Dissemination of the Beet Leafhopper in California

icHNiCAL BULLETIN NO. 1030 ISSUED MARCH 1951

DISSEMINATION OF THE BEET LEAFHOPPER IN CALIFORNIA

FRANCIS It LAWSON JOSEPH C. CHAMBERLIN, and GEORGE T. YORK

Division of Truck Crop and Garden Insect InvestigaticHis Bureau of Entomology and Plant Quarantine Agricultural Research Administration

UNITED STATES DEPARTMENT OF AGRICULTURE, WASHINGTON, D. C. Technical Bulletin No. 1030—1951

'iniK ^^»ggaailfe^^-^ Dissemination of the Beet Leafhopper in California By FRANCIS R. LAWSON, JOSEPH C. CHAMBEBLIN, and GEORGE T. YORK, ento- mologists, Division of Truck Crop and Garden Insect Investigations, Bureau of Entomology and Plant Quarantine, Agricultural Research Administration' CONTENTS Page Page Introduction 1 Factors governing leafhopper Methods 2 flight—Continued Physical factors—Continued Location of work 4 Coalinga district 5 Wind direction 23 Modesto-Hospital Canyon .. 6 Effect of cold weather on Central San Joaquin Valley. 7 day flights 28 Other weather factors.. 29 Seasonal occurrence of the beet Interaction of all factors leafhopper 8 controlling flight 30 Factors governing leafhopper Mechanics of flight 33 flight 9 Direction and speed of flight. 33 Biological factors 10 Host plant relationships 10 Height of flight 34 Egg development 10 Types of flight 38 Parasites 13 Swarming 38 Discussion of biological Primary and secondary factors 13 disseminations 42 Physical factors 13 Effect of weather on routes of Minimum temperature dissemination 46 threshold 13 Spring movements eastward Maximum temperature into the San Joaquin Valley 46 threshold 16 Spring movements westward Light and time of day.. 16 through the Coast Range. . 52 Interaction of light and temperature 21 Fall movements 54 Day-to-day variations in Summary 56 temperature 22 Literature cited 57 INTRODUCTION The beet leafhopper (Circulifer tenellus (Baker)) is the only vector of the curly top disease of sugar beets, tomatoes, beans, melons, and other plants in the western part of the United States (Ball 3, Carsner 6, 7, Severin 31, Severin and Henderson 32).^ ^ Received for publication September 13, 1950. ^ The studies reported in this bulletin were conducted under the direction of W. C. Cook. Alfred A. Barney and Charles H. Feltes gave assistance in the field camps. The Spreckles Sugar Company made some of its records available, and W. Suttie of that company took certain population counts in 1935. Heber C. Donahoe, formerly of this Bureau, and Dwight F. Barnes, of the Division of Fruit Insect Investigations of this Bureau, cared for the traps at Fresno, Calif., during the spring of 1937. ^ Italic numbers in parentheses refer to Literature Cited, p. 57. 2 TECHNICAL BULLETIN 1030, U. S. DEPT. OF AGRICULTURE

Most of the damage to crops results from infection carried by leafhoppers that move in from more or less distant breeding grounds. A thorough understanding of leafhopper movements is therefore of great importance, and was the principal objective of the studies reported here. Most of the field work was done in the San Joaquin Valley of California during the years 1935 to 1937. The literature on insect movements has been reviewed by Felt (15), Click (19), McClure (2i), and Uvarov (36), and no general discussion of it will be attempted here. The terms "migration" and "dispersal" have been used by various authors to designate the movements of the beet leafhopper and other insects (Hills 21, Dorst and Davis i^, Romney 27, Severin 29, Tutt 35). "Dispersal," according to the dictionary, means a scattering in all or many directions. Because any single movement of the beet leafhopper, or a series of movements, often takes a very definite direction, the use of this word as a general term appears inappropriate. The word "migration" is well established but has a specialized meaning in connection with the movement of birds, mammals, and fishes. As the movements of the beet leafhopper bear little resemblance to the migration of verte- brates but are similar in many respects to those of the wind-borne seeds of plants, the term "dissemination" is preferred. "Dissemination" as used in this bulletin means the transfer of a population of insects from one locality to another. As the term implies movement it is convenient to use another term, "flight." "Flight" as used here means only that insects were in the air and implies nothing with respect to speed, direction, or destination of travel. "Magnitude of flight" refers to the abundance of insects in the air at any given time.

METHODS Previous investigations of leafhopper disseminations have in- cluded a number of observations of the insects in flight (Severin 29), Most studies of flights have attempted to trace movements by means of population samples, on the theory that the infesta- tions vary inversely with the distance from the source. Sex ratio has been used as an indication of distance from source, as the percentage of females increases with increasing distance. More recently determinations of the fat content of leafhoppers have been employed to supplement population counts (Fulton 16, Fulton and Romney 18). These methods have yielded valuable data, but all of them have very definite limitations. They do not indicate sources when leafhoppers of different origins are involved. Fat content, and probably sex ratios, may also be a measure of the time spent on poor hosts rather than of the distance traveled. Interpretation of data derived from these methods is made difficult because leafhoppers may be delayed by adverse winds or forced to follow circuitous routes. In 1932 J. C. Chamberlin and R. A. Fulton conducted tests (un- published reports) in which leafhoppers in certain breeding areas BEET LEAFHOPPBR IN CALIFORNIA 3 of southern Idaho were spotted with colored lacquers. Marked leafhoppers were found at the place of marking or within a few hundred yards of it for a week or 10 days, but nene were re- covered at a greater distance. Pulton and Chamberlin (17) used an air-maze trap in Idaho and some useful data were obtained (Annand et al. 1), but this trap catches leafhoppers only when there is a fairly strong wind. In view of these limitations, the first project undertaken in these studies was the development of a device to count the leafhoppers actually in flight at any given time and place. Sticky traps of various sizes, shapes, and designs were tried first. They are referred to as bil-screen traps, since they were constructed of i^-inch mesh screen covered with oil as a sticker. An ordinary automobile transmission oil with a viscosity of S.A.E. 120 was used. It had an advantage over otheF* stickeirs, in that it could be put on at ordinary temperatures with a paint brush and yet would not run off at temperatures as high as 100° F. Insects caught could be easily removed and cleaned in kerosene or gasoline without mutilation. The oil-screen traps caught large numbers of males but very few females, and were therefore discarded. A car trap was made in 1935 by mounting a cone of wire screen on an automobile, so that insects could be collected from the slip stream when the vehicle was in motion. This device proved valuable in tracing and intercepting movements, but required the full time of one man and a car. For that reason it was adapted for use as a stationary trap by mounting the net on a motor-driven, horizontally revolving arm 6 feet long. Descriptions of the origi- nal device and the motor-driven one have been published (Cham- berlin and Lawson 10, Anonymous 2, Barnes et al. A, Cody 11,

FIGURE 1.—Rotary trap equipped with gasoline motor. 4 TECHNICAL BULLETIN 1030, U. S. DEPT. OF AGRICULTURE and Stage and Chamberlin S3), Figure 1 shows one of the earlier models. In 1937 the rotary trap was built into a stationary tower so that the nets were operated at 2i/^, 15, and 32 feet above the ground. Traps similar in principle to these have been developed by other investigators (Bonnet 5, Davies IS, McClure 2iy Wil- liams and Milne 37, Thomas and Vevae 3Í), Both traps have proved very useful in studies of the beet leafhopper and many other insects. The chief advantage of the automobile trap is its mobility. This trap makes possible the determination within a short time of the specific location over which insects are flying. Its chief faults are the time required for op- eration and the difficulties in using it except on good roads. The rotary trap, on the other hand, can be operated in rough country and is very economical to run, but lacks mobility. Little is known about the efficiency of these machines in terms of the proportion of insects removed from a given volume of air. Insects cannot dodge the moving nets, with the possible exception of drangonflies and a few other swift-flying, keen-sighted species. There is no apparent reason why visibility or weather conditions, except wind velocity, should bias trap catches. The rotary trap seems to be indirectly influenced by wind velocity because the machine in itself attracts some insects (including the beet leafhopper) as a resting place when wind velocities are low. Despite their limitations as a means of detecting leafhopper disseminations, population counts have provided some useful data. All samples on host plants were taken with the spear or sampling pan (Lawson et al, 22), a pyrethrum-oil spray being used as an activating and killing agent. These methods give rea- sonably accurate estimates of leafhopper populations. On nonhost plants an ordinary insect net was used. This is an unreliable method for leafhopper counts but was used when populations were very low because other methods required a prohibitive amount of labor. For measuring weather conditions the following instruments were used: hygrothermographs; a standard three-cup anemome- ter; an aneroid barometer; a home-made model of the Forest Service evaporimeter ; and a specially designed direction anemo- graph. LOCATION OF WORK Nearly all the work reported here was done in the San Joaquin Valley of California, a broad flat plain approximately 50 miles wide and 300 miles long. A number of field stations were established near the winter breeding grounds of the beet leafhopper and within the cultivated section of the valley. Exact locations are given in figure 2 ; a brief description of each station follows. BEET LEAFHÖPPER IN CALIFORNIA

FIGURE 2.—Topographic map of California, showing location of principal areas where studies of dissemination were made: a, Coalinga District; 6, Hospital Canyon; c, central San Joaquin Valley; d, Modesto.

COALINGA DISTRICT In the-^ring of 1935 the base camp was located in a branch of Jacalitos Canyon, near the town of Coalinga and in the foothills about 1 mile from the edge of the plain. The vegetation of this section consisted mostly of winter annuals and a scattered stand of desert saltbush (Atriplex poly carpa (Torr. Wats.)) and bunch- grasses. This area is a major breeding ground, and at the time of the study, leafhoppers and their host plants were very abundant. Detailed studies of flight were made here with oil-screen traps. In addition, the car trap was used to study movements in the general area as far south as Belridge and west to the Salinas Valley. 6 TECHNICAL BULLETIN 1030, U. S. DEPT. OF AGRICULTURE

MODESTO-HOSPITAL CANYON Modesto was the home station during the years this work was done, and a rotary trap was run here in the summers of 1935 and 1936. The town lies near the bottom of the valley at its northern end and is surrounded by cultivated fields. In 1935 the trap was set in a small patch of mixed Russian-thistle {Salsola kali var. tenuifolia Tausch.) and bract scale (Atriplex bra&teosa, S. Wats.), two important summer host plants of the leafhopper. During most of the 1936 season the trap was placed in a field of tomatoes. The tomato is not a host plant, but Russian-thistle and bract scale grew nearby. Hospital Canyon is on the west side of the valley near the northern end of the spring breeding grounds of the leafhopper. Studies were made here in the fall of 1935. From its mouth to a point 2 miles up, the canyon runs northeast by southwest and is % to i/4 mile wide, with steep walls 100 to 400 feet high. One trap was placed well within the canyon mouth on the valley floor about 25 feet from the nearest green plants. The vegetation consisted of a strip of Lepidospartum squamatum Gray from 100 to 500 yards wide in the dry stream bed^ which formed a 10- to 25-percent stand. Over the rest of the valley floor and the surrounding hills there was a very sparse and scattered stand of tar weed (Hemi- zonia sp.) and bluecurls or vinegar weed (Trichostema lanceola- tum Benth.). The general appearance of the terrain and vegetation is shown in figure 3. A second trap was operated on the north

FIGURE 3.—View of Hospital Canyon, showing trap and instruments used in the fall of 1935. A second trap is located on top of canyon wall. BEET LEAFHOPPER IN CALIFORNIA 7 rim of the canyon during part of the period of study and then moved to a barren plowed field just outside the mouth "where the nearest green vegetation was approximately i/i mile to the west.

