The Scale Insect and Mite Enemies of Citrus Trees
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THE SCALE INSECT AND MITE ENEMIES OF CITRUS TREES/ By C. L. MARLATT, First Assistant^ Division of Entomology. INTRODUCTION. Any consideration of the insect enemies of citrus plants must give large importance to the scale insects, or bark lice, which are not only very destructive on the orange and lemon and other citrus fruits, but are also the chief insect enemies of most other tropical and subtropical plants. Scale insects are, as a rule^ small and inconspicuous singly, but they multiply so rapidly that very soon an entire plant becomes infested— trunk, limbs, leaves, and fruit. The infested tree is rarely killed out- right, but its growth may be almost completely checked and its fruit products rendered valueless. Next in importance to the scale insects are the mite enemies of the orange and lemon, as represented by the mite which causes the rusting of the orange in Florida and the silvering of the lemon in California, and also the leaf mite, known from its coloring as the six-spotted mite of the orange. These mites, occurring along with the scale insects and being subject to similar remedies, may properly be considered in the same connection. Of very great importance to the Florida grower of citrus fruits is the so-called wdiite fly, the latter not representing a scale insect in its ^ No one can discuss the insect enemies of citrus plants without acknowledging indebtedness, as the writer now does most heartily, to the very comprehensive and valuable work on this subject prepared by the late H. G. Hubbard and published by this Department in 1885, and which for many years has been the chief authority on the subject covered. One is struck to-day, fifteen years after its publication, with the sound, practical ideas contained in it, particularly on the subject of the control of the scale and other insects treated. Mr. Hubbard was a successful orange grower in Florida, as well as a thoroughly trained scientific man, and one of the closest observers of insects this country has produced. The practical side of Mr. Hubbard's work is especially to be remembered also, because he was the deviser of the kero- sene-soap emulsion, which, with allied washes, has for many years been the lead- ing means of controlling scale insects. His work, entitled "Insects añ'ecting the orange,'' but really covering the whole subject of citrus insects, having long been out of print, Mr. Hubbard was, just prior to his unfortunate illness and death, col- lecting data for a new edition, to be published by the Department. It will be a source of lasting regret that he was not able to accompHsh this undertaking. 247 248 YEARBOOK OF THE DEPAETMENT OF AGRICULTURE. ordinary acceptation, but in the practical features of life history and habits coming in the same category, and hence properly considered with the true scale insects. There are a great many insect enemies of citrus plants other than the scale insects and mites, but, for this country, at least, these others, in the main, have no great economic importance, or are only very occasionally abundant enough to be especially destructive. In this category are the various leaf-feeding insects and some wood-boring species. At rare intervals some of these leaf-feeding species appear in numbers sufficient to defoliate trees more or less completely, or wood-borers may attack and hasten the death of frost-injured or other- wise weakened trees; but none of these insects calls for the constant attention and treatment which is necessary to prevent annual loss from scale insects. Occurring about the orange and other citrus trees will be seen also a great many other insects which play a beneficial rôle, preying upon or parasitizing the scale insects living on these trees. It is very important to make the acquaintance of these beneficial species, more particularly to avoid, whenever possible, killing them in the warfare waged against the injurious ones. SOME GENERAL CONSIDERATIONS BEARING ON CITRUS INSECTS. Before taking up the consideration of the several species to be treated in this paper some general topics may be discussed, such as the inñuences which determine the destructiveness of these insects, the species characteristic of different regions, and the nature of the injury occasioned by them, natural enemies, remedies, and means of control. INFLUENCE OF CULTIVATION, PRUNING, AND CLIMATE. It is just as true in the case of the orange and lemon as it is with other plants that negligent cultivation and improper care, or any unfavorable conditions of climate which weaken the vitality and vigor of the tree, encourage the presence and multiplication of the insect enemies. On the other hand, there is something in the vigor of growth and condition of the sap of a healthy tree, living