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How can I control aphids? PDF Print E-mail

Aphids include a variety of small, soft-bodied sap sucking pests about 1-2mm long. Species feeding on conifers reach 4-5mm long. They are widespread and abundant in horticulture, spreading incurable plant viruses through collections and crops. Sometimes aphids are called greenfly and blackfly, although there is a range of body colours including grey, pink and yellow.

Most aphids feed on the above ground parts of plants although some attack roots, such as Phylloxera on grapevines. Aphids can be identified by the pair of slender tubes, called cornicles, projecting from the back of the abdomen. Woolly aphids feed on roots, stems and fruit and protect themselves by a covering of white waxy threads and can sometimes be mistaken for mealybug.

Damage includes wilting and yellowing of leaves and shoot tips, distorted flowers and leaves. Aphids excrete excess sugars harvested from sap and this sticky substance falls on and adheres to leaves below, encouraging sooty mould. Infestations usually begin in growing points, leaf undersides, bark fissures and shaded parts of plants. Spread is most rapid on soft new growth in spring and autumn. Overfeeding plants with nitrogen-rich fertilisers and regular irrigation stimulates the production of soft, sappy growth which encourages attack by aphids and mildews.

Aphids breed all year round in frost-free climates. Females usually reproduce asexually by parthenogenesis, although sexual reproduction does occur. Population growth is explosive and when populations peak on a plant winged females fly to infest new host plants. In frosty districts over-wintering eggs are laid on plants in protected areas, such as in bark crevices, during autumn.

Cultural controls:

There are 400 known species of Ladybirds in Australia. Many are voracious aphid predators at all stages of their lives. Juvenile ladybirds of some species superficially resemble mealybug and woolly aphid, but are more active and less numerous. You can learn to identify them by observing them feeding. They take time to locate and control aphid numbers.

Supporting strategies include:

  • feeding plants with a low nitrogen ‘flower and fruit’ fertiliser or reducing the frequency and amount of fertiliser to prevent the production of soft, sappy growth;
  • regular hand weeding. Weeds, especially milk thistle (sow thistle), are favourite aphid food plants;

Other controls:

  • rubbing aphids off plants, using fingers and thumb
  • using the jet of a hose to wash aphids off plants;
  • spraying plants with soapy water. Use horticultural soap or soap made from vegetable oils;
  • control over-wintering eggs on deciduous fruit trees, roses and other deciduous shrubs by spraying when they are leafless during mid-winter. Spray once only with lime sulphur. Ensure the spray saturates stems and any bark fissures;

For serious outbreaks indoors or in shade houses where ladybirds find entry difficult, it is advisable to control ants as well.

Lime sulphur:

  • was the first artificial pesticide and is a permitted input for organic gardens;
  • is used for spraying deciduous trees to control fungal spores, bacteria and pests and their eggs on the surface of plants. Lime sulphur will burn foliage;
  • Bonsai enthusiasts use undiluted lime sulphur to bleach tree bark to accelerate their aged look;

Caution:

  • Lime sulphur is strongly alkaline. With a pH of 11 it is corrosive and reacts with acids producing toxic hydrogen sulphide. Solutions should be agitated to prevent settling and blocking spray nozzles during use. Wash spraying equipment throughly afterwards;
  • Safety goggles and gloves should be worn while handling lime sulphur;
 
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