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Metamorphosis of an alien invader
My inspiration for Nature Notes is whatever I happen to come across in the Rec that particular month; it’s very serendipitous. At the beginning of July, I was working in the back bed and noticed that there were lots of ladybirds on the goat willow, at various stages of growth. On closer inspection, I saw a couple of the native Seven Spot Ladybirds, but the majority were that infamous invader, the Harlequin Ladybird.
The Harlequin Ladybird originates from Central Asia but is thought to have spread further and faster than any other invasive species. It now occurs in many countries; sometimes introduced intentionally, as a means of controlling aphids and other agricultural pests, sometimes inadvertently, in imported goods. It arrived in the UK in 2004 and has since spread across the whole country. The concern is that they are voracious predators that can outcompete native species for food, or actually feed on their eggs and larvae. However, it’s recognised that they are now so ubiquitous that nothing can be done to control them. But whereas some people are concerned about their impact, others feel that an ecological balance, with them included, will be possible. Time will tell.
Harlequin Ladybirds do share something in common with our native species, in that they all undergo complete metamorphosis, and I saw various stages of this on the leaves of the goat willow. Complete metamorphosis comprises 4 stages; egg, larva, pupa and adult (imago) and the larvae are different in appearance to the adults. By contrast, incomplete metamorphosis, has no pupal stage. The larva (usually known as a nymph in this instance), is a smaller version of the adult and grows through repeatedly shedding its outer skin, until it reaches maturity.
Remains of eggs and Harlequin larva (later instar)
Ladybirds lay their eggs, usually on the underside of leaves, near to a larval food source such as aphids. The eggs are in clumps, or rows, and are yellow. I was too late to get a photo of any eggs but the discarded remains can be seen on this leaf, after the larvae have emerged. These larvae exist as eating machines and scoff their way through innumerable insects and sometimes even eggs of their own species. As a result, they grow quickly and they have to cast off their rigid outer skin (exoskeleton) to accommodate this. Ladybirds have 4 growth stages, or instars.
A slightly earlier stage (instar) of a Harlequin larva
A pupa of a Harlequin ladybird
The photos here show an earlier and later instar of the Harlequin. At the final stage the Ladybird larva becomes an inert pupa.(see photo) It is here that the truly amazing transformation takes place from larva to fully grown Ladybird. The process by which this happens isn’t fully understood but involves at least a partial breakdown of the larval body and reconfiguration of the cells. After about a week the adult Ladybird emerges from the pupa, as seen in these photos. Their body is initially pale coloured but gradually acquires it’s more familiar pattern and colour. In the case of the Harlequin Ladybird, this is highly variable as it comes in a number of forms; a feature that accounts for its name.
An adult Harlequin emerging from the pupa (head at the bottom of photo)
The adult Ladybirds continue to feed in preparation for hibernating overwinter, and often gather in outbuildings or even houses; anywhere they can remain safe and sheltered. They emerge the following spring to seek out a mate and start the life cycle over again.
Newly emerged Harlequin and husk of pupa
Metamorphosis is a remarkable process; an incredible alchemy that is common in the insect world. It has evolved to enable each stage within the creature’s life cycle to be uniquely adapted to its role. Nature is wonderful and it happens right here under our very noses!
Words and photographs by Denise Long