Aug. 11, 2003 COLLEGE STATION – A parasite masquerading as its host to avoid detection may sound pretty unfair. But then again, all's fair in love and war – at least in the war against red imported fire ants.
Recently, researchers at Oxford University in the United Kingdom and at Texas A&M University discovered that members of the insect order Strepsiptera pose as their hosts. That may open doors for management of fire ants and other insects. It has also led to more avenues for research.
While serving as a Research Fellow at Oxford a couple of years ago, Dr. Spencer Johnston, entomologist with the Texas Agricultural Experiment Station, was observing work by Dr. Jeyaraney Kathirithamby, who is a researcher with Oxford's department of zoology.
Kathirithamby studies Strepsiptera, a small insect which spends part of its life as an endoparasite or a parasite that develops in the abdomen of its insect host.
"Only the first instar and the short-lived male are free-living," Johnston said. Insects shed their shells in order to grow and then form new ones, he explained. These changes are called in-stars.
"The developing male and the female are wholly endoparasitic in insects."
Strepsiptera "are poorly understood and so exotic," Johnston said. "The male and female look completely different."
Both use the same mechanism – the tiny, first instar stage – to attack hosts. The ones that attack ants develop into males, and the ones that attack crickets, grasshoppers and mantids turn into females. All of the hosts of the female, and in particular, crickets, are a favorite part of the fire ants' diet, he said.
During his visit to Oxford, Kathirithamby showed Johnston some sections of Strepsiptera DNA. He noticed extremely large nuclei and what appeared to be bags surrounding the parasite. Johnston suggested she should check it out.
After Johnston's return to the United States, Kathirithamby sent him samples of the bags, parasite and the host. He and one of his technicians, Larry D. Ross, sequenced the DNA of all three. Sequencing, which was first developed in 1975, allows analysis of genes at the nucleotide (the basic structural unit of the nucleic acids DNA or RNA) level. Johnston and Ross found the DNA sequence from the bag surrounding the parasite was that of the host, rather than the parasite.
Meanwhile, Kathirithamby observed Strepsiptera actually entering a host. She found that the first-instar infective larva of the Strepsiptera jabs itself between the outer shell and the skin of the host. Once the entire body has penetrated its layers, it remains in constant motion for 24 to 36 hours until it is enclosed by the host.
The host's skin forms a bag, which at this point is suspended by a thin stalk. Later, the larva develops into another stage and the stalk eventually pinches off from the overlying epidermal layer. The second-instar larva moves passively through the host, eventually residing into the abdomen. It goes through two more molts and develops into the fourth-instar larva, all the while deriving its nutrients from the host, Johnston said.
The female develops no further, except to harden an anterior portion (the cephalothorax), which squeezes out between abdominal segments and emits a male attractant.
The male develops within the ant, and the day it is to emerge, it elicits a strange behavior in its host: The ant will leave the colony and climb to the top of any grass or twig it encounters.
"I'd love to know how a pupae enclosed in a bag in the abdomen does that," Johnston said.
The short-lived male Strepsitera emerges from the ant, finds and mates with a female. She develops up to 800,000 eggs and lays live young, which emerge from the female to start the cycle all over again.
The research gets to a fundamental problem, Johnston said. "How does a parasite attack – whether it is you or I or any other organism – without our immune systems protecting us?" he said. "In the case of Strepsitera, it masquerades as the host."
Parasites have many ways to avoid an immune reaction by the host, "but none of them encapsulate themselves into the host tissue" like the Strepsitera.
Johnston said this discovery is an important step to understanding a species that is a parasite of the red imported fire ant.
In most, if not all, infected species, such as crickets, the parasites actually extend the life span of their hosts, Johnston said. But he added researchers don't know if the Strepsiptera extends the life span of the fire ant.
"Do I care? No," Johnston said. "Once they're parasitized, they're sterile anyway...they're out of the picture," he said. "In fact, I'd love for the parasite to extend the life span because those drones are a drain on the colony. If drones, or fire ant males, would stay in the colony and suck up nutrients for another two years, it would be wonderful. It's the same thing with the workers: once they're parasitized, they don't sting, they don't contribute, they just drain the colony."
He and Kathirithamby want to study Strepsiptera further to find out whether half the live young produced are male and the other half female, or whether the young are capable of being either male or female, depending on the host.
Johnston also was excited that, 97 years after the male Strepsiptera was first described by scientists, Kathirithamby recently identified the female – alive – in crickets in Mexico.
"These are really basic questions that we haven't been able to answer," Johnston said. "We've never had the females alive before, we've always had them caught in alcohol traps. With the discovery of the females – and using DNA to match the tiny, free-living adult male with the larvae-like female that never leaves the cricket – we can culture the males in ants and females in crickets and study them more."
Culturing, or raising Strepsiptera in mass numbers, could provide an alternative to pesticides. Kathirithamby has used them with success as parasites of long-horned grasshopper – a pest in palm oil plantations – in Papua, New Guinea.
Using Strepsiptera as a predator to fire ants would provide "an option for people who don't want to use an insecticide," Johnston said. For instance, this type of biological control could be used near water or near schools, he explained.
To control fire ants, crickets could be mass-reared, contaminated with the parasite, and released, or the first infective stage – if it is the right sex – could be sprinkled on mounds, he said.
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