UNIVERSITY OF CALIFORNIA COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION CIRCULAR No. 223 November, 1920 THE PEAR THRIPS 1 By E. O. ESSIG The pear thrips was first recorded in California as a new species 2 in 1904 and for more than ten years it has been an orchard pest of considerable importance in the counties adjacent to and in the San Francisco Bay region. It first became serious in Santa Clara, Alameda, and Contra Costa counties, but in later years developed importance in Solano, Napa, Sonoma, Monterey, San Benito, San Joaquin, Sacramento, Yolo, Marin, and San Mateo counties. The appearance of the pear thrips in the orchards is sporadic; no two successive seasons are alike in any particular locality. It becomes destructive in small or in large, widely separated areas and may be very unevenly distributed in these. It is seldom that a large body of contiguous orchards is found uniformly infested; there may be a small district here, an orchard there, or a number of orchards having serious outbreaks, while neighboring trees escape injury. It is true that some individual insects may be found in practically all orchards, but in many they are not found in sufficient numbers to injure the crop. This fact has not been entirely explained, but the chief factors responsible therefor probably are climatic conditions, general orchard practices, artificial control, and natural enemies, each of which will be considered separately below. LIFE HISTORY The pear thrips spends the larger part of its life cycle in the soil beneath the plow sole as an immature "white thrips" in aestivation and hibernation. It enters the soil in May and June and remains there until February and March. In early November, a change in development takes place from the white larval to the pupal stage i Originally known as Euthrips pyri Daniel, then as Taeniothrips pyri (Daniel), now considered by European entomologists as synonymous with Taeniothrips incoiisequens Uzel, a widely distributed European species. The common name is * 'thrips," never "thrip, ' ' as one often hears. 2 S. M. Daniel, Entomological News, vol. 15, Nov., 1904, pp. 293-294. Original description. Z UNIVERSITY OF CALIFORNIA EXPERIMENT STATION and finally to the adult stage of winged "black thrips, " which emerge from the soil with the first warm days when the buds of the trees are beginning to burst. They continue to emerge until the trees are in full bloom and even later. The adults are mostly very dark brown or almost black and are not easily confused with other species of thrips common in the orchards. In some seasons they emerge before the buds open and may then be found in great numbers on the limbs and twigs of the trees. In the case of the prune and the pear, which Fig. 1. — a, Prune twigs, showing condition of fruit buds when adult thrips begin to attack them. French prune (left), Imperial prune (right). B, Portion of a stem or petiole of a prune flower, showing hatching of young pear thrips. a, Egg exposed to view by removing epidermis; b, young "white thrips" emerging from egg under epidermis; c, exit hole of young thrips; d, small nearly-hemispherical swelling caused by inserting the egg under the thin epidermis. Greatly enlarged. C, Cherry leaf, showing characteristic perforations resulting from attacks of young or "white thrips" when the leaf was unfolding. (Original.) of all fruit trees, are probably the most seriously injured by thrips, the blossom buds open in small clusters, and in these the thrips collect in great numbers. After their long fast, they are voracious feeders and may within a few days destroy the flower buds entirely, even before the blossoms begin to open. The average orchardist associates thrips with blossoms and does not think of examining his trees for them until the most serious damage is done. It is true that the thrips continue to emerge from the soil, and to infest and even seriously injure the blossoms as well Circular 223 THE pear thrips 3 as the leaf buds even to the extent of making control measures neces- sary, but the strategic time in thrips control is before the blossom buds are fully open. Besides feeding, the adults deposit countless numbers of minute bean-shaped, transparently white eggs just under the epidermis of the blossom and leaf petioles. The small distorted areas caused thereby may be easily noticed with the naked eye. The egg-punctures may be so numerous as to cause a serious dropping of