Citrus scale pests are tiny insects that suck sap from the citrus tree and then produce honeydew. The honeydew is then feasted upon by ant colonies, further adding insult to injury. Citrus Scale in Australia are usually either Red Scale or Soft Brown Scale.
Appearance: Red Scale start as yellow crawlers, then whitecaps before turning reddish–brown and growing to 0.5–2 mm in diameter.
Brown Scale are flat, oval, 3–4 mm long, yellow–green or yellow–brown and often mottled with brown spots.
Heavy infestations cause leaf drop and twig dieback. Scale infestations attract ants and cause Sooty Mold, which is a fungal disease that will further stress your tree.
Plant Part: Leaves, twigs and sometimes fruit
Season: Early Summer - Autumn
Symptoms: Flat brown or red oval wax like shapes on the underside of twigs, leaves and sometimes visible on the fruit.
Also, look for a black soot like substance found on the leaves and branches.
Control: Biological control: there are numerous predators and parasites of Scale. The most common predators are lacewing larvae and scale‑eating ladybird beetles. The most common parasites are the Aphytis wasp and Comperiella bifasciata wasp. If you are lucky enough to have enough of these in your area, scale will not be a problem for you :)
Chemical Control: Horticultural mineral oil generally provides adequate control. Ant activity can exacerbate the problem and should therefore be controlled.
Note: By removing the aphids; the sooty mold will eventually dry and flake off.
Symptom: Ants in your plants
Control: Generally, removal of the honeydew producing pest (ie Scale) will do the trick. If the infestation is quite bad then consider ‘tree-trunk banding as easy method to stop ants in their tracks. (see your local nursery).
Prevention: Keeping your tree orchard clean: remove old fruit and branches as these attract and harbour pests. Keep your orchard free of weeds and encouraging beneficial pollinators and insects like bees, parasitic wasps, ladybirds and hoverflies to your garden will keep these pests at bay. Companion planting a few tansy herbs nearby will also deter the ants.
References: Sustainable Gardening Australia, NSW Department of Primary Industries, Gardening Knowhow, Orchard Plant Protection Guide 2020 and the FST team.
