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Study of Wasps Imported to Hawaii Shows Risks of "Biocontrol"

By John Roach
for National Geographic News
August 21, 2001
 
Bugs don't pay much attention to human-drawn battle lines.

That's the lesson scientists learned when they went to Hawaii to look at what happens when farmers import specialized insects to combat invasive weeds and pests—a practice known as biocontrol.


Over time, they found, the non-native insect may go AWOL (absent without leave), leaving the farm and settling into local ecosystems far afield.

More than a century ago Hawaiian farmers imported parasitic wasps from China and Texas to prey on moths and butterflies that were devouring fields of sugarcane. Those wasps, the study revealed, infiltrated a pristine boggy forest on Kauai, many miles from where they had been deployed to do battle, and are now a dominant species in the forest's habitats.

The results shows that land managers need to carefully consider the possible implications of introducing non-native species as biocontrol agents, said Jane Memmott, an ecologist at the University of Bristol in England and co-author of the study, which was reported in the August 17 issue of the journal Science.

"We need to be aware there are risks, and we need to weigh them up," she said. "Very few things in life are 100 percent safe."

Controversial Practice

To control weeds, bugs, and even diseases, some agricultural experts, foresters, and conservationists see the introduction of non-native species as environmentally friendlier than the use of chemicals such as insecticides and pesticides.

Parasitic wasps, for example, are commonly deployed to kill moths and butterflies. The wasps lay eggs inside moth and butterfly caterpillars, causing the victims to die before they metamorphose into winged insects.

The use of biological controls, however, is controversial. Some scientists argue that species introduced as biocontrols can run amok in the ecosystem and do more damage than the pest or weed they were enlisted to destroy.

"At least chemicals are not a permanent addition to the environment, as biological organisms are," said Laurie Henneman, a co-author of the paper and a colleague of Memmott's at the University of Bristol.

The researchers went to Kauai's Alakai Swamp to see if they could find any of the more than 122 species that have been introduced as biological controls in Hawaii over the last century. The swamp is higher, wetter, and cooler than the lowland agricultural fields, making it a distinct ecosystem.

Henneman and Memmott constructed food webs containing Hawaiian plants, butterflies, moths, and parasitic wasps. The webs provided a snapshot of who eats what in the ecosystem and how much the non-native species may have infiltrated the local habitats.

In the swamp, where it can rain more than 32 feet (10 meters) in a year, the researchers collected 2,112 moth caterpillars, then documented the 52 types of plants the insects were feeding on.

Back in the laboratory at the Kauai Agricultural Research Center, Memmott and Henneman reared the caterpillars and watched to see if young wasps emerged from the caterpillars' bodies. About 20 percent of the caterpillars were found to have wasp eggs.

An overwhelming 83 percent of the wasps that had laid the eggs were originally introduced as biocontrol agents. Fourteen percent of the wasp species had been accidentally introduced to the islands over the years. Only 3 percent of the wasps were native to Hawaii.

Caution Urged

In an article accompanying the study in Science, Pauline Syrett, a biocontrol expert at Landcare Research, a government-funded institute in Lincoln, New Zealand, said many ecologists and managers of natural areas would be shocked to discover the extent to which non-native species have infiltrated many ecosystems.

Henneman said it's highly likely that biocontrol agents have infiltrated ecosystems all over the world. "Given that we found what we found the first place we looked, odds are it would be found in most places people actually looked," she said. "So few people have looked at so few places at this point, though, that we just can't say much."

While the data clearly show that biocontrol agents can become dominant players in ecosystems far from where the outside species were introduced, Memmott and Henneman said it's important to remember that all of the non-native wasps observed in the study had been introduced before 1945.

Today, Memmott said, scientists and resource managers have better knowledge and are more careful about which insects are introduced as biocontrol agents. Many of those imported into Hawaii in the 1930s and 1940s would not have been introduced there today.

"It is important for people to realize that there is simply not a control method available that is a panacea and that is without environmental effects," said Henneman. "We need to look at each case individually and figure out what makes sense in that case."
 

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