the blossoms. In such cases, however, the adults are numerous enough to have previously destroyed most of the buds. The young "white thrips" begin to appear nearly two weeks after the blossoms start to open, at about the time the last blossoms are opening, or more often, when the young leaf buds are unfolding. They may damage the blossoms seriously, but the chief injury is to the young fruit, still in the jacket, and to the developing leaves. After two or three weeks of feeding, or about the middle of May or first of June, the "white thrips" drop to the ground and go into the soil to remain during the summer, fall, and winter months. From this review it will be seen that there is but one generation a year; the adults appearing in February, March, and April and the young "white thrips" leaving in May and June. A few straggling adults as well as young may be found much later, especially on young nursery trees. TREES ATTACKED The following kinds of orchard trees and vines are attacked : prune, pear, cherry, plum, almond, peach, apple, fig, English walnut, and grape, but only the four first named are seriously injured. Earlier varieties usually suffer most, but there are times when the later ones are greatly injured. It must not be overlooked that other species of thrips 3 are also commonly found upon fruit trees. These are often associated with the pear thrips but may occur without them. 3 The other species common upon deciduous fruit trees are : The wheat thrips, FranMiniella tritioi (Fitch), which is a rather slender, pale yellow species with a noticeable orange-colored prothorax. It may be associated with pear thrips in the blossoms, but is more often found in midsummer and fall, having migrated from the drying grasslands, grain, and alfalfa fields. It is responsible for considerable injury to young peaches in various parts of the state. The bean thrips, Heliothrips fasciatus Perg., is distinguished by its black color and the dark markings on the noticeably light-colored wings. It may be associated with the pear thrips or may appear later in the season. The greenhouse thrips, Heliothrips haemorrhoidalis Bouche, is a dark brown species easily confused with pear thrips, but not often of any consequence. The western thrips, Euthrips ocoidentalis Perg., is a small, pale yellow species often associated with pear thrips in blossoms, but not important as it does no injury. UNIVERSITY OF CALIFORNIA EXPERIMENT STATION CLIMATIC CONDITIONS INFLUENCING THRIPS DEVELOPMENT No rules or regulations based on climate can be formulated to determine in advance the abundance or destructiveness of pear thrips because little is known respecting just how much this insect is influenced by rains or droughts, and by cold or hot weather, but from past observations the extreme of any of these over a period of several years has a noticeably detrimental influence on development; while under what appear to be the climatic conditions most unfavorable for thrips, some sections are seriously affected. In commercial fruit grow- ing it is not a safe practice to rely upon any extreme of climate to control the insect. GENERAL ORCHARD PRACTICES AGAINST THRIPS Under general orchard practices we include cultural methods which are designed primarily for other purposes than insect control, but which have nevertheless proved to be of great value in checking the ravages of pear thrips. Among the practices are : thorough fall irrigation, chiefly by flooding from late September through October and November, deep fall plowing, and a good heavy cover crop. FALL IERIGATION On a small scale, thorough irrigation by flooding has been prac- ticed as a control measure against pear thrips in Yolo, Solano, and Santa Clara counties, and the results obtained have been excellent in practically all cases. One farmer who has followed this plan in part of his prune orchard, leaving an adjoining part of the orchard as a check, has for several years demonstrated conclusively the truth of the foregoing statement. Each year the non-irrigated trees have suffered heavily from thrips while those irrigated have produced normal crops. In some sections it is claimed that fall irrigation has not proved satisfactory, but in most cases this has probably been due to a lack of sufficient water to reach the young thrips before transfor- mation to the adult stage has taken place. "Winter rains and spring irrigations come too late to have any effect since the adult