THE United States Department of Agriculture (USDA) has imported a tiny
European fly which preys on an American thistle. The fly, Cheilosa corydon,
bores into the stems of the plants. Next spring, when researchers introduce
the fly on the thistle, Carduus nutans, will mark the climax of an effort
that began more than 20 years ago. Ironically, the thistle itself was introduced
from Europe in the 19th century as an ornamental plant.
Paul Boldt, a research entomologist for the USDA, has been looking for
insects that eat C. nutans, also known as the nodding thistle or musk thistle.
‘Carduus is a pest everywhere, and an important problem around the Mediterranean
and in Central Europe. It infests severely overgrazed pastures and range
land, crowding out grass.’ Cows and sheep will not eat, or even go near,
the thistle.
In 1973, Boldt went to the USDA’s laboratory in Rome, Italy, to look
for potential agents against C. nutans, and surveyed the plant from France
and Italy eastwards to Iran. He collected insects that fed on them. ‘After
three or four years,’ Boldt says, ‘we had 115 species of predators and parasites
that lived on Carduus.’
Boldt and several technicians in Rome tested all of these insects to
see what they ate, eliminating all that were polyphagous (eating many species)
or oligophagous (eating relatively few species). They tested rigorously
the ‘finalists’ in this selection process to make sure that they ate nothing
except the nodding thistle.
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Boldt says that the first successful introduction of a predator on C.
nutans occurred before he arrived in Rome. Biologists found that Rhinocyllus
conicus, a seedhead weevil, ate seeds from the thistle. The researchers
brought it to the US in 1969. ‘It occasionally eats other plants,’ says
Boldt, ‘but this is a minor problem. It has been successfully established
in several states and we have noted a strong decline in the thistle in Virginia,
Montana and Missouri.’
Boldt says that of the original 115 insects that live on C. nutans,
seven, including Rhinocyllus, were declared good candidates to fight the
thistle. In 1974, biologists introduced Trichosirocalus horridus, a tiny
weevil that eats the rosette of the thistle, into pastures in Virginia and
Montana. ‘It is now doing damage to Carduus,’ says Boldt.
Boldt emphasises the importance of finding insects that attack the thistle
at every stage of its development. ‘Carduus begins in the winter as a flat
rosette, which may spread up to 20 inches in diameter. In the spring it
sends up stalks, and these primary stalks will sometimes be followed by
secondary and tertiary stalks.’ T. horridus larvae eat the rosette stage,
C. corydon larvae attack the growing stems, and R. conicus larvae eat mature
seeds of the musk thistle.
Finding natural controls for pest plants such as the thistle is important,
not just because chemical pesticides cause environmental problems, but because
ranchers with thousands or hundreds of thousands of acres of range land
cannot afford to spray herbicides. ‘But,’ says Boldt, ‘these insects, once
they are established and reproducing themselves, are free.’
The process of selecting, testing and importing predators is, however,
expensive. According to Boldt, the USDA spends an average of 15 scientist-years
on each plant it wants to control, and an average of three scientist-years
per insect species it introduces. But the payoff can be spectacular. Norman
Rees, a scientist working for the USDA in Bozeman, Montana, estimates that
he has collected and distributed 2.8 million Rhinocyllus, sending them to
21 states and to Argentina, too.
Boldt says that scientists in many countries cooperate in the search
for biological control insects. ‘We have labs in Argentina and Rome, and
a single scientist in South Korea. We currently have an Australian entomologist
here (at Boldt’s lab in Temple, Texas) looking for bugs that eat cocklebur
(Xanthium), Parthenium, and Baccharis, all of which were accidentally introduced
into Australia from North America. We have had visitors from China, and
sent ragweed beetles to Yugoslavia. It turns out that there are a couple
of people in each country working on introducing predators to control introduced
pests.’
Boldt is not sure whether any additional insects will be brought in
to control C. nutans. Of the USDA’s seven original candidates, one, a leaf
beetle called Psylliodes chalcomera, is being tested in Rome. Another promising
bug was rejected when it was found eating artichokes, a close relative of
the thistle family.
