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Last Updated: Tuesday, 3 June, 2003, 12:16 GMT 13:16 UK
Bizarre bug's bag attack
By Jonathan Amos
BBC News Online science staff

Parasites that construct unusual sacs out of their host's own tissues could point scientists to more effective methods of pest control that do not involve agrochemicals.

Strepsiptera, Image by Jeyaraney Kathirithamby
Adult males have just a few hours to find a female protruding through a host's abdomen
Members of the insect order known as Strepsiptera parcel themselves up in a piece of their victim's own skin to avoid detection.

Once inside the body, the invaders and their offspring consume their hapless target before the new generation bursts out to find further hosts.

The parasites' remarkable means of evading their victim's immune defences has been described in detail for the first time by Dr Jeyaraney Kathirithamby, from Oxford University, UK, with colleagues.

The research team says this "bizarre but stunning" group of parasites - which live off a wide range of insect species, such as ants, grasshoppers, wasps, and crickets - can be used in an environmentally friendly way to protect important commercial crops.


"They can be used as biocontrol agents to keep pest numbers down," Dr Kathirithamby told BBC News Online, "but to do that one has to know something about their life history and biology, and that is what we are trying to establish."

Too close

Like so many parasites in nature, Strepsiptera have curious life cycles.

There are more than 596 described species, but scientists confess they know far more about the adult males than the females.

For many species, the females have yet to be identified because they assume a tiny grub-like form and never leave their host, making them difficult to find and describe.

Strepsiptera bag, Image by Jeyaraney Kathirithamby
The bag protects the larva from immune attack
Only the males will take to the air. Their short, stubby, front wings and very large, fan-shaped hind wings have led to the order being called twisted-wing parasites.

Most of the life cycle is spent inside the host as a female or developing larva. They will move across to new victims when insect hosts brush up against each other in nests and hives, or perhaps on plants.

Their success across such a large range of hosts has impressed scientists and Dr Kathirithamby says their strategy of dressing themselves inside their host's own skin would help to explain their versatility.

Inside out

The entomologist's Oxford team watched as larvae of Stichotrema dallatorreanum prodded their way through the tough outer skeleton, or cuticle, of the long-horned grasshopper (Segestidea novaeguineae).

Continuing to jab and writhe downwards, the larvae then enclosed themselves in the underlying softer tissue, or epidermis.

These bag-like structures floated off into the abdomen, allowing the parasites to absorb nutrients but, crucially, still evade detection by the host.

As time went by, the females and their hundreds of thousands of larvae consumed the grasshopper's insides and also destroyed its reproductive system.

Short hop

The bag mechanism is very clever and it must be so that they can evade the host's immune system.

Segestidae novaeguineae, Image by Jeyaraney Kathirithamby
The long-horned grasshopper's abdomen is totally overcome by the invading parasites
Dr Kathirithamby told BBC News Online: "Normally, an insect would try to put a tough capsule around an invader to starve it but I've never seen the parasite encapsulated. This must be some kind of mechanism to tell the host, 'I am self - don't do anything to me'.

"I think this explains their diversity. By using the host's own epidermis in this way, they don't need to evolve so many mechanisms to parasitize insects."

Dr Kathirithamby said efforts to control the long-horned grasshopper in Papua-New Guinea with the parasite had been tremendously successful.

"The hosts become sexually useless when they are parasitized; they completely lose their reproductive organs."

She believes the biocontrol strategy could possibly be applied to leafhopper and plant-hopper insects that destroy rice crops in southeast Asia.

Her team's latest research is published by the journal Proceedings of the National Academy of Sciences.

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