Insects on Fruit

The Oriental Fruit Fly

Walter Carter.

While making routine observations on the Mediterranean fruit fly in May 1946, Mabel Chong, a member of the Territorial entomology staff of Hawaii, noticed a stranger in the collection. The strange fly was Dacus dorsalis, the oriental fruit fly. It soon became evident that the newly arrived immigrant was thoroughly established on the principal Hawaiian Islands.

Within 2 years it became a major pest of almost every economic variety of fruit in the Islands. It was also found in many wild fruits and berries. It even invaded fruit stands inside stores and laid its eggs in imported fruits. Some garden flowers, including orchids, attracted hordes of the flies.

It is an attractive, clear-winged fly about the size of a house fly. Its body is light brown with bright-yellow markings on its middle section, or thorax. The female has a long egg-laying organ protruding from the end of the abdomen.

The fruit is "stung" with this ovipositor and eggs pass through it into the tissues below the surface. The eggs are laid in groups. Many different females may use the same sting hole for oviposition.

After about 2 days, depending on temperature, the eggs hatch. White, legless maggots emerge and begin to feed on fruit, in which decay soon starts. The maggots pass through three growth stages in about 10 days before they leave the fruit and enter the soil to pupate. They rarely pupate on the fruit if it remains on the tree. The pupa is enclosed in a brown cylindrical puparium. This stage of the insect's life, a quiescent one, lasts 8 to 12 days. Lowered temperatures markedly affect the length of the pupal stage. The adult fly, which emerges, is not sexually mature, but at the temperatures normally prevailing becomes so in 6 to 10 days. The adult life of the fly is conditioned by so many factors that it is difficult to set a normal longevity. Flies maintained at high levels of nutrition and fully active live much shorter lives than those kept at lower temperatures and on a low-grade diet. A fully active female probably has a normal life of about a month.

The losses the flies cause are of three kinds. The first is reduction of grade caused by the stings, which spoil the appearance of the fruit. This can occur in many fruits that are not favorable for complete development of the fly. Second, the entire fruit can be lost from damage by maggots and resulting decay. The third, an indirect loss, comes from quarantining any infested area to prevent dissemination of the fly.

ACTION AGAINST THE FLY was promptly taken by Territorial and private agencies. Since control of insect pests by means of parasites and predators has become traditional in Hawaii, the biological method received first attention. Work was started by the Territorial Board of Agriculture and Forestry in 1947 and extended in 1948. A cooperative project added the resources of the Hawaii Agricultural Experiment Station, the Bureau of Entomology and Plant Quarantine, and two private research institutions, the Experiment Station of the Hawaiian Sugar Planters' Association and the Pineapple Research Institute of Hawaii.

Agricultural interests in California had become thoroughly alarmed, and in 1948 a committee from the California legislature visited Hawaii. The committee returned to California convinced that the situation presented grave implications to the agriculture of the mainland.

In the meantime, the depredations of the fly reached a climax of economic loss when the flowers of the orchid Vanda Miss Joaquim were found infested with eggs. Shipments of this flower and, as a precaution, all other Vanda flowers were stopped by embargo. The orchid industry had begun to assume considerable importance in Hawaii, and its abrupt cessation pointed up the necessity for more scientific work on the problem of the fruit fly.

Congressional appropriations were made available on July 1, 1949, in addition to funds from the Research and Marketing Act. The Bureau of Entomology and Plant Quarantine proceeded to assemble its enlarged staff in Hawaii. California made an appropriation for research in Hawaii and in California, and enlarged contributions from Territorial agencies also became available for a concerted attack.

When a new insect arrives in an area and follows the typical pattern of such arrivals in a favorable environment and without hindrance from parasites, a research program must attack the problem from all practicable angles. The approach to the problem in Hawaii appeared to offer five possibilities: Biological control, chemical control, area control, ecology-biology, and commodity treatments. A group began work on each aspect. Later a project in insect physiology was set up.

