Popular Science Monthly/Volume 45/September 1894/Parasitic and Predaceous Insects
|PARASITIC AND PREDACEOUS INSECTS.|
By C. V. RILEY, Ph. D.
THE importance to man, and especially to the horticulturist, of the parasitic and predaceous insect enemies of such species as injure vegetation has been recognized by almost all writers on economic entomology. Indeed, it is a question whether the earlier writers did not attach too much importance to them, because while in the abstract they are all essential to keep the plant-feeding species in proper check, and without them these last would unquestionably be far more difficult to manage, yet in the long run our worst insect enemies are not materially affected by them, and the cases where we can artificially encourage the multiplication of the beneficial species are relatively few. While fully appreciating the importance of the subject, therefore, it is my purpose in this paper to point out the dangers and disadvantages resulting from false and exaggerated notions upon it.
There are but two methods by which these insect friends of the farmer can be effectually utilized and encouraged, as, for the most part, they perform their work unseen and unheeded by him, and are practically beyond his control. These methods consist in the intelligent protection of those species which already exist in a given locality, and in the introduction of desirable species which do not already exist there.
In a few cases like this there is no reason why the farmer should not be taught with advantage to discriminate between his friends and his foes, and to encourage the multiplication of the former; but, for the most part, the nicer discrimination as to the beneficial species, some of the most important of which are microscopically small, must be left to the trained entomologist. Few of the men practically engaged in agriculture and horticulture can follow the more or less technical characterization of these beneficial species, and where the discriminating knowledge is possessed it can, as just intimated, only exceptionally be turned to practical account.
In other cases much good may be done without any special knowledge of the beneficial forms, but as a result of a knowledge of the special facts, which enable the farmer materially to encourage the multiplication of parasitic species while destroying the plant-feeding host. Very good illustrations of this kind of work are afforded by the rascal leaf-crumpler and the common bag worm, both of which in the larva state live in cases, and are much affected by parasites, and neither of which can survive if the cases are plucked in winter and placed away from any trees or shrubs, while under these circumstances the parasites will perfect and escape.
It is quite different with the second method of dealing with beneficial insects which I have mentioned, for here man has an opportunity of doing some very effective work. It is only within comparatively recent years that the importance of this particular phase of the subject has been fully realized. Various more or less successful efforts have also been made, and the transmission from one place to another of certain parasites of the plum curculio; of certain parasites of the common oyster-shell bark-louse of the apple; the successful colonization in France of a certain mite which attacks the grape phylloxera; the efforts to send parasites of plant-lice from Europe to Australia; the introduction into this country of Microgaster glomeratus, a common European parasite of the cabbage worm, and of Entedon epigonus, a common European parasite of the Hessian fly—are matters of record in State and Government publications.
In 1887 and 1888 the now well-kuown importation of Vedalia cardinalis from Australia and New Zealand to California to prey upon Icerya purchasi was successfully carried out. The history of this striking example of the beneficial results that may in exceptional cases flow from intelligent effort in this direction is now sufficiently well known to American economic entomologists, but anticipating that we shall have foreign delegates among us, and that our proceedings will be published more widely than usual, it will perhaps be wise to give the salient historical facts in the case, even at the risk of some repetition of what has been already published.
The fluted scale, otherwise known as the white or cottony-cushion scale (Icerya purchasi Maskell), is one of the largest species of its family, and up to 1883 had done immense injury to the orange groves and to many other trees and shrubs of southern California. From Australia, its original home, it had been imported into New Zealand, South Africa, and California—the evidence pointing to its introduction into California about 1868, and probably upon Acacia latifolia.
In my annual report as United States Entomologist for 1886 will be found a full characterization of the species in all its stages; but the three characteristics which most concern the practical man and which make it one of the most difficult species to contend with are its ability to survive for long periods without food, to thrive upon a great variety of plants, and to move about throughout most of its life.
