The Journeys of Trees by Zach St. George

Author:Zach St. George
Language: eng
Format: epub
Publisher: W. W. Norton & Company
Published: 2020-04-27T00:00:00+00:00

What could go wrong? This is perhaps a question better asked in the negative: What couldn’t go wrong? Recall the two carnivorous snails Charles Elton mentioned in The Ecology of Invasions, introduced into Hawaii as counterpests in the mid-1950s: Neither succeeded in eliminating their intended mark, the giant African snail, but they caused the decline and possible extinction of numerous species of native Hawaiian snails. Between 1905 and 1929, authorities released more than forty-five counterpests of the gypsy moth into the eastern United States. Some of them attacked the gypsy moths, but they also attacked native moths, including the luna and Polyphemus moths that Étienne Léopold Trouvelot had tried to breed for silk. In the 1800s, people introduced mongooses from central and southwestern Asia into Hawaii, the West Indies, Mauritius, and Fiji. They were to be a counterpest of the rats infesting sugarcane fields. It didn’t work. Rats are nocturnal, and mongooses are diurnal. The mongooses devoured rare creatures of all sorts, and drove at least one bird species to extinction. They sometimes come down with rabies.

Things have gone wrong, badly and often. These mistakes are still what the field is best known for, but Leah Bauer and other scientists I spoke with insisted that biological control is on the whole a safer, more regulated, more predictable activity than it used to be. Before the scientists in Michigan could release parasitoid wasps, they had to test whether they might attack other species besides the emerald ash borer.

The worst disasters of biological control have come from creatures like carnivorous snails and mongooses. These animals eat many different things. Scientists call them “generalists.” Parasitoid wasps are pickier. They tend to lay their eggs on (or in) just a few species. Scientists call them “specialists.” But the USDA scientists had to be sure. First, entomologists in China tried to get the wasps to attack the larvae and eggs of various moths and beetles, including two that feed on ash trees. The wasps ignored them. Next, in Massachusetts, Bauer’s USDA colleague Juli Gould presented the wasps with nine of the emerald ash borer’s sister species, members of genus Agrilus. In these “no-choice” tests, Gould gave the wasps only one option: Lay eggs either on the strange insect or nowhere at all. The wasps showed a deep commitment to the future of their race. Presented with no other option, they sometimes laid their eggs on or in the larvae and eggs of several other Agrilus species, among them the two-lined chestnut borer, Agrilus bilineatus, and the bronze birch borer, Agrilus anxius.

The question, then, is of risk and reward. In the post-mongoose, post–gypsy moth world, it’s hard to gain approval to release counterpests of invasive plants. Many invasive plants are closely related to important crops. “That’s really dicey,” Bauer said. The potential for unintended consequences is too high. But if the unintended victim of a counterpest is another bug? “Most of the public doesn’t really care if we kill native insects,” she said. (She was careful to note that she personally doesn’t feel that way.

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