Leslie Massey
If you're noticing damage to your plant leaves, it could be due to pests, diseases, incorrect watering, light issues, transplant shock, chemicals, or even natural leaf death. Common pests in Australia include aphids, beetles, slaters, caterpillars, and locusts. Aphids, for example, are tiny, sap-sucking insects that breed quickly and can get out of hand if left untreated. They can be identified by their pear-shaped bodies, long antennae, and abdominal tubes. On the other hand, beetles can damage leaves, flowers, and roots, with the Christmas beetle known for chewing gum leaves and the hibiscus beetle damaging hibiscus flowers. Slaters, also known as pill bugs or woodlice, are usually beneficial but can become pests if they shift to crop seedlings. Caterpillars are often associated with rapid leaf damage and lots of droppings, while locusts can enter home gardens and quickly build up to high numbers, causing severe damage.
| Characteristics | Values |
|---|---|
| Cause | Pests, diseases, watering or light issues, incorrect nutrients, soil pH issue, transplant shock, or chemicals |
| Pest type | Insects, slaters, snails, slugs, caterpillars, beetles, gnats, mice, rats |
| Pest characteristics | Long antennae, pear-shaped bodies, bumps on undersides of leaves, tiny, sticky liquid substance, webbing or silk, discolouration, holes in leaves, droppings |
| Pest examples | Aphids, mealybugs, leafhoppers, psyllids, whiteflies, scale insects, citrus leafminer, lace bugs, cabbage moth, cabbage white butterflies, locusts |
| Plant symptoms | Curling or wilting leaves, yellow leaves, sunburn, stunted growth, stretched growth, leaf drop, poor flowering, dry crispy leaves, brown leaf tips, shrunken potting mix, dieback, root rot |
| Plant types | Fruits, vegetables, ornamentals, seedlings, shrubs, trees, flowers, ferns, buds, seeds, roots, lawns |
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What You'll Learn
Caterpillars, beetles, and other insects
Caterpillars are a common cause of rapidly appearing holes in leaves, with lots of droppings left behind. They also produce webbing or silk, along with some mites and spiders. The presence of caterpillars can be identified by discolouration of the heads of cauliflower and broccoli, caused by their excrement. To control caterpillar infestations, you can enhance the environment for their natural predators, such as ladybirds, assassin bugs, lacewings, and paper wasps. Providing nest sites for insect-eating birds can also help.
Beetles are another common pest in Australia. Adult beetles can damage many types of plants. For example, Christmas beetles chew on gum leaves, hibiscus beetles damage hibiscus and abutilon flowers, and staghorn beetles feed on the leaves of staghorn and elkhorn ferns. However, it is often beetle larvae that cause the most damage. Curl grubs, for instance, are C-shaped grubs found in soil and potting mix that eat roots. The larvae of the African black beetle are known to damage lawns. To control beetle larvae, you can handpick them from potting mix or turf, or use products containing eucalyptus and tea tree oil.
Other insects that may be eating your plant leaves include aphids, mealybugs, and mites. These sap-sucking insects cause the leaves to curl and stems to wilt as they feed on plant juices. A sticky liquid substance called "honeydew" may also be present and is produced by aphids, mealybugs, leafhoppers, psyllids, soft scales, and whiteflies. To control these pests, you can encourage their natural predators, such as ladybirds, lacewings, and parasitic wasps, by growing suitable flowers.
In addition to the pests mentioned above, there are a variety of other insects that can cause damage to plant leaves in Australia. These include locusts, slaters, snails, slugs, thrips, and scale insects. Proper identification of the pest is essential to determine the best course of action for control and removal.
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Aphids, mealybugs, and mites
Aphids, also known as plant lice, are tiny, pear-shaped insects with long antennae and a pair of abdominal tubes called cornicles. They vary in colour and may be white, green, yellow, brown, black, or even pink. They are slow-moving and can be found on the tips of new growth and the undersides of leaves. A sign of their presence is a sticky substance called "honeydew", which is produced by the aphids and can lead to the growth of a black fungus called sooty mould. To control aphids, you can use a jet of water, squash them with your fingers, or spray them with organic garlic and chilli sprays, horticultural soap, or horticultural oil. However, avoid using horticultural oil when the temperature is above 32°C as it can burn the leaves.