The injuries of this insect, notwithstanding the efforts to check it, kept on increasing, and some ten years ago I felt that the work of this particular species and of others which seriously affected the fruit-growing interests of southern California, justified the establishment of agencies there. Up to this time no special entomological efforts have been made by the Government on behalf of the fruit-growers of the Pacific coast. Through agents stationed—the one at Los Angeles, the other at Alameda—a course of elaborate experiments was undertaken as to the best means of treating the insects affecting the orange there, and more particularly this fluted or cottony-cushion scale. During the progress of these investigations, however, the fact impressed itself upon my mind that we had here an excellent opportunity of calling to our aid its own natural enemies; for while there were at first some doubts as to the origin of this icerya, the question was finally settled to my satisfaction that it was of Australian origin, that in its native home it was not a serious pest, but was kept subdued by natural checks.
A clause in the bill appropriating for the division of entomology prohibited the sending of agents abroad and prevented at the time independent action by the Department of Agriculture; but with the co-operation of the Department of State an arrangement was finally made by the Hon. Frank McCoppin, United States Commissioner to the Melbourne Exposition, whereby two agents of the Division of Entomology were sent to Australia, one of them specially charged with the study and importation of the natural enemies of this insect.
It was thus that Mr. Albert Koebele, in the fall of 1888, was sent to Australia for this special purpose. The history of Mr. Koebele's efforts has been detailed from time to time in Government publications and in the press, especially that of California. It suffices to state that a number of living enemies, both parasitic and predaceous, were successfully imported, but that one of them (Vedalia cardinalis) proved so effective as to throw the others entirely into the shade and render their services really unnecessary. It has so far not been known to prey upon any other insect, and it breeds with surprising rapidity, occupying less than thirty days from the laying of the eggs until the adults again appear. These facts account for its exceptionally rapid work, for in point of fact within a year and a half of its first introduction it had practically cleared off the fluted scale throughout the infested region. The expressions of two well-known parties may be quoted here to illustrate the general verdict. Prof. W. A. Henry, Director of the Wisconsin Agricultural Experiment Station, who visited California in 1889, reported that the work of the vedalia was "the finest illustration possible of the value of the department to give the people aid in time of distress, and the distress was very great indeed." Mr. William F. Channing, of Pasadena, son of the eminent Unitarian divine, wrote two years later:
"We owe to the Agricultural Department the rescue of our orange culture by the importation of the Australian ladybird (Vedalia cardinalis).
"The white scales were incrusting our orange trees with a hideous leprosy They spread with wonderful rapidity, and would have made citrus growth on the whole North American continent impossible within a few years. It took the vedalia, when introduced, only a few years absolutely to clean out the white scale. The deliverance was more like a miracle than anything I have ever seen. In the spring of 1889 I had abandoned my young Washington navel orange trees as irrevocable. Those same trees bore from two to three boxes of oranges apiece at the end of the season (or winter and spring of 1890). The consequence of the deliverance is that many hundreds of thousands of orange trees (navels almost exclusively) have been set out in southern California this last spring."
In other words, the victory over the scale was complete, and will practically remain so. The history of the introduction of this pest; its spread for upward of twenty years and the discouragement which resulted; the numerous experiments which were made to overcome the insect; and its final reduction to unimportant numbers by means of an apparently insignificant little beetle imported for the purpose from Australia, will always remain one of the most interesting stories in the records of practical entomology.
The vedalia has since been successfully colonized at the Cape of Good Hope and in Egypt, and has produced the same results in each case. In Egypt the vedalia was introduced to prey upon an allied species of icerya (I. ægyptiacum). We hope soon to be able to send the same insect to India, where it has recently transpired that Icerya ægyptiacum occurs; while recent information received from Phra Suriya, Royal Commissioner of Siam, at Chicago, would indicate that its introduction into Siam for the same or a closely allied insect will be desirable in the near future.
In fact, the success of the experiment was so striking and so important, and resulted in the saving to California of an industry of so great a money value that it has given rise, not only in the popular mind but in the minds of a certain class of entomologists also, to the idea that remedial work against injurious insects should be concentrated upon this one line of action, and that our best hope for their destruction lies with the parasitic and predaceous species, not to mention fungous and bacterial diseases. From an extreme of comparative incredulity the farmer and fruit-grower have gone, perhaps, to the other extreme of too great faith. The case of icerya and vedalia, as I have frequently pointed out, was exceptional and one which can not easily be repeated.
