Denis CrawfordA balance between garden pests & beneficial insects

Early one morning I was shooting a video of cabbage aphids (Brevicoryne brassicae) and their primary wasp parasite (Diaeretiella rapae) on a broccoli plant – as you do – when along came a very different wasp. The cabbage aphid parasite (Diaeretiella rapae) is a tiny little thing about 3 mm in length, but these other wasps were three times that length. They were obviously attracted to the aphids and ‘appeared’ to be stinging them. But were they really?

The wasp appears to be stinging aphids

The wasp appears to be stinging aphids

Before I answer that question, we need to go back a step or two. My garden in the Grampians region of western Victoria is designed to attract as many insects as possible – good, bad and benign. Some gardeners might be horrified at the notion of a garden teeming with insects, even though their own garden probably is too. It’s a good thing to have a little bit of everything in the garden, creating a balance between pests and their parasites and predators. Many plants will attract pests which will try to eat them, and those pests will then attract predators and parasites. Having flowering plants in the garden will keep the parasites there, because they need to have a feed of sugar-rich nectar to lay fertile eggs.

Using cabbage aphids as an example, here is how it works. You plant your brassicas (cabbage, cauliflower, broccoli, etc) to grow through the winter during which time they remain relatively untouched by pests. As soon as the weather warms up in spring the aphids arrive, and as the weather warms further the aphid population increases rapidly. Plants under attack often release volatile chemicals which attract predators and parasites, in this case the cabbage aphid parasite (Diaeretiella rapae).

Thryptomene calycina Grampians thryptomene

Thryptomene calycina, Grampians thryptomene

As mentioned above these tiny little wasps need to get fuel from somewhere. When they arrived there was a flowering Grampians Thryptomene (Thryptomene calycina) planted nearby. These native shrubs are covered in tiny nectar-rich flowers during winter and into early spring. I have several nectar-rich and pollen-rich plants of various species scattered through my fruit and vegetable garden for parasitic wasps and other insects to feed on. The little wasps didn’t have far to go for their food, so their activities soon began to take toll on the aphid population.

Now here is the gory part! After she mates and fills her belly with nectar, a female wasp lays a single egg into the body of a cabbage aphid. The wasp larva feeds on tissues within the aphid body, killing the aphid after a few days. By the time the wasp larvae are fully grown, the empty aphid bodies turn into hardened, bronze-coloured shells called ‘mummies’. An adult wasp later emerges through a circular hole cut in the back of the mummy. This whole lifecycle takes about two weeks. These female wasps can lay about 100 eggs over their lifespan of a couple of weeks, which means about 100 aphids turned into mummies. This is what I was trying to film for my blog.

Cabbage aphids parasite and aphid 'mummies'

Cabbage aphid parasite and aphid ‘mummies’

I took some samples of cabbage aphid mummies along to show people at a pest control workshop I conducted recently. One of the participants told me she had pulled out all the brassica plants that had mummies on them in her garden because she didn’t know what they were. If you see these aphid mummies on your plants leave them alone – they don’t need to be controlled because the aphids are already dead. Even if only about 10% of the aphid population has turned into mummies today, most of the remainder of the colony will follow suit later.

Now some of you may be thinking: “It’s all right for you living out there in Western Victoria, you don’t have the pest pressure that we do here in… (insert name of tropical or subtropical location)”. The idea of encouraging beneficial insects to control pest insects is not unique to temperate Australia. Those of you who do live in tropical or subtropical locations certainly have more species of insects in their gardens than I do, but that includes more beneficial species. This isn’t confined to domestic gardens either, for example there are several very successful biological control programs taking place right now in commercial horticulture in Queensland.

An important thing to understand is that not all insects are pests – some are beneficial insects as we have seen here – but the vast majority are simply benign (neither good nor bad) in horticultural terms. I am often asked how to control insects which are simply just there and not doing any harm. Take blowflies for example. “How do I get rid of blowflies?” asked someone during a workshop I was giving. If I only had a dollar for every time someone asked me “how do I get rid of….” I would be a millionaire! Blowflies don’t need to be controlled because they are beneficial in two ways, their larvae help decompose small dead animals and the adult flies help pollinate plants while they are feeding on flowers.

So let’s get back to the mystery wasp. To recap: we had cabbage aphids, cabbage aphid parasites, and an unknown wasp that was also active amongst the aphids. Examination of the footage I shot showed that the mystery wasps were poking their abdomens between aphids and not into aphids. Things were starting to get really interesting so I emailed an image off to a colleague at Museum Victoria to see if he could identify it for me. The answer? It was a hover fly larva parasite (Diplazon laetatorius).

Hover fly larva feeding on aphid

Hover fly larva feeding on aphid

I’ll explain quickly before your head explodes! The volatile chemicals being released by the plants not only attracted cabbage aphid parasites but also a major cabbage aphid predator – hover flies. These are the flies you see hovering around flowers in the garden and may be mistaken for bees. Adult flies feed on pollen and nectar and are important pollinators, while their larvae are predators that are particularly fond of aphids.

Female hover flies lay their eggs in or near an aphid colony so larvae don’t have far to go to get food when they hatch a few days later. They remain as larvae for about two weeks and can consume up to 30 aphids per day each before pupating. That means each larva is capable of consuming hundreds of aphids. Even better, hover flies have several generations per year. You can tempt these excellent predators/pollinators to stay in your garden with flowering plants for the adult flies, and low numbers of aphids for their larvae to feed on.

Hover fly larva parasite

Hover fly larva parasite

Hover fly larva parasites were also attracted to the plants’ volatile chemicals. I watched them for about an hour working up and down the plant looking for hover fly larvae and then trying to lay an egg inside them. That’s got to be a bad thing right? A parasitic wasp trying to kill a really important aphid predator! Not really. The aphids were already well under control by the cabbage aphid parasitic wasp, and adult hover flies were continuously laying new eggs. So as one hover fly larva was being parasitised another one was hatching from an egg. This is what I call balance. For every insect out there, there is another one trying to eat it or parasitise it – if they are in balance there is no problem.

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Denis Crawford

About Denis Crawford

Denis Crawford has studied, photographed and written about insects for more than 35 years. His background includes a decade in entomological research, and many years collaborating with an integrated pest management consultancy. Denis is author of Garden Pests Diseases & Good Bugs: the ultimate illustrated guide for Australian gardeners and co-author of Backyard Insects (soon to be released in an updated edition. See more posts on garden insects on his blog One Minute Bugs.

6 thoughts on “A balance between garden pests & beneficial insects

  1. Inspiring post, Denis! It’s captivating when you look closely and see the wonderful dramas being played out in the micro scale! I don’t have your entomological knowledge but I love it when I come across creepy crawly interactions in my garden. One spider – a flattened, pale huntsman-type – is very prolific in my patch, but I never see it in the garden, only when it comes out of the salad greens or silverbeet when I wash the leaves in the kitchen. Which of course necessitates a trip out the back to return him to the garden!

    • Thanks for your kind comments Helen. I also have to rescue spiders from the kitchen and put them back in the garden. One Badge Huntsman went for a swim in the sink and a full spin in the lettuce spinner before I noticed it. Poor thing had vertigo!

  2. Interesting post, Denis, thank you. I have paper wasps building nests outside my back door…I know they are predatory insects, but may have to move them on for the sake of the household when they come indoors! A dilemma indeed,

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