CENTRAL SAN JOAQUíN VALLEY Headquarters in the central San Joaquin Valley was a camp on the plain about 2 miles from the edge of the hills opposite the mouth of Big Panoche Canyon. A trap was run here in the spring of 1936 (flg. 4) and again in 1937. The vegetation surrounding

;;*«

FIGURE 4.—General view of trap and instrument setup at Big Panoche Camp, spring of 1936.

the camp consisted of scattered bushes of desert saltbush and match weed {Gutierrezia sp.) plus a ground cover of winter annuals in which leafhopper host plants were abundant. A thicket of tree tobacco (Nicotiana glauca Graham) and scattered cottonwood trees grew along the creek near camp. In 1986 leafhoppers and their host plants were abundant in and around the camp, but in 1937 the annuals were almost dry and most of the leafhoppers had moved out or perished, although high populations were present in adjacent areas. In both years when work was done at Panoche Camp a trap was also run in the adjacent cultivated district near the town of Mendota. In 1936 this machine was located on the Coit ranch in the edge of a barley ñeld about i^ mile from a field of sugar beets. In 1937 it was placed in the corner of a plowed field on Gales ranch. The only host plants were smotherweed {Bassia hyssopifolia (Pall.) Kuntze) in a small patch in a nearby barn- yard and sugar beets in a large field 100 yards to the southwest. 8 TECHNICAL BULLETIN 1030, U. S. DEPT. OF AGRICULTURE

In 1937 a trap was placed at Fresno, near the eastern edge of the valley about 50 miles from Mendota. Located in the back yard of a residence, it was surrounded by grass and ornamentals. The only leafhopper host plants were a few plants of lambsquarters (Chenopodium album L.) and narrow-leaf goosefoot (Chenopo- dium leptophyllum Wats.) growing in gardens and along alleys. In the fall of 1936 studies were conducted at a base camp 4 to 5 miles southwest of Ora Loma, on the open plain about 1 mile from the edge of the hills. One trap was run here; another was set 4 to 5 miles northeast of Ora Loma in the edge of the cultivated district south of South Dos Palos and on November 1 was moved to a location well within the cultivated district southeast of Ora Loma. Both traps were in pasture land bearing a good growth of newly germinated winter annuals including varying numbers of leafhopper host plants. Near the camp at Ora Loma small plants of Russian-thistle were scattered all over the plains surrounding the trap. In 1936 a third trap was located near El Nido in the approximate center of the valley and south-southeast 15 to 20 miles from a large acreage of Russian-thistle in Merced County. This trap was set in the east edge of a pasture. Other surrounding fields were growing grain and alfalfa. The only summer host plants near the trap were a few Russian-thistle plants, which were cut down a day or two after work was begun.

SEASONAL OCCURRENCE OF THE BEET LEAFHOPPER Severin (28, 29) showed that in the San Joaquin Valley of Cali- fornia the principal concentrations of the beet leafhopper in the winter and spring are on the uncultivated plains and foothills bordering the west side of the valley from the Altamont Pass (near b, in fig. 2) southward to the upper end of the valley. The host plants are typical winter annuals growing abundantly on overgrazed range (Piemeisel and Lawson 26), These annuals germinate in the fall or winter at the beginning of the rains and die in April or May. One generation of the leafhopper develops on the winter annuals, sometimes more. Adult leafhoppers in flight are dispersed by winds. Thus the leafhoppers that develop in the winter and spring breeding areas are spread over the green cultivated areas of the valley. Here they feed on weeds and cultivated crops, spreading curly top disease. The summer host plants are principally weeds growing in cultivated fields, abandoned land, or heavily overgrazed pasture (Severin 29, Lawson and Piemeisel 28). Three or more generations develop on the weeds before they dry up or are killed by frost. In the San Joaquin Valley there are two major movements of the leafhopper each year—to the cultivated area in the spring and to the winter-spring breeding area in the fall. Minor movements take place as the generations mature and as small areas of host plants are destroyed or become unfavorable. Although the leafhopper can subsist for a while on almost any BEET LEAFHOPPER IN CALIFORNIA 9 green plant, its survival in damaging numbers depends on the movements between the summer and winter breeding areas. Sever in (29) reported that in the spring the Western San Joaquin Valley is the principal source of leafhoppers that infest the cultivated portions of the Sacramento Valley, and may con- tribute enough leafhoppers to the Salinas Valley and other coastal areas to cause serious damage. The spring breeding grounds cover a very large area, extending approximately 250 miles, and vary greatly in productivity. The summer breeding grounds, which supply the overwintering areas with leafhoppers, also oc- cupy portions of a very large area. The ultimate purpose of these investigations was to determine which sections of the large breeding grounds were most important, so that control measures aimed at the leaf hopper and its host plants could be applied intelligently. Previous attempts to trace leafhoppers directly by means of population surveys and by the marking of specimens on the breeding grounds had either failed or given indefinite results. Therefore, in these studies the first procedure was to investigate the basic characteristics of leafhopper movements with a view to develop- ing means of tracing them indirectly. This paper deals primarily with these characteristics and only secondarily with the sources and routes of movements. FACTORS GOVERNING LEAFHOPPER FLIGHT Four or five factors acting simultaneously govern leafhopper disseminations. To enable the reader to orient each factor in the whole picture, the following brief outline of the results of this investigation is given before the data are presented. The major disseminations of the beet leafhopper in spring and fall are clearly associated with the maturity of the broods and with the drying of host plants over wide areas. Beyond this we know little of the stimuli to movements except that they are modified by certain physiological conditions. Parasitized leafhoppers and females having mature eggs in the ovaries rarely travel very far. The method of movement is by flight. There can be no major movement until a large number of leafhoppers mature. The particular time of movement is determined largely by temperature and light. Flight is inhibited below a.certain threshold of temperature. Above the threshold the number of leafhoppers in the air increases with temperature when other factors are constant. Light conditions, however, strongly modify the effect of temperature, and the number of leafhoppers in flight is very much greater during the crepuscular period before sunrise and after sunset than in the full light of day or the darkness of night. When leafhoppers are in flight their routes and destinations are determined by the air currents. This does not mean, however, that prevailing winds determine these routes. Atypical air currents associated with local topography and particular times of day or with high temperatures may be far more important. 10 TECHNICAL BULLETIN 1030, U. S. DEPT. OF AGRICULTURE Leafhoppers making flights may land of their own accord or be forced down by low temperatures. They may alight in almost any place—on host plants, nonhosts, or bare ground. Leafhoppers in unfavorable situations and some on good host plants will fly again as soon as the weather permits and will continue flying until they find host plants or die.

BIOLOGICAL FACTORS HOST-PLANT RELATIONSHIPS There is no direct experimental evidence that poor conditions of host plants stimulate leafhoppers to make flights. However, major disseminations occur in the fall, when large areas of summer host plants are mature and dying, and again in the spring, when the winter annuals dry up. At these times, movement to other plants is necessary for survival. The major movements, however, appear to be only enlarged portions of a continuous dissemination. Leafhoppers can be caught in flight at any time when temperatures are favorable. For instance, a trap at Mo- desto in 1936 was run 51 days between July 31 and October 1 in a field of tomatoes (tomato is not a breeding host). The average daily catch for the entire period was 2.7 leafhoppers per day, and one or more were caught on 36 days. Most of the 15 days on which none were taken came during periods of cool weather. Host plants in fields nearby were in good condition throughout this period. A dissemination probably occurs whenever a brood matures but the exodus is larger if the host plants are unsuitable for further oviposition or feeding.

EGG DEVELOPMENT The preoviposition period in the beet leafhopper is several days (Carter 9), and an experienced observer can determine from the size and appearance of the eggs whether or not they are mature and ready to be laid. In the springs of 1936 and 1937 female leafhoppers caught on the wing in flight traps and on the ground in various situations were preserved and later dissected to determine egg development (table 1). Of the leafhoppers caught on the spring breeding grounds at the source of the migration, about 36 percent had mature eggs in 1936 and 2 percent in 1937. Of those caught in flight in both years, only 10 percent had mature eggs ; but of those caught on beets and other summer host plants at the destination 55 to 80 percent were gravid. These differences suggest that most of the disseminating leafhoppers are nongravid and that eggs develop shortly after summer host plants are reached. A more detailed examination of the data supports this conclusion. Day-to-day records on females captured in beet fields in 1937 (table 2) show sharp increases in the percentage of females without mature eggs at the time of the first two major influxes, on April 25 and May 2, and a gradual decline in the intervening BEET LEAFHOPPER IN CALIFORNIA 11 periods. The third influx on May 6 was not accompanied by a marked change in the percentage of females without eggs, although the rate of decline was checked on this date. An increase did occur 2 days later, when there was a significant decrease in female populations. The situation on this date is somewhat con- fused, but it is probable that a heavy influx was masked by a simultaneous efflux.

TABLE 1.—Mature eggs found in beet leafhoppers collected from various sources. All leafhoppers ivere taken with a sweep net, except in the trap collections.

Females with— Females Source of leafhoppers dissected 2 or more 1 QSS eggs

1936 Number Percent Percent Beet (summer host) 93 17 64 Spring breeding grounds 210 10 26 All trap collections 166 8 2 1937 Summer host plants 1,088 15 40 Nonhosts and bare ground 384 7 11 Spring breeding grounds 285 1 1 All trap collections 699 5 5

TABLE 2.—Numbers of female beet leafhoppers on sugar beets and percentages of nongravid females, near Gales Ranch, 1937

Date Leafhoppers Females Females dissected per beet without eggs Number Number Percent Apr. 24 11 0.06 7.^ 25 71 ^ .98 82 26 76 .84 62 27 28 3 .76 67 29 40 1.00 30 30 24 1.08 29 May 1 .82 2 107 '1.72 44 3 114 1.88 55 4 155 1.91 42 5 161 1.70 32 6 30 '2.50 30 7 62 2.36 19 8 102 '1.78 50 9 47 2.00 28 ' Significant population change. 12 TECHNICAL BULLETIN 1030, U. S. DEPT. OF AGRICULTURE

Females caught on nonhost plants and bare ground in 1937 were similar to those from the traps, but with a somewhat higher percentage of gravid females. The difference may be of no significance, as unnoticed host plants may occasionally grow in a field of nonhosts. These leafhoppers had probably moved recently but had not yet found suitable host plants. The differences between trap collections and collections made on the ground are in reality greater than would be inferred from table 1, because the sweep net, which was used to make ground collections, catches too low a proportion of gravid females. In collections made on May 4 and 5, 1937, at nearly the same time of day, the number of females with one or more eggs caught in the net was 12 percent less than the number caught in the sampling pan, and the number with 2 or more eggs was 21 percent less. Because the pan method is much less dependent on leaf hopper activity than the net method, and nearly all insects are caught, it is apparent that the net gives a biased result. The difference between counts in 1936 and in 1937 on the spring breeding grounds was probably due to the fact that when collections were made in 1937 the spring host plants were nearly dry and breeding had ceased, but in 1936 the hosts were still green and part of the leafhopper population was still breeding. Although most of the females caught in night were not gravid, a few had 1 or 2 eggs in the ovaries and an occasional individual had more than 2. In 1937, out of 699 females taken in the traps and dissected, 3 had 3 mature eggs, 2 had 4, 2 had 5, 1 had 7, and another 9 eggs. Some of these leafhoppers may have been dis- turbed by men working near the traps. However, when compari- sons are made between catches of the traps at Panoche on the breeding grounds, at Mendota in the cultivated district 10 miles from Panoche, and at Fresno 50 miles away, there is a progres- sive increase in the percentage of females carrying eggs (table 3). The numbers caught at Mendota and Fresno were low and definite conclusions cannot be drawn, but these data show that gravid females can make flights and suggest that under certain conditions they may travel fairly long distances.