One of the numerous phases of the vedalia experiment is that the wide newspaper circulation of the facts—not always most accurately set forth—has brought me communications from all parts of the world asking for supplies of the renowned little lady-bird for use against injurious insects of every kind and description, the inquiries being made, of course, under a misapprehension of the facts.
While this California experience thus affords one of the most striking illustrations of what may be accomplished under exceptional circumstances by the second method of utilizing beneficial insects, we can hardly expect to succeed in accomplishing much good in this direction without a full knowledge of all the ascertainable facts in the case and a due appreciation of the profounder laws of Nature, and particularly of the interrelations of organisms. Year in and year out, with the conditions of life unchanged by man's actions, the relations between the plant-feeder and the predaceous and parasitic species of its own class remain substantially the same, whatever the fluctuations between them for any given year. This is a necessary result in the economy of Nature; for the ascendency of one or the other of the opposing forces involves a corresponding fluctuation on the decreasing side, and there is a necessary relation between the plant-feeder and its enemies, which normally must be to the slight advantage of the former, and only exceptionally to the great advantage of the latter. This law is recognized by all close students of Nature, and has often been illustrated and insisted upon by entomologists in particular, as the most graphic exemplifications of it occur in insect life, in which fecundity is such that the balance is regained with marvelous rapidity, even after approximate annihilation of any particular species. But it is doubtful whether another equally logical deduction from the prevalence of this law has been sufficiently recognized by us, and this is that our artificial insecticide methods have little or no effect upon the multiplication of an injurious species except for the particular occasion which calls them forth, and that occasions often arise when it were wiser to refrain from the use of such insecticides and to leave the field to the parasitic and predaceous forms.
It is generally when a particular injurious insect has reached the zenith of its increase and has accomplished its greatest harm that the farmer is led to bestir himself to suppress it; and yet it is equally true that it is just at this time that Nature is about to relieve him in striking the balance by checks which are violent and effective in proportion to the exceptional increase of and consequent exceptional injury done by the injurious species. Now, the insecticide method of routing this last, under such circumstances, too often involves, also, the destruction of the parasitic and predaceous species, and does more harm than good. This is particularly true of those of our Coccidæ and Aphididæ, and those of our lepidopterous larvæ, which have numerous natural enemies of their own class, and it not only emphasizes the importance of preventive measures which we are all agreed to urge for other cogent reasons, and which do not to the same extent destroy the parasite; but it affords another explanation of the reason why the fight with insecticides must be kept up year after year, and has little cumulative value.
But the problem of the wise encouragement and employment of the natural enemies of injurious insects in their own class is yet more complicated. The general laws governing the interaction of organisms are such that we can only in very exceptional cases derive benefit by interference with it. The indigenous enemies of an indigenous phytophagous species will, cæteris paribus, be better qualified to keep it in check than some newly introduced competitor from a foreign country, and the peculiar circumstances must decide in each case the advisability of the introduction. The multiplication of the foreigner will too often involve the decrease of some indigene. If a certain phytophage is generally disastrous in one section and innocuous in another, by virtue of some particular enemy, it will be safe to transfer and encourage such enemy, and this is particularly true when the phytophage is a foreigner and has been brought over without the enemy which subdues it in its native home. Icerya had some enemies in California, presumably American; but they were not equal to the task of subduing it. Vedalia in the icerya's native home, Australia, was equal to the task, and maintained the same superiority over all others when brought to America. The genus was new to the country, and the species had exceptionally advantageous attributes. But there is very little to be hoped from the miscellaneous introduction of predaceous or parasitic insects for the suppression of a phytophage which they do not suppress in their native home or in the country from which they are brought. The results of the introduction by Mr. A. D. Hopkins of Clerus formicarius to contend with the scolytids, which were ruining the West Virginia pines, were doubtful, for the reason that the indigenous species of the genus were already at work in America. Yet the experiment was safe and desirable because the European clerus is more active and more seemingly effective than our indigenes. The gypsy moth was evidently introduced into Massachusetts without its European natural enemies, and as in some parts of Europe it is often locally checked by such natural enemies, a great number of which are known, a proper study of them and the introduction of the most effective could result in no possible harm and might be productive of lasting good.
