Popular Science Monthly/Volume 65/August 1904/The Discovery of the Native Home of the San Jose Scale in Eastern China and the Importation of its Natural Enemy
|THE DISCOVERY OF THE NATIVE HOME OF THE SAN JOSE SCALE IN EASTERN CHINA AND THE IMPORTATION OF ITS NATURAL ENEMY.|
By C. L. MARLATT,
U. S. DEPARTMENT OF AGRICULTURE.
THE insect which has had the greatest international importance and has been the subject of more interstate and foreign legislation than all the other insect enemies of plants together is a Chinese bark-louse of deciduous trees, known from its first point of colonization in America as the San Jose scale. This insect has been so thoroughly exploited in the publications, scientific and popular, in this country, and in horticultural and agricultural journals, that a general account of it is not necessary. It was first discovered in San Jose, Cal., in the grounds of Mr. James Lick, in the early seventies. It soon spread throughout California and the Pacific coast and became the most notable enemy of such fruits as the pear, apple, peach, prune and certain small fruits. Its name of perniciosus was given it by Professor Comstock, who studied it in California in 1880 and reported it to be the most destructive scale enemy of fruits known to him. It has more than maintained its reputation in this regard since that time.
Up to 1893 it was only known on the Pacific coast, but in that year it was discovered in a small orchard in Virginia, and the investigation which followed developed the fact that it had got into some large eastern nurseries a number of years before on plum trees from California, and had been spread from these nurseries unwittingly over much of the southern and eastern states. The damage which soon developed from this scale insect in the orchards of peach, pear and apple of the east aroused very considerable excitement, and the alarm thus caused was transmitted to foreign countries with the result that, beginning with Germany, one after another of the European powers adopted measures prohibiting the importation of American plants and fruits, or requiring rigid inspection before such admission. Canada adopted similar restrictions, and even such remote countries as the Cape of Good Hope, New Zealand and Java followed suit. Within the different states of the union also various prohibitions on traffic in nursery stock and plants were put in operation. Since 1893 this scale insect has steadily extended its range in the United States and also in Canada, where it soon gained foothold, and it now occurs in practically all the important fruit districts of North America. It has not gained lodgment in Europe so far, and the chances arc that it' it should do so the climate of Europe is so unfavorable that it would not be nearly so mischievous there as in America.
The explorations, which are briefly narrated in this paper, wen; undertaken to discover the native home of this scale insect, which was, prior to 1901, a mere matter of conjecture. The desirability of discovering the origin of this scale pest arose from the now well-known fad that wherever an insect is native it is normally kept in check by some natural means, and as a rule by some predaceous or parasitic enemy. In the chance importation of foreign destructive insects to our shores it very often happens that the natural check or enemy is left behind, and the imported pest becomes in consequence much more injurious here than in its native place. In a number of notable instances in this country very great benefit has been derived by the discovery and introduction of such natural enemies, thus reproducing the conditions which obtain in the native home of the injurious insect. The fluted scale and the Australian ladybird in California is the most notable instance. The importation of this ladybird from Australia has made citrus growing possible in California, and saves annually many millions of dollars to that state. This and other similar cases indicated the desirability of discovering the native home of the San Jose scale, and the importation, if possible, of whatever natural means were found there keeping it in check.
Prior to this investigation there was a pretty well founded belief, shared by Dr. Howard and the writer, that the San Jose scale was a native of eastern Asia. "Without going into detail, this belief was based chiefly on the ground that most other quarters of the world had been fairly well investigated without any evidence of this scale insect being found. It was known to occur in Japan, but the evidence rather indicated that it had been recently brought to that country from the United States in connection with the large shipments of nursery stock from California to Japan during the last twenty-five or thirty years. The itinerary, therefore, planned by the writer, with the advice of the chief entomologist of the Department of Agriculture, Dr. L. 0. Howard, was to include Japan, China and any other countries in eastern Asia which it should prove desirable to visit. Six months were devoted to a very thorough exploration of the different islands of the Japanese empire, and three months to China, with shorter periods in other regions. The explorations in China and Japan are the only ones which bear especially on the San Jose scale problem.
Explorations in Japan.