TABLE 3.—Percentage of nongravid leafhoppers caught in traps at different distances from the source of dissemination

Minimum distance Location traveled by Females Females of trap leafhoppers dissected without eggs

Miles Number Percent Panoche Camp ... 0 229 92.6 Gales Ranch 10 77 87.0 Fresno 50 712 BEET LEAFHOPPER IN CALIFORNIA 13

There is some evidence that gravid females fly lower than those without eggs. In catches made with the tower trap at Pa- noche in 1937, the percentage of females with eggs was 7.4 at 2.5 feet, 8.1 at 15 feet, and 4.6 at 32 feet. All these data indicate that females without eggs in the ovaries are much more active than gravid individuals. Gravid females may fly under some conditions; but in the area studied most of the flying population was nongravid, although gravid females were present on the breeding ground and eggs developed soon after summer host plants were reached.

PARASITES When dissections were made for egg development during the 1937 season, a record was kept of the parasites seen, although no search was made for them and very small individuals may have been missed. The totals, which are given in table 4, therefore may be somewhat low. The percentage of parasitization in both sweep-net collections and trap catches was higher on the spring breeding grounds than elsewhere. Evidently parasitized females tend to remain behind. Most of the few caught in flight were near the ground, but one was taken at 32 feet in the air and three at 15 feet. DISCUSSION OF BIOLOGICAL FACTORS In brief, it seems that leafhoppers tend to move to other areas as they become adults, but that the exodus is greater when the host plants are unsuitable for further oviposition or feeding. Gravid females and those that are parasitized are less likely to move. All these biological factors affect the number of leafhoppers available for dissemination and may have an important influence on the numbers that can or will move at any given time. How- ever, these factors tend to change slowly and do not have much effect on hour-to-hour and day-to-day fluctuations in the magnitude of flight. Such variation is due primarily to physical factors.

PHYSICAL FACTORS MINIMUM TEMPERATURE THRESHOLD When traps were used to measure leafhopper flights, the insects caught were usually removed at intervals of 5 minutes to 1 hour, the interval depending on the type of information sought. As temperature records were taken at the same time, the short- period observations, when made at low temperatures, can be used to determine the minimum threshold for flight activity. The low figures for each season are given in table 5. The threshold appears to be 62° to 64° F. in the spring and 57° to 58° F. in the fall, although there are indications that it may vary a degree or two within a single season. (See October 21 and 24, 1936, compared with November 4 and 6, 1936.) The variation between sea- O TABLE 4.—Parasitization of disseminating female leaf hoppers, Panoche Camp, 1937 O SWEEP-NET COLLECTIONS > Leafhoppers Parasites td Source Parasitization Dissected Parasitized Pipinculids Dryinids Streptsiptera a

Number Number Number Number Number Percent Spring breeding grounds 285 8 5 3 0 2.81 Summer host plants 1,088 9 8 1 0 0.83 Nonhosts and bare ground 219 0 0 0 0 0 o oCO FLIGHT-TRAP COLLECTIONS Tower trap at elevation of— 32 feet 1 0 0 1 0.73 15 feet 3 3 0 0 1.86 2.5 feet 8 6 1 1 3.49 Total, all nets' 12 10 1 2 2.16 Gales Ranch trap 1 1 0 0 1.30 O Fresno trap 0 0 0 0 0 > ' Totals are not always equal to the sums of the 3 individual nets, because in one collection specimens from all 3 levels were inadvertently mixed. o BEET LEAFHOPPER IN CALIFORNIA 15

sons or even within a few days would be expected. Temperatures at which leafhoppers are active on the ground depend to some extent on previous temperatures. The writers and other workers have repeatedly observed that the threshold of activity on the ground increases in the spring and decreases in the fall. A similar conditioning probably affects flight activity also.

TABLE 5.—Lowest temperatures at which leafhoppers were caught on days when flight apparently stopped because of low temperatures

Spring 1 Fall Date Males Females Date Males Females

°F. °F. °F. 1935 1935 Apr.-May^ 64 Oct 17 62 * 24 61-63 1936 30 58-60 Apr 6 62-64 Nov. 6 60-62 60-62 7 62-65 "¿2-65* 6 57-61 57-61 12 63-64 7 58-60 13 59-64 Probable threshold 60-61 60 17 62-65 Probable threshold' 62-64 63-64 1936 Oct. 21 60-63 1937 24 57-59 57-59 Apr. 29 64-65 30 61-62 59-62 30 64-68 60-64 31 56-59 May 3 65-67 Nov. 1 56-61 56-61 5 60-65 *6Ô-65* 2 51-58 58-61 7 54-62 3 58-65 9 56-64 4 57-62 Probable threshold 64 ¿0-62' 6 56-58 Probable threshold 57-58 57-58

^ Temperatures given for the spring of 1935 were the lowest at which leafhoppers were caught in the car trap, and were determined by a thermometer read at some time during a run of 5 to 10 miles. Other figures were based on the rotary trap, and temperatures were read from instruments under standard shade or equivalent. Only leafhoppers caught at the 1.5- to 3.0-foot level were considered, as temperatures were taken at" approximately that height. ^ When temperatures are low the catch is also low and the exact time and temperature at which the last leafhopper is caught becomes a matter of chance. The exact threshold cannot be determined from such data, but it can be estimated from the day-to-day ranges of temperature. Thus, for the series April 29 to May 9, 1937, the maximum for May 7 indicates that the threshold for females was below 62. The minima on May 5 and April 30 indicate that it was above 60, so that the most probable figure lies between 60 and 62. Either the minimum of 64 for April 29 was too high or there was a drop in the threshold after this date.

There is evidence that females v^ill fly at slightly lower temperatures than males. In the spring of 1935, when the car trap was used to make short-period observations, the flrst and last leafhopper caught at low temperatures was nearly always a fe- 16 TECHNICAL BULLETIN 1030, U. S. DEPT. OF AGRICULTURE male. The lowest temperatures at which females will fly are approximately 2° lower than those at which males will fly.

MAXIMUM TEMPERATURE THRESHOLD Because all organisms become less active if the temperature becomes too high, it would be expected that above certain maximum temperatures the leafhopper would cease flight. Our data, however, do not indicate any such upper threshold, probably because temperatures were not high enough when the observations were made. Although flight usually stopped during the middle of the day, a comparison of the temperatures prevailing at that time show them to have been extremely variable, whereas the time was relatively constant. Time rather than high temperature appeared to be the critical factor. The highest temperatures at which leafhoppers were actually taken in flight were 90°-92° F. near Coalinga in the spring of 1935, 86^-88^ at Modesto in Sep- tember 1935, 82° at Hospital Canyon in October 1935, 86°-90° at Panoche in April 1936, 79°-85° at Ora Loma in October 1936, and 95^-98° at Panoche in May 1937.

LIGHT AND TIME OF DAY At certain seasons on warm calm evenings large numbers of beet leafhoppers can be seen swarming around any conspicuous object. This tendency to swarm at sunset is also well known in other species. Meunier (25) (see also Uvarov 36) has shown that the evening swarming of the cockchafer (Melonlontha melonlontha L.) keeps pace with sunset. When systematic observations were begun with the traps, it was found that the flights of the leafhopper followed a similar pattern. Day after day the catch in all types of traps began to increase shortly before sunset, rose to a very high peak, and decreased almost to zero, all within an hour. On many days the catch during this short period was several times that taken during the rest of the day. On some days there was another peak in the morning near sunrise, but the catch during this period was almost always smaller than in the evening, and very often there was no morning flight at all. When the trap catch on successive days is plotted a recurring diurnal pattern appears. The nature of the pattern is best shown when the records for a whole season are combined into an average curve (figs. 5, 6, 7). The curves show clearly that, regardless of season or locality, there is a high peak of flight in the evening, and in the spring another peak in the morning. The stimulus re- sponsible for this aspect of leafhopper behavior appears to be time or light. If the evening peaks of activity for successive days are plotted (fig. 8), it will be seen that they fall at nearly the same time each day in relation to sunset. However, on very cold days the peak occurs much earlier than normal. (See data for November 6 and 7, 1935, and May 3 and 4, 1937, in table 6.) It has been suggested therefore that temperature might be a more important factor than light in determining the time of flight. Severin (29) suggests that the lowering of the temperature at sunset may cause increased leafhopper activity. BEET LEAFHOPPER IN CALIFORNIA 17

71.7 Males ^^ Females Temperature

3 18 O X 16 û: UJ a. 14 H X 1? O 3 < 10 O i/i o: 8 o.ÜJ o. b o X Ü-< 4 -« 2

5 6 p.m. FIGURE 5.—Leafhoppers caught per hour and mean temperatures for periods when temperature was above apparent minimum threshold of 63° F. at Panoche Camp, April 5 to 17, 1936.

51.7

FIGURE 6.—Leafhoppers caught per hour and mean temperatures for periods when temperature was above apparent minimum threshold of 57° F. at Ora Loma Camp, October 21 to November 6, 1936. 18 TECHNICAL BULLETIN 1030, U. S. DEPT. OF AGRICULTURE

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CJ 00 u> ^ csj o « jnou Jddm6nD3 sjaddoq^oai BEET LEAFHOPPER IN CALIFORNIA 19 p.m. 6:30 Modesto 1935

illlll Hospital Canyon 1935

24 26 28 30 2 4 6 8 10 12 14 16 18 20 22 24 26 28 September October

Panoche 1937 7:30 Panoche 1936 7:00 Sunset

6:30 rAWh-

6:00 11 13 15 17 29 30 4 6 April April May FIGURE 8.—Evening peak of beet leafhopper flights (vertical bars) in relation to time of sunset. TABLE 6.—Time and temperature at sunset and at the peak of flight MODESTO Time (p.m.) Temperature CF.) Date Sunset^ Peak of flight Sunset Peak of flight 1935 Sept. 24 5:56 6:00-7:00 83 80-73 25 5:54 6:05-6:15 80 79-79 26 5:52 6:00-6:15 76 75-75 27 5:51 6:00-6:15 79 77-75 28 5:49 6:00 6:15 75 74-73 29 5:48 6:00 6:15 77 75-73 30 5:46 6:00-6:15 80 79-77 Oct. 1 5:44 5:45-6:00 73 73-71 2 5:43 5:49 6:00 71 69-69 3 5:41 5:45-5:55 71 71-70 4 5:39 5:45 5:52 73 73-72 5 5:38 5:45-5:50 75 74-73 6 5:36 5:40-5:50 73 73-72 7 5:35 5:40-5:50 77 75-73 8 5:33 5:30-5:40 73 73-71 9.. 5:32 5:30-5:40 75 75-73 10 5:31 5:40-5:50 71 71-70 11 5:29 5:18-5:30 63 63-63 12 5:27 5:30-5:45 66 65-64 13 5:26 5:15-5:30 76 67-65 14 5:25 66 20 TECHNICAL BULLETIN 1030, U. S. DEPT. OF AGRICULTURE

TABLE 6.—Time and temperature at sunset and at peak of flight—Continued

HOSPITAL CANYON

TimeÎ (p.m.) Temperature (° F.) Date Sunset' Peak of flight Sunset Peak of flight

17 5:20 5:15-5:30 67 73-66 18 5:19 5:30-5:45 71 69-68 19 5:17 5:30-5:45 75 73-71 20 5:16 5:15-5:30 70 70-69 21 5:14 64 22 5:13 64 23 . . 5:12 5:00-5:15 65 69-65 24 5:10 5:15-5:30 m 65-63 25 5:09 5:15-5:30 70 68-65 26 5:08 5:00-5:15 72 74-70 27 5:06 5:18-5:30 76 72-70 28 5:05 4:45-5:15 67 67-67 29 5:04 56 30 5:03 56 31 5:02 56 Nov 1 5:01 48 2 5:00 46 3 4:59 49 4 4:58 52 5 4:56 56 6 4:55 4:00-4:30 60 68-64 7 4:54 4:15-4:30 62 70-70