By far the greater part of the cooperative investigations are for the benefit of mainland agriculture. We hope the biological-control project will give immediate relief in Hawaii, but even that project, by reducing fly numbers, will help reduce the opportunity for chance migrants to reach the mainland. It will also provide a reservoir of beneficial insects in local fruit, which will be available for use should a mainland infestation be discovered.

Commodity treatments also have an immediate value to Hawaiian economy, although few commodities are of sufficient quantity or quality for export. The success of these treatments no doubt will influence the future production of such agricultural commodities as papayas, since before the first approved commodity treatment there was little incentive for small farmers to produce for export. The greatest value of commodity treatments, however, will be to any infested mainland area whose fruits would have to be treated before shipment.

The chemical-control project is designed primarily as a proving ground for old and new insecticides against this one insect. Its data will be essential to orchardists and other producers on the mainland, should the fly reach there.

Area control is primarily for the development of large-scale emergency techniques, equipment, and methods of application to meet a mainland infestation.

The chief purpose of the ecology-biology project is the study of the effect of climate on the insect to permit a, carefully based prediction of the possible distribution of the fly on the mainland. Should an infestation develop, the data will help in establishment of safe areas, thus contributing immeasurably to the free movement of commodities.

The term ecology-biology requires some explanation. Ecology is primarily concerned with the study of the insect in its natural environment or surroundings. Biology is intended to cover the detailed life history of the insect as studied in the laboratory. All ecology is biology and all good biology recognizes the necessity for interpretation of data in terms of the insect's environments.

In nature, the fly feeds on "honeydew" deposited by other insects and on decaying fruit that has been broken down by micro-organisms. Slight changes in composition of the insect's diet, particularly in protein and vitamin constituents, may profoundly affect egg-laying capacity. For example, flies fed a diet of macerated papaya, honey, and yeast in a laboratory laid a total of 655 eggs, as against 21,538 eggs from the same number of flies fed the same basic diet plus a source of protein and vitamins. One female in the latter group produced 3,062 eggs. Diet also affects the length of time necessary for the fly to develop its eggs. In the basic-diet group, this was from 21 to 71 days; with the fortified diet, from 6 to 14 days.

CALIFORNIA FRUITS have been tested as hosts of the fly. This study is of critical importance if the significance of an incipient infestation in California is to be properly evaluated.

For example, some varieties of plum are very susceptible, others not at all. Cotton bolls, in the young, firm stage, are susceptible. Citrus fruits strongly attract egg-laying females, but very little larval development takes place in them; perhaps the rag of the citrus interferes with the progress of the maggot into the flesh. Apparently citrus is not a good host and is not likely to be responsible for the development of sizable fly populations. Apples also become heavily infested but are poor hosts for larvae, maybe because they rot slowly.

The ideal host is one that is fleshy and succulent, with a firm but not too thick epidermis, and developed beyond the early immature stage. Bruising and damaging sometimes makes a poor host into a good one. More than 150 Hawaiian hosts have been recorded. When large numbers of flies are present, the pressure to lay eggs is terrific. Eggs are laid in strange places: A ping-pong ball punctured with a few pinholes; a knothole in a wall; even on an entomologist's thumb. Host records, therefore, require careful evaluation, especially if serious economic consequences might result.

The orchid Vanda Miss Joaquim and its record as a host is a striking example of the necessity for careful evaluation. Eggs have been found on these flowers and, under very special circumstances, larvae have developed. As a result, an embargo against the flowers coming into the mainland was placed by the Bureau of Entomology and Plant Quarantine. Later, flowers were allowed to come in after being fumigated. A subsequent study has shown that the commercial flower does not present any hazard as it is only in unopened buds, kept in a saturated atmosphere and allowed to rot, that larvae could develop. When this was learned, the requirement of fumigation was discontinued.

Guava is the most important host fruit in the Islands. It covers many thousand acres of gulch land and is responsible for the greater part of the fly infestation. It fruits heavily, and the two main fruiting seasons a year give a carry-over of late fruit to connect them up. It is an ideal host fruit, and in the absence of parasitism or other interfering factors, enormous populations can build up. A biological control for guava fruiting would be of importance in the control of the oriental fruit flies, but recent build-up of parasite populations has reduced the need for such drastic action.