Cottony scales

This post was originally published in May 2016. Since then it has clocked up a very surprising 8750 hits! I wonder what makes it so popular? I am republishing the post (with minor edits) for those that missed it the first time.

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 the ovisac. Can you see it? This wasn’t a planned shot of the crawler – it happened by chance as I was shooting a series of images for a focus stack.

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 will write more on this topic in a future one minute bugs.

Share this:
Facebooktwitterlinkedinmail

Sap-suckers & sooty mould

Sooty mould is not a plant disease as such but various species of fungi, which grow on the sticky honeydew excreted by many species of sap-sucking insects. A very thick mould layer on leaves may cause a reduction in photosynthesis in plants, which can result in leaves falling prematurely. The main concern to gardeners (apart from the sap-sucking pests which caused it) is that a thick covering of sooty mould on a plant’s leaves is downright ugly.

Sooty mould is a serious problem for the horticultural industry. Ugly plants are unsaleable. Similarly in the citrus industry, the main economic damage caused by mealybugs is by the downgrading of fruit quality due to sooty mould growing on mealybug honeydew. The picture below shows a leaf infested by soft scale insects and the sooty mould they cause – pretty ugly eh?

Sooty mold

Continue reading Sap-suckers & sooty mould

Share this:
Facebooktwitterlinkedinmail

Sex, gender & difference

Men are from Mars and women are from Venus or so the saying goes. However the anatomical differences between male and female humans are relatively minor when compared to the differences between male and female insects.

“Why is this ‘ant’ attacking a ‘fly’ via its ovipositor?” an insect enthusiast asked me in an email recently (the insects in question can be seen below). The image clearly illustrates sexual dimorphism. My Dictionary of Entomology, a massive tome only owned by insect nerds like me, defines sexual dimorphism as ‘differences in size, shape, anatomical features, colour or behaviour between males and females of a species’.

Flower wasp Continue reading Sex, gender & difference

Share this:
Facebooktwitterlinkedinmail