Aphids are a member of the order Hemiptera, which contains many damaging sucking bugs. Most pest aphids in Australia are introduced, but we gave about 50 native species. Major pest aphids are Toxoptera aurantii or black aphid, Brevicoryne brassicae or grey aphid and Erisoma Langerum or woolly aphid.
Aphids feed by inserting their piercing mouthparts into the sap flow and sucking. However the damage caused by aphids is not limited to stealing sap juices. They also transmit plant disease in their salivary secretions.
Identification and life cycle
Small, 2mm pear shaped, soft-bodied insects, living in colonies. Widely adapted to host species, they are found on many commercially important crops. They secrete a honeydew, which is the plant sap after the insect has extracted the nitrogen and other nutrients which it needs. They honeydew may cause sooty moulds to grow on the plant. They are often tended by ants which feed on the honeydew and protect the aphids from their natural enemies. Aphids can be wingless, or winged individuals may be produced, especially where population numbers are very high. They can reproduce sexually or asexually, depending on population density and the availability of food.
Polyphagous species attack a wide range of hosts. Oligophagous species are specific to one host.
Young tip growth is often distorted and mis-shaped by high numbers of aphids. Fruit growing on severly infected branches may also be distorted.
Mainly insecticides, especially Dimethoate, Maldison, Rogor and Malathion.
Ladybird larvae and adults are effective and are available commercially by mail order. Insect eating birds are also useful oredators to have active in the garden or orchard. Many generalist predators such as hoverflies, lacewings young mantids, predatory mites, spiders and earwigs also take some aphid.
Many wasps will parasitise aphids, especially Aphelinus, Aphidius and Trioxys.
Entomophthora fungi also invade and kill aphids, in suitable seasons control can be significant.
Aphids will transmit viruses with their sucking tubes, causing secondary problems worse than direct attack by the aphid.
Organic chemical controls
In the home garden use a string jet of water or soapy water. Pyrethrum or derris can be used for commercial applications with caution, as they may kill the many very useful predators of aphids. Insecticidal soap (potassium soap), bordeaux or spraying oil is preferable. Garlic, diatomaceous earth, wood-ash or dilute-clay sprays and potassium permanganate also work.
There are anecdotal reports of rhubarb, tomatoe leaf, quassia, onion, seaweed and wormwood sprays being effective.
Wash, brush or pick aphids from plants. Reflective mulches made from aluminium foil deter aphids. Prevent ants from protecting aphids against predators by placing a ring of petroleum jelly or polybutene glue around the stem of the plant.
Yellow-painted boards with non-drying glue can also be used.
Ensure that plants are well fed and watered, and not over-supplied with nitrogen.
As aphids are mainly a problem on flush growth in spring, delay planting were possible, until later in the season.
Selection of control method
Chemical control, even with organically acceptable products, is too often the first choice of action. In a well balanced environment chemical intervention is rarely required. Aphids are readily susceptable to cultural and biological controls. Introducing or encouraging local predators by providing habitat, including flowering plants and shrubs, and reducing the pesticide pressure is the best long-term solution. Chemical control may be necessary in prolonged periods of cool weather as aphids operate at lower temperatures than most predators. It is usually possible to isolate spraying to effected parts, usually growing tips. With an eye to the weather, consider waiting for predator populations to build up. Do use chemical before severe leaf distortion is caused by the aphids, as the will shelter in the curled leaves and will be harder to eradicate. Try to avoid pesticide use when temperatures are above 8 degrees C.
Pyrethrum is an extract from the flowers of Chrysanthemum cinaria folium it is a useful organically acceptable chemical control of last resort. It is not a preferred treatment due to its effects on beneficial insects, especially hymenoptera (wasps and bees).
Derris is the ground root of the Rotenone plant. It is very toxic to fish and should not be used near streams or dams. Possibly toxic to eartworms.
Treat all poisons with care - including organic pesticides.