Mealybugs are small, oval-shaped insects that belong to the Pseudococcida family. They are covered in a powdery wax substance that acts as a protective coating, giving them a distinctive white, cottony appearance. Mealybugs can be found on all parts of a plant, including the roots, and they particularly favour the nodes. They feed on plant sap and can cause leaves to turn yellow and eventually drop. To control mealybugs, you can use a cotton swab dipped in rubbing alcohol and water, or treat the plant with insecticidal soap or commercial products.
Mites are another type of sap-sucking pest that can cause damage to plants. They leave behind sticky black blobs on the underside of leaves and are known to produce webbing. While they do not have a specific colour, they are small enough to be considered microscopic. Mites can be controlled by encouraging their natural predators, such as ladybirds, lacewings, and parasitic wasps, to inhabit your garden.
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Slaters, snails, and slugs
Slaters, also known as pill bugs, woodlice, or roly-polies, are usually considered beneficial to gardens because they feed on decaying organic matter and help build soil. However, in high numbers, they can become a problem for gardeners by feeding on crop seedlings. They are found all over Australia and can be identified by their armoured, flattened, segmented bodies, which are grey or brown in colour. To prevent slaters from eating your plant leaves, you can use traps such as damp newspaper, corrugated cardboard, or a short piece of hose placed near plants before dark. Dispose of the trapped slaters in the morning by shaking them into a bucket of soapy water.
Snails and slugs are troublesome pests that can cause serious damage to gardens. They feed on both living plants and decaying plant matter and are particularly attracted to young leaves and seedlings. They tend to feed at night or on rainy days and hide in damp places during hot, dry weather. To identify snail and slug damage, look for holes in the middle of leaves and shiny silver slime trails.
To protect your plants from snails and slugs, you can try several methods:
- Grow sacrificial plants, such as horseradish, to distract snails from other plants.
- Use physical barriers, such as crushed eggshells, lime, wood ash, coffee grounds, or copper banding around vulnerable plants.
- Spray a strong coffee brew on plants and soil to repel snails and slugs.
- Set traps, such as inverted flower pots or citrus halves, to catch snails and slugs.
- Mulch the area with strongly-scented herbs like wormwood, mint, tansy, or lemon balm, which snails and slugs dislike.
Slug pellets containing metaldehyde are effective but have been banned as they also kill wildlife that prey on slugs, such as birds and hedgehogs. Organic growers now use pellets made from ferric phosphate, which are considered safer for wildlife. Biological controls containing microscopic nematodes can also be watered into the soil to infect and kill slugs.
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Locusts, moths, and butterflies
Locusts
The Australian plague locust is a major insect pest that affects crops and pastures throughout Australia. Active from spring to autumn, locusts can quickly build up in numbers in home gardens due to the availability of irrigated plants, causing severe damage. They prefer to feed on young, tender green shoots and leaves, with wheat, barley, and oats being the crops most at risk. Locusts will also eat a wide range of horticultural crops, including vegetables, grapevines, and trees, if there is a lack of green grass. The presence of locusts can be identified by the loss of leaves on plants, which can impact the plant's ability to fully ripen, affecting its quality and productivity.
To control locusts, ground spraying with chemical or biological insecticides is the most successful method. One such biological control agent is derived from an Australian fungus called Metarhizium anisopliae, which attacks locusts and grasshoppers. Mowing or applying herbicides to grass near crops may reduce the food supply for locusts, but it can also encourage them to climb and damage nearby plants.
Moths
Moths, such as the cabbage moth, can be a common garden pest in Australia. The adult cabbage moth is greyish and about 10mm long, while its caterpillars are green. They feed on plants from the family Brassicaceae, including broccoli, cabbage, Chinese cabbage, watercress, mustard, and Brussels sprouts. The presence of these pests can be identified by discolouration of the heads of cauliflower and broccoli due to the caterpillars' excrement.
