,B 741 
 
 P3 C2 ■ STATL HORTICULTURAL COMMISSION 
 iopy 1 
 
 LLLWOOD COOPER, Commissioner 
 
 THE PLAR THRIP5 
 
 (Luthrips pyri) 
 
 BY 
 
 DUDLLY MOULTON 
 
 Entomologist for Santa Clara County, California 
 
 ILLUSTRATED WITH ORIGINAL DRAWINGS BY W. 5. ATKINSON 
 AND PHOTOGRAPHS BY A. P. HILL 
 
 SACRAMENTO 
 
 W. W. SHANNON, - SUPT. STATE PRINTING 
 
 1905 
 

 ' 
 
 CALIFORNIA 5TATL COMMISSION OF 
 HORTICULTURE.. 
 
 ELLWOOD COOPER Commissioner Santa Barbara 
 
 JOHN ISAAC Secretary . San Francisco 
 
 ED. M. EHRHORN ..... Deputy.. Mountain View 
 
 E. K. CARNES Assistant, Deputy Riverside 
 
 O. E. BREMNER Second Assistant Santa Rosa 
 
 GERTRUDE BIRD Stenographer Sacramento 
 
 OFFICE: 
 
 Room 41, State Capitol, Sacramento. 
 
 Branch Office, Ferry Building, San Francisco. 
 
 
PRLFACL. 
 
 Although our present knowledge of the life-history, general habits, and 
 distribution of the thrips pest (Euthrips pyri) of our fruit trees is by no 
 means complete, yet we have made such progress in getting acquainted 
 with this insect that it seems advisable to issue in bulletin form a con- 
 cise account of what we already know about it. It is obvious to every 
 thoughtful person that the necessary basis for a successful fight against 
 any insect pest is a sound knowledge of its life-history and habits. 
 Where are the vulnerable points in this life-history? How can we best 
 take advantage of the conditions under which the pest normally lives 
 in order to attack it? In egg or immature or adult stage? To answer 
 these questions we must put all of our observations and experiences 
 together ; we must combine on the one hand the knowledge of the 
 actual orchard grower, and on the other that of the working entomolo- 
 gist, in order best to undertake a systematic war on the pest. 
 
 As fast as any considerable progress is made in getting acquainted 
 with our enemy and its ways and actions, we should all know about it, 
 in order better to gain more knowledge and more speedily to devise 
 means and methods of attack. Therefore, what we have already learned 
 about the fruit thrips is now published and disseminated among fruit- 
 growers, so that all may understand the exact nature and mode of life 
 and work of this serious menace to our orchards. 
 
 The insect is apparently one not heretofore generally recognized as a 
 pest. Indeed, it was first discovered by entomologists only a few years 
 ago. Its habits are unique when we compare them with those of other 
 thrips, such as the familiar onion and wheat thrips. The pear thrips 
 is not confined alone to the Santa Clara Valley. It occurs in other 
 parts of the State, but so far nothing or but little has been done else- 
 where in the study of its life-history or in the devising of remedies. 
 This bulletin should, then, interest others besides the growers of the 
 Santa Clara Valley, and we hope it may be a spur to stimulate study of 
 the pest all over the State. 
 
 I wish to acknowledge the careful and accurate observing done in con- 
 nection with my study of the fruit thrips by Mr. W. F. Budlong, who 
 assisted me for a time in the work. I wish also to acknowledge my 
 obligations to the Board of Supervisors of Santa Clara County for its 
 encouraging attitude toward my work and its liberal aid given me in 
 carrying it on. 
 
 D. M. 
 
CONTENTS. 
 
 Page. 
 DESCRIPTION OF THE INSECT - - 
 
 OCCURRENCE AND DISTRIBUTION --------- 6 
 
 NATURE AND EXTENT OF INJURY - - - - - - - - - 7 
 
 LIFE-HISTORY AND HABITS 10 
 
 METHODS OF CONTROL ------------ 14 
 
 SUMMARY --------------- 17 
 
THL PELAR THRIP5. 
 
 Luthrips pyri.) 
 
 
 During the months of February, March, ami April, if one were to 
 examine closely the branches of many of the fruit trees in the Santa 
 Clara Valley, he would find numerous brown insects, about one 
 twentieth of an inch long, hidden within the buds, blossoms, or leaves, 
 and if these are disturbed by being shaken out over one's hand, they 
 suddenly become active, raise up the tip of the abdomen, and lifting 
 and disentangling their wings, fly away. They are so small and their 
 wings so transparent, that unless one watches very closely they dis- 
 appear even before one's eyes. 
 Such, in brief, is the thrips pest of 
 our orchards — a rather attractive 
 and for many reasons a very in- 
 teresting insect. 
 