insects in the soil are more resistant to unfavorable conditions than the imma- ture forms. Circular 223 THE pear THRIPS DEEP FALL PLOWING While not so important as either of the other recommended practices, this is the only one that can be employed by a great many farmers who are unable to irrigate, or to retain a cover crop until after the trees bloom. Deep fall plowing disturbs the young insects sufficiently to cause a considerable mortality not obtainable by early or late spring cultivation when the insects have reached the adult stage. It is a practice which should be followed by orchardists where the other methods are impossible. COVER CEOP Any kind of a heavy cover crop allowed to remain until the trees come into full bloom has given excellent results in thrips control when the practice was followed for a number of successive years. The large number of fine cover crops which have been grown in the Santa Clara Valley orchards in late years have been accompanied by a decrease in pear thrips injury. Next in importance to growing the cover crop is permitting it to remain if possible until the blooming period is passed before plowing under, but this will depend upon late rains or other available water to insure proper tillage. Early barley has not proved so successful as a cover crop as have other plants. The cover crop does not serve as food for the thrips, as many believe, but on the contrary produces a cold, unfavorable soil con- dition which destroys many of the insects. It is of most importance because it retards their emergence until after blooming time when the period for the most serious damage is past. During the spring of 1920 many orchards were studied with this factor in mind and where cover crops were turned under early the thrips appeared in great numbers. Orchards partly plowed early showed sharply the difference between the numbers of insects present in plowed and unplowed parts and the only orchards which were entirely devastated were those with no cover crops at all. There are exceptions to the favorable results occurring from cover cropping and other practices described, but all the data thus far gathered indicate that these are excellent orchard practices for thrips control. UNIVERSITY OF CALIFORNIA EXPERIMENT STATION CONTROL BY SPRAYS The work done by such competent entomologists as Dudley Moulton, Paul Jones, S. W. Foster, Earl Morris, D. L. Currier, and others on the control of pear thrips has not been without excellent results. They not only made possible the so-called "Government Formula," but worked out the practical field-spraying operations Fig. 2. — A suitable hand-duster for applying nicodust to control pear thrips. Note condition of flowers at time of making this particular application. A high wind prevented proper spreading of the dust. (Original.) which insure adequate control. The government formula is the following : Distillate emulsion 10 gals. "Black leaf 40" 1 pint Water to make 200 gals. When this formula was first prepared there were few commercial oil sprays on the market so that it was first necessary to make a distillate emulsion. 4 In recent years there have appeared the miscible oils Distillate 27° Baume 20 gals. Fish oil soap 24 lbs. Caustic soda 12 lbs. Hot water to make 20 gals. 4 Formula for homemade distillate emulsion for stock use : Circular 223 THE pear THRIPS 7 which may be used as follows : Miscible oil 5 gals. "Black leaf 40" 1 pint Water to make 200 gals. These formulae are for use against the adult "black thrips" and should be applied liberally as soon as any considerable number of insects is found upon the trees. Do not fail to make daily inspections after buds begin to swell! Applications should be repeated if neces- sary until the adults begin to lessen in numbers. In case the "white thrips" appear in destructive numbers later, the same formula may be safely used if the oil content is decreased and the nicotine increased as follows: Distillate emulsion 6 gals. "Black leaf 40" iy 2 pints Water to make 200 gals. Miscible oil 3 gals. "Black leaf 40" 1% pints Water to make 200 gals. For work with thrips, the greater force and volume delivery of the spray gun has rendered it superior to the spray rod, insuring not only a better, but a quicker and easier job. The loss in material occasioned thereby is more than overcome by the rapidity and ease of operation. CONTROL BY DUST SPRAYS The development of the nicotine dust sprays for