New Zealanders spend a fair bit of spare time in their gardens. If your roses are looking like the one below, covered in aphids, or your citrus trees have gone black with mould, you could spray insecti- or fungicides. Or, you could add some plants into your garden to encourage hoverflies and try let them fix the problem instead.
The fly family Syrphidae is known as hoverflies. As the name suggests, they tend to hover around flowers but they can fly any direction they like, even backwards! Unlike house flies they aren’t out to annoy you. They eat pollen and nectar and some are important pollinators. They might mimic the colours of wasps and bees but they’re harmless, they’re only pretending to be a wasp in order to be left alone.
New Zealand has more than 40 species of hoverfly, most are endemic, some are introduced. The main beneficial hoverflies in New Zealand are the creatively named large hover fly(Melangyna novaezealandiae) and the small hoverfly (Melanostoma fasciatum).
What do they have to do with aphids? The larvae of some hoverfly species are voracious predators that specialise on aphids. For example, each larvae of Episyrphus balteatus, a European species of hoverfly, has been found to eat between 400-1000 aphids a day for up to a week.
Aphids suck the sap out of plants and, given the right conditions, they can explode in numbers to become a major problem. Aphids do this with a clever life-cycle trick of giving live birth to clones when conditions are good. Sucking enough sap can stunt a plant and aphids also spread viruses and excrete sugar onto the surface of the plant. The fungi that grow on that sugar are known as sooty moulds and while a little mould doesn’t really affect the plant, a lot of mould blocks photosynthesis. The mould doesn’t look great either.
Sadly you can’t buy hoverflies in NZ, like you can in Europe, but if you have flowers around then they’ll probably turn up. Fewer may show up if you’re a prolific weeder. Flowers are a major food source for the adults. Like us, they have their preferences. They like tiny flowers that suit their tiny mouths, so things like coriander, buckwheat or Queen Annes Lace are good choices. But not all plants are equal. International research has been done into finding what offers the best chance at attracting and keeping hoverflies. Lincoln University’s Steve Wratten and colleagues examined flower choice on hoverflies (this study).
The researchers grew their own Episyrphus balteatus hoverfly individuals, a species not found in New Zealand. The results are still applicable to New Zealand as our hoverflies seem to have similar preferences. They placed these hoverflies with a broad bean plant covered in aphids (to induce egg laying) and some flowering plants from the species they were testing. These plants were either phacelia, coriander, mustard, buckwheat, alyssum or marigold. The researchers recorded how long the hoverflies lived, how many eggs were laid and the length of time that egg laying lasted for. There were some differences in performance, depending on the flower.
Coriander was the plant species with the most eggs laid. Coriander is also a herb, so there’s no reason to not have it in the garden. Unless you are a coriander hater. Parsley is another plant in this family which will still bring hoverflies in when it’s flowering and is edible.
Phacelia gave some of the best overall results and is brilliant to have, as it is popular among many beneficial insects, including honeybees and bumblebees. It has purple pollen, which looks a bit odd when the bees are carrying it. It is also said to attract lacewings, which will help deal to aphids.
Buckwheat kept the hoverflies alive the longest. It is an attractive plant and, if you’re really keen, you can harvest a highly nutritious seed from it. I’d recommend planting it separately as opposed to a flower mix if you’re planning on this. In thick plantings it outcompetes most other plants. It’s relatively easy to get seeds online or you could maybe try sprouting some from the local grocery store (buy raw or sprouting seed, not toasted).
Alyssum is an ornamental plant that forms a carpet made of masses of small flowers that the hoverflies like. It comes in many different colours but I suggest white for beneficial insects. Alyssum flowers are edible and while they smell like honey the taste is described as peppery.
Mustard and marigold did bring hoverflies in but their results weren’t as good. If none of the plants listed above appeal to you there are certain traits you could look for in your planting or you could try something from this website.
A lot of research has been done into what makes a plant attractive to hoverflies. It has been found that they prefer the colour yellow and smaller flowers and plants with lots of nectar (like phacelia). If you want a larger plant then something like Euphorbia mellifera, a 1.5m plant with honey smelling flowers, might work. You will have to be careful about the toxic sap. Yarrow, dill and fennel all work too. It’s best to try planting a few different flowers. NZ natives plants that may work include manuka, kanuka, hebes and tauhinu.
The study concluded that if you could only plant one flower then phacelia was the best choice. My recommendation is to plant a mix of flowers. This will mean that your flowering period will be a bit more spread out, giving you more variety in the garden and hoverflies a longer feeding period.
Having hoverflies may not completely eliminate your aphids but it will keep their numbers manageable. Either way, it’s less effort (and less damage) than spraying insecticides and at least the flowers look nice. If you’re giving planting flowers a go, I wish you good luck and I hope you see a difference.
Edwin Spencer is a postgraduate student completing a Postgraduate Diploma in Applied Science. He wrote this article as part of his assessment for ECOL 608 Research Methods in Ecology.