Invasive Ladybird Beetles Escape Parasitism

The world of insects is full of diverse and violent blood-curdling relationships. With more than a million species described, and an estimation of ten quintillion insects alive at any given time, the diversity of these odd relationships is less surprising. A particularly interesting relationship is of that between a parasitoid and its host. A parasitoid normally lives as larval parasite upon or within another organism (usually another insect), and eventually kills the host during development. This relationship can be found throughout the insect world, and has been extremely effective in agricultural biological control programs.

Braconid wasp pupating on surface of a pest species – Tomato Hornworm. Photo by Max Wahrhaftig: Wikimedia Commons

Many are familiar with the popular Alien series. The series follows interactions between humans and a violent race of alien parasitoids that utilise humans as their hosts. The series of films has been extremely popular throughout the world, and provide a perfect example to explain a biological phenomenon to any audience. Although the Aliens in this film are fictitious, a recent study led by Oxford D.Phil Candidate Richard Comont takes a closer look at a common ‘alien’ found across the UK, a native species, and the armies of parasitoids that attack them both.

The harlequin ladybird beetle (Harmonia axyridis) is common throughout the UK and many other parts of the world. Originally introduced from Asia as a biological control agent, this beetle is spreading quickly across the country. Alongside this population explosion, Britain’s native ladybird species have been experiencing a steady decline.

The many different looks of harlequin ladybird beetles – Photo by Entomart

Richard and his team set out to understand the rates at which H. axyridis were parasitised by a natural enemies. Most ladybirds found parasitised had been attacked by Dinocampus coccinellae – a parasitoid specialising in ladybird hosts. This small wasp lays an egg within an adult or pupal ladybird. As the egg develops, it will eventually tunnel out of the ladybird through a small hole chewed in the abdomen. The larvae will then spin a cocoon underneath the ladybird. The colourful warning of the ladybird will hopefully keep predators far from the vulnerable D. coccinellae pupa. Occasionally the ladybird will not die despite the invasive procedure, and will instead stand guard – paralysed as a sort of ‘zombie-body-guard’ shaking from side to side above the pupa.

The ladybird Coccinella septempunctata with a parasitoid cocoon, probably of Dinocampus coccinellae (Braconidae). Photo by Giles San Martin

Richard and his team examined wild H. axyridis, along with a native species – the seven-spotted ladybird (Coccinellae septempunctata). Populations of species were monitored across Southern England, where H. axyridis has become recently established. Through collecting mature pupae through visual searches, the team were able to collect thousand of pupae, and brought them back to the lab to determine rates of parasitism.

Richard and his team found that native species C. septempunctata is more than ten times more likely to be parasitised than the alien species H axyridis. This allows allows the population to grow without the significant control experience by native parasitoids including D. coccinellae. The Enemy Release Hypothesis predicts higher success of invasive alien species as a consequence of reduced mortality from natural enemies; this seems to be exactly the case for harlequin ladybird.

Remember to keep your eyes peeled this summer during your excursions into the great outdoors for these fascinating relationships between aliens, zombies, parasitoids, and good ol’ fashioned ladybirds. Sometimes the most fascinating relationships can be just under your nose.

References

Comont, R. F., Purse, B. V., Phillips, W., Kunin, W. E., Hanson, M., Lewis, O. T and Roy, H. E. 2013. Escape from parasitism by the invasive alien ladybird, Harmonia axyridis. Insect Conservation and Diversity.