Insects in your garden: Beneficial or bad?

Insects are a vital component of a spray-free garden. But which ones? Duncan Smith describes what various insects do, why you need them, and how to get them.
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Maintaining an ecological balance in our gardens involves understanding and respecting the interconnectedness of all living organisms.

Insects play a crucial role in our ecosystems, with positive or adverse effects. Not all insects are harmful, except those that feed on desirable plants or transmit diseases. There is a distinction between good insects (beneficials) and harmful insects (pests). Promoting a diverse and thriving ecosystem in our gardens allows pest populations to be managed more naturally, creating a healthier environment.

ECOSYSTEMS are communities of living and non-living components of their environment, interacting as a system. These components include water, air, sunlight, soil, plants, microorganisms, insects and animals. Ecosystems may be on land or aquatic. The whole surface of Earth is a series of these connected ecosystems. Ecosystems, like rock pools on a beach, can be small or very large, such as the Amazon Rainforest in South America. Other examples are forests, grasslands, tundra, deserts, rivers, lakes and oceans.

BIOSYSTEMS comprise a living organism or any complete system of living things that directly or indirectly interact with others. Examples include our own respiratory and digestive systems, but also within our soils. One cubic metre of soil can contain insects, bacteria, fungi, protozoa, nematodes, algae, worms, larvae, centipedes, woodlice, spiders, slugs, and snails.

What adversely affects our ecosystems and biosystems?

Insensitive gardening and large-scale farming activities destabilise the balanced equilibrium of ecosystems, stripping away or poisoning the habitats and plants that beneficial insects need for food and shelter. Using artificial pesticides and fertilisers is particularly bad, killing or mutating soil microflora/fauna and destabilising ecosystems. Even homemade or natural sprays, and admissible organic sprays can be unsafe, e.g. copper is very toxic to beneficial soil fungi, bacteria, and earthworms.

Insect pests

Most insect damage results from directly feeding on plant parts. The type of feeding damage caused by pests is related to the type of mouthparts they have, i.e. suckers, chewers and raspers.

  • Suckers – These have syringe-like mouthparts that they thrust into plants and suck sap, e.g. aphids, green vegetable bugs, leafhoppers, mealybugs, soft scales, and whiteflies. Their feeding can also transmit plant viruses and bacteria.
  • Chewers – Adults and larvae of these insects have mandibles (teeth), e.g. locusts, grass grubs, Japanese beetles, the caterpillars of cabbage white butterfly, and cutworms. Japanese and grass-grub beetle larvae also feed on the plant roots.
  • Raspers – Some insects have rasping mouthparts that scrape the surface of plants like sandpaper and then suck up the spilt leaf contents they’ve damaged, e.g. thrips. Slugs and snails are raspers.

Pest: Green vegetable bug adult and nymph (Nezara viridula)

The ‘good guys’

There are three beneficial insect groups: predators, parasitoids and pollinators.

  • Predators – Eat large numbers of pest adults, larvae and eggs. Some feed on certain pest species, e.g. hoverfly larvae only feed on aphids, while others feed on a wide range of pests. Predators include lacewings, ground beetles, minute pirate bugs, big-eyed bugs, and ladybirds. Spiders, predaceous spider mites and centipedes are also important insect predators.
  • Parasitoids – Mainly minute wasps, and harmless to humans. They lay numerous eggs into or onto the outside of their insect hosts, e.g. caterpillars, aphids, and scales. Parasitoids are host-specific, only attacking one or a few closely related host species. Once the eggs hatch, the larvae consume the pest from within, usually killing it. Weeks later, they emerge from the hosts’ bodies to pupate, and their cocoons may be found beneath host plants or on foliage near the dead prey.
  • Pollinators – Our most essential pollinators are bees, hoverflies, butterflies, moths and some birds.

Beneficial: Green lacewing larva (Chrysoperia carnea) feeding on aphids.

Creating a healthy ecosystem

A biodiverse garden overflows with life and has abundant plant species that attract and support a diverse range of insect species and other wildlife. To encourage beneficial insects in our gardens, it has to be an inviting and thriving ecosystem, which may take time to establish.

Some adult predators and parasitoids don’t just feed on pests; some also feed on plant nectar and pollen. Planting various species of different shapes, heights, colours, sizes, and fragrances will encourage beneficials in your garden. Choosing plants that flower throughout the year will attract beneficials from spring onwards. Plant them in clumps or throughout your garden. A swath of mixed wildflowers is pleasant on the eye and a food source for beneficials.

Most beneficials have small mouthparts and need plants with small flowers so they can easily feed on them, e.g. the Umbelliferae (carrot family) including Queen Anne’s lace, dill, fennel and coriander; Compositae (aster family) with sunflowers, cosmos, coneflowers, coreopsis; Brassicaceae (mustard family) and many common vegetable crops, e.g. radish, broccoli, and cabbage. Leaving these to go to flower will attract beneficials. Phacelia, buckwheat and milkweeds are also valuable food sources for beneficials.

Predators and parasitoids won’t give 100 percent control of insect pests but will sustainably reduce their numbers. A few pests must remain to feed and encourage predators and parasitoids to stay. Blanket elimination of a pest removes the food source for the controllers, upsetting the ecological balance and creating a safe void for pests to repopulate.

