158 hectare
Insects and Man—An Exotic Dilemma
ByTimothy L. McCabe, Ph.D.
Insects have long been recognized for their benefits as pol-
linators, honey producers, predators, and parasitoids, as
well as for pure aesthetics, such as admiring the grace and
beauty of a butterfly. They have also been recognized as
vectors of disease, agricultural pests, defoliators of forests,
bloodsuckers, and as having other characteristics unwant-
ed or not admired by humans.
Insects have only recently come to be appreciated by
the scientific community and public for what they can tell us
about the health of the environment. Their sheer diversity,
estimated at 30 million species, and numbers (the world’s
ant biomass equals the total human biomass) are evidence
of their significance. In recent times, insects have been
used for biocontrol, i.e., using one species to control the
population of another. Klamath weed (Hypericum perfora-
tum,also known as St. John’s-wort) control by a leaf beetle
is a famous example. The Klamath weed leaf beetle,
Chrysolina quadrigemina,intentionally introduced in 1946,
feeds on the foliage of this weed in both the adult and larval
stages. The Klamath weed leaf beetle has dramatically
reduced the abundance and density of this weed in areas
where fall temperatures are mild. This biological control
agent is responsible for contributing to the impressive
decline in St. John’s-wort on the Pacific coast.
Introductions have led to mistakes that are equally
dramatic. Another leaf beetle, the Colorado potato beetle
(Leptinotarsa decemlineata), was introduced from the Unit-
ed States to Europe in the late 1920s, where it has become a
serious potato pest. A parasitic fly (Compsilura) was inten-
tionally introduced to the United States in the 1920s to con-
trol a newly established and very serious forest threat, the
gypsy moth (Lymantria dispar). It took a long time for the fly
to become established, and now it is an integral part of our
fauna. However, this species of fly has five broods per year,
while the gypsy moth only has one. The fly survives for the
remainder of the year by attacking hundreds of species of
native moths and butterflies, unrelated to the gypsy moth,
and has suppressed their numbers to unprecedented low
levels. The population of this fly has been estimated at
16,000 individuals per acre in Connecticut. Clearly this
“cure” for the gypsy moth was worse than the “disease,”
having totally unexpected side effects.
The honeybee is widely acclaimed for its ability to pol-
linate and for its honey production. However, this intro-
duced bee, originally an Old World species, has displaced
and probably caused the extinction of several bees native
to the New World. The tracheal bee mite, a recent acciden-tal introduction, possibly from the Philippines, has had a
catastrophic effect on the honeybee and honey production.
The secondary impact of tracheal mites on native bees is an
ongoing concern. The “killer” bee, an introduced strain of
the honeybee, is another matter entirely.
Not all introductions need be derived from another con-
tinent. Geographic barriers, such as the Rocky Mountains
and the Great Plains, prevent the natural spread east and
west on the North American continent. The western conifer
seed bug (Leptoglossus occidentalis) was accidentally intro-
duced from the West to a tree nursery in Iowa in 1956. From
Iowa, the bug spread throughout the northeastern United
States, reaching New York in 1992, where it has become a
nuisance due to its proclivity to overwinter in houses.
Insects are continually on the move. Even native popu-
lations exhibit periodic instability and can rapidly expand (or
contract) a range that had been stable for decades, even
centuries. The plain ringlet butterfly (Coenonympha tullia)
has been moving south from Canada at a pace of nearly 40
miles per year with no end in sight (except eventually an
ocean).
Even exotic plants have a profound impact on native
plants and consequently on the associated insects. Glaciers
“erased” the flora and fauna of the northern states thou-
sands of years ago. Species with poor dispersal mecha-
nisms have still not reinvaded since the glacial retreat
15,000 years ago. Glaciers wiped out earthworms, but intro-
duced earthworms have filled the void. One exotic earth-
worm species is so adept at removing organics that it
actually mineralizes the soil in the process of feeding. Garlic
mustard, also an introduction, loves mineralized soils and
follows the earthworms. The Virginia white butterfly (Pieris
virginiensis), a species threatened by competition with the
introduced cabbage white butterfly (Pieris rapae), has a
caterpillar that feeds on a native mustard, but the adult but-
terfly demonstrates an ovipositional preference for the
introduced garlic mustard. Its caterpillar cannot survive on
the garlic mustard, and as a consequence of this oviposi-
tional preference, this species is disappearing in the wake
of the encroaching garlic mustard. As an added insult, the
diminished quality of the forest litter beneath the garlic
mustard has led to a dramatic (more than 90 percent)
decline in ground beetles, native denizens of the forest
floor. This impact makes its way up the food chain as we
witness declines in red-backed salamanders that feed on
insects in the forest litter.
Ladybugs have been intentionally introduced with wild
abandon because of their reputation as voracious aphid
eaters. Nearly half of North America’s ladybugs are not
natives, but introductions. Our native nine-spotted ladybug