 The pear thrips was described in 
 "Entomological News" for Novem- 
 ber, 1904, by Miss D. M. Daniel, of 
 the University of California. Her 
 type specimens were taken from 
 pear blossoms near San Leandro, 
 Alameda County, hence the name 
 "pyri," the pear thrips. Miss 
 Daniel's original description is as 
 follows (see Fig. 1): 
 
 Female. — Length, 1.26 mm.; width of mesothorax, .32 nun.; general color, dark 
 brown. Head about as long as. broad ; cheeks convexed ; anterior margin broad, acutely 
 angular; back of head transversely wrinkled, and bearing a few minute spines. Eyes 
 medium, black with light borders, rounded or oval in outline, coarsely faceted, hairy. 
 Ocelli yellow, margined inwardly with reddish-brown crescents, widely separated, 
 posterior ones contiguous, with light borders around eyes ; one very long, slender spine 
 on each side midway between ocelli. Mouth-cone pointed, tipped with black ; maxillary 
 palpi 3-segmented. Antenna? 8-segmented, approximate, slightly over twice the length 
 of head. Length of segments: 33, 43, 55, 52, 35, 50, 8, 10. Antennae brown, except seg- 
 ment three, which is yellow. Spinespale, conspicuous, special sense organs on segments 
 three and four. 
 
 Prothorax longer and wider than head; bears many prominent spines, the one at 
 each anterior angle and the two at each posterior angle are longest. Color, yellow- 
 brown ; faintly cross-striated. 
 
 Mesothorax approximately as wide as antennae are long; front angles obtusely 
 rounded; metanotal plate bears four spines close to front edge, middle pair equal in 
 
 Fig. l. Pear Thrips, greatly enlarged. 
 
 (Original.) 
 
6 THE PEAR THRIPS. 
 
 size and prominence to those at the angles of prothorax, the others are small; ptero- 
 thorax yellow-brown, transversely wrinkled. 
 
 Wings present, extending slightly beyond abdomen, about twelve times as long as 
 wide, pointed at ends; surface of wings thickly covered with minute brown spines; 
 both longitudinal veins and costa of fore wings thickly set with quite long, brown-colored 
 spines, placed regularly on costa and bind vein; costa has from 29-33 spines, fore vein 
 12-15, and bind vein 15-16; veins not prominent; costal fringe of fore wings about 
 twice as long as costal spines. 
 
 Legs moderately long, scarcely thickened; femora and all except the terminal part 
 <if tibia brown ; terminal part and tibiae and tarsi yellow, a double row of twelve strong 
 spines on the inner side of hind tibia, several inconspicuous spines on fore and middle 
 pairs. 
 
 Abdomen about two and one half times as long as width of mesothorax, cylindrical 
 to eighth segment, then abruptly pointed. Spines on sides and around tip of abdomen 
 dark brown, conspicuous; those on last two segments are long and approximately equal. 
 Color of abdomen dark brown, connective tissue yellow. 
 
 OCCURRENCE AND DISTRIBUTION. 
 
 We know now that the pear thrips is not confined to Alameda County, 
 where it was first found, and to the Santa Clara Valley. We have 
 learned, through Prof. C. W. Woodworth of the University of California, 
 that during the present year it has been injuring fruit trees around 
 Vacaville and Suisun and along the Sierra foothills. During a recent 
 trip through the southern part of California, I learned of its presence 
 in Los Angeles County. Other than this we do not know how wide- 
 spread the pest is, nor where it originated, whether here in California 
 or whether it is an introduced species. The insect may have originated 
 on some of our wild plants, such as the wild plum or cherry, and later 
 as our valleys have developed into large fruit-growing districts, the 
 insect, finding more and better food, may have changed its feeding 
 habits from wild to cultivated plants. This would be a not unnatural 
 change. On the other hand, it may have been imported and here find- 
 ing conditions favorable without any effective natural insect enemies 
 or fungous diseases, it has spread and developed accordingly. 
 
