the meat, a big step toward cheaper fattening of meat animals with hormones.

The use of tranquilizers has been proposed for reducing the customary loss of weight in cattle being shipped to market. These chemicals, also, are used in such minute quantities that residues could not be detected except with radioactive tracers. Tritium can now be used by health officials to study the effects of tranquilizers.

Insects

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.

Aphids are made radioactive.

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.

Radioisotopes as Radiation Sources

Earlier in this booklet radioisotopes were compared to fireflies because they emit flashes of “light.” Isotopes serve research in another important way, other than as tracers.

Suppose you collected all the fireflies within a 100-mile radius and put them into a glass jar. Instead of an occasional twinkle, you would now have a steady glow of light. Similarly immense numbers of radioactive atoms can be compressed into a small volume to produce steady, intense sources of radiation. Agricultural research has answered many questions with the use of such radiation sources.

Can Radiation Produce New Plants?

Perhaps no biological aspect of atomic energy has so caught the fancy of the public as the prospect of creating new plant varieties. There is something mysterious about pouring invisible energy into seeds or buds and watching for changes in the emerging leaves and flowers. There is also the challenge of the lottery in being unable to predict where, when, or in what form the alterations will appear.

Although the claims of over-enthusiastic gardeners and seed dealers about astonishing new plants “created” by atomic radiation are doubted, clear proof exists even in the restrained scientific journals of hereditary changes caused by radiation.

From more than 30 years of scientific study, certain conclusions have emerged. High-energy radiations can cause sudden hereditary changes (mutations) in any living thing: man, animal, microbe, or plant. Any feature of a plant subject to hereditary control—root, shoot, leaf, flower, or fruit—can be altered by radiation. Most of these changes are undesirable; they interfere with the normal state of biological affairs. A very small percentage of mutated organisms is improved in some way. So far changes cannot be controlled or predicted.

To date fourteen new strains of crop plants improved by radiation have been put into production in various parts of the world. These varieties with their places and dates of release follow:

1. “Primex” white mustard, Sweden, 1950

2. “Chlorina Mutant” tobacco, Indonesia, about 1950

3. “Shafer’s Universal” bean, Germany, about 1950

4. “Regina II” summer oil rape, Sweden, 1953

5. “Weibull Stralart” fodder pea, Sweden, 1957

6. “Sanilac” navy bean, Michigan, 1957

7. “Pallas” barley, Sweden, 1958

8. “N. C. 4X” peanut, North Carolina, 1959

9. “Florad” oats, Florida, 1960

10. “Seaway” bean, Michigan, 1960

11. “Alamo-X” oats,