Red scale
Scientific name
Aonidiella aurantii
Description of adult
Red scale has a thin, circular, leathery covering over the soft, flattened, shield shaped, creamy-yellow insect. The reddish-brown scale cover of the adult female is about 2mm across. The cover of the male scale is elongated, smaller and paler than that of the female.
Immature stages
The female produces a mobile reddish-yellow crawler stage that can just be seen with the naked eye. The smaller immature female and male scales are similar in shape to the adult female.
Life history
The adult female gives birth to 100–150 mobile young called crawlers, at the rate of 2–3 per day over a 6 to 8 week period. The crawlers emerge from under their mother's scale cover, and search for a suitable feeding site on leaves, shoots or fruit. The wind can blow crawlers wandering on the tree canopy into neighbouring trees or orchards.
Once a crawler settles, it inserts its mouthparts into the plant and starts feeding on the sap. It secrets a white waxy covering and at this stage is called a 'whitecap'. After a period of feeding and growth the insect moults. The cast skin is attached to the scale cover, giving the cover its typical red colour.
After the second stage, scales can be identified as male and female. The scale cover of males is elongated, while the scale cover of females is circular.
The male develops through a pre-pupal and pupal stage under a scale cover, before emerging as a delicate, winged insect. A pheromone attracts the male to the female, and he dies without feeding after mating.
There are 2 to 5 generations per year below latitude 29oS and 5 to 6 above.
Distribution
Red scale is found in all areas where citrus is grown in Australia.
Host range
Red scale occurs on a wide range of hosts including citrus, passionfruit, roses and carob.
Damage
Minor and sporadic on carob.
Severe infestations can cause leaf drop and dieback in carob.
Control
Monitor regularly by sampling five leaves per tree on each of 20 randomly selected trees per 1 to 5 hectare block. Once, or twice if necessary, during November–December examine 100–200 virgin adult female scale to determine parasitism by Aphytis lingnanensis. If A.lingnanensis parasitism is less than 20%, make a parasitoid release of 10,000 per hectare in November to December. Comperiella bifasciata is another important parasitoid. Spray if infestation levels are high and A. lingnanensis parasitism is less than 20% (during December) or 50% (during January).
Cultural
Avoid excessive dust from roads or cultivation.
Biological
Parasitoids play a crucial role in the control of the pest and they are described in some detail to assist in recognition. The best way to determine if a parasitoid is present is to lift the scale covering with a pair of tweezers and then examine the body of the pest with a x10 or x15 hand lens, or preferably with a small stereo-microscope. The 2 most important parasitoids of the scale are Aphytis lingnanensis, a small yellow wasp 1mm long, and Comperiella bifasciata, a small black wasp 1.5mm long.
Aphytis lingnanensis attacks virgin adult female scales (also second instar and male prepupae). Its eggs, which are white and teardrop-shaped, are laid on or under the scale body as distinct from the scale covering or cap. They hatch into yellow rotund larvae that feed on the scale body, eventually growing almost as big as the body of the scale itself. When fully grown, the parasitic larva forms a yellow pupa about 1mm long. It is readily recognised because it is surrounded by a number of brown larval faecal pellets. The adult parasitoid also destroys many scales by mutilating them to feed on their body fluids. At 26oC the life cycle takes 16–17 days and the adults live for 2 to 3 weeks.
Comperiella bifasciata lays its egg entirely within the scale body, parasitising most stages of the scale. At 26oC the life cycle takes 3 to 6 weeks depending on the age of the scale at oviposition. The larva is elongate and white, and the pupa black. Parasitised scales sometimes have a bloated appearance and the scale covering becomes darkened around the edges. Parasitism levels of 50% to 90% occur where insecticide usage is restricted and providing these levels are present from December on, effective control is obtained.
Chemical
Spray if scale infestation is high and parasitism rate is low. Good coverage is essential as the scales do not move once they go through the initial crawler stage. The insecticide must therefore hit the scale when applied.
Check the Australian Pesticides and Veterinary Medicines Authority database for chemicals registered or approved under permit to treat this pest on the target crop in your location. Always read the label and observe withholding periods.