To control and get rid of cabbage moths, you can enhance the environment for their natural predators, such as ladybugs, assassin bugs, lacewings, and paper wasps. Providing nest sites around your garden can also attract insect-eating birds, which will help control moth populations.
Butterflies
Butterflies, such as the cabbage white butterfly, can also be a pest in Australian gardens. The adult butterfly is white with black spots on its wings and a wingspan of about 45mm. The larvae are velvety green with faint yellow stripes and spots along the body. Similar to moths, they feed on plants from the family Brassicaceae.
To control butterfly populations, similar methods to those for moths can be employed. This includes encouraging the natural predators of caterpillars, such as ladybugs, and providing nest sites for insect-eating birds.
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Rust disease, root rot, and sooty mould
Rust disease
Rust disease is characterised by orange, yellow, or white pustules on the underside of leaves. It typically occurs in mild, moist conditions and can affect both ornamental and productive plants. Some plants that are commonly affected by rust include fuchsias, roses, calendulas, geraniums, hollyhocks, frangipani, raspberry, and blackberry. To prevent rust disease, it is recommended to regularly inspect plants for any signs of the disease and remove infected parts or plants. Pruning tools should be sterilised between plants to prevent the spread of the disease. In areas where rust is prevalent, it is advisable to grow rust-resistant plant varieties if available. Maintaining healthy plants by using fertilisers, proper watering, and appropriate light exposure can also help reduce the risk of rust infestation.
Root rot
Root rot is a condition that occurs when plant roots are unhealthy and unable to absorb necessary nutrients and water. It is often caused by overwatering, which leads to dense soil that does not drain properly or containers without adequate drainage holes. The first signs of root rot are yellow leaves and stunted growth. Upon closer inspection, the roots will appear soft and brown instead of firm and white. In advanced stages, the roots become mushy and black. To identify root rot, gently remove the plant from its container and examine the roots and soil. An unpleasant smell and soggy soil indicate the presence of root rot.
To minimise the risk of root rot, it is crucial to ensure proper watering practices. Water plants thoroughly but allow the soil to dry slightly before watering again. Always empty any excess water from the cachepot or plant saucer. To treat root rot, carefully remove the plant from its pot, wash the roots, and cut away all rotten parts, leaving only the healthy roots. Sterilise the pruning tools before and after this process to prevent the spread of fungal spores. Finally, repot the plant with fresh potting soil specifically formulated to promote healthy root growth.
Sooty mould
Sooty mould is a fungal disease that affects plant leaves, stems, and twigs. It is identified by a black, sooty, and sticky coating on the plant surfaces. Sooty mould is a secondary disease caused by sap-sucking insects such as aphids, scale, mealybugs, whiteflies, psyllids, and leafhoppers. These insects excrete a clear, sticky substance called honeydew, which serves as a food source for the mould. While sooty mould does not directly feed on plants, it blocks sunlight and reduces photosynthesis, hindering plant growth. The best way to control sooty mould is to manage the pest population that produces honeydew. Various sprays are available to control sap-sucking pests and sooty mould on plants.
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Frequently asked questions
There are many insects that eat plant leaves in Australia, including:
- Aphids, which breed quickly and can get out of hand if left unchecked.
- Mealybugs, which are tiny but can do a lot of damage.
- Caterpillars, which leave behind lots of droppings.
- Citrus leafminer, which is a persistent pest that will come back yearly to lay its eggs on the leaves of your citrus trees.
- Beetles, which can damage many types of plants.
- Slaters, which are usually beneficial but can become a problem for gardeners if their numbers get too high.
- Locusts, which can quickly build up to high numbers and cause severe damage.
To get rid of these pests, you can try a variety of methods, including:
- Handpicking beetles and larvae.
- Using products containing eucalyptus and tea tree oil.
- Applying a registered fungicide.
- Encouraging natural predators such as ladybirds, lacewings, and parasitic wasps.
- Using horticultural soap.
- Providing nest sites for insect-eating birds.