PANOCHE CAMP

1936 Apr. 7 6:38 6:00-6:30 64 71-65 8 6:39 9 6:40 68 10 6:41 6:30-7:00 11 6:42 6:30-7:00 77 79-74 12 6:43 6:30-7:00 78 79-77 13 6:44 6:30-7:00 74 76-72 14 6:45 6:45-7:00 71 71-69 15 6:45 6:30-6:45 72 74-72 16 6:46 6:45-7:00 71 71-70 17 6:47 6:30-6:45 70 71-70 1937 29 6:57 6:05-7:00 62 68-62 30 6:58 6:35-7:00 68 70-68 May 1 6:59 7:00-7:30 75 75-71 2 7:00 7:00-7:30 81 81-78 3 7:01 6:30-7:00 68 72-68 4 7:02 6:30-7:00 67 69-67 5 7:03 7:00-7:30 73 73-69 6 7:04 62 7 7:05 62 8 7:06 6:00-7:00 71 74-72

^ Sunset times are taken from the almanac. Observed sunset times at Modesto are practically equivalent. At Hospital and Panoche actual sunsets are somewhat earlier, owing to the proximity of the hills to the west. BEET LEAFHOPPER IN CALIFORNIA 21

If this were true, there should be a significant correlation between evening temperatures and the time of the peak, because low temperatures occur earlier on cold days than on warm. With sunset temperatures as a measure of day-to-day variation in evening temperatures, the coefficient of correlation with the time of the peak is only 0.016 for 45 days. This figure is not significant and is so low that it lends no support to the hypothesis that temperature is a dominant factor. On the other hand, if time is the factor controlling the peak, and if temperature is only a negative influence in stopping flight earlier than normal on cold days and does not otherwise influence the timing of the evening flight, there should be a positive correlation between the temperature at the peak and evening temperatures from day to day. That is, if the peak occurred at a definite time near sunset, the temperature at that time and at sunset would be nearly the same and would vary together from one day to the next. The coefficient of correlation for these two temperatures is 0.840 and highly significant. We conclude, therefore, that time or a closely related variable is the principal cause of the greatly increased leaf hopper activity in the evening. Actually the real factor probably is light intensity. The fact that the peak falls a little earlier each day in the fall and a little later each day in the spring, paralleling the change in sunset time, is evidence of this. Cloudy days were usually cold with little or no activity, but that which occurred seemed to be earlier than normal. Morning flights are probably governed by the same factors, but this is not so well shown by our data, owing to the effects of temperature in modifying the characteristic curve.

INTERACTION OF LIGHT AND TEMPERATURE During the morning flights the light was nearly always much stronger than during the evening flights. The temperature sel- dom reached the minimum threshold until well after sunrise, and flight was therefore inhibited at the time when light intensities were theoretically most favorable. This is supported by the data graphed in figure 7, in which the peak of the leafhopper catch curve falls at or very near the beginning of flight in the morning. A detailed examination of the records reveals that when morning flights occurred the rate of catch per hour was invariably highest during the first period. That is, flight was at maximum intensity as soon as temperature crossed the minimum threshold, and the curve apparently represented the tail end of what would have been a much larger flight had temperatures been suitable. The same phenomenon was often observed in reverse in the evening, when a large flight was quickly reduced by sub- threshold temperatures. Inhibition of morning flights when temperatures are low is probably the reason why such flights were made in the spring but not in the fall and were more prominent in 1937 than in 1936. On days when temperatures are high, the threshold is reached 22 TECHNICAL BULLETIN 1030, U. S. DEPT. OF AGRICULTURE

much earlier in the morning and leafhoppers are able to fly when light conditions are more favorable. The mean maxima for the days covered by figures 5 to 7 were 81.7 for the spring of 1936, 83.3 for the spring of 1937, and 73.5 for the fall of 1936. The interaction of light and temperature may also explain why the morning flight consists mostly of females. As already noted, females fly at a slightly lower temperature than males. Although the difference was not accurately determined, it appeared to be about 2° F. In the morning at near threshold temperature the time required for a rise of 2° is generally about 30 minutes. Be- cause the morning peak of flight occurs as soon as the threshold is reached, a considerable flight of females could take place while light was relatively favorable and before males would leave the ground. In the evening the threshold is usually reached after the peak of flight has passed, when activity is at a low level for both sexes. Furthermore, the temperature drop in the evening is much faster than the rise in the morning. Thus the differential between sexes is important in the morning but not in the evening. In brief, the typical diurnal pattern of flight observed in these studies appears to be basically a light reaction modified by temperature. If temperature were always high the primary flights of the leafhopper would be made near sunrise and sunset. Under actual conditions morning flights are usually inhibited by low temperature ; and when they occur they are limited mostly to females, because the slightly lower temperature threshold permits them to take wing half an hour earlier than males when light conditions are more favorable. During the hours of full daylight there is very little flight, although temperatures are usually suitable. In the evening, when light begins to fade, both males and females take to the air in large numbers. This flight gradually subsides as darkness falls, although a few leafhoppers may continue flying all night if temperatures permit. Usually, the threshold is reached shortly after the peak of flight and all movement stops until suitable temperatures are reached the next day.

DAY-TO-DAY VARIATIONS IN TEMPERATURE As leafhoppers fly only when the temperature is above a certain minimum, it might be expected that above the threshold the rate of activity would increase with temperature. However, no such increase is apparent in the hourly records because of the effect of light, which so greatly magnifies the flight during the crepuscular periods when temperatures are comparatively low. Nevertheless, temperatures have a very definite effect on flight, as can readily be seen when the total catches from day to day are compared with concurrent mean temperatures. That is, a comparison of day-to-day variations in temperature during the favorable crepuscular period with similar fluctuations in catch shows that more leafhoppers fly on warm days than on cold. The basic data on this point are given in tables 7 and 8, but a clearer picture is shown by the correlations in table 9. The coefficients of correlation between mean temperatures and evening BEET LEAFHOPPER IN CALIFORNIA 23

catches are significant for all localities and seasons, and with the exception of Modesto in 1935 are all quite high. Temperature is thus a major factor in determining the number of leaf hopper s making nights. WIND DIRECTION When the data on temperature and catch were plotted it was apparent that on certain days the catch was far higher than would have been expected from the temperature alone. This phenomenon is shown by table 10, which gives the deviations of the actual catch from that expected from the temperature for all days for which complete records are available. The table shows that extreme plus deviations coincided with days on which there were favorable winds. At Ora Loma the days marked as having such a wind are those on which the prevailing wind during the evening was from the southeast or south-southeast. There was an extensive area of summer-breeding hosts 4 to 10 miles from the trap in this direction. At Panoche the situation was somewhat different. Here the principal concentrations of leafhoppers were in and near the hills 2 to 3 miles southwest of the trap. The prevailing wind was from the nprthwest during the daytime and southwest off the breeding grounds at night. Although the night wind was from a favorable direction, the change did not normally occur in the evening until after the temperature had dropped below the threshold for night. The evenings when the wind changed before the threshold was reached are marked in table 10. Every day on which the catch was greatly in excess of that expected had a favorable wind. Except for October 30 and April 12, 1936, every day on which there was a favorable wind showed an excessively high catch of leafhoppers. October 30 was not really an exception because the maximum temperature was only 62° F., too low for flight except for a brief period during the middle of the day. On April 12, however, both temperature and wind were favorable, and about the same as on April 11, but the flight was much smaller. The reason is unknown. Possibly the heavy flights of the previous day had temporarily exhausted the supply of the leafhoppers in condition to move. The importance of changes in wind direction under different temperatures and at different times of day is well illustrated by a comparison of the records for April 11 and 17, 1936. At this season the evening flight reached a peak between 6:30 and 7 p.m. and ended between 7:30 and 8. On April 11 the wind changed to a favorable direction about 7:30, but the temperature threshold was not reached until late at night; so that, although there was a total of 6 hours and 45 minutes of favorable wind, most of it occurred after the usual time of the evening flight. The number of leafhoppers caught was higher than normal, but not so high as on April 17, when the wind changed direction at 6 p.m. and a favorable wind blew during most of the evening flight. This flight was much greater than on April 11, although the temperature was lower and a favorable wind blew for only 2 hours and TABLE 1 .—Day-to-day variations in leafhopper flight under principal meteorological conditions during the spring disseminations at Panoche Camp. Records from the rotary net at elevation of ^14 feet

Maxi- Mean temper- Mean humidity, Period of favorable Total daily ature, ° F. percent Baro- Wind velocity winds^ O Date mum catch temper- metric B ature Day Evening Day Evening pressure Evening Day Evening Day and night Day Evening ^ > F. 6 a.m, to 5:30 to 6 a.m,. to 5:30 to 1936 3 p.m.'' 7 p.m. 3 p.m."" 7 p.m. Inches M. p. h. M. p. h. Hr. Min. Hr. Min. Number Number td Apr. 5.. 64 56.4 55.7 41.7 42.5 29.88 6.9 0 0 0 6.. 72 60.2 63.0 34.4 26.2 29.80 7.5 1.2 0 0 0 15 3 7.. 80 65.8 67.7 30.7 30.8 29.80 2.8 2.7 40 0 0 35 15 8.. 79 69.0 69.5 36.3 34.5 29.64 8.1 4.9 05 2 2 9.. 82 71.3 73.2 28.9 28.2 29.57 4.6 45 20 11 10.. 73.0 30.5 23.5 3.6 3.1 25 '13 154 11.. 92 78.0 80.2 35.7 31.8 4.4 3.2 05 45 24 116 o 12.. 92 79.0 81.7 CO 40.2 36.0 29.50 5.0 5.2 15 0 10 52 o 13.. 88 77.9 77.5 35.0 31.0 4.7 4.7 15 9 14.. 129 82 69.6 73.7 37.1 32.8 29.49 4.7 3.5 0 2 50 15.. 85 70.0 76.0 42.8 37.0 29.45 4.2 4.5 0 35 16.. 83 69.9 74.2 39.0 35.3 29.50 4.0 4.6 0 40 CO 17.. 82 71.4 72.0 42.6 31.2 29.40 4.6 5.8 25 7 170 6 a.m. to 6:30 to 6 a.m. to 6:30 to ü 1937 6 p.m. 10 p.m.. 6 p.m. 10 p.m. Apr. 28.. 71 59.2 50.6 75.0 83.7 0 '0 0 29.. 78 66.0 57.6 53.0 76.1 0 12 7 O 30... 83 72.1 62.8 54.2 69.6 0 0 28 May 23 1... 88 77.4 68.5 52.5 55.5 35 0 32 32 2.., 97 83.9 76.4 46.9 50.1 > 50 20 48 92 O 3.. 90 80.1 63.4 56.7 72.0 0 50 ^6 48 4.. 80 70.3 62.7 64.4 71.4 t—I 11 8 Cl 5... 87 77.2 68.4 55.0 55.0 21 35 6.., 86 75.1 58.6 58.8 79.7 17 3 7.., 76 66.7 59.3 76.2 91.5 4 8.., 81 71.8 67.6 60.3 60.1 '6 20 83 71.7 58.1 58.6 83.4 4 2

^ Favorable winds reported are those from highly populated breeding grounds. 2 Averages of hourly readings; all others are averages of half-hourly readings. ^ Record incomplete. TABLE 8.—Day-to-day variation in leafhopper flights in the fall under principal meteorological conditions. Records from the rotary net at 3- to 4V2-foot elevation MODESTO Mean temperature/ Mean humidity,' Maxi- Wind velocity Wind direction Total daily percent Baro- catch Date mum metric temper- pressure, Day Evening td ature Day Evening Day Evening inches Day Evening (pre- (peak of Day Evening vailing) flight)