 Euthrips pyri first came particularly to the notice of the Santa Clara 
 County public in February, 1904, when several orchardists noticed that 
 their trees were not coming into blossom in the usual way. The buds 
 appeared blasted, turned brown and fell without opening, and trees 
 which did blossom dropped their petals early. In either case the trees 
 looked much as though a hot blast had passed over them. This, as 
 orchardists remember, had happened in places for several years and 
 the injury had not been explained. Last year an attentive observer 
 actually found certain tiny insects in the blossoms, and the cause of 
 the injury was at once apparent. The insects were brought to the 
 attention of Mr. Ehrhorn, former Horticultural Commissioner of the 
 county, now first deputy State Horticultural Commissioner, who deter- 
 mined them to be thrips. Immediately on learning of the damage 
 
THE PEAR THRIPS. I 
 
 caused by this ''new insect' 1 . in the orchard of R. K. Thomas, where it 
 was first observed, many people made special visits to his place to find 
 out what the insect was like, not realizing that by shaking a few blos- 
 soms from almost any tree in their own or a neighbor's orchard the 
 dreaded insects could be found. 
 
 To those people who are not well acquainted with the general appear- 
 ance and habits of this group of insects, the pear thrips look very much 
 like those found ordinarily in grasses, flowers, etc.; but on close exam- 
 ination one finds very strong individual characteristics as regards their 
 structure and life habits. The thrips which are commonly found in roses 
 and other Mowers are not the destructive forms which injure our fruit 
 trees. 
 
 NATURE AND EXTENT OF IN.) TRY. 
 
 It is the adult thrips which first comes to our trees in countless num- 
 bers, and which does the great and serious injury; it is this stage, also, 
 which is most conspicuous and which is best known to orchardists. 
 
 The manner in which thrips attack trees is, generally speaking, 
 
 much the same. They arrive just before or 
 
 while the buds are opening, and enter the bud, 
 
 if possible, otherwise they begin to feed on the 
 
 tip and eventually work their way in. When 
 
 once inside they attack the tenderest of the 
 
 flower parts and in many cases reveal a rather 
 
 fastidious taste, preferring some particular part, 
 
 such as the stamen, petal, or pistil. 
 
 The mouth parts of thrips project from the fig. 2. Month parts of Thrips, 
 . . , , , , ■ , , greatly enlarged, 
 
 lower posterior side 01 the head and have some- (Original.) 
 
 what the shape of an inverted cone (Fig. 2). 
 
 The mouth opening, the rim of which is strongly chitinized and rough- 
 ened, rasp-like, is in the smaller distal end of the cone, and through it 
 project two stylets or needles. The insect, when feeding, first pierces 
 the plant tissue with the stylets, thus making an opening; then moving 
 the mouth-cone back and forth it rasps and tears the opening larger, 
 and, pushing the tip of the mouth-cone down into this puncture, sucks 
 in the juices of the plant. 
 
 The specific injury to blossoms varies in different plants. Trees may 
 be divided into two groups as regards the structure of the blossom bud: 
 (1) in which a single bud produces one blossom, such as the almond, 
 apricot, and peach; and (2) where one bud opens out into a cluster of 
 blossoms, which later produce a cluster of fruit, such as the prune, 
 cherry, pear, and apple. In the development of the bud, the winter- 
 protecting scales are the first to spread, thus permitting the insects to 
 enter at the tip. In the apricot and peach the insect forces its way in 
 
8 
 
 THE PEAR THKirS 
 
 at the tip and down between the sheath scales and the blossom; it feeds 
 
 lirst on the outer side of the blossom, and especially on the very tender 
 blossom stalk in the center at the base; later it feeds on the inner parts 
 of the flower. When the insect is once inside the bud it takes but a 
 comparatively short time to destroy the blossom. Many buds on peach 
 and apricot trees, thus injured, will fall with a slight tapping of the 
 limb. 
 
 Many instances can be cited where thrips were especially numerous 
 
 in almond blossoms, 
 yet the trees set and 
 matured a large crop 
 of nuts. The prob- 
 able explanation of 
 this is that the trees 
 were almost in bloom 
 before the insects ar- 
 rived in dangerous 
 numbers. The thrips 
 did not get a chance 
 at the newly open- 
 ing buds, and, after 
 the blossoms had 
 opened, the nature 
 of the insects' feed- 
 ing was such that no 
 serious injury to the 
 fruit followed. The 
 insects did not at- 
 tack a vital part of the blossom, but seemed to prefer the inside of the 
 calyx-cup at the base of the stamens, and they did not seem to injure 
 any other part of this particular blossom, although they were often 
 present in very large numbers. Soon after pollination the calyx-cup 
 is sloughed off and the fruit left apparently no worse for the presence 
 of the insects. 
 
