Where and How Fast Do Insects Travel?
Radioactive isotopes have been used to study insects, their life cycles,
dispersion, mating and feeding habits, parasites, and predators. Several
hundred such studies have been made on dozens of insect species.
With radioactive tracers even the smallest insect becomes more easily
followed. As one example, nearly half a million mosquito larvae were
tagged with radioactive phosphorus in Canada. Some of the adults from
these larvae were later found as far as seven miles away, but most were
recovered within one-eighth mile.
In a companion study grasshoppers were labeled with the same isotope.
Their average rate of movement was only twenty-one feet per hour, and
after seven days their position was based entirely on random motions
plus prevailing winds. It seems that grasshoppers have no ability to
move toward food.
How Far Do Insects Carry Pollen?
This question is of practical importance in knowing how far to separate
seed-fields to maintain pure varieties of plants. In the past it was
studied by the laborious method of growing a plant having a dominant
“marker” gene for some visible trait surrounded by plants without the
marker. Seeds from plants at various distances from the marked plant
were grown the following year to see how far the genetically marked
pollen had been carried. Since such plants are normally cross
pollinated, it was difficult to obtain strains genetically pure for
presence or absence of the marker gene. Also, considerable testing and
bookkeeping were involved.
With tracers the answer may be found in a few days. A plant is injected
with radioactive phosphorus; after a few days its pollen is highly
radioactive. Flowers at various distances from the tagged plant may be
checked daily for radioactivity. In one study with alfalfa, radioactive
pollen was carried as far as thirty feet by bees, but more than
one-third was deposited on plants adjacent to the labeled one.
Are Predators Used to Destroy Insects?
With insect pests, as with plant diseases, biological control is more
economical than artificial control. The use of insecticides too often
results in destruction of helpful insects along with pests. Limited
success has been achieved in breeding certain plants for resistance to
insects.
Two important uses of biological control in agriculture have been made
in recent years: importing an insect from Australia to eradicate a weed
in California and disseminating ladybird beetles to control certain
scale insects.
Fig. 5—Identifying predators that destroy unwanted insects.
[Illustration: Aphids are made radioactive.]
[Illustration: Larger insects found nearby are checked for
radioactivity.]
Bumblebee has not eaten radioactive aphids, but mantis has.
Helpful parasites and predators must first be identified before they can
be used. In the case of small or nocturnal insects, this can be
exceedingly troublesome. Tagging the pests with radioisotopes in order
to identify the predators which consume them is much simpler because the
most efficient predators contain the most activity.
With such techniques entomologists have studied insects and animals
which prey on unwanted aphids, mosquitoes, blackflies, and roaches. Such
experiments may lead to a deliberate increase of certain predators to
control injurious insects.
Radioactive labeling is also valuable in studying helpful insects. In
one case the indolence of drone bees was indicated by finding that even
with adequate syrup in their cage they still received identical syrup
from worker bees in an adjoining cage!
Can Tracers Measure Spray Residues?
Any material used on plants or animals to kill insects or disease
organisms must pass rigid inspection to be sure it does not accumulate
in foodstuffs. This is particularly true of the “systemic” poisons,
those which are fed to plants via leaves or roots and are carried
internally to all parts of the plant. Such chemicals can be used widely
with nonfood plants such as cotton to kill insects feeding on them.
Combining radioactive labeling with other techniques has permitted the
researchers to show that some compounds are soon broken down into
harmless chemicals—a big step toward acceptance for their use on food
plants.