° F, 10 a.m. to 5:30 to 10 a.m. to 5:30 to 5:30 to 1935 5 p.m. 7 p.m. 5 p.m. 7 p.m. 8 p.m. M. p. h. M. p. h. Number Number Sept. 24., 91 81.2 76.8 15.6 11.7 36 25.. 90 85.4 79.2 13.2 23.5 2 36 26.. 87 81.8 74.8 21.5 26.2 0 18 o 27.. 88 80.9 75.5 21.2 29.2 0 19 28.. 86 79.1 72.5 18.2 29.2 0 42 29.. 86 78.6 74.5 24.0 34.2 0 46 S3 30.. 89 84.0 78.0 7.5 14.0 2 57 10 a.m. to 5 p.m. to 10 a.m. to 5 p.m. to JfP.m. 6:30 p.m. U p.m. 6:30 p.m. o Oct. 1.. 82 n.s 72.8 63.7 2 17 > 2.. 78 73.9 70.5 38.6 50.0 ^1 38 3.. 78 74.7 71.2 40.5 48.0 29.88 17 56 4.. 82 77.6 73.0 38.8 45.7 29.83 6 31 O 5.. 83 77.4 72.5 46.5 29.84 4 18 6.. 83 78.9 72.2 36.4 46.0 29.84 4 61 !.. 86 81.6 75.8 40.0 29.8 5 13 8.. 82 77.3 71.0 40.1 55.5 29.8 2 12 9.. 86 79.0 73.2 36.8 53.0 29.78 ^0 29 10.. 89 84.4 71.5 23.6 45.0 29.80 ^2 54 11.. 72 67.0 63.5 59.3 78.0 1 1 12.. 74 71.4 65.2 38.8 49.2 29.93 21 4 13.. 76 71.0 66.0 35.6 52.2 30.00 2 3 14.. 67 63.7 57.5 62.3 68.2 29.82 21 0 TABLE 8.—Day-to-day variation in leaf hopper flights in the fall—Continued to

HOSPITAL CANYON Mean temperature/ Mean humidity,^ O Wind velocity Wind direction Total daily Maxi- ° F. percent Baro- catch M Date mum metric temper- pressure, Day Evening ature Day Evening Day Evening inches Day Evening (pre- (peak of Day Evening O vailing) flight) > td 9 a.m. to Jl^ :30 to 10 a.m.. to 5 to 5 to °F. M. p. h. 1935 4 p.m. 6:30 p.m. Up.m. 6:30 p.m. 6 p.m. M. p. h. Number Number Oct. 17.. 77 72.1 67.2 21.2 26.0 29.80 7.4 2.2 NW SW 1 67 18.. 80 75.2 70.6 26.3 27.1 29.60 3.5 1.4 E S-SW 9 122 19.. 82 76.1 73.2 26.6 31.2 29.47 3.6 2.7 N-NE NW 6 97 20.. 82 75.5 70.0 25.0 30.0 29.51 7.0 9.0 NW w 2 17 21.. 70 65.9 63.4 18.6 14.6 29.70 13.4 9.6 NW NW 0 0 22. . 71 66.2 63.6 22.8 21.4 29.78 12.4 13.8 O NW NW 3 0 CO 23.. 74 67.6 63.8 15.3 18.6 29.82 7.2 3.5 NW SW 0 10 a> 24.. 75 70.9 64.6 18.7 22.2 29.79 3.1 3.4 NE SW 2 13 25.. 78 71.6 67.4 20.8 19.8 29.78 3.2 3.3 NE-E SW 5 31 26.. 82 75.6 70.4 19.2 21.2 29.70 2.8 2.5 NE-NW SW 7 50 Ul 27.. 84 77.6 72.4 18.6 21.2 29.60 4.1 4.9 N SW 5 82 28.. 83 75.1 66.0 21.7 34.4 29.38 4.0 15.2 N SW 2 4 Ü 29. . 62 59.6 52.6 31.5 45.4 29.60 8.3 11.4 NW SW 1 0 30. . 61 56.6 53.4 37.0 38.2 29.72 7.8 7.6 N SW 3 0 31.. 62 57.0 53.4 35.1 33.6 29.73 6.0 7.2 NW NW 0 0 9 a.m. to Uto O 3 p.m. 6 p.m. Nov. 1. . 56 54.3 48.8 44.4 77.7 29.42 6.7 4.5 E SE 0 0 > 2.. 54 50.3 46.4 58.5 46.2 29.67 9.9 11.0 SW SW 0 0 3.. 55 50.7 48.8 30.5 32.8 29.72 10.1 NW SW 0 0 I—I 4.. 59 54.3 52.0 26.1 31.6 29.78 6.7 **** 4.Ó* N-NW SW 1 0 o 5.. 67 61.0 56.6 30.8 31.4 29.70 2.8 1.6 NE NE 17 0 6.. 69 63.4 60.4 26.4 22.8 29.62 3.8 2.9 NE SE 15 36 !.. 72 66.7 63.0 25.0 26.2 29.64 3.9 2.4 NE NE 5 23 TABLE 8.—Day-to-day variation in leafhopper flights in the fall—Continued ORA LOMA Mean temperature,' Mean humidity,^ Wind velocity Wind direction Total daily Maxi- ° F. percent Baro- catch Date mum metric W temper- pressure, Day Evening ature Day Evening Day Evening inches Day Evening (pre- (peak of Day Evening vailing) flight)

8 a.m,. to 3:30 to 8 a.m. to 3:30 to 1936 3 p.m. 6 p.m. 3 p.m. 6 p.m. M. p. h. M. p. h. Number Number > •pi Oct. 21.. 74 '70.8 67.5 52.5 2.5 1.8 E-SE-W 5 129 M 22.. 77 67.9 65.5 2.4 N-NW N 3 o 23.. 75 67.0 63.5 34.3 45.2 N 333 24.. 78 66.8 68.0 36.8 40.7 2.7 2.1 NE-N WNW 28 25.. 76 67.0 68.2 47.4 48.2 7.2 3.1 N N 'i 17 26.. 83 71.2 73.2 34.5 36.3 3.3 1.5 N-NNW NNW 1 ^^ 27.. 81 73.0 70.5 31.4 36.7 2.5 2.8 NE N-NW 6 18 28.. 82 71.7 73.8 32.1 34.2 2.9 2.0 NE-N E-NE 6 35 29.. 86 77.1 76.3 23.7 16.7 3.0 3.6 E-NE SE 3 335 30.. 62 59.3 56.2 56.0 83.0 5.9 3.8 S-SE SSE 5 0 > 31.. 64 57.9 58.7 59.2 57.0 4.7 1.1 N SSE 26 66 Nov. 1.. 61 58.2 53.0 55.0 58.2 10.7 6.9 N N 10 0 2.. 61 55.6 52.2 41.4 38.2 8.6 7.8 NNW N 10 2 O 3.. 69 60.2 59.0 24.5 36.7 4.4 1.9 SE SE 43 108 4.. 72 61.6 60.3 33.6 45.8 2.9 1.8 N WNW 25 12 5.. 72 61.9 59.8 37.8 46.3 2.7 3.1 N NNW 2 15 6.. 76 68.4 64.7 30.8 38.7 4.9 4.2 ESE NNE 5 16 ^ Mean temperatures for day periods are averages of hourly ^ Mean humidity determined as for mean temperature. readings except at Ora Loma which were half-hourly; for ^ Incomplete record. evening periods all readings were half-hourly. '11:30 a.m. to 3:00 p.m.

to 28 TECHNICAL BULLETIN 1030, U. S. DEPT. OF AGRICULTURE

TABLE 9.—Correlations between mean temperatures and total evening catches in the rotary trap

Days of r Locality Season record^ Number Modesto Fall, 1935 21 '0.555 Hospital Canyon Fall, 1935 14 ' .709 Ora Loma Fall, 1936 10 ' .824 Panoche Camp Spriner, 1936 7 ' .804 Panoche Camp Spriner, 1937 9 ' .876 ^ Days when the temperature was below the threshold or when wind directions greatly influenced catch were excluded from these calculations. ^ Significant at the 5-percent level. ^ Significant at the 1-percent level. 25 minutes. It is evident that the number of leafhoppers passing- through any particular locality is greatly influenced by winds blowing off heavily populated areas, and the timing and duration of such winds are therefore important.

EFFECT OF COLD WEATHER ON DAY FLIGHTS Although most of the leafhoppers were caught during the evening flight period, appreciable numbers were occasionally caught during the day. The day flights appear to have been governed by the same factors that controlled the numbers flying in the evening-, except that after a period of cold weather the number of leafhoppers taken in the traps was much higher than expected. For instance, at Modesto there were only 3 days—October 3, 12, and 14—in which daytime flights were of any importance. Each date was within 1 to 3 days after the beginning of a period of cold weather. At Hospital Canyon heavy day flights occurred on November 5 and 6, the first 2 days on which the temperature was above the threshold during a cold period. At Ora Loma day flights were above normal from October 31 to November 4, the second to sixth day after the beginning of a cold period (table 8). The flight of April 6, 1936, was much higher than would have been expected from the temperature and wind (table 7). This was the first day on which temperature was above the threshold after a period of cold weather. It seems generally true that abnormally heavy day flights are made on the second or third day of a period of cold weather or as soon as the temperature is above the threshold. Leafhoppers are probably forced by lack of food or water to move under unfavorable conditions. All the heavy day flights observed in cold weather were preceded by abnormally heavy evening flights on the last warm days before the cold period. It will be shown later that large numbers of leafhoppers making flights are forced down in unfavorable situations, on poor host plants or even on bare ground. After heavy evening flights the next favorable opportunity for movement is usually the following evening, but if BEET LEAFHOPPER IN CALIFORNIA 29

TABLE 10.—Effect of favorable winds on the number of leafhoppers caught in the rotary traps in the evening

PANOCHE CAMP Deviation of Date Expected catch^ Wind^ actual from expected catch

19S6 Apr. 5 0 0 6 8 —5 7 19 —4 11 48 X +68 12 51 X + 1 14 33 +17 15 38 —3 17 29 X + 141 1937 Apr. 28 0 0 30 17 +5 May 1 28 +4 2 43 X +49 3 18 X +30

ORA LOMA

19S6 Oct. 21 19 X + 110 24 20 +8 25 20 —3 26 26 —11 27 23 —5 28 26 +9 29 29 X +306 30 7 . X _7 31 10 X +56 Nov. 1 3 —3 2 2 0 3 10 X +98 4 11 + 1 5 11 +4 6 16 0 ^ Expected catch was calculated from regression of number of leafhoppers caught on mean temperature for each period shown. See tables 8 and 9 for basic data. ^ On days marked *^X" the wind blew from an area containing high populations. this flight is inhibited by low temperatures, hunger or thirst apparently forces abnormal activity as soon as the temperature again rises above the threshold, which is usually in the middle of the day. OTHER WEATHER FACTORS Although temperature and wind direction are the most important of the weather factors controlling the magnitude of flight, other variables have some effect. Part of the apparent 30 TECHNICAL BULLETIN 1030, U. S. DEPT. OF AGRICULTURE

effect of other factors is, however, due to correlations with temperature. For instance, if correlation coefficients are run on the figures given in tables 7 and 8, the within-seasons correlation between catch and humidity is —0.288, a low but significant value. However, the coefficient of partial correlation between catch and humidity when the effect of temperature is removed is +0.133, which is not significant. When the effect of humidity is removed the coefficient of correlation between catch and temperature (+0.588) is highly significant. In other words, the apparent relation between catch and humidity is explained by the correlation of both factors with temperature. There is a similar relation between barometric pressure and catch. When temperature is held constant the coefficient of correlation between barometric pressure and catch is low and not significant. There is an independent relation, however, between wind velocity and leafhopper flight. The correlation coefficient between this factor and catch, with temperature constant, was —0.414, a significant value. High winds tend to reduce the catch. Males are probably attracted to traps as resting places but in high winds are less able to reach them. Thus the catch is decreased at such times to a greater degree than it would be with the same aerial populations flying on a calm day. A few records were made of evaporation rate at Hospital Can- yon in 1935. No relation to leafhopper flight was noted, but this factor doubtless has an indirect effect on leafhopper flights by increasing or retarding the rate of drying of host plants.