 Thrips do even more serious injury to blossoms of the second 
 group (prunes, cherries, etc.) than they do to those of the first. The 
 insect enters at the tip of the swelling bud and forces itself down the 
 center of the cluster, injuring each blossom on the inner, contiguous 
 side When thus affected, natural growth ceases. For a time the 
 untouched outer side of each budlet grows in the natural way, while 
 the injured inner side has ceased growing. The deformity thus pro- 
 duced causes each budlet to turn in toward the center, and all of the 
 cluster eventually fall. The injury done by thrips during this early 
 opening of the bud is by far the most serious. As blossoms develop 
 
 Fig. ■ <. imperial Prune branches. Blossoms and leaf buds 
 affected. Every blossom on these twi^s lias been killed. 
 On the two inner branches many of the clusters have 
 fallen. 
 
TIIK PEAR TIIHIPS. 
 
 9 
 
 thrips display their preference for certain flower parts; in cherries and 
 prunes, the tips of the petals and the stigma and style are the parts 
 that suffer. 
 
 The feeding of thrips in leaf buds and on tender foliage is almost as 
 disastrous as when they attack blossoms. In many cases where trees 
 have suffered injury for three or four days it takes several months for 
 them to again put forth new buds and foliage and to again assume 
 their natural growth, and then often they appear sickly for the entire 
 year. The rapidity 
 with which whole or- 
 chards have been in- 
 jured marks tins as 
 one of the worst pests 
 which we have ever 
 had to fight. 
 
 The insect is known 
 to feed on the follow- 
 ing plants, and it is 
 probable that even 
 this list is not com- 
 plete: Apricot (sev- 
 eral varieties), apple, 
 almond, cherry, rig, 
 grape, pear ( Doyenne 
 du Cornice and Bart- 
 lett preferred), prune 
 (Imperial preferred), plum, walnut (English), and peach (Muir and 
 Clings (Nichols) preferred). As mentioned, the insect gives preference 
 to certain varieties of prunes, peaches, and pears, and of the other 
 fruits all seem to be affected alike. The Imperial prune is espe- 
 cially susceptible to the attacks of thrips. Last year it was generally 
 known that the thrips could absolutely clean an orchard of its fruit. 
 Tins year, when the insects began to appear, of course every one was 
 alarmed, fearing that wherever a few thrips were present more would 
 appear and all orchardists would suffer the same damage which a few 
 had experienced the year before. This overanxiety was stimulated 
 also by the lack of knowledge on this particular thrips. No one seemed 
 to know where it came from, how many broods there were, how long it 
 would feed, or where it would go at the end of the season. 
 
 It is impossible to estimate the loss caused by thrips during the 
 season of 1904, though we know of a number of orchards where the crop 
 was a complete failure. This year, while the injury is very serious, we 
 are again unable even to estimate the loss. The insect is much more 
 widespread, but during the last blossom season there occurred unusu- 
 
 Fig. 1. Black Tartarian cherries, showing dead blossoms, 
 the work of mature thrips; and deformed leaves, the 
 effect (if the feeding of young thrips. 
 
10 THE PEAK Til IMPS. 
 
 ally heavy driving rains, and we were unable in many cases to deter- 
 mine the amount of injury done by the thrips and the amount caused 
 by the rain, except in those places where injury done by the insects 
 was noticed before the rains came on. The damage, however, will 
 aggregate many thousands of dollars. 
 
 LIFE-HISTORY AND HABITS. 
 
 Tht Egg, the Ovipositor, and the Placing of the Egg. — The egg is bean- 
 shaped, is almost transparent, and when seen within the body of the 
 adult female is very large in proportion to the 
 size of the abdomen. It can be seen only with a 
 powerful lens or microscope. 
 
 The ovipositor — the organ fitted for cutting 
 an incision in the plant tissue and for convey- 
 ing eggs from the abdomen of the adult female 
 into this incision — is made up of four distinct 
 
 Fl °of ttSTSSrlnroT? P lateS > each 0f Which haS a Serrate outer ed S e > 
 positor, greatly enlarged, is pointed, and is operated by powerful muscles 
 
