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On a scale of 1 to 10

April 4, 2019

Those who read this blog regularly will know that the Order of insects known as Hemiptera, insects with sucking mouthparts, feature regularly. We have featured cicadas, shield bugs, gumtree hoppers and psyllids.

Another common but rarely noticed Hemipteran is the scale insect. Scale insects are parasites of plants sucking their sap. The female is wingless and usually immobile, attaching itself onto a plant and then secreting a wax covering to protect itself. A common scale insect locally is the Gum-tree Scale (Eriococcus coriaceus), recognised by their housings, groups of white sacs on eucalyptus twigs and branches (see below).

gumtree scale 1-DSCN2302

The sac is the house of an adult female scale insect. The insect itself has its mouthparts attached to the plant and the opening at the top of the sac is blocked by its abdomen. A by-product of feeding, as with many Hemipterans, is a sugar-rich substance called honeydew. This is extruded from the hole in the top of the sac (see picture above). Scale insects have developed a symbiotic relationship with ants which collect the honeydew as a food source in return for protection from predators and parasites.

After each moulting both male and female scale insect instars find new positions on the plant and construct new wax sacs. After the final moulting the adult female insect constructs the final sac which takes up to two weeks. During this time the female is fertilised. Several hundred eggs are laid in the sac and when hatched the resultant nymphs emerge from the hole in the sac and crawl or are blown to other plant locations or plants to which they attached themselves.

Adult male scale insects do not have mouthparts with which to feed nor do they construct wax sacs. Their sole purpose is to mate with females, after which they die. This occurs over a few days.

Natural predators include birds such as Blackbirds and Silvereyes, the caterpillars of some moths and the adult and larval forms of Ladybird Beetles.

On balance, scale insects are quite interesting.

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