Ten garden pests: an A(phids) to S(nails) guide

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Sustainability & Environment

A garden bursting with biodiversity is a wonderful place to be and beneficial insects and invertebrates in the garden perform vital roles such as pollinating, composting and even keeping pests under control. Yet a small number of insects and invertebrates can cause damage to plants. We asked Dan Hall, our Randwick City Council Bushcare Officer, to explain which pests are most likely to be found where and how to deal with them.

1. Aphids
Aphids are small sap-sucking insects that gather in groups on young shoots. The aphids excrete a sweet substance called honeydew that attracts ants, which protect the aphids from attack by other insects, and a black sooty mould that can cover the leaves and branches.

The symptoms of aphid attack include wilting and sometimes deformity of new shoots and leaves. The presence of ants and sooty mould can indicate an attack but can also indicate attack by scale or other sap-sucking pests.

Small aphid infestations can be squashed by hand or hosed off but larger infestations may need to be sprayed with a soap solution. Ladybirds and lacewings both predate aphids. Grow plants such as marigolds, coriander, dill and fennel to attract these beneficial insects to your garden.

2. Borer

Borers are generally weevil, beetle or moth larvae that tunnel into the stems, trunks and sometimes roots of a wide range of plants, including Callistemon, Grevillea and Banksia species. Indications of borer attack include dieback and collections of resin and a substance called frass, the woody excreta of the borer.

Where larger trees have been attacked it is important to remove dead or damaged branches. Borers can sometimes be killed by poking a piece of soft, flexible wire into the hole, or squirting a kerosene or soap solution into the hole, forcing the borer out so it can be killed. But a better strategy is to maximise plant health to avoid borers, as they mainly attack, damaged or unhealthy plants. Improving plant health through regular watering and feeding will help prevent attack.

3. Bronze Orange Bug

Bronze Orange Bug can cause serious damage to lemon and other citrus trees. The bugs suck sap and cause shoots to wilt and die and flowers and fruit to fall.

The young bugs, called nymphs, are pale green and appear in winter. Their colour changes to orange and then bronze as they age. The nymphs can be sprayed with a soap spray that you can make yourself or buy commercially. The adults can be knocked into a bucket of hot water but wear goggles, gloves and a long-sleeved shirt as they squirt a caustic fluid.

4. Caterpillars

Caterpillars are the larvae of moths, butterflies, beetles and wasps. A bad infestation can defoliate a tree or plant. The key to minimising damage is early detection. It is useful to know which type of caterpillar you are dealing with so you know when it is likely to be active. The lily caterpillar, for instance, which can strip the native Swamp Lily, is more likely to occur in warm, humid weather.

Caterpillars can be hand removed, but it’s a good idea to wear gloves as some have stinging or irritating spines or hairs.

Many caterpillars can be treated with pyrethrum-based sprays, which kill on contact and break down in sunlight. Other products, such as Dipel, are based on a naturally occurring bacteria that interferes with the caterpillar's digestive system but is not harmful to bees or other beneficial insects. It is a non-contact pesticide, so the caterpillar doesn't have to be present when you spray. The powder is mixed with water and sprayed on the plant, then the caterpillar ingests the bacteria when it eats the leaf.

5. Curl Grub

Curl grubs are the larvae of scarab beetles that are found in the soil and feed on the roots of a wide range of plants, including grasses. Unfortunately the first indication of curl grub is often the death of the plant, which can occur quite quickly. Where the grub is in a pot, the pot can be submerged in a bucket of water for 24 hours. When planting or digging in the garden if you come across curl grubs remove these by hand. Soil drenches are available for a severe infestation in the garden.

6. Mites

Mites are in the same family as spiders and one indication of their presence is webbing on the underside of the leaf. The tiny mites are easier to see with a hand lens or microscope. The mites pierce the leaf cells and suck out the contents, leaving the top of the leaves silvery and mottled.

Mites have an extremely short life cycle and have become resistant to a wide range of miticides. They can be sprayed with a pesticide such as Natrasoap and predatory mites (available commercially) have been found to be highly effective. Ladybirds will also predate mites.

7. Psyllids

Psyllids are a small insect related to the cicada that commonly attack lilly pillies such as the Brush Cherry (Syzygium panicu/atum) and the Weeping Lilly Pilly (Waterhousea floribunda). To treat psyllids you can prune off affected foliage, which is often the softer, newer leaves. The best way to deal with psyllids is to grow one of the many lilly pillies that are not attacked or are highly resistant to psyllids. Lilly pillies in the genus Acmena and many other Syzygium are not attacked.

8. Thrips

Thrips appear in mid to late summer and attack the lower surface of leaves first. Signs of their presence include a silvering of the leaves and distortion of shoots and other growing points. In addition, they excrete a reddish fluid that gradually turns brown black and can eventually cover both surfaces of the leaf.

Treat using a soap spray, spraying again after 10 days to control newly hatched 'nymphs'. Adult thrips can be found in leaf litter so cleaning up garden waste can help control this pest. Predators include ladybirds and lacewings.

9. Scale

There are two main types of scale - soft scale and hard scale. Both scales appear as small rounded bumps on the stems or leaves of plants. The insect lays its eggs under the 'bump' and immature scale 'crawlers' emerge.

The hard scales do more damage and are more difficult to control. However, the soft scales excrete large quantities of honeydew, which attract ants and unsightly sooty mould. With the soft scales, to control the ants, apply a grease band around the trunk. This will allow birds and insect predators such as ladybirds to help control the pest.

In the early stages, many scales can simply be rubbed off. The best time to spray scales - hard and soft- is when the crawlers emerge, many of which emerge in mid-summer. Use horticultural oil or a soap spray.

10. Slugs and Snails

Slugs and snails can cause serious damage to young plants. Look for chewed leaves and their distinctive silvery trails.

Hand remove, preferably with a torch at night, or use a small amount of beer in a container to trap and drown slugs and snails. Alternatively, sprinkle sawdust, coffee grounds, egg shell or wood ash around plants to deter the pests.

Controlling weeds and removing places where slugs and snails can shelter such as upturned pots will help to control numbers.

Reference: David Jones and Rodger Elliot, Pests, Diseases and Ailments of Australian Plants, Lothian Books, Melbourne, 2000

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