aphis control suggested the use of dusts in combating thrips. Dust sprays offer advantages over liquids in cost, saving of time, and ease of applica- tion. The power equipment necessary is not expensive, and suitable hand-dusters may be had at a very low cost. A power-duster with one operator and a driver can cover from 20 to 30 acres a day, while one man with a hand machine can dust only from 3 to 5 acres a day. These are important factors in thrips control where only limited time is allowed. Where rainy weather conditions are likely to make it impossible for a power sprayer to be moved over the soft ground, a light power-blower or hand-blower can be used effectively. The owner of a small orchard can well afford to have at least one hand- blower to care for his own trees. With these factors in mind a large number of tests were made with 5 per cent "Nieodust," containing 5 per cent "Black leaf 40" 8 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION or 2 per cent of pure nicotine. The dust used for carrying the nicotine is finely pulverized kaolin, an inert clay. A power-blower and hand-dusters were used and widely separated orchards treated. Experiments were also made next to sprayed trees. The results of the work demonstrated clearly that the 5 per cent "Nicodust" readily killed all exposed thrips. It was more efficient against the adult "black thrips" than against the "white thrips" Fig. 3. — Applying nicodust with a power-dusting machine to control pear thrips. A good distribution of the dust is being accomplished. (Original.) hidden in the tightly curled unfolding leaves. After dusting, the thrips usually drop from the twigs immediately so that but few dead individuals can be found on the trees. The most efficient killing occurs during the heat of the day because of the rapid volatilization of the nicotine, so that an early start in the morning when the temperature is low may prove to be a waste of time and material. It required about *4 pound of "Nicodust" per average sized tree and, at a cost of fifteen cents per pound, this would amount to three and three-fourths cents per tree for material. The cost of application may be estimated by a calculation based upon an average treatment Circular 223 THE PEAR thrips 9 of 20 acres per day with a power machine and two men and three acres per day with a hand machine and one man. Dusting for adult " black thrips" gave practically as good results as spraying. Dusting for "white thrips" was effective, killing all exposed insects, but was not quite so effective as a single spraying, owing to the greater penetrating power of the oil sprays, but two dustings can be made at almost the same cost as one spraying, and if properly timed would give much better results than a single spraying. 5 NATURAL ENEMIES The natural enemies of the adult pear thrips are few. The young thrips in the soil appear to be practically free from parasites and predators, but those on the trees are preyed upon by the insidious plant bug, Triphleps tristicolor White, the larvae of the California green lacewing, Chrysopa oalifornica Coq., those of the brown lace- wing, Hemerobius oalifomicus Banks and H. pacificus Banks, the larvae of the common ladybird beetle, Hippodamia converges, syrphid fly larvae, etc., but as yet natural enemies appear to be an unimportant factor in the control of thrips. 5 This work will be continued by the writer during the coming season, working under the State Deciduous Fruits appropriation through the University of Cali- fornia Deciduous Fruit Experiment Station, at Mountain View, in cooperation with County Horticultural Commissioner L. E. Cody, Deputy Horticultural Com- missioner D. L. Currier, and Professor W. L. Howard, all of whom have aided very greatly in securing the information contained in this article. 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Preliminary Report on Kearney Vine- 312. yard Experimental Drain. 313. The Common Honey Bee as an Agent in 316. Prune Polination. 317. The Cultivation of Belladonna in Cali- 318. fornia. 319. The Pomegranate. 320. Grain Sorghums. 321. Irrigation of Rice in California. 322. Irrigation of Alfalfa in the Sacramento 323. Valley. Trials with California Silage Crops for 324. Dairy Cows. The Olive Insects of California. 