Table 1 – Beneficial insects and pests they control

Lacewings – larvaeAphids, leafhoppers, spider mites, beetle larvae, caterpillars
Ladybirds – adults and larvaeAphids, mealybugs, scales, spider mites and other soft–bodied insects
Hoverfly – larvaeAphids
Minute pirate bugs – adultsAphids, spider mites, thrips, whiteflies, and immature stages of many other insects
Big-eyed bugs – larvaeAphids, leafhoppers and spider mites
Hymenoptera wasps (e.g. Ichneumonids,
braconids, chalcids) – larvae
Aphids, cutworms, butterfly and moth caterpillars
Tachinids (Type of small fly) – larvaeButterfly and moth caterpillars, aphids, cutworms and beetles

Green Vegetable Bugs (Nezara viridula)

These are the bane of gardeners and are challenging to control.

In 1949, a parasitoid wasp, Trissolcus basalis, was released in New Zealand to control crop damage caused by this pest in sweet corn and green beans. It only parasitises their eggs and has been a very effective beneficial insect.

A parasitic fly (Trichopoda pennipes) was also introduced early on, but most were predated by German wasps (Vespula germanica).

Green vegetable bugs can be repelled from our gardens by planting heavily scented plants, e.g. catnip, garlic, chives, lavender, chrysanthemums, rosemary, mint and basil. They don’t like their scent.

Planting trap/sacrificial plants of sunflowers and cleome will attract these bugs away from our vegetables and fruit. When numbers build up on them, pick them off by hand and drop them into soapy water. However, a lot usually drop off, if just one is touched. A container held underneath or sheet under plants will catch some of them. As a last resort, spot spraying trap plants with neem oil, two to three times a week, is quite effective.

These bugs lay a raft of yellow eggs underneath leaves and can be easily seen and removed by hand.

Grass grubs (Costelytra givini – formerly C. zealandica)

Grass-grub beetles eat the young leaves of a wide range of plants, and their larvae decimate the roots of grassland and other plants. They have few insect predators, but birds will eat the adult and larval stages.

There are few recordings of parasitoids of grass grubs, but in 2012, the larva of a native parasitoid fly, Ostenia robusta, was discovered in a single grass grub larva. There has been ongoing research on it since then.

A naturally occurring soil bacteria, Serratia entomophila, has been proven to kill their larvae (Bioshield® Lawn Care).

Getting the goodies

Some predators and parasitoids can be purchased online, e.g. ladybirds, lacewing eggs, and parasitoid wasps. However, they can migrate out of your garden even if an attractive ecosystem exists, as their needs can be very specific. Purchased beneficials are generally used in greenhouses.

Discourage insect pests by not planting the same variety of plants en masse. Mixing and spreading combinations makes it harder for pests to find what they are looking for. This method also helps prevent plant diseases from spreading easily.

Limit the use of any pesticides. (‘Pesticide’ is an umbrella term for insecticides, herbicides, fungicides, acaricides, etc.) Even organic pesticides are not safe or healthy for soil flora, fauna, or the environment. Most insecticides are broad-spectrum, killing a wide range of insects, including our beneficials.

If you must spray, choose the least toxic and spot spray when insects are less active, i.e. early in the day or late evening. NEVER spray when plants are in bloom; bees may be present. Companion planting is an excellent way to discourage pests and attract beneficials, e.g. marigolds deter aphids and are also a valuable food source for beneficials.

Maintain healthy plants through proper watering, fertilising, weeding and pruning. It reduces plant stress and susceptibility to pests and diseases.

Table 2 – Garden flowers that attract beneficial insects

Carrot family (Umbelliferae)
CorianderHoverflies, Ladybirds,
Tachinids, Wasps
Dill and FennelHoverflies, Ladybirds,
Tachinids, Wasps
LovageHoverflies, LadybirdsTachinids, Wasps
YarrowHoverflies, Ladybirds,
Lacewings, Mites
Tachinids, Wasps
Daisy family (Asteraceae)
ChamomileHoverflies, LadybirdsWasps
CoreopsisLadybirds, Lacewings,
Earwigs, Pirate bugs
CosmosHoverflies, Ladybirds,
MarigoldHoverflies, Ladybirds,
Lacewings, Pirate bugs
SunflowerHoverflies, Ladybirds,
Lacewings, Big-eyed bugs
TansyLadybirds, Pirate bugsWasps
Cabbage family (Brassicaceae)
MustardsHoverflies, Pirate bugs,
Big-eyed bugs
Sweet alyssumHoverfliesTachinids, Wasps
Scabiosa family (Dipsacaceae)
Pincushion flowerHoverfliesWasps
Legume family (Fabaceae)
Lucerne (Alfalfa)Hoverflies, Ladybirds,
CloverHoverflies, Ladybirds,
Pirate bugs
Buckwheat family (Polygonaceae)
BuckwheatHoverflies, Ladybirds,
Pirate bugs
Tachinids, Wasps

Duncan Smith studied Agriculture and has an MSc in Plant Pathology. He was a research agronomist and with his wife, Judy, created a certified organic block in the Waimata Valley, Gisborne, where they grew a wide range of vegetables and fruit, chardonnay, and olive oil.

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