INTERACTION OF ALL FACTORS CONTROLLING FLIGHT As shown in the preceding discussion, the major factors that control the magnitude of leafhopper flights are as follows : Num- ber of adults present, host-plant conditions, physiological condition of the leafhoppers, temperature, light or time of day, and wind direction. Another factor, interaction, might well be added to the list, as the way in which the factors combine may often be more important than any single influence. This is best shown by a study of the hourly and daily variation in the magnitude of flight in a series of successive days. Perhaps the best illustrative record was obtained from Oc- tober 28 to November 3 at Ora Loma in the fall of 1936. The record of flight for these 7 days is plotted in figure 9, together with temperatures and the time of favorable winds. October 28 resembled the days immediately preceding it in that temperatures were approximately normal for this time of the year and the lower threshold was not reached until too late in the day for the sunrise flight. Day flight was slight, but there was a marked evening flight, the peak coming shortly after sunset. This flight was cut short by low temperatures. The next day, October 29, was unusually warm for the time of year, with a maximum of 86° F. (table 8). The temperature was 85° during most of the period between 1 and 4 p.m. Flight during the daylight period was low, being comparable to that of the previous BEET LEAFHOPPER IN CALIFORNIA 01

FIGURE 9.—Total leafhoppers (male and female) caught at Ora Loma, October 28 to November 3, 1936. Daily temperature curve and periods of favorable winds are also indicated. Horizontal dotted line represents approximate temperature threshold.

days. A favorable southeast wind off the summer-breeding area began to blow about noon. This wind was intermittent until 3:30 p.m., when it began to blow steadily and continued until 5 p.m. The trap was run continuously during the period of the evening flight (4 to 5:30 p.m.), and 324 leafhoppers were caught. This catch is at the rate of 216 per hour and is one of the highest ever obtained. The rates during parts of this period were undoubtedly much higher. The results of this catch constitute a striking illustration of the effects of high temperature and favorable winds occurring together at the most favorable time of day. From 5:30 to 7 p.m., the rate of flight was very much lower, although temperatures were still well above the threshold. The favorable wind had ceased to blow, however, and the time of the evening flight was mostly past. 32 TECHNICAL BULLETIN 1030, U. S. DEPT. OF AGRICULTURE

During the night of October 29 the weather turned cold. Octo- ber 30 was a cold day, with a maximum temperature of 62° F., only slightly above the prevailing threshold of flight. No morning or evening flight was made. The only leafhoppers caught were taken in the middle of the day during the warmest period, and only a few then, despite the fact that a favorable wind blew nearly all day. October 31 was slightly warmer, with a maximum of 64° F. Leafhoppers were caught between 10 a. m. and 3 p. m., although flight probably did not begin until about 11 a. m., when the temperature crossed the threshold. During this period wind direction w^as not favorable, but the catch was much higher than might have been expected. From 3 to 4:30 p. m. the flight was rather heavy and the wind was favorable. According to the graph, this flight resembles a normal evening flight, but it occurred much earlier than usual. This flight may have been a very heavy day flight resulting from the starvation of leafhoppers forced down in unfavorable situations during the heavy flight of October 29. Be- ginning at about 2 p. m., the sky was cloudy and the light thus dimmed, so that part of the flight may have been an abnormally early evening flight. Certainly the rate was unusually high, con- sidering the temperature. November 1 was another cold day with temperatures lower than on the preceding 2 days. The maximum, only 61° F., pre- vailed for a very short period. From 11 to 3 o'clock the temperature was only 60°, which is very close to the threshold, and there was no favorable wind. However, there was some flight during the middle of the day—in fact, greater than the flight at much higher temperatures on October 28 and 29, probably owing to the effect of a preceding cold period. November 2 was similar to November 1. On November 3 the weather was considerably warmer. A favorable wind blew most of the time from 10 a. m. to 4:15 p. m. All the factors that tend to make for heavy day flights were present ; that is, this day followed a period of cold weather, there was a favorable wind, and temperature was well above the threshold. The day flight was unusually heavy, and an evening flight of considerable magnitude began about 4 p. m. The favorable wind stopped blowing about 4:15, however, and the temperature began to drop rapidly. From 4:30 to 5 p. m. the temperature fell 12°, and in the next half hour another 9°, which cut the flight off sharply before the usual time of the evening peak. The examples just described illustrate the interaction of the various factors influencing flight. Any other series of examples selected would show similar interactions, except that in the spring, when temperatures are higher, morning flights sometimes occur. As already shown, morning flights are similar to evening flights, but in reverse order. If the available populations of leafhoppers are known, any flight can be readily understood when the co- incidence of temperature, wind direction, and time of day are taken into consideration. BEET LEAFHOPPER IN CALIFORNIA 33

MECHANICS OF FLIGHT DIRECTION AND SPEED OF FLIGHT Many workers have reported increased populations of the beet leaf hopper in cultivated crops or on other host plants when winds are blowing off breeding grounds or immediately afterward, and most investigators have considered movement with the wind to be an established fact. Data previously given, showing greatly increased trap catches when winds blew from known areas of high populations, strongly support this conclusion. However, Severin (29, p. 326) says "During the autumn they have frequently been observed flying against light breezes in canyons and mountain passes." The authors have been unable to cor- roborate this observation in their investigations. Although it was common to see leaf hoppers swarming around the observer or some other conspicuous object when flights were taking place, it was difficult to determine the direction of flight. "Swarming'' leafhoppers may dart around an object in any direction and even when seen some distance away often quickly change direction before alighting, so that definite identification is difficult. In an effort to overcome this difficulty leafhoppers were put in a vial and released against a dark background. Such individuals could be followed for several feet in the air and always fiew with the wind and at approximately the same speed as did particles of "cotton" from a nearby cottonwood tree. This observation agrees with Severin's (29) on released insects. To check the possibility that captured and released leafhoppers might behave abnormally, further observations were made on free specimens. In the fall of 1936 and the spring of 1937 near sunset, when heavy flights were expected, observers were stationed on the open plain in an area where leafhoppers were abundant. At such times insects of many kinds could be seen flying through the air and even very small individuals could be seen 10 to 15 feet away. However, it was not possible to distinguish the beet leafhopper from other species of comparable size while they were in flight. To make a positive identification it was necessary to observe an individual insect, note its direction of flight, and then catch it in a net. A very small proportion of the insects actually in flight were leafhoppers, and the highest catch made in one evening by two observers was 10 males and 10 females. A total of 24 females and 36 males were captured in this way in about 10 days. Although this is a relatively small number the results were extremely con- sistent. The typical flight of the leafhopper was somewhat erratic and tended to be a series of arcs, but the general course was always in the same direction as the wind. This was often strikingly illustrated when one specimen was caught going in one direction and a few minutes later another specimen was caught going in a different direction. In such cases the wind had invariably changed direction. A particular effort was made to catch insects going against the wind and, although some species behave in this manner, no beet leafhoppers were observed to do so. Many of the 34 TECHNICAL BULLETIN 1030, U. S. DEPT. OF AGRICULTURE insects captured were 10 to 15 feet away when ñrst seen and were traveling at right angles to the observer's line of sight, so that it is unlikely that the course was influenced by the observer. All these observations were made when wind velocities were low. In stronger winds leafhoppers were traveling too fast to be seen and caught. To further determine leafhopper reactions to air movement a small wind tunnel was set up and leafhoppers were released in the leeward end. The results are given in table 11. The tunnel was 3 feet 4 inches long and 10% inches in diam- eter. A fan placed opposite one end at various distances produced different air velocities. Air speed was measured by an anemom- eter placed inside the tunnel. A band of oil was smeared on the inside of the tunnel near the center to keep leafhoppers from crawling or hopping along the sides, and thus force them to fly. This band was 10 inches wide in experiment 1, and 19 inches wide in experiments 2 to 4. A beet was placed in the windward end of the tunnel as food for leafhoppers that crossed the barrier. Except in experiment 4, leafhoppers reaching the beet were re- turned to the leeward end until all had been caught in the oil. In experiments 1 and 2, with air velocities slightly under 1 mile per hour, many leafhoppers crossed the barrier against the wind. Proportionately more females than males succeeded. In experiments 3 and 4, with a wind of not quite 2 miles per hour, only 1 male and 1 female were able to cross. Several individuals flew short distances against the wind, but could not fly across the 19-inch barrier. These experiments indicate that the leafhopper cannot make progress against a breeze stronger than 2 miles per hour. Winds as low as this are uncommon. The trap at Panoche Camp was operated 164 hours in 1936, and the wind velocity was less than 2 miles per hour during only 4.2 percent of the time. Of 894 leafhoppers caught, only 1.9 percent were taken when the wind velocity was less than 2 miles per hour. At Ora Loma Camp, where there was an unusual amount of light wind, the trap was run 147 hours. During 13.6 percent of the time the wind velocity was less than 2 miles per hour. Out of a total catch of 919 leafhoppers, 17.0 percent were taken at these low velocities. Thus, although leafhopper flights against very light winds are possible, this cannot be the usual method of movement. The speed at which leafhoppers travel is probably about the same as the wind velocity. Leafhoppers observed in the air appeared to travel at about the same speed as inert particles. It is concluded that leafhoppers travel with the wind and at about the same speed or slightly faster, and that disseminations of significant proportions are brought about by a more or less passive drifting.

HEIGHT OF FLIGHT During the early work, when sticky screens were used to catch leafhoppers, records were kept on each 12-inch section of a 6-foot cylinder. Later the rotary traps were operated with two nets, one td

TABLE 11.—Results of wind-tunnel experiments on leafhopper flight, Modesto, 1986 > Leaf hoppers flying M Experiment Wind velocity Leaf hoppers used O No. Date Duration to windward end Range Average Males Females Males Females •-d 50 Hours Minutes M. p, h. M. p. h, Number Number Number Number 1 May 18-21... 73 11 0.614 to 0.962... 0.769 23 14 2 4 2 May 28 4 7 0.873 to 0.986.. . .958 19 17 4 12 3 May 28-29... 17 4 1.844 to 2.011... 1.857 12 17 1 0 > 4 June 17 1 14 1.700 to 1.930. .. 1.880 5 10 0 1 O

CO OÍ 36 TECHNICAL BULLETIN 1030, U. S. DEPT. OF AGRICULTURE

above the other, at heights from 1 to 4I/2 f^^t above the ground. The tower model of this trap extended the range of observations up to 32 feet. The Fulton-Chamberlin air-maze trap (17) and sticky screens used in Idaho were placed at heights from 8 to 50 feet. The 1930 results of this study were published by Annand et al. (1). Comparable data for other years were obtained from reports of the Twin Falls, Idaho, station. Hills (21) gave the catches obtained in air-maze traps up to 127 feet above the ground. The data from all these sources are summarized in table 12. By assuming that different traps should catch proportional numbers of leaf hoppers at the same level, and by interpolating where necessary, it was possible to reduce all these data to a comparable basis and fit a curve to the figures. The results are plotted in fig. 10. The coefficient of regression of relative number of leafhop-

Y 100 '- "

2 80 o. Q. ¿ 70 «*- o «»-■s 60 O

I 50 ■■ ^ 40 Y. 11.772 ^^^'f'^^ o 30 - • DC 20 •

10 — • • • • 1 I 1 1 1 1 1 -1 \ 1 1 1 1 10 20 30 40 50 60 70 80 90 100 110 120 130 X Height above the ground in feet

FIGURE 10.—Composite curve data from all traps showing relative number of leafhoppers in the air at various elevations.

pers caught (Y) on the reciprocal of height (X) is 44.12844 dz 2.0402 with 24 degrees of freedom. This is highly significant, and indicates that the reciprocal curve gives a true description of the relative number of leafhoppers at varying elevations. The chief features of this curve are the sharp rate of decline near the ground and the slow rate at higher elevations. The the- oretical upper limit of flight would be infinity, but, because temperatures normally decline with increasing altitude, the actual BEET LEAFHOPPER IN CALIFORNIA 37

