The Mexican Fruit Fly

P. A. Hoidale.

The Mexican fruit fly is an international traveler that pays no attention to boundary lines and is as unwelcome abroad as at home. Each fall and winter large numbers of the flies move from northeastern Mexico to southern Texas and infest grapefruit and oranges in groves in the Rio Grande Valley. Shipments of infested fruit can easily spread the fruit flies over wide areas and cause infestations in fruit-growing regions. It is therefore imperative that every precaution be taken to prevent their spread from the rather isolated infested area in southern Texas.

The Bureau of Entomology and Plant Quarantine is charged with that responsibility. Over a period of several years, through its quarantine regulations, it has supervised the movements of citrus fruit from the lower Rio Grande Valley of Texas to the rest of the country. Although many thousands of carloads of fruit are shipped from that area annually, the Mexican fruit fly has not been able to spread outside the regulated area. The area regulated in Texas on account of the fly consists of the southern part of Jim Wells County and all of Brooks, Cameron, Dimmitt, Hidalgo, La Salle, Webb, and Willacy Counties.

The Mexican fruit fly is the principal citrus-infesting fruit fly native to Mexico. Entomologists first believed it was a native of the Tropics and had spread northward, but research has indicated that its original home was in northeastern Mexico in the States of San Luis Potosi, Tamaulipas, and Nuevo Leon. With the coming of modern transportation and through its own migratory habits, it has spread throughout most of Mexico to southern Texas and as far south as Panama. It destroys large quantities of fruit in Mexico and Texas annually and is a serious threat to other American fruit-growing sections. Not being a tropical insect, it can stand freezing weather and still live and infest fruit. It can adapt itself to a dry country as well as to rainy areas, mountains as well as the coastal plains.

The adult fly is a brightly colored insect with beautifully marked wings. It is considerably larger than a house fly, but the female particularly is noticeably different from flies of that group. The ovipositor sheath of the Mexican fruit fly is nearly as long as its thorax and abdomen. The ovipositor proper is a sharp, needlelike organ, capable of depositing eggs even beneath the thick rinds of citrus fruit. When the gravid female is ready to lay her eggs she selects suitable fruit on the tree and deposits I to I or more eggs in the pulp of the fruit. The incubation period of the eggs, length of larval life, and days spent in the pupal stage vary with the temperature, variety of fruit, and other factors. The shortest period from egg to adult appears to be about 36 days, while the maximum is close to 150.

Besides tropical fruits like the sapodilla, white sapote, and mangoes, the fly infests oranges, grapefruit, apples, peaches, pears, pomegranates, quinces, and yellow chapotes in the field. The yellow chapote, Sargentia greggi, is the wild host of primary importance in Mexico. It abounds along the mountain ranges in San Luis Potosi, Tamaulipas, and Nuevo Leon. It is a close relative of citrus and resembles an orange tree in size, shape, and color of foliage. The mature fruit is nearly the size of an olive and is single-seeded. The seed constitutes most of the fruit, which is said to be edible but is of small economic importance in Mexico. The plant is generally distributed for a distance of over 200 miles along the mountains wherever moisture conditions are favorable. It provides an important reservoir from which flies originate and migrate to establish infestations in Texas citrus groves.

In southern Texas, the flies begin to move into the groves in numbers in the late fall. The influx of files continues during the winter and the infestation builds up to its height in late March and April. Then it begins to diminish. By early summer infested fruit is difficult to find. Some infestation may be present throughout the summer, if fruit is left on the trees, but it frequently happens that no infestation can be found in a grove in June that was heavily infested in April. The operation of a large number of traps over a period of years has established that there is a general exodus of flies from the groves in Texas in the late spring. We do not know just where the flies go. That they may leave the groves in search of their preferred wild host, the yellow chapote, is one possibility, but it may be that the flies do not prefer an environment where there is no opportunity for oviposition and that the harvesting of the fruit causes them to move out in search of a place to lay their eggs. Because the flies do not move into the groves until late fall and as they leave in the late spring, Texas citrus plantings are relatively free of infestation over a large part of the year.

The fruit flies prefer heavily foliaged groves in which to rest as well as to infest fruit. No grove is immune to infestation, but larvae are rarely found in fruit on young trees or trees with sparse foliage. Flies apparently prefer to lay their eggs in fruit on the lower branches. Few larvae are ever found in fruit high in the tree or on outer exposed branches. Flies do not normally infest fallen fruit.

