A Beneficial Orgy

Organic gardens are safe havens for all sorts of activities, but I’ve never witnessed a scene like this. Last summer they did it amongst my pineapples. Last autumn they were hard at it in my sweet potatoes.

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This week it’s the cardamom that are the silent, involuntary witnesses. I can’t water these plants without wetting the participants, knocking them down so they have to dry off. But they don’t seem worried about getting wet, they’re so engaged in an amazing, frenzied orgy of reproduction. They are black Hairy Flower Wasps (Scolia soror).
This latest bout follows some rain we had about two weeks ago. Holes, big enough to put your little finger in, started appearing overnight in the warm, wet, rain-softened soil. Adult cicadas are emerging from their earthy nursery. By day, Cowboy beetles, a type of cockchafer, also emerged and started flying. They don’t so much navigate as crash around, bumping into people, fences, even the side of the house. Like my flower wasps, they’ve emerged to reproduce, but the larvae of Cowboy beetles are one of various kinds of root and tuber-eating curl grubs.

Brisbane has about six beetle species that produce sizeable curl grubs, including African Black beetle, Rhinoceros beetle and Cowboy beetles. After the Cowboy beetles, I can expect the Christmas beetles. Christmas beetle larvae eat roots and tubers and adults eat leaves, especially of  Eucalyptus, a preferred food plant. But the stars in this whirl of winged activity are my beneficial friends, the Hairy Flower Wasps.

Today, there’s between 50 and 100 of them zooming around the cardamom, busily searching for curl grubs. They’re almost impossible to photograph as they’re fast moving, but on a warm day last May I got lucky. Pictured is a female dragging the paralysed grub of a Rhinoceros beetle (Xylotrupes gideon), which produces the largest curl grubs in my garden, to a nearby burrow. There she will lay an egg on the grub so her offspring has fresh food. According to Museum Victoria, these wasp larvae begin dining on non-essential parts of the grub so they don’t kill it outright, a grisly, practical way of extending its meal of fresh meat.

When the Christmas beetles emerge to breed, the black Hairy Flower wasps will be joined by another Hairy Flower Wasp (Campsomeris tasmaniensis), which has a very long, stripy body. Together they help control those pesky curl grubs.

Encouraging Flower Wasps: Integrated Pest Management

In my garden, Flower wasps create egg tubes under fruit trees that have been mulched with chopped sugarcane. Why? It seems it’s easier for them to create egg tubes and, since no sensible gardener digs amongst the roots of fruit trees, the soil within the drip line is not disturbed. This enables Flower wasp larvae to completed their annual life cycle and emerge as adults the following spring, usually as the storm season begins, which can be from late October to December.

* If Curl grubs are found in compost – leave them alone. Their role in compost heaps is to help you make compost. Along with Soldier fly larvae, they eat coarse material and turn it into manure. They are part of a fleet of micro-shredders which, though unappealing to the eye, support the work of compost worms.

* If you have curl grubs seriously damaging your lawn and you must have a quick fix, water in a solution of Dipel (Bacillus thuringensis) using a watering can and rose.

The best solution for turf grazed by curl grubs is to feed it with organic poultry manure (apply before rain, or water it in). Well-nourished turf growing in pH adjusted soil of between pH 6.5 to 7, minimises the recovery period.

Dipel is also used to control caterpillars. While approved for use in certified organic properties, Dipel is a non-selective caterpillar/ beetle larva killer, so you can also eliminate beneficial insects – like the larvae of carnivorous, pest-killing beetles and beautiful butterflies – along with the pests.

* If curl grubs are in pots, take the plant out of the pot, remove the curl grubs and feed them to poultry. Or squash them. It is here, amongst the roots of a pot plant, where they can do lasting harm.

If you can’t tease out the curl grubs, drench the potting mix with a Dipel solution. Often these pots are near night lighting: outside front or back doors, on verandahs or near windows. Beetles are attracted to night lights and often end up crashing into pots. Egg laying females are often desperate to lay – and that’s how high concentrations of eggs are laid inside a pot. Moving the pot away from night lights is part of the natural  solution.

Elsewhere, try doing what I have been doing for years here at Bellis – encourage a beneficial orgy in your garden.

You will find that some years are better than others. The relationship between biocontrol (Flower wasps) and prey (Curl grubs) is dynamic. Two years ago the Flower wasps in my garden were so effective they eliminated the Curl grubs and lost their source of food. The following year, my Flower wasp population crashed and is now recovering.

This is the most active time of year for Flower Wasps so make your garden as attractive to them without delay: reposition container plants, mulch fruit trees with chopped sugarcane, and be kind to your lawn.

Jerry Coleby-Williams
Originally published 15.11.07, Revised 22.1.15