TABLE 12.—Beet leafhoppers caught in traps at various elevations in Idaho, California, and Oregon

Portion State Year Type of trap Elevation Catch of total Females catch

Feet Number Percent Percent 8 434 36 57 16 404 Idaho 1930 .... Flat oil screen"^ . 34 57 25 354 30 60 Total.. 1,192 100 58 10 1931-32 . Fulton- 217 35 72 20 200 33 70 Chamberlin 30 199 airmaze 32 76 Total.. 616 100 73 10 69 13 73 20 ..... 138 27 59 30 120 1931-34 . do 23 70 40 103 20 59 50 89 17 73 Total.. 519 100 66 0-1 .... 92 43 5 1-2 .... 43 20 7 1-3 .... 21 California . 1935 .... Cylindrical 10 10 oil screen .... 2-3 .... 18 8 0 4-5 .... 27 13 4 5-6 .... 14 6 7 Total.. 215 100 6 riy2-3 .. 1,133 57 17 1935-36 . Rotary flight .. 3-41/2 .. 859 43 19 Total.. 1.992 100 18 Í21/2 .... 469 45 53 15 296 1937 .... do 29 57 32 268 26 57 Total.. 1,033 100 55 38 .... 18 39 74 .... 12 26 Oregon ... 1931 .... Air-maze ...... 108 .... 6 13 127 .... 10 22 Total.. 46 100

^ Most of this season's catch was taken on flat oil screens, the remainder with the Fulton-Chamberlin airmaze trap. (See Annand et at. (1).) 38 TECHNICAL BULLETIN 1030, U. S. DEPT. OF AGRICULTURE upper limit would probably be at the point where temperature reaches the lower threshold of flig-ht. Under some conditions this might be at very high altitudes. Severin (29) discussed at some length the question of temperature inversions, which are relatively common in the San Joaquin Valley, and speculated on the possibility of leafhoppers being carried in the upper air currents. Carter (8) also suggested that the leaf hopper might be carried into the upper atmosphere and transported for long distances. This is an important question, for winds aloft may blow in a different direction from those at the surface. Glick (19) caught a large number of insect species at high altitudes with airplanes. Felt (15) and Uvarov (36) have reviewed the literature on the possibility of insects being carried in upper air currents. Ball (S) reported finding beet leafhoppers on the snow on top of Pike's Peak at an altitude above 14,000 feet and suggested that they were deposited there by downdrafts from the upper atmosphere. The greatest height above the ground at which leafhoppers have been caught is 127 feet (Hills 21). Glick (19) collected many insects of the size of leafhoppers as high in the air as 5,000 feet. When the automobile trap was used to trace migrations, leafhoppers were never caught on a high mountain, although they were frequently found flying in the pass below. Dorst and Davis (H) reported that the routes traveled by the leafhopper in long-distance movements in Utah follow the valleys and passes. Dana (12) reported that in the Willamette Valley in Oregon the greatest incidence of curly top is at the mouth of the Columbia River Gorge. The leafhopper does not breed in this valley in the winter, but migrates into it from beyond the Cascade Mountains to the east. It seems unlikely that leafhoppers would exhibit such a definite tendency to follow mountain passes if significant numbers were carried by the upper air currents. At least the bulk of the flying population must travel below 10,000 feet on surface winds. TYPES OF FLIGHT SWARMING Swarming of the beet leafhopper was noted by the early observers (Ball S and Severin 31) and has been observed many times in the course of these studies. On warm calm evenings "swarming" leafhoppers alight in numbers on any object projecting above the ground surface and dart erratically around it. Severin noted that such activity is associated with sexual activity and that the swarms consist mostly of males, an observation confirmed by data from the oil-screen traps used at Coalinga in 1935. These traps were large stationary devices around which leafhoppers swarmed, and the catches consisted almost entirely of males. The rotary traps were also conspicuous objects on a barren plain, and it is quite probable that swarms of males were also attracted to them and caught, although considerable numbers of females were taken at the same time. The data on the sex ratio expressed as the percentage of females are given in table 13. BEET LEAFHOPPER IN CALIFORNIA 39

When the trap was located near vegetation on which leafhoppers were present the catch sometimes showed a high percentage of males. For instance, at Hospital Canyon the catch in the trap placed in a plowed field, where leafhoppers must have come some distance, showed a sex ratio of nearly 50 percent, while the machine in the canyon nearby but near a patch of green vegetation on which leafhoppers were present caught a much higher proportion of males than was present on the ground. At Panoche in 1936, host plants and leafhoppers were abundant in the area immediately surrounding the trap and the catch was very high in males, but in 1937 at the same locality, and at a time when host plants were dry and local populations were very low, the sex ratio was nearly 50 percent. Low percentages of females were also caught at Ora Loma and Dos Palos, where host plants surrounded the traps, although local populations were low and many of the leafhoppers caught probably came from some distance. The catches in the trap at Mo- desto in the summers of 1935 and 1936 proved to be an exception to the rule. Although this machine was located in the midst of host plants the first year and only a few feet away the last year, it caught a few more females than males. This suggests that males swarm only when major disseminations are taking place, as the Modesto records were made during the summer, when little movement occurred, whereas the other observations were made in spring and fall at the height of dissemination. There seems to be little doubt that swarming males are caught by the rotary trap. Swarming may be a form of dissemination. It has been observed only in calm weather ; yet the traps showed that leafhopper activity occurred at the same time each day if temperatures were above the threshold, regardless of other weather conditions or of locality. Both males and females showed high peaks of activity near sunset even when the wind was strong. It seems probable that they swarm in both calm and windy weather but can be observed directly only when the wind is very low. At high wind velocities swarming leafhoppers presumably are carried along in the air stream too fast to be seen or to alight on conspicuous objects. This hypothesis is supported by the correlation between sex ratio and wind velocity (table 14). For the figures given the coefficient of correlation is +0.606, and highly significant. This means that the proportion of females in the catch declined with decreasing wind velocity. Further evidence that swarming results in movement is given by the data on wind directions. It has been shown previously that at Ora Loma catches were greatly increased when a southeast wind blew from heavily populated areas toward the trap. This wind blew over a barren field at least a mile wide and extending within a few yards of the trap. The high populations caught must have come at least a mile ; yet the percentage of males was as high or higher on October 21, 29, and 31, and on November 3, 1936, when this southeast wind blew as on other days when the wind was off adjacent areas of winter- host plants. O

O TABLE 13.—Sex ratios of leafhoppers caught in traps compared with nearby ground collections a

O Percentage of females >

Date Locality Vegetation near trap In traps On ground w Upper Lower Host Nonhost Bare net net plants plants ground 1935 September and October... Modesto Summer host, Russian-thistle 53 OpfobPT and Novpmbpr Hospital Canyon. fNonhosts 19 47 o Î None, bare ground CO 51 46 o 1936 April fPanoche Camp . . Spring host plants 19 '41 1 Near Mendota .. Grain with beets nearby 76 54 92 June and September Modesto Tomatoes, summer hosts nearby 70 October and November fOra Loma Spring host plants 8 8 50 ö IDos Palos Spring host plants 15 10 52 1937 ÍPanoche Camp .. Spring host plants 53 O /\.prii ana iviay \ Near Mendota .. Bare ground near beets 37 86 8Í 39 Fresno Nonhosts with nearby summer hosts.. 95 100 > ' Average of 2 sweep-net collections taken April 17 at 8:30 a. m. and 3 p. m. BEET LEAFHOPPER IN CALIFORNIA 41

TABLE 14.—Relation of sex ratios during evening flights in the spring and fall to mean wind velocity during flight period

HOSPITAL CANYON Mean wind Date velocity Total catch Females

1935 Number Percent Oct. 17 2.2 64 5 18 1.4 100 7 19 2.7 92 15 20 9.0 13 54 23 3.5 8 50 24 3.4 13 0 25 3.3 16 13 26 2.5 46 22 27 4.9 71 7 28 15.2 3 Ç^Q Nov. 6 2.9 35 34 7 2.4 20 45

PANOCHE CAMP

1936 Apr 6 1.2 3 33 7 2.7 15 13 8 4.9 2 0 9 10 3.1 154 16 11 3.2 116 17 12 5.2 52 12 13 4.7 129 12 14 3.5 50 14 15 4.5 35 11 16 4.6 40 13 17 5.8 170 12

ORA LOMA

1936 Oct. 21 1.8 129 6 22 23 33 5 24 2.1 28 17 25 3.1 17 12 26 1.5 15 7 27 2.8 18 11 28 2.0 35 13 29 3.6 335 4 30 3.8 0 31 1.1 m 3 Nov. 1 6.9 0 2 7.8 2 50 3 1.9 108 3 4 1.8 12 0 5 3.1 15 7 6 4.2 16 0 42 TECHNICAL BULLETIN 1030, U. S. DEPT. OF AGRICULTURE

In conclusion it seems clear that the swarming of leafhoppers so commonly observed is confined almost entirely to males, but this type of activity is closely associated with dissemination and may be essentially a phase of the same phenomenon. Both sexes exhibit greatly increased activity at the same time of the day, the chief difference being that males are attracted to conspicuous objects and at low wind velocities may alight on them, whereas females either alight on the ground or continue flying.

PRIMARY AND SECONDARY DISSEMINATIONS In this discussion the term "primary disseminations'* will be used to mean initial movements from the breeding grounds to any other locality. "Secondary disseminations" refers to later flights by the same insects. The distinction is purely arbitrary and is used for convenience. None of the flights observed in our studies in California were more than 50 miles from the nearest breeding grounds; yet the evidence strongly indicates that even this short distance was covered in two or more stages. In 1937, counts in a beet field at Mendota (table 15) taken each morning reflected changes due to flights on the previous day. These data indicate that in this area heavy flights occurred on April 24 and 28 and on May 1, 5, and 7. At Fresno, about 50 miles to the east, the trap catch indicated major movements on April 25 and 28, and on May 2, 4, 6, and 8. The flight of May 4 at Fresno apparently was not related to movements at Mendota, but all Mendota movements except the one on April 28 were followed 1 day later by flights at Fresno. Since night temperatures were too low for flight, the delay clearly indicates a secondary movement. This is most vividly shown by the flight of April 25. Although leafhoppers arrived at Mendota on the evening of April 24, none were caught in Fresno until the next day, even though the weather was more favorable for movement on April 24. Further evidence of secondary movements was obtained from population counts in beet flelds. This crop is a good host, and females generally accumulate on it in large numbers (table 15), but on May 8, 1937, there was a statistically significant drop of about 25 percent in the population in 24 hours. It seems improbable that such a sudden decrease in young leafhoppers would be caused by mortality, and a movement out is definitely indicated. There may have been other outgoing movements that were ob- scured by incoming populations. In contrast to female behavior on host plants, males show very little tendency to accumulate. In 1936 and 1937 the male population at the end of the migratory period was about the same as that after the first influx (table 15). This may have been due in part to mortality, since in longevity experiments males invariably show a shorter life than females. However, collections on nonhost plants also show that males pass over beet fields or leave them very quickly. The evening of April 24, 1937, was very favor- BEET LEAFHOPPER IN CALIFORNIA 43

TABLE 15.—Leafhopper populations on sugar beets near Mendota (numbers per 50 beets)

