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AIR pollution can wreck trees by culling the predators that normally keep
leaf-munching insects in check, say ecologists in Finland. Though this has long
been suspected as a cause of damage to plants, this is the first time
researchers have found hard evidence for it.

Plagues of insects often devastate plants in polluted areas. Ecologists
reasoned that this could happen either because pollution makes parasites of
these insects less active, or because it kills off predators that eat the
insects. Till now, however, researchers have always failed to detect any signs
that pollution keeps the pests鈥� enemies at bay.

But past studies had only looked at small subsets of the insects鈥� enemies.
Elena Zvereva and Mikhail Kozlov of the University of Turku in Finland decided
to test the theories again by studying the entire troop of enemies of the leaf
beetle Melasoma lapponica. Exploding populations of this beetle have
periodically stripped nearly all the leaves from willow trees in forests near a
nickel-copper smelter on the Kola Peninsula in northwest Russia. This is one of
the world鈥檚 worst black spots for sulphur dioxide and heavy-metal pollution.

Zvereva and Kozlov measured pollution levels at 10 sites up to 36 kilometres
from the smelter each year between 1993 and 1998. They also checked the density
of adult beetles at the sites, and collected evidence of predation and
parasitism of adults, eggs, larvae and pupae.

The results showed that parasites killed more M. lapponica at
polluted sites than at relatively clean ones. But fewer predator attacks at the
polluted sites meant that overall, around 20 per cent more insects survived
there.

鈥淚t鈥檚 clear that reduced predation is one of the most important causes of the
herbivore population increase in polluted regions,鈥� Zvereva concludes. Head
counts of the wood ant, one of the most voracious predators of the beetle,
confirmed that its numbers had dwindled in areas where smog was worst.

Zvereva says their work highlights the importance of long-term studies of the
entire range of natural enemies of animals to pin down the effects of pollution
on plants. 鈥淢ost researchers have only looked at one species or one group of
predators,鈥� she says. 鈥淭his may easily produce misleading conclusions.鈥