Quarantines

How Insects Gain Entry

Ralph B. Swain.

Most of our major insect pests of foreign origin have been brought into the United States by man and chiefly aboard ship. From Mexico and Canada some have come by natural means. An understanding of the ways in which they enter is the basis for sound quarantines.

Insects, like other organisms, occupy well-defined niches in nature. Environmental factors and innate characteristics tend to restrict them to certain areas of the earth. But the variability in animals in time permits individuals of a species to adapt themselves to slight changes in their surroundings and acquire new food habits and immunities to diseases, extremes of climate, and insecticides. We therefore can expect that certain species with centers of distribution in Mexico or farther south, by normal dispersal of the adult insects, eventually will spread north of the Rio Grande, and that certain Canadian species (or European or Asiatic forms that become established in Canada) will descend from the north.

That has happened. The boll weevil and the harlequin bug are two of many injurious insects that have worked northward from Mexico as agriculture reclaimed parts of the southwestern desert, once an effective natural barrier to them. The elimination of the desert barriers has been accelerated with the building of great new irrigation projects.

The satin moth, a European species, spread from British Columbia into Washington and Oregon. It was discovered in eastern Canada and in New England at about the same time, and in that regard has had a parallel history with other forest pests that had no respect for the Canadian border. The gradual dissemination of a pest through adjacent or nearby fields of host plants is much more difficult to combat than are incipient infestations well separated from population centers.

AIR CURRENTS and storms bring insects to us from Caribbean islands, Mexico, Central and South America, and Canada. We have evidence that the pink bollworm moth is brought into the border districts of Texas and New Mexico each year by winds from Mexico. The Mexican fruit fly enters citrus groves in the lower Rio Grande Valley every year, either flying or being wind-borne from south of the border.

Studies of the insect fauna of the upper air have shown that such weak-flying insects as aphids, psyllids, and leafhoppers are found at altitudes up to 14,000 feet. It is conceivable, therefore, that both native and introduced insects from the West Indies, Mexico, and places farther south may be borne to our shores by winds.

Other insects migrate annually, flying on strong wings from tropical areas and no doubt assisted by prevailing winds. One such is the cotton leafworm moth, a widespread pest in South America, which moves northward into the United States and in a few generations may even reach Canada. As with the Mexican fruit fly, a winter destroys our populations of cotton leafworms, but next season a new invasion may be expected. No way exists to keep such migrants out of the country short of controlling them in the lands whence they come.

People who have tramped Florida beaches in the wake of great storms know that in the driftrows are bits of vegetation, seeds, even large branches and logs, which obviously traveled from West Indian islands. Many additions to the Florida insect fauna, especially wood and seed-boring species, must have arrived in such sea drift. Possibly living insects can travel even greater distances in floating plant debris, but by and large the seas are excellent barriers to natural spread, and insects, especially the immature stages, have small chance of survival when deposited on a wind-swept and wave-washed beach, far from food plants and other requisites for their existence.

Man also might introduce insects from contiguous land areas in personal baggage carried by pedestrians or in cargo or personal effects aboard automobiles, trucks, trains, and airplanes. An automobile or truck can transport insects in various ways as adult stowaways or hitchhikers inside the vehicle; as adults, immature stages, or eggs in and on plants, fruits, seeds, and plant products; and as eggs or larvae in mud or dirt caked under the body and fenders, on wheels, or in tire treads.

SOME INSECTS TRAVEL from country to country on or in the bodies of man and his domesticated animals, or on animals brought in for food, processing, or exhibition. Various kinds of lice and mites may be on the bodies of travelers. The important species probably are already of world-wide distribution. It is possible that fly larvae of the sorts that may live in the flesh of man or invade the digestive and respiratory tracts could be brought into the country from abroad, although it is likely that such a sufferer would be detected by Public Health officers.

Most of the insects and mites likely to be introduced with domesticated or wild animals are already cosmopolitan, but some serious pests of domestic fowl have been discovered in this country in recent years. Migrating wild birds carry insect parasites on their bodies from one continent to another, but here again the species involved are not new to our fauna.

