t 608 35 U6 spy 1 U. S. DEPARTMENT OF AGRICULTURE, BUREAU OF ENTOMOLOGY— BULLETIN No. 99, Part I. L. O. HOWARD, Entomologist and Chief of Bureau. PAPERS ON INSECTS INJURIOUS TO CITRUS AND OTHER SUBTROPICAL FRUITS. THE ORANGE THRIPS: A REPORT OF PROGRESS FOR THE YEARS 1909 AND 1910. P. R. JONES and J. R. HORTON, Agents and Experts, Deciduous Fruit Insect Investigations. Issi i;d March 6, 1911. WASHINGTON: GOVERNMENT PRINTING OFFICE, 1911. 7 ib-series ILLUSTRATIONS Page. Plate I. Fig. 1. — Young oranges showing injury by the orange thrips (Euthrips titri). Fig. 2. — Young oranges showing injury to stem and blos- som ends by the orange thrips 4 II. Mature oranges showing injury due to the orange thrips 4 III. Orange foliage showing curled and distorted condition of leaves due to work of the orange thrips 4 TEXT FIGURES. Fig. 1. — Diagram illustrating the relative abundance of orange thrips on oranges, on orange foliage, and on other plants during the season 7 2. — Power spraying outfit in use in spraying for the orange thrips 14 IV U. S. D. A., B. E. Bui. 90, Tart I. D. F. I. I., March 6, 1911. PAPERS ON INSECTS INJURIOUS TO CITRUS AND OTHER SUBTROPICAL FRUITS. THE ORANGE THRIPS: A REPORT OF PROGRESS FOR THE YEARS 1909 AND 1910. By P. R. Jones and J. R. Horton," Agents and Experts, Deciduous Fruit Insect Investigations. INTRODUCTION. The orange thrips (Euthrips citri Moulton), a small, yellow, active insect belonging- to the order Thysanoptera (popularly known as thrips) , scars the fruit and curls and distorts the leaves of the orange. At the present time its control constitutes the chief insect prob- lem confronting the citrus growers of the San Joaquin Valley orange belt of California, which winds along the Sierra Nevada foot- hills, from east of Fresno to south of Delano. This insect, the work of which was first noticed 15 or 16 years ago, has increased in num- bers with the growth of the citrus industry and recently has assumed serious economic importance. At the urgent request of a number of orange growers of Tulare County, an investigation of the insect was begun the latter part of April, 1909. The present paper is a preliminary report of the results obtained during the seasons 1909 and 1910. The writers wish to acknowledge the financial assistance of the Tulare County board of supervisors, the Lindsay Citrus Growers' Protective League, and the Tulare County Fruit Exchange; they desire to acknowledge the kindness of Messrs. P. M. Baier, Harry Postlethwaite, and R. H. Shoemaker in allowing the Bureau of Ento- °The investigation of the orange thrips by members of the force engaged in studies of deciduous-fruit insects appeared desirable, because these men were familiar with a closely related species — the pear thrips — which is very destruc- tive to prunes, pears, cherries, etc., in the San Francisco Bay region. However, in order to keep together the articles dealing with insects damaging citrus and other subtropical fruits, the present paper is published in a series of articles dealing with insects of that class. — A. L. Quaintance, in Charge of Deciduous Fruit Insect Investigations. 1 2 INSECTS INJURIOUS TO SUBTROPICAL FRUITS. mology the use of their orchards for experimental and demonstration purposes; and they would express their indebtedness to the large number of orange growers in Tulare County who have put into effect in their own orchards the recommendations of the Bureau, thereby demonstrating the value of the spraying treatments advised. ORIGINAL HOME AND DISTRIBUTION. The orange thrips is probably native to North America. Its natu- ral habitat is probabty the Sierra Nevada foothills or the adjoining plains of the southern San Joaquin Valley, and it was no doubt attracted from its natural food plants by the more succulent and luxuriant orange trees. This insect is distributed throughout the entire orange belt of the San Joaquin Valley and has been collected in several places in Southern California and at Phoenix, Ariz., by the senior author. The infestation in Arizona embraces orange groves in the Salt River Valley surrounding Phoenix, and was re- ported upon by Prof. J. Eliot Coit in a bulletin of the Arizona Agri- cultural Experiment Station." This gentleman, in sending specimens to Dr. W. E. Hinds for identification, probably did not obtain the true orange thrips (Euthrips citri Moulton), but some specimens of Euthrips occidentalis Pergande, which is found occasionally upon citrus trees, but which rarely causes any serious injury. The true orange thrips was described as a new species by Mr. Dudley Moulton in a bulletin of the United States Department of Agriculture, issued February 11, 1909. & The orange thrips has also been reported from Hermosillo, Sonora Province. Mexico, but the writers have not been able to obtain speci- mens from that locality. The occasional scarring of oranges in the north-central portion of California is caused by the grain thrips (Euthrips tritici Fitch), and not by the orange thrips. FOOD PLANTS. Although the orange thrips. when described, was thought to infest only citrus trees, the writers have taken it from a number of other host plants. The following list shows the wide range of food plants upon which this insect can exist : Of citrus fruits the following are affected: Citrus aurantivm var. sinensis (Washington Navel, Australian Navel (?), Thompson Im- proved. Valencia Late, Mediterranean Sweet, Parson Brown, Ruby Arizona Agricultural Experiment Station, Bulletin No. 5S, Citrus Culture in the Arid Southwest, p. 319, 1908. 