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. 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?