Simberloff said there have been highly successful biocontrol
projects for aquatic plants and crop pests, “but there is no history
of a successful biological control project for a significant forest
pest. I’m not saying it won’t work now, but it hasn’t worked yet.
He notes that five beetles, two flies, and a fungus have been
released since 1995 to fight the hemlock woolly adelgid, an
aphid-like insect that is killing hemlock throughout its range.
And while most of the control agents have survived, none have
slowed the death of the trees or slowed the spread of the adelgid.
Simberloff is especially concerned about the parasitoids being
released to control the emerald ash borer. He said there are
about 100 native jewel beetles in the same genus as the ash borer
that could be at risk.
“They have tested these three biocontrol wasps on very few
of them,” Simberloff said. “Some of these jewel beetles are seen
only once every few years or every decade, so if they disappear,
we’ll never even know.”
He admitted that he knew of no alternative to biocontrol to
fight the emerald ash borer, but he thinks scientists are working
too quickly and not being careful enough in testing biocontrol
agents – particularly for the hemlock woolly adelgid.
Joe Elkinton agreed that biocontrol is not always successful,
noting that each case is unique, and sometimes no natural enemies
exist to stabilize some pests. He has worked for nearly three
decades on biocontrol of the hemlock woolly adelgid, and while
he acknowledged that the project was “limping along” – especially
in the Northeast, where the most promising control agents do
not appear to survive the cold – he believed a breakthrough
might be on the horizon with a predatory beetle from the Pacific
Northwest that has been introduced in North Carolina.
“We don’t have the answer yet, but there are various other
natural enemies being evaluated,” Elkinton said. “It may take a
suite of these things to control the adelgid.”
Most public opposition to biological control emerges from
periodic news reports about biocontrol horror stories from a
century ago, like the early gypsy moth efforts or the release of
mongooses in Hawaii and cane toads in Australia. Biocontrol
practitioners say there are plenty of bad examples, but most of
those species were introduced by non-scientists with no over-
sight at a time when the only insects most people cared about
were honey bees.
“I don’t want to have to defend actions from a century ago,”
said Casagrande. “You don’t think we’ve learned anything since
then? Doctors were practicing bloodletting back then, but
nobody avoids going to the doctor these days because of it.”
They noted that the national and international standards
now in place and required host specificity testing and govern-
ment approval for every insect release have slowed the number
of biocontrol agents that have been released in recent decades.
Since the new rules were implemented, two recent studies
of the safety of biological control concluded that more than
99 percent of releases have had no significant effect on non-
What’s likely to be the next pest that biocontrol researchers
will delve into? The Asian longhorn beetle is an excellent
candidate. It meets at least two of what Casagrande said are the
primary requirements: It’s a serious problem, and it is not easily
controlled in any other way. The third requirement – that it be
controllable by a host-specific agent – is uncertain at this time.
Elkinton traveled to Korea in 2015 in search of the beetle’s
natural enemy but came home empty-handed. Gould went
searching in China and found a parasitic beetle that showed
promise, but host specificity testing is just beginning.
“We have to create an environment in the lab where beetles