During the time spent in Japan, from April to September, 1901, the writer visited some forty-two provinces, and explored all the principal islands, representing a stretch in latitude the equivalent of from northern Maine to Florida. Altogether these explorations enabled him to make a pretty correct judgment on the San Jose scale problem in Japan. Japan is not especially a horticultural country. Her comparatively enormous population of 46,000,000 compels the growth of cereals and other necessities of life wherever possible. Very little land, therefore, is devoted to fruit raising, and fruits are considered as luxuries. Nevertheless, practically every dwelling house in Japan has a little dooryard or kitchen garden in which are single examples of cherry, plum, peach, persimmon and other trees. Furthermore, the roadways and temple grounds and streets are lined with cherry and plum trees, planted for bloom and ornament and not for fruit. There are orchard districts in Japan of limited extent. In northern and central Japan there are a few peach orchards and a few orchards of native pears, and in southern Japan, small orchards of orange, pomelo, walnut and other fruits. In old Japan the chief deciduous fruit is a native pear grown in small patches of from a fraction of an acre to two or three acres in extent. These are trained low on overhead trellises, and at a short distance look like vineyards. There are several districts where such orchards occur in considerable numbers. These orchards are very ancient, many of them having trees more than one hundred years old. If the San Jose scale were native to Japan it would occur in these pear orchards, the pear being one of the favorite food plants of this scale insect.
In northern Japan, including the island of Hokkaido, and the northern end of the main island, Hondo, apple raising has been introduced in modern times very much on the lines of this country. Prior to the opening of Japan to foreign commerce and exploration, the apple as an edible fruit was unknown in that country. The orchards in northern Japan arc chiefly, therefore, of American origin and represent American varieties. Most of the stock came from California, and much of it was undoubtedly infested with San Jose scale when it was received. There is, therefore, throughout these northern apple orchards, a mild infestation with this scale. The Japanese are very enthusiastic in their efforts to gain all the benefits of western civilization, and this is shown in horticultural as well as in other fields. The three leading nurseries, therefore, of Japan have been very active during the last twenty or thirty years in importing the different varieties of pear, peach and apple from America, and all three of these nursery districts have become infested with San Jose scale, evidently from such importations from California, where the scale has been widely distributed for thirty years. Outside these nurseries, however, in central and southern Japan, the San Jose scale did not occur, except where it had been introduced on new stock from the nurseries referred to. The old native pear orchards were free from scale, except where replants had been made of American varieties, or new native stock, to fill in breaks in the orchards. The infestation was very often just beginning and immediately surrounded the replants. In all Japan, therefore, in the little house gardens and temple grounds where were cherry, plum and other trees suitable for San Jose scale, this insect did not occur, except where the evidence was very plain of its recent introduction as indicated. Without going into details of the evidence, it is sufficient to say that the conditions in Japan are essentially the same as in this country. The San Jose scale is a recent comer. It was, in fact, not known in Japan prior to the year 1897, when its presence there was first determined, but it has now been scattered pretty widely by nursery stock, exactly as in this country, and occurs under similar conditions; in other words, only where it has been recently introduced. The investigation showed very distinctly that Japan could not be considered responsible for the San Jose scale.
Explorations in China.
Investigations up to this point, while freeing Japan from the onus of giving the San Jose scale to the world, left the problem unsettled as to the original home of this insect. China remained as the most likely place of origin, and the writer proceeded to China to continue his explorations there. While in Japan a good deal of information was gained relative to fruit conditions in China, from English, German and American residents who were spending the summer months in Japan to escape the rather trying climate of China. In brief, it may be stated that deciduous fruits are grown from the Shanghai region northward, the peach being practically the only fruit grown to any extent about Shanghai. The great apple district of China is the region lying back of the city of Chifu in the north. This apple-growing industry was started many years ago by a missionary, Dr. Nevius, and has assumed very considerable proportions and covers a good deal of the province of Shantung. Below Shanghai, the orange and other subtropical fruits replace the deciduous varieties. North of Chifu native fruits only are grown, consisting of the native pear and peach, and such wild fruits as wild crab apples and an edible haw apple.