Insects certainly were among the first stowaways on sailing ships. Our various roaches of African origin arrived aboard slave ships. A large percentage of our household and storage pests, such as insects affecting fabrics, wood and wood products, and stored foods, came from other lands aboard ships in which they were able to maintain themselves and even breed in litter or filth. On the faster ships of today it is easier for insects, even rather delicate ones, to survive an ocean passage. The monarch butterfly and others have traveled from the continental United States to Hawaii and Europe as stowaways, and European forms have come to us in the same way.

Many factors work against the successful establishment in this country of an insect arriving as an adult stowaway aboard ship. Its chance for survival, particularly in the larger port areas, may be relatively slight even if it succeeds in gaining the shore. Much depends on the sex of the insect, its distance from food plants and good egg-laying sites, and on prevailing weather. From the standpoint of quarantine enforcement, it is fortunate that the commoner stowaways are already of nearly world-wide distribution and hence of significance from the standpoint of control rather than exclusion. The problem of ridding a ship of stowaway insects is one that has not been solved satisfactorily. No easy solution is in sight.

THE AIRPLANE potentially is a major distributor of insect stowaways because of its speed. Almost 3,000 species belonging to 293 families and most of the orders of insects, many of them alive, have been intercepted inside aircraft. Partly because of the precautions now taken to disinsectize airplanes from areas of greater pest risk, it is not possible to make a long list of economically important airborne insects. But at that the list is long enough to be distressing. The oriental fruit fly, one of the most destructive pests ever to strike the Hawaiian Islands, gained entrance during the Second World War at a time when proper precautions regarding military aircraft from the Marianas could not be taken. At Brownsville, Tex., a living Japanese beetle was found in a military plane that had arrived from New York City by way of Panama; evidently it had hitchhiked from an airport in the metropolitan area. We have two records of tropical, malaria-carrying mosquitoes taken near military airports in Florida. In one instance, a living larva was taken from the canal in which the stowaway parent evidently had laid its eggs.

Adult insects travel almost exclusively within the fuselage. The exterior surfaces and wheel housings are of no importance as carriers. It is thus relatively easy to eliminate insects aboard aircraft by properly applied combinations of space and surface insecticidal sprays. An interesting though minor problem has been posed by the egg masses of several species of moths found on outside surfaces. Possibly the moths, night fliers, were attracted to the bright exteriors of the craft as they rested on illuminated aprons or simply flew toward lights aboard. Frequently such eggs are alive. Even freshly hatched caterpillars have been found crawling about. Lights inside parked aircraft attract insects just as they do elsewhere and often are responsible for hundreds of insects finding their way in through open doors, hatches, and windows.

Military movements, especially during war, can undo in minutes what quarantines and control programs have accomplished in years. Then expediency rules, the usual civilian control may be impossible, and invading armies usually have little regard for the regulations of an enemy country. The Hessian troops that landed at New York during the Revolutionary War are believed to have brought the hessian fly in their straw bedding. It is well that we have never since suffered a large-scale invasion of foreign troops. Americans today are fortunate that the hazards to health and agriculture resulting from the bringing of foreign insects to this country are appreciated by the military branch, which cooperates with our quarantine agencies.

SHIP'S BALLAST has brought many insects from other lands, sometimes with the seeds or roots of their host plants. Ballast is of little consequence as a hazard if dumped far out at sea, but it can be dangerous when used for fills in a port area. Most modern ships no longer use earth ballast instead they use water, which can be disposed of more inexpensively by pumping into the harbor at the end of the voyage. Earth as ballast is still in use, however, and at one time was used almost exclusively. It may consist of beach sand, gravel, rock, or topsoil. The soil would be most apt to contain insects, since it may hold any of thousands of soil-inhabiting species in all stages of development and the eggs and larval stages of innumerable others, which as adults live above ground. Quarantine inspectors, working directly with shipping companies, make recommendations and arrangements for the proper disposal of dangerous ballast.