Parasites and Predators

C. P. Clausen.

The cottony-cushion scale, a small, inactive insect that feeds on the sap of the leaves and twigs of citrus trees, was first found in California in 1872. Within 15 years it had spread over the entire citrus-producing area of the State and threatened to destroy the industry. In many orchards the fruit crop was a complete loss, and in some the trees themselves were killed. The situation was desperate, as no method of control was known, and many growers gave up hope of relief and pulled out their trees.

It was known that the scale occurred in Australia. It probably originated there and had reached California by unknown means, possibly on nursery stock. That knowledge yielded one ray of hope a parasite was known to attack it and appeared to hold it in check in its native home.

C. V. Riley, entomologist of the United States Department of Agriculture, became keenly interested in the problem and laid plans to send a qualified entomologist to Australia to obtain the parasite. He selected Albert Koebele, at that time engaged in studying other insect problems in California. Koebele had studied the cottony-cushion scale, attempting to control it by various means, and it was he who first concluded that the pest must have come from Australia. Difficulties arose, however. At that time it was almost impossible to obtain funds for foreign travel. Finally Koebele went to Australia, but as a representative of the State Department to the Melbourne Exposition.

Koebele arrived in Sydney late in September 1888 and immediately with the aid of an Australian entomologist was able to find the parasite, a tiny fly, Cryptochaetum iceryae. Many. thousands were dispatched to California. Fortune favored him still more, however, when, a few weeks later, he discovered the previously unknown but now famous vedalia. Both the beetles themselves and their larvae were found to feed greedily upon the eggs and the larvae of the scale, and on them only.

The first shipment of vedalia by Koebele reached California November 30, 1888. It comprised 28 beetles. Additional shipments followed. By the end of the following March a total of 514 had been received.

The beetles thrived. In less than 2 years after the arrival of the first shipment, the scale was under complete control throughout the citrus-growing sections of the State. It has remained so ever since. This highly successful outcome was due to the beetle rather than to the parasitic fly, the original object of the search, though it likewise became established and abundant. Koebele's trip cost less than $5,000; it has saved the citrus industry millions.

Koebele's later investigations in New Zealand and Australia in 1891-92 and to other Pacific regions were financed by the Department of Agriculture and the California State Board of Horticulture. He found and forwarded several valuable mealybug and scale insect predators, four of which adapted themselves to California. Among them was the well-known Australian lady beetle which during the 1920'S was reared and distributed to the number Of 40 million or more each year for the control of the citrophilus mealybug, another citrus pest in California closely related to the cottony-cushion scale.

Koebele was the pioneer among entomological explorers who search the far places of the world for parasites and predators to be used in controlling insect pests in this country. This method is now termed "biological control" to distinguish it from chemical control, which involves the use of insecticides. All available natural enemies, including parasitic and predaceous insects and disease-producing organisms as well, whether native or of foreign origin, are used in this program. In biological control the first cost is usually the only cost; the application of chemicals, on the contrary, must be repeated year after year and often several times each season.

Most of our destructive insect pests are not native to this country. They have gained entry in various ways, some as long ago as Colonial days. Nearly every injurious insect is attacked in its native environment by one or more parasites and predators which hold it in check. When a pest gains entry into a new country, its natural enemies are usually left behind. The pest therefore can increase unhampered by their attack. That is why many insects of foreign origin are more destructive in the United States (or any new habitat) than in their home country.

ONE PRECEDENT for Koebele's work was not entirely successful. In 1883-84 and following years, C. V. Riley imported a small wasp, Apanteles glomeratus, from England to combat the imported cabbageworm, a European pest that appeared first in Canada about 100 years ago. The parasite became established and abundant in all sections, but failed to control it.