325. The Milch Goat in California. Commercial Fertilizers. Potash from Tule and the Fertilizer Value of Certain Marsh Plants. The June Drop of Washington Navel Oranges. Seedless Raisin Grapes. The Use of Lumber on California Farms. Commercial Fertilizers. California State Dairy Cow Competition, 1916-18. Control of Ground Squirrels by the Fumigation Method. Grape Syrup. A Study on the Effects of Freezes on Citrus in California. Fumigation with Liquid Hydrocianic Acid. II. Physical and Chemical Pro- perties of Liquid Hydrocianic Acid. The Carob in California. II. Nutri- tive Value of the Carob Bean. Plum Pollination. Investigations with Milking Machines. Mariout Barley. Pruning Young Deciduous Fruit Trees. The Kaki or Oriental Persimmon. Selections of Stocks in Citrus Propagation. The Effects of Alkali on Citrus Trees. Caprifigs and Caprification. Control of the Coyote in California. Commercial Production of Grape Syrup. The Evaporation of Grapes. Heavy vs. Light Grain Feeding for Dairy Cows. Storage of Perishable Fruit at Freezing Temperatures. Rice Irrigation Measurements and Ex- periments in Sacramento Valley, 1914- 1919. CIRCULARS No. 70. Observations on the Status of Corn Growing in California. 76. Hot Room Callusing. 82. The Common Ground Squirrels of California. 113. Correspondence Courses in Agriculture. 114. Increasing the Duty of Water. 115. Grafting Vinif era Vineyards. 117. The Selection and Cost of a Small Pump- ing Plant. 124. Alfalfa Silage for Fattening Steers. 126. Spraying for the Grape Leaf Hopper. 127. House Fumigation. 128. Insecticide Formulas. 129. The Control of Citrus Insects. 130. Cabbage Growing in California. 131. Spraying for Control of Walnut Aphis. 135. Official Tests of Dairy Cows. 136. Melilotus Indica. 137. Wood Decay in Orchard Trees. 138. The Sdo in California Agriculture. 139. The Generation of Hydrocyanic Acid Gas in Fumigation by Portable Machines. 140. The Practical Application of Improved Methods of Fermentation in California Wineries during 1913 and 1914. 143. Control of Grasshoppers in Imperial Valley. 144. Oidium or Powdery Mildew of the Vine. 148. "Lungworms". 152. Some Observations on the Bulk Handling of Grain in California. 153. Announcement of the California State Dairy Cow Competition, 1916-18. 154. Irrigation Practice in Growing Small Fruits in California. 155. Bovine Tuberculosis. No. 156. 157. 158. 159. 160. 164. 165. 167. 168. 170. 172. 173. 174. 175. 176. 177. 178. 179. 181. 182. 183. 184. 185. 187. 188. 189. 190. 191. How to Operate an Incubator. Control of the Pear Scab. Home and Farm Canning. Agriculture in the Imperial Valley. Lettuce Growing in California. Small Fruit Culture in California. Fundamentals of Sugar Beet Culture under California Conditions. Feeding Stuffs of Minor Importance. Spraying for the Control of Wild Morning- Glory within the Fog Belt. The 1918 Grain Crop. Fertilizing California Soils for the 1918 Crop. Wheat Culture. The Construction of the Wood-Hoop Silo. Farm Drainage Methods. Progress Report on the Marketing and Distribution of Milk. Hog Cholera Prevention and the Serum Treatment. Grain Sorghums. The Packing of Apples in California. Factors of Importance in Producing Milk of Low Bacterial Count. Control of the California Ground Squirrel. Extending the Area of Irrigated Wheat in California for 1918. Infectious Abortion in Cows. A Flock of Sheep on the Farm. Beekeeping for the Fruit-grower and Small Rancher or Amateur. Utilizing the Sorghums. Lambing Sheds. Winter Forage Crops. Agriculture Clubs in California. Pruning the Seedless Grapes. CI RC UL ARS— Continued No. 193. A Study of Farm Labor in California. 198. Syrup from Sweet Sorghum. 201. Helpful Hints to Hog Raisers. 202. County Organization for Rural Fire Control. 203. Peat as a Manure Substitute. 204. Handbook of Plant Diseases and Pest Control. 205. Blackleg. 206. Jack Cheese. 207. Neufchatel Cheese. 208. Summary of the Annual Reports of the Farm Advisors of California. 210. Suggestions to the Settler in California. 213. Evaporators for Prune Drying. 214. Seed Treatment for the Prevention of Cereal Smuts. No. 215. Feeding Dairy Cows in California. 217. Methods for Marketing Vegetables in California. 218. Advanced Registry Testing of Dairy Cows. 219. The Present Status of Alkali. 220. Unfermented Fruit Juices. 221. How California is Helping People Own Farms and Rural Homes. 222. Fundamental Principles of Co-operation in Agriculture. 223. The Pear Thrips. 224. Control of the Brown Apricot Scale and the Italian Pear Scale on Deciduous Fruit Trees. 225. Propagation of Vines. 226. Protection of Vineyards from Phyl- loxera.