Insect vs. Insect

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Insect vs. Insect

Copyright © 1958
by
The American Museum of Natural History

How scientists find insects that will kill
other insects thus save fruit growers and
farmers millions of dollars each year.

By WELDON D. WOODSON

Success in controlling pests by marshalling beneficial insects to "battle" them was first recorded in 1775. In that year, a report by the Swedish naturalist Peter Forskal told how the date growers of Yemen had been observed to use one kind of ant to kill another which was destructive to the trees.

The use of beneficial ants to protect orchards from insect pests has long been practiced in Asiatic countries. A report in 1936 says citrus growers of south China put bamboo runways between the trees to aid the ants in their movements from tree to tree

Almost a century passed after the publication of Forskal's report before pest-killing insects tested by bona fide scientists were transported from one country to another. In 1873 a mite (Tyroglyphus phylloxerae Riley ) was shipped from the United States to France for the control of the grape phylloxerae. ln the next year the lady bird (Coccinella undecimpuctata L. ) was introduced into New Zealand from England. A third insect Apanteles glomeratus L. a parasite of the cabbage worm was sent to the United States from England in 1883 and likewise proved successful.

Nevertheless these efforts were scattered over ten years and the true beginning of the sustained biological control movement did not come until l888 and 1889 when Albert Koebele traveled to Australia for the U.S. Department of Agriculture to secure natural enemies of the cottony cushion scale. At this time this pest was killing so many large branches of citrus trees in California that the industry was threatened with extinction. Banks refused to accept infested groves as collateral for loans.

Albert shipped back some ladybirds (genus Rodalia ) which readily killed cottony cushion scale and thrived over the winter in the groves. Propagating, they quickly reached adequate numbers to check the pest and to this day their descendants - together with those propagated in the insectaries and released - have kept the scale under control. All told, the introduction of this insect probably did not cost the government more than $5000 including the salaries of the scientists.

For 25 years, Koebele, the first of the entomological explorers, was engaged in importing parasites. He worked for the U. S. Department of Agriculture, then for the California State Board of Horticulture, and finally for the Hawaii Board of Agriculture and Forestry.

Contemporary with him was George Compere of the California Board of Horticulture, who from 1899 to 1901 traveled to the Far East and Australia in search of scale insect parasites and predators. Following this, from 1901 to 1904 he served the Government of West Australia in finding and importing parasites of fruit flies. So valuable were his services that from 1904 to 1910, California and Australia employed him jointly. His quests for insects beneficial to crops took him around the world.

Both Koebele and Compere were self-trained naturalists. Not so Frederick Muir, the remaining pioneer, who had a hand in setting biological control on a firm footing. Technically trained, he began exploratory work in 1905 for the Hawaiian Sugar Planters' Experiment Station. He was especially eager to find shock troops that would attack the sugar-cane beetle borer, Rhabdoscelus obscurus. Tracing down this and that clue, he found at long last at Amboina, in the South Sea Islands, the insect Macroceromasia sphenophori. This, he had reason to believe, would curtail the pest, and after prolonged effort, he was able to transport it alive to Hawaii, where it met with great success in the purpose intended.

In turn, those who have followed Muir have been specialists, devoting themselves quite closely to specific problems. For instance, in California, the Citrus Experiment Station took a special interest in a shipment in 1927 of two parasites (Tetracnemus pretiosus and Coccophagus gurneyi ) from Australia for the control of the citrophilus mealy bug. Already it had spread to 70,000 acres of Valencia oranges.

By l930, the imported parasites had put this pest out of the picture as an economic factor. They have probably saved citrus growers more than $1,000,000 a year. Yet the total cost to the university of California for the search, importation, and establishment of the beneficial parasites did not exceed $10,000.

On September 30, 1942, agricultural officials who were making a routine inspection of a peach orchard in Southern California's Orange County found an insect larva that differed from any they had heretofore collected. When they identified it as the oriental fruit moth, the State's economic entomologists and agricultural administrators were alarmed.

They knew well that since 1915, when this moth was first discovered in the United States near Washington, D. C., it had built up a record of increasing destructiveness to peaches. As an indication of the extent of its damage, in South Carolina it has rendered 12 per cent of the peaches unmarketable. In some eastern orchards the losses to individual growers have reached 50 to 100 per cent.

