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Weevils help save Jurassic plant in Australia
May 25, 1999
Weevils could be helping to protect Australia's Jurassic heritage by prolonging the survival of the ancient Australian Cycad plant, researchers say.
Cycads are relic Gondwana plants whose ancestry dates back to the dinosaurs. Their hard, spiny leaves and seed cones probably developed to deter hungry herbivores. In spite of that, they left the plants vulnerable to the hordes of plant-consuming insects that emerged during the succeeding Cretaceous era.
So it is presumable that the Cycad plant, characteristic of the Australian landscape, has not survived the last 100 million years unassisted. Researchers in Australia say the Cycads have actually employed weevils to help them reproduce.
In order to survive, the Cycads devised a new, and subtle, chemical warfare strategy both to combat and to exploit the new pests.
But the Cycads may have disappeared if they hadn't found a cunning way to make use of a particular group of weevils, says Dr Rolf Oberprieler of CSIRO's Australian National Insect Collection.
Weevils are normally associated in most people's minds with damage to plants and their seeds. To most gardeners they are considered menacing pests. However, in the case of the Cycad, the weevil is both a foe and a friend. Because the weevil has become the plant's main pollinator, it is an essential part of the plant's conservation, Oberprieler says.
The weevils are attracted to male Cycad plants by the warm, strong, appetizing odors that their ripening cones emit. The insects mate, dine and lay their eggs in the male cones, during which time they become covered in pollen.
Then, in one of nature's more impressive strategies, female Cycads employ a subtle form of chemical conmanship, says Oberprieler.
"Studies on Central American Cycads suggest that receptive female cones initially emit an odor that mimics that of the male plants. This dupes the pollen-laden weevils, which are irresistibly drawn to leave the male plants to fly across and alight on the cones of the female plants," he says.
Here they deposit the pollen, as they hunt for the promised banquet. But before the weevils can cause too much damage to the maturing Cycad seed-cone, the female has a second chemical signal a potent form of insect repellant.
This toxic substance in the cone tissues drives away the weevils, which take refuge on the male plants once more, where they are in fact "invited" by the cones' increased starch contents to lay their own eggs and perpetuate the cycle.
"This process also appears to occur in the Australian Cycads and their weevils," said Oberprieler. Although the Australian Cycads are inhabited by different weevils than those elsewhere in the world, they appear to have been equally successful in conscripting these into becoming their pollinators.
"Cycads are such a distinctive part of the Australian landscape that we would wish to preserve them and now we understand that one of the keys to doing that will also be to look after the weevils which are essential to their successful reproduction in the wild."
Studies of modern weevils show that the Cycads were invaded by these highly specialized herbivores no fewer than seven times during their long history, but on each occasion managed to surmount the threat. By killing off sick and diseased plants, the weevils actually contributed to the emergence of the hardy strains which survive today.
Particularly vulnerable were the seed-bearing female cones which gradually became denser, more heavily armored and more toxic in their reactive process to control the invaders.
Over time this caused the weevils to develop their present way of life, laying, hatching and living mostly on the cones of male Cycad plants and traveling across to the females for just long enough to deposit the precious pollen, before being driven off again.
Particular species of weevils are closely associated with particular species of Cycads. In fact, Oberprieler believes the insects may be one way for botanists to distinguish between different Cycad species. Several species of these weevils remain to be studied and described, and more probably await discovery among the 69 Australian species of Cycads.
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