 (Original.) f . . . . , , , , 
 
 and plates within the abdomen. (See Fig. 5.) 
 In placing an egg, the thrips first tears or weakens the plant epi- 
 dermis by means of the mouth parts, takes a step forward, and, arching 
 the abdomen a little, lowers the ovipositor from its sheath in the last 
 two abdominal segments, almost at right angles with the body, and by 
 operating the tiny saws up and down, she enlarges the opening and 
 cuts a quite deep incision. When this is finished an egg is forced down 
 through the space between the four plates and into the cavity below, 
 underneath the plant epidermis. The operation of making the incision, 
 of depositing a single egg, and of withdrawing the ovipositor requires 
 from four to ten minutes, and has been observed many times. One 
 often finds a branch or a whole tree where almost every female will be 
 depositing eggs at the same time. For depositing eggs, the tiny and 
 very tender stems of blossoms and leaf petioles are preferred, and as 
 the leaves develop the midrib and veins on the lower side of the leaf 
 are chosen and later the tissue of the leaf itself. It has been stated by 
 other observers that the feeding and depositing of eggs go together; 
 indeed, the destructive work of the ovipositor is quite as effective as 
 the injury caused by feeding, for the cutting of numerous incisions into 
 a tiny stem greatly weakens it. We have seen the stems of cherries 
 and prunes so injured in this way that, after the fruit becomes almost 
 half grown, the stem weakens and the fruit falls. 
 
 The insect always chooses the tendercst parts of a plant for oviposi- 
 tion, and this with reason. If the tissue is hard there is danger of the 
 ovij)Ositor becoming fastened so that it can not be withdrawn. Also. 
 
THK PEAR THRIPS. 11 
 
 during the development of the egg and the issuing of the larva it is 
 necessary that the tissue be very flexible; the egg must be in close con- 
 nection with the tree sap and must be kept moist, for the egg-covering 
 is elastic and the embryonic thrips within increases in size quite notice- 
 ably before it issues. The egg stage lasts approximately four days. 
 
 As the fully developed eggs are quite large there is space inside of 
 the adult insect's body for but a few at a time -seven or eight. The 
 insect probably places but few eggs during a single day. She feeds for 
 a time and deposits an egg, and then moves to another and still other 
 places; this may be on one or more trees, and thus she spreads her 
 progeny from tree to tree and from place to place wherever she goes. 
 When once set on ovipositing nothing seems to hinder, as we have 
 observed thrips in the act of placing their eggs at all hours of the day 
 and night and under all conditions of weather. The period of oviposi- 
 tion is of several weeks' duration, or practically all of the life of the adult 
 insect; and when oviposition is finished the life mission of the adult 
 has been fulfilled and death follows. It is interesting to note in this 
 connection that all adult insects which we have observed up to the 
 present time have been females, no males having been found. 
 
 Larva, Description and Habits. — The larvae of the pear thrips, of 
 which we have determined two stages, are tiny, white, soft-bodied, 
 wingless forms, with the customary pair of antennae, three pairs of legs, 
 and with mouth parts similar to those of the adult, as already described. 
 It is interesting to watch, with the aid of a strong lens, a young thrips 
 issuing from the egg. The tiny speck of an incision in the stem of a 
 blossom or leaf tells us where an egg has been placed, and the enlarg- 
 ing of the egg within, causing a swelling in the plant tissue at the 
 summit of which is the incision, tells us of the new insect about ready 
 to emerge. The first signs of life are apparent when the tiny head 
 with its bright red eyes appears, pushing out of the incision; little by 
 little, and, swaying backward and forward, the larva works itself out 
 until about half of the body is exposed, when first the antennae, then 
 one by one the pairs of legs, are made free from their resting position 
 against the body. Still swaying backward and forward, with legs and 
 antennae waving frantically about as if glad of the power of action and 
 eager to get free, the tiny insect works itself out from the egg-covering 
 and the cavity in which the egg was placed, almost to its full length, 
 when it leans forward and eagerly takes hold with its newly formed feet, 
 and with a few final efforts it pulls itself free and walks rapidly away. 
 From four to ten minutes are required for the young insect to thus free 
 itself from the egg. A number of leaves and blossom-stems in which 
 eggs had been placed were brought into the office and closely watched 
 to determine the length of time spent in the egg. In many cases these 
 
12 
 
 THK PEAR TIIKFI'S. 
 
 stems would become dry during the four days of confinement, and 
 almost invariably no young thrips issued. The egg seemed to need 
 the moisture for its preservation and development, and the young 
 thrips must have very tender and pliable tissue in order to he able to 
 emerge. The young insect is almost transparent, and when food is 
 taken in the green (chlorophyll) particles can be seen through the body 
 wall. From the beginning the body growth is very rapid, and a few 
 insects are capable of doing great injury, so voracious are they in 
 feeding. 
 
