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My plants look like they're covered in warts PDF Print E-mail

Scale on lime
Scale on lime
Scale insects are widespread and common in horticulture. There are many species of this sap sucking insect and they attack a wide variety of crops and ornamental plants. Larger species include the Chinese wax scale (Ceroplastes sinensis). Adults look like waxy lumps up to 7mm long, 5mm wide and 3mm high. Smaller species include the hard or armoured scale. Adults may not exceed 1mm in length.

Scale insects breed in warm conditions, mostly during spring and summer. Juvenile scale are mobile and when they have found a suitable site to feed begin building a protective scale from waxes and their own cast-off skins. Common garden ant species will transfer scale insects to healthy plants to farm them, harvesting the sugar-rich honeydew they secrete.

Damage includes yellow blotches on leaves, wilting, premature leaf drop, stunted leaves and fruit, disfigured leaves, shoots and fruit. Scale insects excrete honeydew which consists of excess sugars harvested from sap. This sticky substance falls on and adheres to leaves below, encouraging sooty mould.

Scale on Bamboo
Scale on Bamboo
Infestations usually begin in growing points, leaf undersides, bark fissures and shaded parts of plants. Spread is most rapid on drought-stressed plants. Hard scale species tend to be the most difficult to observe and they tend to cause the most damage, including the death of host plants.

Cultural controls:

Many of the 400 known species of Ladybird in Australia are voracious scale predators. Juvenile ladybirds of some species superficially resemble mealybug and woolly aphid, but are more active and usually less numerous. You can learn to identify them by observing them feeding. Ladybirds take time to locate and control scale numbers especially where neighbouring gardens are sprayed with persistent pesticides.

  • A supporting strategy is to feed 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;

Other controls:

Control is rarely completely effective and scale insects have become resistant to many pesticides.

  • If one plant in a collection is infested it may be easier to dispose of it in the compost heap;
  • Control small infestations of scale by hand using a cotton bud dipped in an emulsion made of equal parts of methylated spirits and water. Dab scale individually;
  • Infestations can be wiped off plants using a soft cloth or sponge soaked in soapy water;
  • Control large infestations by spraying with a horticultural oil twice during spring and again during summer;
  • Control scale on deciduous trees and shrubs when they are leafless during winter. Spray once only with lime sulphur. Ensure the spray saturates stems and any bark fissures;

For serious outbreaks 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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