TRAPS ARE SET in groves in the Rio Grande Valley primarily to determine when the migration period begins and ends. Traps do not control the fruit fly, but trapping records form an excellent basis for forecasting the probable amount of larval infestation that can be expected and when the first-treatment period will begin. The records of their operation also have been beneficial in working out the life history of the fly in Texas. Without them it is doubtful if its migratory habits would have been known.

Fruit flies are attracted to various lures for feeding purposes, but no substance has been found that is outstandingly attractive to either sex of the Mexican fruit fly. As a result of work done by A. C. Baker and others, the lure that was found to be the most practical for use in Texas groves comprises 6 pounds of brown sugar dissolved in 5 gallons of water. Traps are filled with the bait as soon as the mixture is made. It is not necessary to wait for fermentation to take place, although fermentation apparently enhances its usefulness.

The trap used in Texas, an adaptation of a Mexican house fly trap, is made of glass. It is somewhat bottle-shaped, with a concave bottom which forms a receptacle for holding the bait. The insects enter through a large opening in the center of the bottom. They are removed through a small opening in the top.

The fruit flies most frequently are trapped in heavy shade. More of them are caught on the outer rows of the grove than in the center. Traps set in trees planted along irrigation canals and close to windbreaks usually take more flies than those set in trees with sparse foliage, where they are exposed to high winds.

Traps are usually set in groves in groups of 20. They are placed about shoulder high and as near the center of the tree as possible. They should not be in contact with foliage but be visible from all sides. They are set out in the grove with at least one line of them near an outer row. They are examined and rebaited once a week. One inspector can examine and rebait approximately 200 traps daily. About 8,000 traps are in constant use in the Rio Grande Valley.

CITRUS FRUIT GROWN in the Rio Grande Valley is inspected in the field before it is certified for shipment. Because the northward movement of flies from Mexico to Texas blankets the whole citrus area, all groves in Texas are considered to be infested with the Mexican fruit fly and all the fruit produced in the area is subject to inspection and treatment and certification before shipment.

The field inspection of citrus fruit consists of examining the fruit on the tree for off-color and cutting all fruit on the tree or the ground that shows any signs of insect injury. Grapefruit usually shows no outside evidence of being infested other than taking on a slight orange color. Oranges, however, may develop an enlarged brown spot, which shows where the larvae have worked within a segment. Newly hatched larvae are hard to find, but their presence frequently is betrayed by small brown spots in the rag underneath the skin on each end of the fruit. Examination includes cutting off a small part of each end of the fruit. If no telltale signs of infestation are visible, no further examination is made. If the small brown spots are visible, larvae, with rare exceptions, are found within the fruit. Fruit in which the female fly has laid her eggs soon falls from the tree, and the larvae complete their development before going into the soil to pupate. Before leaving, the larvae burrow through the fruit and make it wholly unfit for food.

FRUIT- STERILIZATION methods have been developed that will prevent the shipment of infested fruit and still permit the movement of Texas citrus fruits without danger of establishing infestations elsewhere.

When the first infestation was discovered in a grapefruit planting near Mission, Tex., in April 1927, the presence of such a dangerous fruit pest in American groves was viewed with great concern by many growers. Quarantine measures were promptly promulgated to control the shipment of fruit and to eradicate the pest. It was hoped that eradication could be effected within a few years, but the migratory habits of the fly were not known at that time. It was soon found that there was no way of preventing flies from moving into Texas citrus groves from northeastern Mexico and that eradication was impossible. Research efforts were then directed toward developing ways and means of treating Texas fruit which would permit it to be shipped safely to other fruit areas.

One method of sterilization consists of lowering the inside temperature of the fruit to 32 -33 F. and holding it there for 18 days, or keeping the temperature at 33 -34 for 22 days. Either will kill any larvae in the fruit.

The second method is more economical and more widely used in Texas. Known as the vapor-heat process, it involves raising the inside temperature of the fruit. Larvae can be killed much more quickly with high temperatures than with low temperatures. In order to treat large quantities of fruit properly, specially designed rooms are necessary; the rooms also can be used during the regular packing-house procedure for the coloring of fruit. Treatment consists of forcing a large volume of air, saturated water vapor, and water in the form of a fine mist through the load of fruit, at a temperature of not less than 110 . After the inside temperature of the fruit has been raised to that point, it is maintained there for the duration of the holding period. The process is then reversed and a large quantity of dry air is forced through the load of fruit in order to reduce the temperature as rapidly as possible and permit the fruit to be packed for shipping.

P. A. HOIDALE began his career in the Department of Agriculture with the Bureau of Plant Industry in April 1915. In 1917 he transferred to the Federal Horticultural Board in the control of the pink bollworm. When the Mexican fruit fly was first found in n the Rio Grande Valley, he was placed in charge of the control project.