 Young thrips feed almost entirely on tender foliage and fruit, and 
 their manner of feeding is much the same as that of the adult insect, it 
 
 Fig. 6. Bartlett Pear. Head blossom clusters 
 conspicuous; one late straggling blossom 
 left untouched, the adult thrips having all 
 left the tree. Leaves deformed. 
 
 Flo. 7. Pear branch, showing the rolling and the 
 cup-shaped deformities of the leaf; injury 
 paused by the feeding of young thrips. 
 
 being a rasping and sucking combination. They usually prefer the 
 tenderest foliage, such as terminal buds, but often, as in the cherry, 
 they attack the under side of leaves near the prominent veins, causing 
 them to become much contorted and ragged, and full of holes. 
 
 Young thrips are perfectly helpless creatures and subject to the 
 attacks of other insects, but they seem to be able to protect themselves 
 in a remarkable way. They are commonly secreted in the terminal 
 tips of the branches, but in some cases they seem to take advantage of 
 certain tendencies in the growth of the plant on which they happen to 
 feed. Newly opening pear or apple leaves have a tendency to roll from 
 the sides inward. Young thrips find this inner protected surface a 
 most desirable place for food and shelter, and in feeding, the upper leaf 
 
THE PEAR THRIPS. 13 
 
 surface alone is weakened, which causes the leaf to roll up until eventu- 
 ally it becomes rolled up tight (Fig. 6). In doing this the insects get 
 the tenderest part of the leaf for their food, and also secure pro- 
 tection for themselves. With such shelter no ordinary predaceous 
 insect can reach them. Often on more mature leaves the insects in 
 feeding cause a deadening of the margin and the leaf in its develop- 
 ment is forced into an abnormal, cup-shaped growth. This is a very 
 characteristic injury on pear trees (Fig. 7). 
 
 The list of food plants of young thrips is larger than during the 
 adult stage. Aside from all the fruit trees mentioned on which the 
 adults feed and also on which the young are to be found, it often 
 happens that the young, by various means, are carried from the original 
 food plant to some other, being blown, for example, from the tree above to 
 a weed beneath. They have no wings and can not rly 
 back to the tree; a few crawl up again, but most of 
 them adapt themselves to the new food plant until 
 fully grown, when they go into the ground with the 
 others. All of our common weeds have thus been '^ • '» 
 
 found supporting young thrips, although no full-grown 7 4^j£, -~v> 
 
 insects have been seen feeding and depositing eggs on "*" % j ' s | 
 
 such plants. A [ -'pr;- ^7 
 
 It is the young thrips which injures fruit. Prunes % >-^-- \ ■ if 
 especially are affected, although a similar scab is 
 found on cherries, apricots, and pears. Well-set fruit 
 gets to be about the size of a pea before the old bios- A -# 
 
 som is sloughed off, and under cover of this dead W~ J 
 
 blossom, on almost every prune where thrips were TH> 
 
 present, one or more of the young could be found, FlGS Thl . ipsinseC( „ H , 
 also a small abrasion on the skin of the fruit where larval stage, greatly 
 the insect had been feeding. We have followed this en arge " ngina ■' 
 abrasion of the skin with the thrips present to the mature fruit with 
 its scab. Only the skin of the fruit is injured, and the marking enlarges 
 with the growth of the fruit. The scab was especially marked last year 
 where thrips were found, and it was very prevalent again this year. 
 It must be remarked, however, that something other than thrips has 
 caused much of the scab on prunes during the past year. 
 
 During the second larval stage of the young thrips a very decided 
 change takes place. The second larva, like the first, feeds voraciously 
 and after some three weeks from the egg reaches a size often larger than 
 that of the fully matured insect. (See Fig. 8.) At this time it ceases 
 feeding and falls to the ground. We have observed but few deliberately 
 walking clown the tree. Each individual goes into the ground separately, 
 entering by some crack or worm hole, and having reached a secure 
 depth it hollows out for itself a little cell, and in this chamber it remains 
 
 • -- -• 
 
14 THE PEAK 'I'll RIPS. 
 
 quiescent until the following year. Thrips get down into the ground 
 from three to ten inches, according to the structure and condition of 
 the soil; the prevailing depth is four to five inches. They are scattered 
 generally from a few inches to several feet from the trunk of the 
 tree. At the present time (September 30th), though the insects have 
 been in the ground several months, they arc still active if disturbed 
 and do not show signs of wing formation, the first indication of the 
 approaching nymph. Insects taken from the ground on August 28th 
 still showed green matter (chlorophyll) within the digestive tract, pre- 
 sumably food taken in several months before but as yet undigested. 
 
