A Beneficial Orgy

Sustainable 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).

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Newt: My mommy always said there were no monsters – no real ones – but there are, aren’t there?
Ripley: Yes, there are.
Newt: Why do they tell little kids that?
Ripley: Most of the time it’s true.
Aliens, movie, 1986.

Sustainable 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.

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. It was adult cicadas and mole crickets, pushing their way out of their earthy nursery.

By day, Cowboy beetles, a type of cockchafer, also emerged and started flying, looking to reproduce. These beetles 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.

Cowboy beetles spend most of their lives living underground as curl grubs, pallid-looking vegetarian larvae that feed on roots and tubers.

Brisbane has about six beetle species that produce sizeable curl grubs, including African Black beetle, Rhinoceros beetle and Cowboy beetles. After the late spring-early summer Cowboy beetle season, I can expect the early summer season when  Christmas beetles mate. Christmas beetle grubs also eat roots and tubers and adults eat leaves, Eucalyptus especially. 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 difficult to film and photograph 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 meat. According to Museum Victoria, these wasp larvae begin dining on non-essential parts of the paralysed 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 long, stripy body. Together they control my curl grubs so well, I am surprised when I unearth one.

Encouraging Flower Wasps: Integrated Pest Management

In my garden, flower wasps create their egg tubes under fruit trees that have been mulched with sugarcane. Why? It seems it’s easier for them to create egg tubes.

Since no sensible gardener digs amongst the roots of fruit trees, the soil within the drip line is undisturbed. This enables flower wasp larvae to reach maturity over winter to emerge as adults the following spring, just as the storm season begins softening the earth and everyone has to start mowing the lawn every week. That can be any time from late October to the end of 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 excrete it as manure. Here, they are part of a fleet of micro-shredders which, though unappealing to the eye, support the work of compost worms. You might also find them gorging themselves on bagged manure – they like composting horse manure.

* If curl grubs seriously damage your lawn, feeding the turf with poultry manure – a handful per square metre before rain – will help it recover.

* Well-nourished turf growing in pH adjusted soil of between pH 6.5 to 7, minimises the recovery period. Since most soils around town tend to be acidic, the addition of poultry manure helps raise the pH – a minor additional benefit.

* You can water turf with a solution of Dipel (Bacillus thuringensis) using a watering can and rose. Seven litres of solution covers about four square metres. Apply after a good soaking fall of rain, this brings curl grubs to the surface and the solution disperses more effectively in moist soil. The same solution can be watered around potted plants that have been attacked by curls grubs.

* Dipel is also used by organic gardeners to control caterpillars. It 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 pest species of both kinds.

* 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.

* 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 can be desperate to lay and high concentrations of eggs may be laid inside pots. Moving the pot away from night lights is a natural solution.

Some years are better than others because the relationship between biocontrol (flower wasps) and prey (curl grubs) is dynamic. Two years ago, in 2009, flower wasps eliminated almost every curl grub. Losing their source of baby wasp food meant a wasp population crash in 2010. Wasps started reappearing the following year, evidence the grubs were making a population recovery, and by 2015 I was again outside watering my garden amongst the wasps.

The storm season is when flower wasps are most active, so ready the sugarcane mulch, be kind to damaged turf – put out the  welcome sign for wasps in your garden.

In Wynnum 4178, no one can hear the curl grubs scream.

Jerry Coleby-Williams
Originally published 15.11.07, Revised 9.12.16