The wormlike larva of this moth burrows into the tips of growing shoots, causing the terminal leaves to wilt and die. Or it tunnels into the fruit itself, producing the well known "wormy peach." It had now spread from the Atlantic seaboard to the Pacific coast. In view of California's extensive production of late canning peaches, it looked as though the fruit growers were in for a hard time.

Fortunately, as early as 1917 a native ichneumon "fly," Macrocentrus ancy1ivorus, in reality a wasp, was observed to be playing an important part in the natural control of the moth in a limited area on the Atlantic coast. By 1929, this insect had clearly shown its worth from southern Connecticut to southern Virginia by destroying as high as 90 per cent of the larvae infesting the twigs. As the oriental fruit moth spread westward, the United States Bureau of Entomology and Plant Quarantine nurtured the parasite in their laboratory at Moorestown, New Jersey. In co-operation with the various state agencies, they liberated it wherever the fruit moth advanced in sufficient numbers.

Rusty-yellow in color and about equal to a mosquito in size, the female of this ichneumon fly possesses an ovipositor about as long as her body. When ready to lay her eggs, she crawls over the twig or fruit until she finds the web or excrement of the larva. She then unsheathes her ovipositor and with it locates the hole by which the larva burrowed in. Her ovipositor contacts the larva, and she quickly pierces its skin and injects into its blood a minute egg. As soon as the egg is immersed in blood, it begins to absorb food. In time it hatches, and the fruit moth larva is ultimately killed.

The value of this fly can best be realized by the fact that under proper temperatures the female can lay eggs approximately 24 hours after emerging as an adult, and her average life cycle is about 34 days, during which she may deposit over 500 eggs. She may waste more than one egg on one fruit moth larva, but still the destruction is great. When released in an orchard, she searches diligently for infested trees. After depositing eggs in one locality, she may travel as much as six miles to other trees.

A method for producing the beneficent parasites was successfully worked out, and in 1946, almost 29 million of them were produced. About 23 million were shipped to orchardists in 14 counties. Aside from a few backyard trees in one section, the oriental fruit moth vanished as a problem, and it has not reappeared. Should it do so, the University of California now has at its command the "assembly line" to produce a redoubtable natural enemy. And the cost, compared with the potential value, was extremely low.

Another recent contribution of the Citrus Experiment Station had to do with the California red scale. For more than half a century, this scale, Aonidiella aurantii, had annually wrought great damage on the trunks, branches, and foliage of citrus trees. In several test orchards it was shown that it might be possible to reduce the infestation by means of two species of tiny wasps. These were the so called golden chalcid "fly" and its relative from China, Aphytis "A." These wasps parasitized the scale and also killed many of the invaders by sucking the body fluids through a strawlike wax tube formed with the aid of the ovipositor. This was the first indication that the scale might be satisfactorily controlled by methods other than the use of insecticides.

The Station further learned that under favorable conditions the wasps may control the scale much more cheaply than insecticides. When production methods are refined still further, it may cost less than $40 to raise 400,000 wasps - the quota per acre necessary to give biological control in all but heavily infested groves during the first year after the insecticidal treatment is stopped. Control by insecticides costs approximately $50 an acre. In addition, the use of insecticide is a yearly proposition, whereas the insect "police," once they are generally established, will increase through natural reproduction, thus taking the strain off the insectary. In many groves, a natural balance may then ensue.

In the introduction of natural allies from abroad, great care is exercised to avoid releasing any imported species that might possibly prove detrimental. As Dr. Stanley E. Flanders of the University of California puts it, "We make sure that the parasite or predator has the fixed habit of attacking the pest for which it is employed and that it cannot become a problem in some other way. Imported insects, whether noxious or beneficial, are apt to run rampant when introduced into a favorable environment." The answers to over 50 different questions are sought concerning the habits and inclinations of any insect being considered as a possible clean-up corps.

In citing these achievements, scientists freely admit that numerous other projects have not been so successful and that the expense of them must be added to the cost of the successful ones. Nevertheless, they point out that the successes eclipse the failures many times over.

Further, they wish to make it clear that their advocacy of the use of beneficial insects to destroy injurious ones does not imply that this method will supplant altogether the employment of insecticides. They warn rather that in our enthusiasm for sprays and fumigants, we must not overlook the possibilities of utilizing potential pest-destroying insects. They contend that the maximum exploitation of both methods - biological and artificial control - is needed in the war of man-versus insect pests.



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