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Beetles and humans

described as the second biggest threat to

biodiversity after habitat destruction.

Often, when a plant is established in a new

environment, controlling it by the usual methods

becomes impossible, as it establishes a seed bank in

the soil—and it is then that people generally resort

to biological control. The object of biological

control is straightforward. The invasive plant has

become so successful because it has escaped the

usual checks and balances that regulate its

population in its native range, so scientists look

at the wild population where the target plant

originated, to seek specific herbivores and seed

predators, and import them as well, in the hope

that they might regulate the pest without affecting

anything else in the environment. This has to be

carefully managed, since the wrong “biocontrol

agent” might become an invasive species in its

own right, or switch hosts and attack a vulnerable

native relative of the target plant. Nowadays, a lot

of laboratory testing takes place before biocontrol

agents can be released, and only agents shown to

be host-specific to the target weed are selected, to

reduce collateral damage to other plants. Beetles,

as well as Lepidoptera (butterflies and moths), are

most frequently selected against invasive plants,

because many of them are very host-specific,

feeding on only one genus or species of plant.

Among beetles, weevils (Curculionoidea) and leaf

beetles (Chrysomelidae) have been used most

often. Successful examples include Thistle Weevil

Rhinocyllus conicus (Curculionidae), introduced from

Europe to Canada, and Gorse Weevil Exapion ulicis

(Brentidae) from Europe to New Zealand.

BIOLOGICAL CONTROL OF WEEDS

As people traveled around the world they brought

with them exotic plants, sometimes to remind

them of home, or as crops, ornamentals, food for

livestock, to modify the landscape in some way,

or even by accident as seeds in animal feed or

bedding. Some of these plants then established

and proliferated in the new environment and

became invasive weeds. Unchecked by the

usual herbivores and competitors, these plants

can grow faster and denser than usual, crowding

out native habitats and becoming severe pests,

to the extent that invasive species have been

left | Larvae of Oxyops vitiosa

(Curculionidae) An Australian weevil

biocontrol agent for paperbark tree, an

invasive pest tree in Florida’s wetlands.