So spectacular was Koebele's success with the cottony-cushion scale that the Bureau of Entomology began a large-scale search in 1905 in Europe for natural enemies of the gypsy moth and brown-tail moth. Those two destroyed or damaged forest and ornamental trees over a wide area in New England. The explorations from 1905 to 1914 covered all Europe and Japan, and were renewed and completed in 1922-27. Thirteen species of parasites and predators were successfully established in New England as a result of this work. The frequency and destructiveness of the gypsy moth outbreaks have been appreciably reduced as a result of these importations. The brown-tail moth has subsided to a position of little importance.

Other forest and shade-tree pests that have been dealt with in the same way are the satin moth, oriental moth, birch leaf miner, and larch casebearer. The satin moth was a serious pest of poplar and willow in New England and the Pacific Northwest before 1930. Compsilura concinnata, Eupteromalus nidulans, and Apanteles solitaries (which had been imported for use against the gypsy moth and the brown-tail moth) reduced greatly the infestations in New England. Apanteles and Meteorus versicolor gave satisfactory control in the Pacific Northwest.

The oriental moth occurs in a few places in Massachusetts. Its parasite, Chaetexorista javana, obtained from Japan in 1929-30, in most years has effectively controlled the pest, but it apparently cannot withstand the occasional severe winters in Massachusetts; consequently the pest increases during the seasons following such winters.

AMONG THE PESTS of cereal and forage crops, the alfalfa weevil, which had become established in Utah, was the first on which investigations were undertaken. Importations from Italy in 1911-13 resulted in the establishment of a parasite, Bathyplectes curculionis, which destroys a high proportion of the larvae. The real value of this parasite in controlling the pest is difficult to determine, but parasite attack, in conjunction with a change in cutting practices, has given fairly satisfactory control.

The project for biological control of the European corn borer is the largest yet undertaken. It covered the years from 1920 to 1935, with activities centered mainly in the European countries but extending also to Japan, Korea, and Manchuria. Six species of parasites are known to be well established as a result of shipments during the period. Unfortunately four of them are severely restricted by climatic conditions and are established only in limited areas. The two most valuable are Lydella stabulans grisescens and Macrocentrus giluensis. Lydella is widely distributed in the Eastern, Middle Atlantic, and North Central States. In some localities it may parasitize 50 percent or more of the borers. Macrocentrus is common only in southern New England.

Several attempts have been made to find effective natural enemies of the sugarcane borer. A considerable number of parasites were found in Cuba and several South American countries and introductions have been made intermittently since 1915. Two large importations were from Argentina and Peru in 1929-32. Not a single species of the many that have been released in Louisiana has become established, because of climatic conditions, mainly winter temperatures too low for the parasites to survive, and because of the practice of cutting the cane annually, which eliminates most of the parasite population and provides unfavorable conditions for increase in the spring.

Two parasites, Lixophaga diatraeae from Cuba and Basses stigmaterus from Peru, have become established in Florida. These, especially Lixophaga, have been responsible for a considerable degree of control in the Felsmere area.

The oriental fruit moth, which came to this country from Japan, is a destructive pest of peaches in the eastern half of the United States. A large-scale program was undertaken in the 1930's to import its natural enemies from Japan and Korea, More than 20 species of parasites were imported and colonized throughout the infested area. Several showed promise during the season of release, but winter conditions were unfavorable to them and they declined and disappeared in a few years. Only one species has been able to maintain itself, and that in small numbers in one locality in New Jersey.

Before undertaking the importations from the Far East, entomologists knew that a native parasite, Maerocentrus ancylivorus, frequently attacked the pest in New Jersey and Delaware. Its normal host is the strawberry leaf roller, but the new pest was just as suitable. Investigations revealed that the parasite, although limited in its distribution, was adaptable to most of the area infested by the fruit moth. Colonization in the infested orchards in the spring reduced the later fruit injury as much as 80 percent. That benefit continued year after year. Large numbers were reared and distributed widely. It was the most dependable means of control before the development of the new insecticides. It is one of the few instances in which a native parasite has proved to be effective in combating an introduced pest.

Mexican bean beetle.