Eight legs of destruction

A recent social media post of mine about redlegged earth mite got lots of comments so I thought I should write a longer piece about the pest.

The first thing to know about the redlegged earth mite (Halotydeus destructor) is that it is an introduced pest from southern Africa. It is now a pest of southern Australia. It’s also really small – about 1mm long – with a round, velvety black body and eight red legs. Here is a close-up image.

Halotydeus destructor

The second thing you need to know about this pest is that it mostly attacks broadleaf plants including clovers in lawns, and seedlings of a wide range of vegetables and ornamentals. It also feeds on and breeds in a number of broadleaf weed species.

Now let’s join the dots between the ‘first thing’ and the ‘second thing’. One of the weeds it likes to breed in is capeweed (Arctotheca calendula) which is an introduced weed from South Africa – the same region the redlegged earth mite originates from!

This is capeweed.

Arctotheca calendula

The next thing you need to know is what redlegged earth mite damage looks like. Earth mites are sapsuckers, and produce a characteristic silver or white pattern on leaves where they feed. When large numbers of mites attack a plant the leaf tips may shrivel and die. Seedlings are particularly vulnerable, and may die.

This is what redlegged earth mite damage to clover looks like…

Redlegged earth mite damage

and to pea seedlings ….

RLEM damage peas

and silverbeet! Halotydeus destructor – what an appropriate scientific name!

RLEM damage

You also need to know about their life cycle. Redlegged earth mites are active over the cooler months from May to November. They hatch in autumn from over-summering eggs held in the bodies of dead females on the soil surface. Eggs hatch and follow a classic mite life cycle – larva, protonymph, deutonymph to adult. There are about three generations over the cooler months, each lasting about eight weeks. When temperatures increase and host plants dry off in spring, female mites die with about 100 over-summering eggs held in their desiccated bodies!

What can we do to prevent them damaging our plants? The best preventative method is weed control. Remove all broadleaf weeds near where susceptible seedlings grow. You could try placing plastic sleeves made from plastic drink bottles around seedlings. Smear vaseline (or some other sticky substance) around the rim to prevent mites climbing in. This will work (against mites and other seedling pests) but it may look unsightly (to some people).

No pesticides are specifically registered for use against redlegged earth mites in home gardens, but there are several with a general registration against ‘mites’. My preference is horticultural oil (e.g. EcoPest Oil, PestOil, etc.) or soap (e.g. NatraSoap) sprays, or perhaps you could try neem (e.g. Eco-Neem).

If you combine one of these products with weed control you will win the battle. If you are looking for a quick fix they will win!

I’ll leave you with an image that I just snapped on my phone. These pea seedlings have not been sprayed. They are growing in an area where I always control weeds – there is not a redlegged earth mite to be seen. I think the nice fresh mulch might help too.

Undamaged seedling

 

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Art imitates life

The science fiction movie “Alien” (1979) introduced us to the scariest alien of all time, as well as its sinister juvenile forms (facehuggers and chestbursters). It seems to me that’s just art imitating life.

If I had to pick a favourite science fiction movie it would be Aliens (1986), the second movie in the Alien series. Who can forget Sigourney Weaver’s immortal line “Get away from her, you bitch!” as she prepared to battle the alien queen? The scene where the alien’s extendable jaws get very close to Sigourney’s face is downright terrifying. But where did the idea for those extendable jaws come from? HR Giger, the graphic artist who designed the alien and won an Academy Award for his efforts, said “I hadn’t studied any animal. My instructions were that it should be somehow frightening and horrible, and I did my best. We come to the conclusion that we must make the beast blind and give it a terrific set of teeth – something like the detail in Francis Bacon’s Crucifixion triptych.”

I wonder if Giger knew that extendable jaws are old hat in the insect world, because insects with such mouthparts have existed for hundreds of millions of years. I am referring to the nymphs of damselflies and dragonflies (Odonata), sometimes known as ‘mudeyes’. These aquatic predators have elaborate hinged mouthparts which they can extend rapidly. Check this YouTube video out – it even references Aliens! The nymphs achieve this by local increases in blood pressure caused by sharp contraction of their abdominal and thoracic muscles. All odonate nymphs are predaceous and may be ‘ambush’ or ‘attack’ predators depending on the species. The nymphs feed on immature aquatic insects such as midges and mayflies, tadpoles, and small fish. I don’t have a shot of dragonfly nymphs underwater, but I reckon this image of a cobweb festooned nymph skin (below) looks a bit alien!