under the best con- ditions, which is repellant to insect attack; and it will be almost inva- riably found that the unhealthy tree is the one first severely infested with scale insects or mites. This does not mean that vigorous, healthy trees will not be attacked, but on such trees insects seem to be less apt to multiply abundantly and effect the complete investment that is often noted in the case of a weak or improperly nourished plant. Therefore, as a means of protection against scale insects, a proper system of cultivation and pruning is highly important. The value of pruning as a means of preventing scale-insect injury can not be too strongly urged. Scale insects thrive best where they SCALE INSECT AND MITE ENEMIES OF CITRUS TREES. 249 are protected from direct sunlight and free movement of the air, hence trees of dense growth, unpruned, are almost certain to have their cen- ters, at least, scaly. A well-opened and pruned tree, in which free access is given to light and air, is much less apt to be badly attacked than a dense, thick-headed tree, the interior of which is entirely shaded and protected and the moisture held, thus furnishing the conditions most favorable for the well-being of scale insects. As a general propo- sition, therefore, light, air, and dryness are inimical to scale insects, and, conversely, shade and dense habit of growth, protecting from air currents and holding moisture, are favorable. The truth of this is often exhibited in the citrus districts of the Pacific coast. Very frequently the high temperature and drj^ness of the long summers, if allowed to have full action on well-opened and pruned trees, is as destructive to scale insects as would be a thor- ough treatment with an insecticide, in some cases killing the scales out almost completely. Similarly, scale insects are more apt to be abun- dant and troublesome in moist, warm regions than in regions with even higher temperature, but with a very much lower rate of humidity. In general, therefore, the citrus orchards in Florida, Jamaica, and the West Indies suffer more than do those in California. On the Pacific coast, also, the orchards in moist ocean districts are much more apt to be infested than those farther inland with greater elevation and enjoying a lower degree of humidity, with occasional much higher summer temperatures. The black scale in California has, in places, been almost entirely exterminated by the temperature holding for several days above 100° F., and a similar effect is noted with other species also. PERIODICITY IN SCALE INSECTS. With most insects injurious to cultivated plants a periodicity is noted in their occurrence in injurious numbers. In the case of subtropical species, like the scale insects affecting citrus plants, this periodicity is not always as marked as it is with insects affecting deciduous plants and field crops in temperate latitudes. While it is true that scale insects have always occurred more or less on citrus trees in the Old World and wherever these plants are cultivated, investigation will un- doubtedly show that there have been more or less well-marked periods of destructive abundance separated by periods of comparative scarcity. In illustration of this may be noted the epidemic referred to by Hub- bard as prevailing throughout the entire orange, lemon, and olive dis- tricts along the shores of the Mediterranean from Italy to Spain dur- ing the first decade of the present century, to the consternation of the inhabitants, who were dependent upon these fruits. This unusual scale epidemic subsided, however, and very largely of itself, efficient remedies at that date being practically unknown. 1 A1900 17 250 ÏEAPwBOOK OF THE DEPARTMEISTÎ OF AÖEIGÜLTÜKE. In the same way in this country, scale infestation varies consider- ably from year to year. The fluted scale, in California, increased enormously during the first t^n or fifteen years and threatened the very existence of the citrus cultures. Thanks, however, to the Aus- tralian ladybird, and, doubtless also, to the fact that many native pre- daceous and j^rasitic insects became acquainted with it, it is no longer feared in California. The long scale in Florida, also, was much more injurious in the first years of its activity than it has been since. In 1890 the black scale was very abundant and destructive in the orange districts about Riverside. Partly owing to adverse climatic conditions and partly owing also to natural enemies, this insect has almost dis- appeared from this district, which is now one of the least affected by scale insects. These facts are merely cited to give the citrus grower whatever en- couragement they may offer, but not in the least with the idea of belit- tling for an instant the need of remedial operations for the prompt and immediate control of scale insects whenever and wherever they appear.