 Ancient Enemies: Beetles and Leaves
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Oct. 8, 2003 — A fascinating and unique evolutionary war between leaf-eating beetles and their plant hosts has been waged for no less than 112 million years, a new study revealed.
Both combatants have co-evolved in an extraordinarily extended series of defensive and counter-defensive maneuvers, said a report Tuesday in the Proceedings of the National Academy of Sciences.
Among the strategies devised by the plants has been a bizarre capacity to squirt high-pressure streams of toxic liquid when their leaf veins are punctured. In response, the canny beetles learned to sever the veins before making a meal of their host.
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Only a few other such highly host-specific links have been timed and the intimate bond between these two is "the oldest age so far for a specialized plant-herbivore association", said the report by Judith X. Becerra, of the University of Arizona in Tucson.
Becerra's study used independent time-calibrated evolutionary trees to develop a timeline for such adaptations in both groups, revealing that the war began more than 100 million years ago and that the plant defenses and insect counter-measures subsequently evolved at roughly the same times.
She focused on the 70-plus beetle species that belong to the genus Blepharida: about half are Old World types found in Africa and the rest belong to a sister group found mainly in the New World tropics, especially in Mexico. The plants are flowering species belonging to the genus Bursera, which have a similar distribution (Mexico alone has 100 species).
Because the beetles specialize in eating the same plant family in both regions, it had been speculated that the interaction between them started very long ago — before Africa and South America separated from the ancient super-continent of Gondwana, Becerra said.
The study confirms that both plants and beetles first evolved before that separation began about 100 million years ago, but it also shows that most of the diversification of both families into separate species happened over the past 30 million years in response to environmental change, and presumably to each other's changing survival strategies.
Many Bursera species have evolved to have remarkable chemical weapons at their disposal. The squirting varieties store chemical resins under high pressure in their stems and leaves and release them in response to damage: "Besides being repellent and toxic, resins solidify when exposed to air and may entomb small insects completely," Becerra said.
In turn, some beetle species have figured out how to carefully disarm their hosts by methodically gnawing a series of small notches in the central leaf veins to disrupt their flow. Some will spend up to an hour doing this to one leaf, and then take another 10 to 20 minutes to consume it.
"Calibrations indicate that lineages of plants that possess the defensive traits in question are of about the same age as the lineages of beetles with adaptations to counteract those defenses," Becerra said. "Most plant lineages that include highly squirting Bursera species evolved in relative synchrony with the Blepharida lineages that include species with the ability to disarm those defenses."
Non-squirting Bursera species take a different tack, producing complex chemical mixtures designed to repel or poison hungry beetles. Complex and highly specialized interactions have developed in those cases as well: one particularly distasteful plant is eaten by only one particularly hardy beetle, which can metabolize the chemical mixtures without coming to harm.
Becerra was able to trace that the plant species that produce these chemicals and the beetles that specialize in eating them have evolved relatively recently over the past five to 12 million years.
She points out that a related group of beetles, the genus Diamphidia — which includes the renowned poison-arrow beetle of the !Kung San of South Central Africa — are also specialist feeders on another group of plants, the Commiphora.
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