 The pupal changes take place in the ground in the same cell where 
 the larva has spent so much of its time. We have not determined how 
 long this stage lasts. 
 
 Orchardists know the adult insect best. It is the adult which comes 
 to our trees in countless numbers and which does the greatest injury to 
 fruit buds and blossoms. The mature insect, having wings, flies up 
 from the ground to the tree and, if it finds the tree in suitable con- 
 dition, at once begins to feed; but if it does not find the proper conditions, 
 it moves on to other and better places. Thrips remain close during the 
 day, either feeding or depositing eggs. They often leave the food-plants 
 just before sundown, and it is especially at this time that they migrate 
 from place to place. We have distinguished two modes of flight: a 
 hovering and a migrating one. If the food-plant offers suitable food 
 the insect conies out for a few minutes, hovers around and a little later 
 settles back on the same or. a nearby tree. There is, however, a dis- 
 tinctly migratory flight, when the insects in great numbers fly for some 
 distance, and in this way they are spread over a large area. 
 
 Adult thrips appeared in many orchards in alarming numbers on 
 February 22d during the season of 1905, and in 1904 some two or three 
 days earlier than this. Thrips continued to come out from the ground 
 on through March and April and for a short time during May. On 
 May 5th pupal forms were taken from the soil, and if, as we think, the 
 insect is but single-brooded, all of the previous year's forms had not 
 matured at this time. 
 
 METHODS OF CONTROL. 
 
 Natural Enemies. — The subject of natural enemies for the controlling 
 of our insect pests is of prime importance. If the pear thrips has an 
 effective natural enemy, what is it? And if it is not present here, 
 where can it be found? Such questions come to us repeatedly. First 
 of all we must know as much as possible about the life habits of the 
 thrips itself, and this is what we have tried to present in this report. 
 Up to the present time we have found several common predaceous 
 insects feeding on thrips, but none which are parasitic. Predaceous 
 
THE PEAR Til HIPS. 15 
 
 forms attack their prey externally and literally devour them, while 
 parasitic insects, which are considered perhaps the most important as 
 check insects, must live for a time within the body of their host. 
 
 Our pear thrips, from the very nature of its habits, spending by far 
 the greater part of its life concealed several inches beneath the surface 
 of the ground, as has been shown, is very largely protected from 
 ordinary predaceous or parasitic insects. It comes from the ground so 
 suddenly and injures the trees so qnickly that those of its enemies 
 which we have found here can hardly prove an effective check. 
 
 The Raphidians, the commonest feeders on thrips in the Santa Clara 
 Valley, are general predaceous insects, and feed rather on the younger 
 stages of thrips than on the fully developed insects. For the complete 
 control of the thrips pest, they do not appear early enough in the 
 season. Raphidians are distinctly a Western form of insect, being 
 found only in the far West and especially in California; they are 
 unusually voracious, and besides killing almost any insect which happens 
 in their way, they will attack and devour each other if confined together. 
 
 Ants were thought by some to do much good as an enemy to thrips. 
 One gentleman brought in an ant with a thrips impaled in its jaws — 
 the evidence complete. This matter we took up somewhat in detail. 
 Four hundred ants were taken as they descended the trunk of a thrips- 
 infested tree. Twelve carried something in their jaws; four of these 
 objects were thrips. From the observations only one per cent (four out 
 of four hundred) were actually killing thrips. It may be that others 
 of these ants killed thrips, but did not carry them down the tree. It 
 has been a common observation among orchardists that where ants 
 were usually abundant thrips were not numerous. 
 
 Spiders killed many thrips. Breeding-cages were placed in trees for 
 determining various points in the life habits of thrips, and later almost 
 invariably one or more spiders would be found within and most of the 
 thrips gone. 
 
 The most effective natural enemy of thrips in the Eastern States is a 
 bug, Triphleps insidiosus Say, as mentioned by Dr. Hinds; it feeds on 
 both plants and insects, and at times may be quite as destructive as the 
 thrips itself. 
 
 It was noticed by both myself and Mr. Budlong that often a tree 
 would be thickly infested with young thrips and when these disappeared, 
 supposedly going into the ground, only a few could be found; they 
 seemed also to leave the tree before reaching full growth. We believe the 
 explanation for this is that a fungous disease thinned out their number 
 to quite a large extent. This is borne out by the fact that some dead 
 thrips were found whose bodies were penetrated by a fungus mycelium, 
 and in one case small sporangia were seen on tiny stalks projecting 
 from the body. It is possible that these insects were dead before being 
 attacked by the fungus, and that after all the fungus was not parasitic. 
 