A very considerable exploration of the country lying immediately back of Shanghai was made in the course of a long house-boat trip. A
great many peach orchards were examined and a good deal of miscellaneous fruit and other plants growing about house yards were inspected. Nowhere was there any evidence of the San Jose scale, nor were scale insects of any sort much to be seen. The climate of this region is unfavorable for such insects and they are normally killed out by fungous disease. The writer afterwards proceeded by boat to Chifu—a five-day ocean trip from Shanghai, and made a considerable exploration throughout the apple orchards of this region on horseback, visiting, among others, the original orchards planted by Dr. Nevius. In all these the San Jose scale was found scatteringly present, not, however, doing any special damage, and probably not enough to be noticed, if its possibility for evil was not so well established. The presence of the San-lose scale in this region did not, however, have any special significance, since much of the original stock was obtained front California, and doubtless from nurseries which were infested with the scale.
The journey of exploration was continued northward to Tiensin and Pekin. In this region the San Jose scale was also found on native plants, including the flowering peach, a tree grown for ornament solely, and not for fruit, and notably on the native fruits in the markets in these cities.
The markets of Pekin were of especial interest in this connection. Pekin is the center and market for all the region lying to the north and west, and the streets devoted to the sale of fruits and other products in the Chinese city are one of the great show places. The fruit and nut products are brought into Pekin in little two-wheeled carts, or more generally on camelback, great caravans of heavily loaded camels and streams of carts constantly entering the city with the products of the outlying provinces. One finds, therefore, in the markets of the Chinese city the fruit products of all northern China, and can study them at ease. All the district lying between Pekin and the great wall, north, and west, and east, has been most carefully explored and mapped by the foreign military authorities. From various individuals employed in this minute survey a great deal was learned relative to the fruit growing in the district indicated. Much of the fruit found in the markets of Pekin comes from the hill region leading up to the mountains separating China from Mongolia and Manchuria. These fruits are native apples, pears and peaches, and the little haw apple already mentioned. Great quantities of these fruits were examined in the market, with the exception of the peach, which
was then out of season, and later similar examinations were made at Tiensin. A very scanty but general infestation with San Jose scale was found on the different fruits examined. Perhaps one apple in a hundred would have a few of these scales about the blossom end and the same proportion was true of the haw apple and the native pear. Throughout the region where these fruits are grown, there has been no introduction of foreign stock. The occurrence of the San Jose scale on these two fruits was conclusive evidence that in the region whence it came the San Jose scale is native. The scattering occurrence of the scale also indicated, as would be anticipated, that this pest in its native home is kept in check by natural means.
The investigations made at Shanghai, and later southward to Hongkong, the Malay Peninsula and Java, indicated that the San Jose scale in eastern Asia can not survive below Shanghai.
The special district where it is native and thrives is a fairly well shut-off region, which probably accounts for the failure of this insect to become a world pest ages ago. This district is the region leading up to the mountains and comprising the northern and northeastern frontiers of China proper. Beyond the great wall on the north and west lies Mongolia, consisting chiefly of the vast Desert of Gobi. To the northeast and separating the region from Manchuria and Corea is
the eastern Gobi desert. To the south and east lies the great alluvial plain, the product of centuries of mud carried down by the Yellow River, a region where cereals only are grown. These are all effective barriers, and especially so when considered in connection with the political conditions of the past. We have, therefore, as the original home of this insect a naturally shut-off area from which it could not easily escape under the conditions prevailing up to our own times.
The means by which the San Jose scale came from China to America is a matter of interest. This pest reached California on trees imported by the late James Lick, a gentleman who was an enthusiastic horticulturist and an energetic importer of plants from foreign countries. It is the writer's belief that Mr. Lick imported from China,
possibly through Dr. Nevins, with whom tie was probably in correspondence, the flowering Chinese peach, and brought with it the San
Asiatic Ladybird (Chilocorus similis), oviposition and early larval stages; a, beetle in act of thrusting egg beneath scale; b, scale slightly raised, showing edge of egg beneath; c, scale lifted from bark, showing manner of attachment of egg to the inner surface; d, view of egg in the scale; e, egg magnified to show sculpturing; f, three eggs placed under flap of bark; g, same, natural size; h, i, dorsal and lateral views of newly hatched larva; j, larva, first stage, feeding on mature and young scales—all enlarged except g.
Asiatic Ladybird (Chilocorus similis), later larval stages, pupa, and adult insect; a, second larval stage; b, cast skin of same; c, full-grown larva; d, method of pupation, the pupa being retained in split larval skin; e, newly emerged adult not yet colored; f, fully colored and perfect adult—all enlarged to the same scale.