6 IT. S. Department of Agriculture, Bureau of Entomology, Technical Series No. 12, Part VII. THE ORANGE THRIPS. 3 Blood. St. Michael. Homosassa, and seedlings) ; Citrus nobilis (Sat- suma and tangerines) ; Citrus decumana (grapefruit) ; Citrus medica var. limon (lemon) ; Citrus medica var. acida (lime, varieties of) ; and Citrus japonica (kumquat). The following miscellaneous plants are infested: Punica gramatum (pomegranate) ; Vitis vinifera (European grape, varieties of) ; Schinus molle (California pepper tree); "umbrella tree;" Pyrus communis (pear); Primus armeniaca (apricot); Primus persica (peach) ; Prunus domestica (European plum, varieties of) ; Salix sp. (willow) ; Rumex sp. (dock) ; Portulaca oleracea (purslane) ; Olea europi a (olive) ; Rubus idceus (red raspberry) ; Rosa sp. (rose) ; Solatium sp. CHARACTER AND EXTENT OF INJURY. Injury to citrus trees and fruit is caused directly by the feeding of both adults and larva* upon the surface of the parts attacked. This feeding may be on the young fruit (Plate I, figs. 1,2), the nearly mature fruit (Plate II), or the new, tender foliage (Plate III), and generally takes place on all of these. The injury to foliage is gen- erally on young leaves, but may also occur on the axillary buds. The manner of feeding of both the adult and larva of the thrips is identical, and consists in piercing the plant tissues with the sharp mouthparts with which both stages are equipped and then rasping the wound by a " rooting " motion of the head. The vegetable juices thus liberated from the plant cells are sucked into the alimentary canal of the insect. The characteristic marking or scabbing of the fruit, so noticeable at picking time, is started when the fruit is very small— just after the petals have fallen from the blossoms. This scabbed area is small at first, but as the fruit grows and the thrips continue to feed the markings deepen and at the same time the area of injury is enlarged. The continued feeding of a large number of thrips results in the scabbing of nearly the entire surface of the fruit. Often the marking is so large and deep over a portion of the orange that it causes the fruit to be misshapen and aborted. Frequently the entire surface is scarred while the fruit is still small, with the result that it ceases to grow and falls from the tree. Orange trees in the Tulare County citrus belt make about four dis- tinct growths a year, and it is on this tender foliage that the orange thrips multiply in greatest numbers. The feeding of large numbers of these little insects causes the young leaves to curl and become dis- torted and the whole growth to present a sickly appearance. Young trees are often held back a year or more in growth by the prompt destruction of the terminal buds soon after these make their appearance. 4 INSECTS INJURIOUS TO SUBTROPICAL FRUITS. DESCRIPTION AND LIFE HISTORY. THE ADULT. The adult female of the orange thrips is a small, four- winged, orange-yellow insect, which moves very rapidly by running, leaping, and ftying. The mouthparts, which are suctorial in nature, form a sharp cone projecting from the underside of the head. The adult male is smaller than the female and much more rapid in its movements. The original description of the adult female by Moult on a is as follows : Euthrips citri n. sp. Measurements: Head, length 0.75 nun., width 0.15 mm.; prothorax, length 0.09 mm., width 0.18 mm. ; mesothorax, width 0.24 mm. ; abdomen, width 0.25 mm. ; total body length 0.86 mm. Antennae: I, 12/*; II, 36/*; III, 39/*; IV. 39/t ; V. 30/*; VI, 34 M ; VII, 6/t; VIII, 12/*; total, 0.205 mm. Color, yellow to orange-brown, with thorax and segment 2 of antennae more noticeably orange-brown. Head twice as wide as long, retracted considerably into the prothorax, broadly rounded in front, with only slight depressions to receive the basal joints of the antenna?; two spines on anterior margin, other spines not conspic- uous; cheeks almost straight and parallel. Eyes large, occupying almost one- half the length of the head, prominent: pigment deep red to purple: facets of eyes large, eyes pilose. Ocelli subapproximate, margined inwardly with yel- low-brown crescents. Mouth-cone short, reaching almost to posterior margin of prothorax, broadly rounded and with black spot at tip ; maxillary palpi 3-segmented. Antennas 