16 THE PEAR THRIPS. 
 
 Spraying. — It has been stated with regard to killing thrips by means 
 of various spray mixtures that whatever these mixtures are they must 
 lie thoroughly applied to do even fair service. We have learned from 
 this year's experience that for this particular thrips spraying is a very 
 unsatisfactory means of control. The insect is hard to reach because 
 of its manner of hiding in bud or blossom, and difficult to kill without 
 injuring the tree with the spray mixture itself. Because of its habits 
 of leaving the ground, extending over a period of several weeks, and its 
 habits of migrating, one might apply sprays and kill most of the insects 
 on his trees and within a very few days find an infestation as heavy as 
 before. The injury, as we have seen, is so rapid and so fatal-, taking 
 but a few days (four or live) to ruin an entire orchard, that were we to 
 depend on sprays all work would necessarily have to be done in a com- 
 paratively short time and repeated often, and this of course is imprac- 
 ticable. 
 
 It does not seem necessary at this time to give in detail the results 
 of the spraying experiments which we carried on during the past season. 
 Fourteen different formulas were tried, including various forms of soap 
 washes and caustics, tobacco, sulphur, crude carbolic and creosote oils. 
 The sprays were carefully prepared under the personal supervision of 
 .Mr. Budlong and myself, and applied with a Bean power outfit with 
 the Bean Cyclone ami Vermorel nozzles, under pressure of from 140 to 
 170 pounds. The results were very unsatisfactory. Exposed thrips 
 would be killed, but those within the blossoms or buds showed almost 
 no signs of injury. New adults and also young thrips, — for the young 
 continued emerging from the stems and leaves where the eggs had been 
 deposited, — appeared on the sprayed trees within a day or two after the 
 washes were applied, and after four or live days the trees revealed 
 almost as heavy an infestation of both young and mature thrips as 
 there was before any spraying had been done. No one wash proved 
 satisfactory, and apparently under the conditions none can. Sprays 
 will doubtless be tried again, but they can only lessen the number of 
 thrips with little or no appreciable results, and some injury to the tree 
 is almost certain to follow. 
 
 Methods of Cultivation. — From the foregoing it might seem an almost 
 hopeless task to check the thrips by either natural or artificial means. 
 The insect, as we have seen, spends the greater part of its life beneath 
 the soil, and this is probably the best time to attack it. After the larva 
 is fully grown, it leaves the food-plant and seeks a secluded place in 
 the ground. On entering the ground it follows openings such as cracks, 
 or holes made by other insects or worms, and reaching a depth of from 
 four to six inches, though often deeper, it hollows out a small cell on 
 the side of the larger opening, and thus securing itself aw r aits further 
 developments. 
 
THK PEAR THRIPS. 17 
 
 We believe that if the ground be thoroughly plowed and cultivated 
 during November, December, and January, or before the insects leave 
 the ground, which they begin to do by the first of February, many of 
 these young thrips would be killed or injured or so disturbed that but 
 few would ever reach maturity. 
 
 Our present manner of cultivation is well adapted to give the thrips 
 a long, undisturbed period of rest. During the late spring and early 
 summer, when thrips are in the ground, we give the soil nothing but 
 light cultivation, and as the insects are still in an active larval stage they 
 seek another place of hiding, if disturbed. After this no cultivation is 
 usually done until the last of February or March of the following year, 
 by which time the insects have left the ground. While the plowing 
 and cultivating method of control has not been thoroughly tried, we 
 believe it gives more promise than any other yet suggested. 
 
 SUMMARY. 
 
 The insect is single-brooded. Adult females appear and begin to 
 deposit eggs in February. After four days in the egg the young appear, 
 feed for a time on the tree, then drop and enter the ground, where the 
 greater part of the year is passed. 
 
 No effective natural enemies are yet known. 
 
 Owing to the insect's method of feeding, its habits of coming from the 
 ground, extending over a period of several weeks, and its habits of 
 migrating, sprays can be only partially effective, even where thoroughly 
 applied. 
 
 We suggest winter plowing and cultivating, with careful working 
 about the trees, so as to kill or injure the immature forms of thrips. 
 This plowing must be done before the first of February, at which time 
 the mature insects begin to come out from the ground. 
 
LIBRARY OF CONGRESS 
 
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