Jose scale to his premises. Undoubtedly this scale insect came to this country in some such way on ornamental stock from North China.
The Asiatic Ladybird, the Natural Enemy of the San Jose Scale.
Throughout the region investigated in China where the San Jose scale occurs the natural agency keeping the scale in check was evidently a small ladybird (Chilocorus similis), which was feeding voraciously upon it wherever the scale was found in any numbers. A considerable quantity of these beetles was collected in both Japan and China and sent by mail to Washington. Unfortunately, most of the specimens died in transit or during the first winter. Two individuals, however, survived, and during the first summer from these two some five thousand or more beetles were secured. The original stock was kept in cages, but later on the insects were liberated in an experimental orchard attached to the insectary of the department. During the first summer a considerable number of colonies were sent out to various states placing them in charge, in the main, of the entomologists connected with the state experiment stations. During the summer of 1903, the second one since the introduction of this insect, some thirty or forty additional colonies were distributed. A good many of these colonies were liberated under rather unfavorable conditions, and the beetles probably perished. The best success has come from certain colonies sent to Georgia. One of these, located at Marshallville, presents a most satisfactory outcome. This orchard contains some 17,000 peach trees, covering about 85 acres, and has lying immediately adjoining it a much larger orchard belonging to the same owner, containing 250,000 trees, all scatteringly infested with scale. The ladybirds were liberated in August, 1902, in the smaller orchard. An examination of this orchard in July, 1903, indicated that the beetles were rapidly spreading and that they would soon cover the smaller orchard. A rough estimate at this time of the number of ladybirds in all stages places the total at somewhere between 25,000 and 40,000. In Georgia the beetles evidently continued breeding up to January. There is, therefore, in this orchard at least, a very flattering outlook for good results from the imported beetle. Other colonies placed where there were only two or three or but a few trees have not yielded very satisfactory results. This outcome is not especially surprising, as under such circumstances the beetles are very apt to fly away and become lost. We are endeavoring, therefore, to place colonies where there are rather Large bodies of trees infested with scales. After the beetle becomes once well established, distribution can be more general, but until this end is reached it is probably wiser not to waste material in sending specimens for colonization to small orchards or gardens or for liberation on a few trees. Tn Japan and China where the Chilocorus occurs rather generally, it finds food for itself in every dooryard, the white peach scale being very widely distributed in those countries and the San Jose scale also in northern China and portions of Japan. In this country, on the other hand, the San Jose scale is practically its only host insect, and in spite of the wide dissemination of the San Jose scale, it still occurs after all in a very scattering way in orchards here and there, with often twenty or thirty miles between places' of infestation. This imported insect can not, therefore, multiply and extend itself as rapidly and naturally as would be the case if the San Jose scale occurred more generally. For this reason it will be necessary to distribute the imported beetle artificially for some time. In Georgia, Alabama and other regions where there are general orchard districts it will undoubtedly be able to take hold much better than it will in regions where orchards are more scattering and of smaller area. Ultimately it is hoped that it will become established in America and have the same beneficial action which it now has in China and Japan. It probably will not be a complete remedy for the San Jose scale for the reasons already indicated, but if it will keep the San Jose scale in check as much as it does in its native country it will be of very decided service. One of its chief advantages will be the fact that it ultimately will take hold of the scale in many small orchards and gardens, the owners of which would be indifferent and would not undertake remedial operations, and thus furnish centers for additional reinfestation.
The importation of this insect can not work anything but good. It feeds only on scale insects, and ultimately may feed on certain of our native species as well as on the San Jose scale. It is a most voracious feeder, and has been observed to eat as many as five or six scale insects a minute, and even an average of but one a minute would give a total of 1,440 scale insects destroyed per day. The appetite of the larva seems never to he satisfied, and it is eating practically all the time. The adults also feed actively on the scale.
The chief drawback to this importation is the fact that several of our native predaceous insects have at once acquired a rather decided liking for the larvæ of, the Chilocorus. A parasitic insect of our native ladybirds has also been attacking it in considerable numbers. These predaceous and parasitic insects may be sufficient to prevent this imported ladybird from giving the benefits which it ought. They have been especially in evidence in the Washington colony, but do not seem to have done any considerable damage in the more southern colonies referred to, and it may he that the ladybird will not suffer materially from them.