Cottony scales

There are about 80 species of ‘soft scale’ insects of the family Coccidae found in Australia. The ones I find most interesting are the pulvinariine soft scales which are known colloquially as ‘cottony scales’, ‘cottony soft scales’ and (my favourite) ‘cushion bears’. These common names help to describe the cottony egg sacs (ovisacs) of adult female scales. The image below shows adult female Pulvinaria dodonaeae on the leaves of a species of Myoporum.

Cottony soft scaleThe insect itself is brown in colour and the ovisac is the furrowed white mass behind. Pulvinaria dodonaeae is endemic to Australia and is not considered to be a pest. Its species name, dodonaeae, coveniently indicates some of its host plants are within the plant genus Dodonaea. One of the exotic (i.e introduced from elsewhere) cottony soft scales found in Australia goes by the name of Pulvinaria hydrangeae. Can you guess which plant it occurs on?

I recently found a small colony of an introduced cottony soft scale which has the tongue-twisting scientific name Pulvinariella mesembryanthemi (image below). Thankfully it has a couple of more easily  pronounced common names ‘iceplant scale’ and ‘cottony pigface scale’. ‘Iceplants’ and ‘pigface’ are succulent plants within the botanical family Aizoaceae – one genus of this family is Mesembryanthemum.

iceplant scaleThe female insect (the brown disc) is about 3 mm in diameter and the cottony ovisac is about 4 mm long. Females may lay as many as 2,000 eggs each during their short life of a few weeks. Males are rare in these scale insects, in fact they are not required because the females can reproduce parthenogenetically. The image below shows an open ovisac of one the cottony scale insects revealing the eggs inside. That’s a lot of eggs!

insect eggsMobile nymphs known as ‘crawlers’ hatch from the eggs and move away from the adult female to a different part of the plant, or further afield. People often ask me how immobile scale insects can spread from one plant to another – well, this is how. Crawlers may wander from one plant to another using their own six legs, or they may hitch a ride on an air current and travel effortlessly over much greater distances. The animation below shows a crawler emerging from under its ovisac while I was shooting images for a focus stack. Can you see it?

Insect animation

Crawlers eventually settle on a plant somewhere and moult into sessile (immobile) nymphs which plug into the sap flow of the plant with their sap-sucking mouthparts, then moult into a larger nymph, and then moult once more into an adult. And so the cycle goes.

There is one more cottony scale insect which I should mention. That is the cottony cushion scale Icerya purchasi now classified in the family Monophlebidae. This native Australian insect was accidentally introduced into the United States in the 1860’s and within a couple of decades brought the US citrus industry to its knees. I reckon that sounds like a good topic for a future one minute bugs.

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7 Replies to “Cottony scales”

  1. The only time that scale insects would be considered a pest is when too many are on your prized ornamentals. Other wise they are just part of the eco system. Thanks Dennis

  2. Dear Dennis:
    Excellent pictures and very easy explanation of “soft scale” insects. Congratulations! I do agree with Richard statement “they are just part of the ecosystem”, but I think that we most not forget that the physiological condition of the host plant play a major role in the insect plant interaction. Here in the Peruvian coast, where the seasons are well marked, the population growth of “soft scale” is also shared with other sucker insects like aphids and whiteflies, with well-known exceptions of the super pest into each group. Best wishes.

  3. I think that they have appeared long time ago, the plants that they feed must have evolved in order not to be extinct! The problem, as always happen, is to change these species from one location to other! This is usually produced by men. So men are responsible to tranform one species into a pest.

  4. Dear Dennis:
    I also agree with Richard statement “they are just part of the ecosystem”. I have studied the biology, distribution and population density of another cottony cushion scale, Anapulvinaria pistaciae which lives on Pistacia vera and few other plants. Its population was found almost always very low and patchy in pistachio plantations. It never emerged as a pest. It is a good prey for several species of ladybirds too.

    • Thanks for your comments and the very interesting information about the pistachio scale insect.