boring beetle larvae. They exude an enormous amount of sap (see below) in an attempt to defend themselves, giving the appearance the tree is bleeding to death.

Longicorn grub damage

The damage caused by gall-inducing insects is even more obvious – check out my blog post Gall-inducing insects – look for woody lumps and bumps on leaves, twigs and branches. Galls may occur on leaves, stems, buds and roots, and may be caused by certain species of wasps, flies, beetles, psyllids, coccids, thrips, moths or aphids. With most gall-inducing insects, the insect itself is fully enclosed by the gall during most of its life stages.

Some leaf chewing insects damage leaves in characteristic ways, so you can identify the culprit even if they are hiding or have flown away. For example, Christmas beetles leave a characteristic zigzag pattern on gumtree leaves initially, but they may consume leaves down to the midrib if the population of beetles is high. Adult weevils often chew leaf margins resulting in leaves with a typical ‘notched’ appearance (see below), unlike caterpillars which typically cause smooth scalloping of the leaf margins.

Weevil feeding

Did you know that some beneficial insects, particularly parasitic wasps, also leave tell tale signs of their activities. Have you noticed swollen bronze coloured aphids on your plants? These are aphid ‘mummies’ caused by the parasitic wasp Aphidius (see my blog post Revenge of the Mummy).

I met a gardener once who had pulled all the kale plants out of her vegetable garden and destroyed them, because they were covered in ‘pests’. The ‘pests’ were aphid mummies (see below)! Not only were the aphids dead but there were beneficial wasps developing inside those aphid mummies. They are a good thing to have in the garden!

Aphidius

Some beneficial predatory insects may also be confused with pests, if you don’t know what you are seeing. I know of instances where lacewing larvae and ladybird larvae have been sprayed with chemicals because they were mistaken for pests! Do you know what the larvae of lacewings and ladybird beetle look like?

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Spitfires revolt

Posted on September 22, 2020 by Denis Crawford — 14 Comments ↓

This post was inspired by audience reactions to an image I saw on Instagram. The image showed a cluster of sawfly larvae sometimes known as ‘spitfires’.

Many people are afraid of insects because of their “creepy crawly” habits but few insects disgust entomophobes more than the sawfly larvae known as ‘spitfires’. This is due to their habit of rearing up en masse and vomiting thick yellow fluids (hence the name ‘spitfire’) when disturbed. This is a defence mechanism against animals which might try to eat them. What a predator (such as a bird or possum) sees is a revolting Medusa-like writhing vomiting mass. That’s enough to put any predator – or Instagram viewer – off their lunch!

The most commonly seen ‘spitfires’ in urban areas are the larvae of the steelblue sawfly Perga affinis, and the black sawfly Perga dorsalis. All told there are about 20 species of Perga, as well as several species of the less common Pergagrapta spp. and Pseudoperga spp. sawflies, whose larvae display similar defence mechanisms.

Sawflies are not true flies (i.e. members of the insect order Diptera) but are insects related to wasps and bees (Order Hymenoptera). The name ‘sawfly’ comes from the delicate serrated, saw-like ovipositor (egg-laying appendage) at the tip of the abdomens of female sawflies. They use this ‘saw’ to cut slits in leaves or stems of suitable plants into which they lay their eggs. The image below shows a female Pergagrapta polita laying her eggs.

Studies have shown that spitfires possess specialized mouthparts which separate toxic oils from food plant leaves and pass them into a special internal pouch known as a diverticulum. The liquid is known to contain essential oils such as monoterpenes and, in some species, concentrations of sideroxylonal. Sideroxylonal in eucalypt foliage significantly influences the amount of leaves that some possums ingest – the higher the concentration of sideroxylonal the less likely the possum is to eat those leaves. It makes sense then that a possum may be deterred from a meal of juicy grubs if those grubs rear up and regurgitate a high concentration of a chemical the possum dislikes.

Now it gets interesting. Spitfires regurgitate these stored oils just before feeding or during feeding as well as when provoked by predators. The oils are toxic to the sawfly larvae themselves, so it makes sense that the diverticulum would have to be emptied when full. Is the purpose of the diverticulum a defence from toxins or defence from predators? Who knows?

Young Pseudoperga spp. sawfly larvae are defended by the mother sawfly (image below) for as long as it takes the larvae to build up defensive oil reserves. Studies have shown that it takes several days to fill the diverticulum with oil. A fair bit of oil it is too – studies carried out back in the 1970s showed that the oil accounted for nearly 20% of body weight in fully grown larvae.

Sawfly larvae also use stored oil when they spin their cocoons and pupate in the soil. By incorporating the oil (and faeces) into the silken wall of the cocoon they chemically shield their pupae.

Sawfly larvae are sometimes responsible for defoliating native trees, especially Eucalyptus spp. and related Myrtaceae. Depending on the species, larvae can grow to about 60 mm in length and are nocturnal feeders. Larvae are present over spring and summer and their feeding usually starts with the youngest leaves at branch tips. They can completely strip small branches of their leaves. Trees usually recover.

Larvae rest in groups during the day as a defence, and also climb down from trees in large groups to pupate in the soil, sometimes migrating along fences and across pathways in the process. Spitfires communicate with each other by using a kind of Morse code. When they spread out at night to feed they tap on the branches with their abdomens to let the rest of the group know where they are, so they can re-form their protective cluster before daylight.