Recently an isotope of hydrogen (tritium or H³) was linked to hormones, and these were fed in normal amounts to cattle. Tests 90 days afterward showed less than one part per billion in 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.