8-segmented, with segment 2 orange-yellow, other seg- ments uniformly light brown: segments 2, 4, 5, and 6 almost equal in length; style about one-half the length of segment (i. All spines inconspicuous; sense cones transparent. Prothorax about twice as wide as long, posterior angles broadly rounded; with long brown and outer small spine at each posterior angle, other spines not conspicuous. Mesothorax largest and with anterior angles broadly rounded. Legs light yellow-brown, with tarsi lighter but dark brown at the tip ; spines on legs brown. Wings present and fully developed, forewings broadest near base and pointed at tips; with the ring vein and a single longitudinal vein which divides at about one-third the length of the wing from the base, the anterior part running parallel and approximate to the anterior part of the ring vein, and ending abruptly near the tip, the posterior paralleling and approach- ing the posterior part of tbe ring vein and ending about one-half the wing's length from the end, each branch with a dark-brown marking immediately at its tip. The costa bears a row of about 29 regularly placed spines. Other spines placed as follows: A group of 5 near base of median longitudinal vein; 2 on either side of where second vein branches from the first, and 3 scattered spines about equidistant on each branch vein and in each case one of these spines immediately at the end of the vein; several rather long spines on scale. Veins of the forewing unusually strong and conspicuous, somewhat orange colored near base but fading to yellow near tip. Membrane of wings trans- parent. a Loc. cit. Bui. 99, Part I, Bureau of Entomology, U. S. Dept. of Agriculture. Plate I. Fig. 1.— Young Oranges, Showing Injury by the Orange Thrips (Euthrips citri). Somewhat Enlarged. (Original.) Fig. 2.— Young Oranges, Showing Injury to Stem and Blossom Ends by the Orange Thrips (Euthrips citri). Somewhat Enlarged. (Original.) Bui. 99, Part I, Bureau of Entomology, U. S. Dept of Agriculture. Plate II. Mature Oranges, Showing Injury Due to the Orange Thrips. (Original.) Bui. 99, Part I, Bureau of Entomology, U. S. Dept. of Agriculture. Plate III. Orange Foliage, Showing Curled and Distorted Condition of Leaves Due to Work of the Orange Thrips. (Original.) THE ORANGE THRIPS. 5 Abdomen ovoid, tip conical, all spines, excepting a very few at tip, incon- spicuous. Described from many female specimens collected from orange foliage and fruit at Exeter, Tulare County, Cal. The males are similar to the females, but smaller and more active, with the orange-colored testes prominent. THE EGG. The egg is a bluish white, bean-shaped object measuring from 0.2 mm. in length to about O.OTo mm. in width, with a very thin shell. THE LARVA. First-stage larva. — Length 0.041 mm.; width of mesothorax 0.011 mm.; general shape fusiform. The antennae, head, and legs are large and unwieldy in proportion to the rest of the body. Color translucent white. Antenna-, length 0.015 mm. ; distinctly 4-segmented ; I short, cylindrical ; II more than twice as long as I, slightly urn-shaped, longer than wide; III about as long as II. obtusely fusiform; IV about as long as the other joints combined, fusiform, very finely drawn out at the distal end. Segments II, III, IV (II very ob- scurely) ringed, the distal rings on segment IV appearing as segmental divisions. A few fine hairs present on all segments, most numerous on IV but not very conspicuous on any of the segments. Head subquadrate ; eyes reddish- brown. Abdomen gradually tapering, 10-segmented, first 8 segments subequal; IX and X large and more abruptly tapering, hairs inconspicuous. Legs stout, femora and tibia? nearly equal in length, tarsi one-jointed, ending in a single claw. Second-stage larva. — Length 0.9 mm.; head length 0.1 mm.; width 0.0S3 nun.; length of antennae 0.175 mm. ; width of mesothorax 0.200 nun. : widtli of abdomen 0.3 mm.: Antennas, I, 2.u ; II, 3/x; III, 9/*; IV, 45/*; V, 9fi; VI, 15/m; color orange-yellow. In shape similar to first-stage larva except that the abdomen is oval to ovate and generally more robust. //(•*/(/ quadrate, small in proportion to body, eyes reddish. Antennae apparently 4-segmented under 2/3 objective, but under 1/6 objective distinctly 6-segmented, the chitin not extend- ing into the fifth and sixth segments; I short, conical, about as broad as long; II cylindrical, broader than long and slightly longer than I; III obtusely spindle-shaped, about twice as long as broad and about as long as I and II combined: IV obtusely spindle-shaped but blunt on the distal end, about as long as III ; V very short and thick, slightly broader than long, about one- fifth as long as IV ; VI cylindrical, longer than broad, about one-third as long as IV. Abdomen oval to ovate. 