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.

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An accidental discovery

Many insects are difficult to identify at the larva/nymph or pupa stage of their life cycle, so I often rear juvenile insects through to adulthood to be certain of what they are. Sometimes the insect which emerges inside the container is not what I was expecting!

A few years ago I was sent some insect specimens to be identified. The insects were causing considerable damage (lots of distorted and shriveled leaves) to a Murraya hedge (Murraya paniculata) in a garden on the NSW central coast. Peering down my microscope I could see what looked like psyllid nymphs (Psyllidae), but I couldn’t see any psyllid adults to confirm it. In the image below you can see a nymph with wing buds (indicated by the arrow). I must admit it’s not the best image I’ve ever shot!

psyllid murraya

I put the nymphs, and the leaves they were on, into a sealed container and waited. A few days later there were tiny adult insects about 2mm long flying around inside the container. They were not psyllids; they were tiny wasps – the parasites of psyllids! I identified the little wasps as being in the genus Psyllaephagus (Hymenoptera: Encyrtidae). There are about 60 wasps in that genus found in Australia, and they are exclusively parasites of psyllids – so the insects damaging the Murraya hedge had to be psyllids! After a few days my jar was full of these little wasps, indicating that the psyllid infestation on that Murraya hedge was heavily parasitised.

Psyllaephagus

So what to do with the hedge? Psyllids can be controlled with a couple of horticultural oil sprays. But an oil spray would kill parasitic wasps, especially ones this small, as efficiently as killing the psyllid nymphs. The specimens in the jar indicated that the majority of the psyllids were parasitised, and would cause no further damage. Personally I wouldn’t spray at all in such a situation, because I would be confident of the wasps winning the overall battle. However it is difficult to convince other people to let nature take its course – yep, the hedge was sprayed by the landowners. I need to be more convincing next time!

I have had unexpected guests in my insect containers on other occasions. For example, I collected a mantid egg case (ootheca) and kept it in a container on my desk so I could watch the nymphs hatch. Unfortunately I didn’t put the lid on the container properly. One morning I caught a movement out of the corner of my eye and looked up to see several tiny mantid nymphs lined up on the top of my computer monitor. There were also several on my desk lamp, my phone, book shelves, the slatted blind, computer speakers, the printer, and so on. It took a while, but I managed to wrangle them all into a container and released them outside.

mantis nymph

Later that day I was able to take some photographs (like the one above) and videos (I’ll use that footage sometime) of other mantid nymphs emerging from the egg case. But it was what happened next that was more interesting. When I examined the container a couple of days later what did I found flying around inside it? You guessed it – several parasitic wasps (about 3-4mm body length). I identified them as Podagrion sp. (Hymenoptera: Torymidae) wasps – known parasites of mantid egg cases.

Podagrion

As you can see from the photograph above the female wasp has a long ovipositor (it’s as long as her body), but she must insert it in a mantid egg case before it hardens. To achieve this she either follows the mantid female, or hitches a ride (a behaviour known in zoology as phoresy). As soon as the mantid egg case is complete, she inserts her eggs inside. Later, the wasp eggs hatch into wasp larvae which feed on the mantid eggs within the protection of the mantid egg case. In the end about 100 mantid nymphs and 50 Podagrion wasps emerged from that single mantid egg case. Male Podagrion wasps are pretty cool too (below). They look just like the female but without the long ovipositor.

Podagrion male

As mentioned above, the mantid egg case yielded a mixture of mantid nymphs and Podagrion parasitic wasps. In other words the mantids weren’t wiped out entirely. Some people think that parasitism is a bad thing. I think it’s incredible that these tiny wasps have evolved to occupy an ecological niche such as the inside of a psyllid nymph, or a mantid egg case! The wasps’ lives are so entwined with that of their host insects, that the wasps can’t survive without that other insect. If the wasps killed 100% of their hosts 100% of the time, the wasps would die out. That would be a tad counterproductive!

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The Cherry Tree & The Meat Ant


This is a video about ants which visit my cherry tree. The story is primarily about meat ants (Iridomyrmex purpureus), but a couple of other species make cameo appearances. Why do the ants climb into my cherry tree? You’ll have to watch the video to find out!

I hope you enjoy the video – as usual, being ‘one minute bugs’, it’s short and sweet! Let me know what you think. Please hit the subscribe button in the widget if you would like to receive email alerts about new posts. The subscription process is more than one click, but it won’t take too long!

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