Dragonfly exuvia

What of Alien facehuggers? Surely that’s fanciful? A facehugger is the second stage (after the egg) in the Alien’s life cycle. It has eight spindly legs, which allow it to crawl rapidly, and is somewhat similar in appearance to an arachnid or perhaps a horseshoe crab. The similarities end there because the facehugger is a parasitoid – its only purpose in life is to implant an embryo (which eventually becomes a chestburster) within a host – invariably in the movies the host is a human!

Note that word parasitoid. A parasitoid spends most of the juvenile stages of its life cycle inside or attached to a host, and eventually kills that host. A parasite feeds on or within its host without killing it. Eight-legged parasitoids are a Hollywood invention but six-legged parasitoids are abundant. Check out my earlier blog post I love parasitoids. I usually refer to them as parasitic wasps but the correct entomological term is wasp parasitoid. These beneficial wasps kill their hosts upon emergence – which leads us neatly to ‘chestbursters’.

‘Chestburster’ sounds like a particularly silly concept that could only have been dreamt up in Hollywood but, alas for entomophobes, it happens regularly in the insect world. For example, there is not much left of a cabbage white butterfly caterpillar (Pieris rapae) when 50 Cotesia glomerata wasp larvae burst from it to pupate.

Some wasps even recruit what is left of the host as a guardian for their pupae. For example, the wasp Dinocampus coccinellae lays an egg in a ladybird beetle and the hatching wasp grub develops through four instars. Just prior to emerging from the host beetle, the wasp grub secretes chemicals which paralyses the ladybird. The wasp grub then emerges and pupates between the legs of the still twitching ladybird beetle (picture below). There is no safer place for an insect at its most vulnerable stage than between the legs of a not quite dead predatory ladybird. Apparently if you move the ladybird beetle away from the wasp pupa it will stagger, zombie-like, back to its original position.

Dinocampus coccinellae

The connection between insects and Hollywood is quite direct with certain movies. At the Joint Annual Meeting of the Entomological Societies of America, Canada, and British Columbia in November 2018, graphic artist Ryan Church shared his journey from a youthful fascination with insects to a career in movie design. Church has worked on numerous Star Wars movies, Star Trek, Avatar, War of the Worlds, and John Carter. He says he finds his inspiration in the world of insects and you can clearly see that in the gossamer winged airships featured in John Carter – think adult dragonflies.

There are plenty of other sci-fi movies inspired by insects such as Them! (1954), The Fly (1986), Starship Troopers (1997) and Mimic (1997). Just in case you think this article is merely a nerdy ode to Hollywood Sci-fi I should point out that I have mentioned some pretty important beneficial insects here. Dragonflies and damselflies are predatory insects both as adults and as nymphs, and are particularly important around waterways. As for wasp parasitoids, horticulture and agriculture as we know it would collapse without them. Predatory and parasitic insects are also vital in the natural environment and some of them have been around for a long time. To illustrate that, here is a story from yesteryear to finish with.

130 million years ago, on a tree trunk deep in the conifer forests of the Levant, some strange looking insect eggs were showing signs of life. The eggs were vaguely ‘egg shaped’ and each one was perched above the tree bark on its own stalk. Faint scratching could be heard, until the eggs began to split and a dozen Tragychrysa ovoruptora emerged into the light. They were hatchling green lacewings of the family Chrysopidae – but they were about to die.

We know they died because these particular insects are locked inside pieces of amber – frozen in time, eggs and all. They were the subject of a recent scientific paper. The paper describes how the sap, which eventually set to amber, caught the hatchlings in various stages of egg emergence. Researchers were then able to study the mechanism which the insects used to break out of their eggs – a method which has changed little in 130 million years.

In the words of one of the researchers, “Modern green lacewing hatchlings (image below) split the egg with a ‘mask’ bearing a jagged blade. Once used, this ‘mask’ is shed and left attached to the empty egg shell, which is exactly what we found in the amber together with the newborns.” The name usually given to such an egg-splitting mechanism is “egg burster”. Hello Hollywood!

Green lacewing larva

This article was first published in the February 2019 issue of Hort Journal.

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Cup moth larvae video

I promised a couple of posts ago to load a video of cup moth larvae – here it is (linked from my YouTube channel)!

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