10-segmented, the last segment tubular. Legs short and stout, hiud femora and tibiae about equal, hairs everywhere incon- spicuous except a few under 1/6 objective, which are the most prominent on last segments of antennae. THE PUPA. First-stage pupa. — Length 0.56 mm.; width of head 0.15 mm.; width of mesothorax 0.18 nun.: width of abdomen 0.25 mm.; antenna 1 , length 0.2 mm. Color pale translucent yellow; antennae, legs, and wing-pads lighter. Shape similar to advanced first-stage larva ; abdomen elongate ovoid. Antenme pro- jecting cepbalad, 4-segmented; I short, thick, slightly wider than long'; II ob- tuse, urn-shaped, about as wide as long; III obtusely spool-shaped, about as 7S562°— Bull. 99, pt 1—11 2 6 INSECTS INJURIOUS TO SUBTROPICAL FRUITS. long as I and II combined and about twice as long as wide; IV about as long as III, tapering to obtuse apex. Wing-pads extending to distal margin of the second abdominal segment, those of hind wings slightly longer. Legs stout, hind femora and tibia? about equal. Hairs present on live specimens but not prominent, short, slightly longer on tip of abdomen. Second-staye pupa. — Length 0.GG6 mm.; width of head 0.133 mm.; width of prothorax 0.133 mm. ; width of mesothorax 0.16G mm. ; width of abdomen 0.133 mm. Shape similar to that of the adult. Color translucent white to pale yel- lowish ; eyes reddish, more prominent than in first-stage pupa. Antenna? 4-seg- mented, projecting backward over the head and thorax and reaching to the middle of the prothorax, second segment forming a kind of elbow from which 3 or 4 long seta? project cephalad. Prothorax nearly twice as broad as long; wing-pads in pupa? just entering the second pupal stage extending to the distal margin of the sixth abdominal segment; in pupa? in which the adults are nearly ready to emerge the wing-pads extend to the distal margin of the ninth abdominal segment. Abdomen similar in shape to that of the adult. Legs stout, hind femora and tibia? about equal in length, seta? more prominent than in first-stage pupa, longer at the tip of the abdomen ; conspicuous in fresh specimens but not in mounted ones. Tip of abdomen often with a cremaster- like formation resembling in shape a fork with 4 tines. Male pupa? smaller, resembling the adults, their wing-pads usually reaching past the tip of the abdomen. Seta? usually not so prominent. SEASONAL HISTORY. The orange thrips passes the winter in the adult state, and it is generally the adult form which first becomes conspicuous upon the orange trees in the spring. Although no large number of adults has been collected in hibernation, these undoubtedly pass the winter in sheltered places, such as the dead leaves and twigs forming the trash under most orange trees; they are occasionally found on living plants and on citrus nursery stock in midwinter. Adult thrips appear in limited numbers during March. They deposit very few eggs in the early part of April, prior to the blossom- ing of the Navel orange trees, but soon after most of the petals have fallen larva? become quite numerous. Oviposition has not been observed, but it is probable that it takes place mostly at night. Examinations for eggs revealed the fact that most of them are placed in the new, tender growth, being inserted into both upper and lower leaf surfaces, and also in the shoots. They are also placed in the receptacles of the blossoms after the petals have fallen and in young fruit and fruit stems. The larva? are wingless and when full grown are orange colored. When ready to pupate they fall from the trees, get into a curled dead leaf, amid cobwebs, dust, and leaf particles, and hide until the trans- formation is completed. Pupa? are not found in numbers propor- tionate to the larva' and adults, since it is in this stage that the mortality rate of the insect is greatest. The pupa? are very soft- bodied and less active than larva? and adults. They move readily, however, when disturbed. THE ORANGE THRIPS. Eggs, larvae, and adults are found on the trees, and pupa? in the dead leaves under them, from early May until early November, all four forms being present during the entire period. The broods thus overlap so closely that it is very difficult to separate them. INTERRELATION OF ABUNDANCE OF THRIPS AND FOOD PLANTS. The orange thrips feed only on very tender plant tissues, namely, the young leaves, shoots, and tender fruit. This makes it necessary for them to pass from foliage to fruit and from plant to plant as the suitability of the tissues as food changes. They first make their appearance in April and May on the new growth of the Navel orange, reaching the first maximum of abundance about the time four-fifths of the petals are off'. When most of the petals have fallen a few thrips pass to the more advanced fruit and the number feeding on the latter rapidly increases as the first growth of foliage becomes hardened and distasteful. The thrips continue feeding on the fruit until the latter, in turn, becomes somewhat tough, and reach a second APR. 1~MAY | JUNE I JULY I AUG. I SEPT. I OCT. 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