There are two other laws which it is worth while to consider in this connection. One is, that while a plant-feeder's natural enemies are apt to cause its excessive abundance to be followed by a corresponding decrease, yet this alternation of excessive abundance and excessive scarcity will often be produced irrespective of such natural checks. An injurious insect which has been on the destructive march for a period of years will often come to a sudden halt, and a period of relative and sometimes complete immunity from injury will follow. This may result from climatic conditions, but more often it is a consequence of disease, debility, and want of proper nutrition, which are necessary corollaries of undue multiplication. Frequently, therefore, it may be inaccurate and misleading to attribute the disappearance of a particular injurious species to some parasitic or predaceous species which has been let loose upon it, and nothing but the most accurate observation will determine the truth in such cases. The past year furnished a very graphic illustration in point. Throughout Virginia and West Virginia, where the spruce pines have for some years suffered so severely from the destructive work of Dendroctonus frontalis, not a single living specimen of the beetle has been found during the present year. This has been observed by every one who has investigated the subject, and particularly by several correspondents who have written to me: by Mr. E. A. Schwarz, who was commissioned to investigate the facts, and by Mr. Hopkins, who has made the study of the subject a specialty. The clearest explanation of this sudden change is, that the species was practically killed out by the exceptionally severe cold of last winter, since such was the case with several other insects. Now, following so closely on the introduction by Mr. Hopkins of Clerus formicarius, how easy it would have been to attribute the sudden decrease to the work of the introduced clerus, had not the decrease been so general and extensive as absolutely to preclude any such possibility! In like manner a certain scale-insect (Aspidiotus tenebricosus) had become exceedingly destructive to the soft maples in the city of Washington last year, whereas the present year it is almost entirely killed off, evidently by the same exceptional cold. Many of the affected trees were painted with whitewash, with a view of destroying the aspidiotus, and the death of this last might have been attributed to the treatment (and naturally would be by those employing it) were it not that the same result was equally noticeable on the trees not treated. Reports from southern California would indicate that the red scale (Aspidiotus aurantii) is in many orchards losing its destructiveness through agencies other than its insect enemies, and in this case the facts are particularly interesting, because of the ease with which its disappearance may be attributed to some of the recent introductions from Australia.
The other law that is worth considering in this connection is that, as a rule, the animals and plants of what is known as the "Old" World—i. e., of Europe and Asia—when introduced into North America have shown a greater power of multiplication than the indigenous species, and in a large number of instances have taken the place of the native forms, which have not been able to compete with them in the struggle for existence. This is still more true of the species introduced from the Old World, as well as from America, into Australia, where the advantage of the introduced forms, as compared with the indigenes, has been in many cases still more marked.
There are some instances in which there can be no doubt whatever as to the good which will flow from the introduction of beneficial species, and an illustration is afforded in the caprifig insect (Blastophaga psenes). There can be no question as to the good which would result from the introduction of this species from Smyrna into those sections of California where the Smyrna fig is grown without its intervention, and there are other similar instances which promise well and involve no risk. But I have said enough to show that the successful utilization of beneficial insects is by no means a simple matter, and that discriminating knowledge is required to insure success or prevent disaster, especially in the second category dealt with in this paper. The danger attending introductions of beneficial species by unconsciously accompanying them with injurious forms, or by failure to appreciate the facts here set forth, is well illustrated by the introduction to Europe of our Peronospora viticola, of the English sparrow to America, and of the mongoose to Jamaica.
Wherever the importance of the matter leads to legislation what are denominated "political" methods are apt either to control or in some way influence the resulting efforts—too often with unfortunate consequences. We should, as economic entomologists, be on the alert for the special cases where the introduction or dissemination of beneficial species promises good results, and do our best to encourage an intelligent public appreciation of such special cases, while discouraging all that is of unscientific or sensational nature, as likely to mislead and ultimately do our profession more harm than good.
- Condensed from a paper read before the Association of Economic Entomologists at Madison, Wis., August 15, 1893.