Date Males Females

1936 ADr. 9 0 1 10 0 6 11 1 7 12 0 10 13 5 12 14 2 21 15 2 25 16 1 29 17 1 26 1937 Anr 24 0 3 25 ^14 '49 26 13 42 27 28 11 38 29 ^21 50 30 ^10 43 3 May 1 13 41 2 14 '86 3 12.5 96 5 4 13 95.5 5 9 85 6 12 '125 7 8 118 8 11 '89 9 ^21 106 Difference required for significance in 1937 at 5-DercpTit. level . 9.9 23.7 ' Significant population change. able for movement into the Mendota area. Temperatures were high and there was a wind off the breeding grounds. Sweep-net collections made on this evening on grass immediately to leeward of a large beet field total 19 males and 2 females. Counts in the beets the next morning showed a substantial increase in populations of both sexes (table 15) ; 71 females and 22 males were taken by sweeping. Collections on the same patch of grass the same morning yielded 6 males and 1 female. The striking difference in sex ratios on the beets and on the adjacent grass to leeward shows clearly that a higher percentage of females than males stopped over on the beets. In 1936 the Mendota trap was set in a grain field near the edge of the cultivated district. Of the leafhoppers caught, 54 percent were females ; in a beet field nearby 92 percent of the leafhoppers caught were females. In 1937 a trap placed in a comparable situation, except that there was a large field of beets between the trap and the breeding ground, caught only 37 percent females—17 percent less than in the previous year. Beet-field collections showed only 6 percent fewer females than males. Apparently a greater 44 TECHNICAL BULLETIN 1030, U. S. DEPT. OF AGRICULTURE

proportion of females than males dropped out when the flight passed over a ñeld of host plants. Thus, it is evident that secondary movements of both males and females from good host plants do occur. However, it is unlikely that the few scattered patches of host plants are the major source of secondary movements. Collections made on nonhosts in 1937 show that both males and females were present continuously from April 24 to May 8 in numbers of 1 to 40 per 500 sweeps. Although collections on nonhosts were made with a sweep net, an unreliable method, they indicate that the numbers present after known movements were higher than before, and that they gradually declined. The prevalence of curly top in such nonhost crops as tomatoes, melons, and beans is additional evidence that large numbers of leafhoppers alight on such plants. Severin also found leafhoppers on nonhost plants in the spring. He says (29 p. 306), *'When the immense swarms of beet leafhoppers flew into the cultivated regions on April 14, 1919, they were found the next day generally distributed on green vegetation, but later congregated on their most favorable host plants for the purpose of feeding and egg-laying." It is clear that disseminating leafhoppers infest nonhost plants in considerable numbers and rest even on bare ground after movements. They apparently come down almost anywhere when conditions for flight become unfavorable. The populations found in such unfavorable situations are low compared with those that accumulate on host plants, but the total area covered is many times greater than the area of host plants, so that the leafhoppers found on nonhosts immediately after a dissemination must constitute a large proportion of the total numbers present in any area. Some of these leafhoppers may die, particuarly the males, but females can live many days on water alone. It is unlikely that the large populations present on nonhosts after major flights would merely sit there until they die. It seems much more likely that they would take off again at the earliest opportunity, and such flights may account for a large proportion of secondary movements. It was shown in a previous section that females in flight are mostly without eggs in the ovaries. The tendency for females to remain on host plants in the spring may be due to the fact that they develop eggs soon after arrival. The overwintering females that disseminate in the fall do not develop eggs until spring. Thus in the fall both sexes continue to move until they are grounded by cold weather, even when good host plants are found. The more or less continuous fall movement is shown by counts made in the fall at Ora Loma and Hospital Canyon (table 16). Day-to-day fluctuations at both places were erratic and the number of leafhoppers present was about the same at the end of the period of study as at the beginning. At both places the traps showed large numbers of leafhoppers in flight, and they were known to be leaving summer host plants, so that there is little doubt that a dissemination was in progress. At Hospital Canyon the counts themselves prove it. When the area was sprayed, popu- BEET LEAFHOPPER IN CALIFORNIA 45 lations dropped to almost zero, but the next day they were approximately the same as before. Hence other leafhoppers must have moved in during the interim.

TABLE 16.—Leafhopper populations in the fall on perennials at Hospital Canyon and on winter annuals at Ora Loma, Counts at Hospital Canyon were made with the sampling spear; at Ora Loma with the sampling pan

Hospital Canyon, 1935 Ora Loma, 1936 Date (number per 5 square feet) (number per 50 square feet) Males Females Males Females

Oct. 17 5 6 18 7 9 19 14 9 20 8 4 21 22 3 1 23 10 6 3 2 24 12 6 0 0 25 13 6 4 4 26 9 7 0 2 27 12 13 5 4 28 7 6 0 2 29 9 8 30' 0 1 1 4 31... 5 10 5 1 Nov. 1 3 7 4 1 2 10 2 0 5 3 2 2 0 3 4 3 4 2 1 5 3 3 1 1 6 9 6 3 0 7 7 4 8 2 7 ' Plants sprayed with pyrethrum in oil before the count was made.

The only reasonable explanation of the failure of ground populations to increase is that leafhoppers were constantly shifting about and spreading over wider and wider areas. From the above observations it appears that leafhoppers disseminating from the spring breeding grounds may land almost anywhere—on host plants, nonhosts, or even on barren soil. Fe- males that find good hosts, such as beets, tend to remain there, although under certain undetermined conditions they may move again. Those landing in unfavorable situations probably take to the air again and again, until they find suitable host plants or perish. Males that find good hosts may remain for a while, but often they either do not stop or they move again immediately. In the fall both sexes continue to disseminate even when good host plants are found, although finding food plants may curtail or delay movement. 46 TECHNICAL BULLETIN 1030, U. S. DEPT. OF AGRICULTURE EFFECT OF WEATHER ON ROUTES OF DISSEMINATION SPRING MOVEMENTS EASTWARD INTO THE SAN JOAQUíN VALLEY The San Joaquín Valley is surrounded by mountains except for a broad gap at its northern end, where the Sacramento River cuts through the Coast Range to the sea. The v^est v^inds that normally prevail over the ocean blow through this gap and are deflected up the valley. Thus, the prevailing winds blow toward the southeast, and it might be expected that the main routes of leafhopper dissemination would be with the prevailing winds towards the southeast. In the spring, however, most of the crop damage from the leafhopper is in the valley to the northeast or northwest of the breeding grounds. These observations supple- mented by ground collections indicated that the most signiñcant movements in the spring were either at right angles to the direction of the prevailing wind or directly counter to it. It seemed possible, however, that, because of the scarcity of host plants to the southeast, movements of greater magnitude could have occurred with the prevailing winds and not have been detected by the ground collections. Rotary traps were therefore operated in order to determine the relative numbers of leaf hoppers that were moving in different directions. The data on direction of movement from the rotary traps is summarized in table 17. The table shows quite clearly that, although an appreciable part of the spring disseminations at Pa- noche in 1936 and 1937 did travel on the prevailing northwest wind, the total number of leafhoppers caught on winds from other directions was considerably greater. This is easier to see in ñgure 11, which shows 1936 data plotted on a map. In this year the catch taken on northwest winds was greater than from any other point of the compass, but the number traveling at right angles to this direction straight out into the valley or back into the hills was a larger proportion of the total. In 1937 no leafhoppers were caught moving west toward the hills, but again very large contingents went northeast into the valley (55 percent as against 45 percent caught on the prevailing winds, table 17). The difference in the number caught in the 2 years is partly due to the fact that in 1936 leafhoppers were abundant in all directions from the trap but in 1937 this area was dry and large populations were found only toward the west. Although the traps may not have been located to the best advantage, the data are sufficient to establish that a large proportion of the leafhoppers originating on the west side of the San Joaquin Valley do not follow the prevailing winds, but move at right angles to them. Further consideration of the trap data also reveals why this happens. It has already been noted that the catch at Panoche was greatly influenced by the temperature and time of change of the evening wind from northwest to southwest. Trap catches were heaviest when the wind changed direction before the temperature threshold of flight in the evening or after this threshold in the morning. The night wind at Panoche is a local phenomenon associated BEET LEAFHOPPER IN CALIFORNIA 47

1^/

^.A'i»^ \1-

V' %

I LITTLE PANOCHE CREEK

J^BIG PANOCHE CREEK ^M ^^. "f^.

•%«

FIGURE 11-—Map of central San Joaquin Valley showing direction of leafhopper flights, spring of 1936. Length of arrows indicates proportion of females caught traveling in the direction indicated. 00

TABLE 17,—Percentage of leaf hoppers from rotary traps caught on winds from different directions^ O

Valley trap Ridge above Ora Loma Camp Panoche Camp Hospital Canyon Hospital Canyon > Wind Apr. 28 to May 3, direction Apr. 5 to 15, 1936 Oct. 16 to Nov. 8,1935 Oct. 16 to 27, 1935 Oct. 4 to Nov. 6, 1936 1937 Cd Males Females Males Females Males Females Males Females Males Females

N 36 12 0 0 0 3 0 0 2 0 NNE 6 11 0 0 4 2 0 3 1 6 NE 7 12 0 0 1 5 0 0 2 7 ENE 2 2 0 0 1 7 2 7 0 0 3 7 26 7 4 0 O E 0 0 0 0 CO ESE 0 0 0 0 1 0 20 14 0 0 o SE 0 0 0 0 3 10 0 0 30 21 SSE 0 0 0 0 1 2 0 0 0 0 10 1 0 0 0 0 S 0 0 0 0 U2 SSW 0 1 0 0 20 18 0 7 0 0 sw 2 16 48 55 27 11 0 0 37 ö WSW 1 6 0 0 2 8 0 0 1 0 w 0 0 0 0 3 1 0 0 7 6 •-d WNW 8 11 0 0 0 2 11 21 1 1 NW 26 21 52 45 10 3 28 21 4 13 O NNW 12 8 0 0 14 20 13 20 1 9 > Total number of leaf- o hoppers caught .. 461 127 301 470 417 93 46 29 757 70 ^ Directions approximate since the period between collections was often an hour or more, and a relatively steady wind may vary as much as 45° in that time. td BEET LEAFHOPPER IN CALIFORNIA 49 with topography. The air currents flowing out of the hills do not normally reach more than a few miles into the valley, but on hot days they are stronger and reach much farther. They also begin earlier in the evening and blow later in the morning at such times. High temperatures also greatly increase the number of leafhoppers flying, partly by directly stimulating activity of the insects but mostly by causing the lower threshold for flight to fall earlier in the morning and later in the evening, thus increasing flight time during the very favorable crepuscular periods near sunrise and sunset. The same factors that cause heavy leafhopper flights are also associated with west winds. The result is that large numbers of leafhoppers are carried out into the valley. The fact that in the spring the air masses moving out of the hills are warmer than the valley air increases this tendency. This is strikingly illustrated by a record obtained near Coalinga during the period the car trap was being used. On May 4, 1935, at 7:53 p. m. the temperature on the plains at the edge of the hills was 62° F., which was near or below the threshold for flight. In a nearby canyon at 9:15 p. m., an hour and a half later, it was 68°, well above the threshold. On the plains at 9:33 the temperature had dropped to 56° and at 9:53 p. m. it was 54°, while in the hills at 9:41 it was 66°. When these warm-air masses from the hills flow out across the valley, large numbers of leafhoppers are carried with them. The connection is well shown in table 18. High temperatures and favorable winds in the evening or morning were concurrent on April 24, and May 1, 2, and 5. Significant increases in beet-field populations were found on April 25 and May 2 and 6 (table 15). There was also an increase on May 3, although it was not statistically significant. In 1936 the wind records at Mendota were incomplete, but the data for nearby Panoche show only one favorable period for movement on west winds—between April 7 and 16. During these 10 days beet-field populations reached 29 females per 50 beets. In 1937 there were three favorable periods between April 24 and May 9. In these 16 days populations in beets rose to 106 females per 50 beets, although conditions on the breeding grounds were not so favorable in 1937 as in the previous year. In other words, there were fewer leafhoppers on the breeding grounds in 1937 than in 1936, but four times as many females entered the beets in a period less than twice as long. It appears that the number of leafhoppers that reach the beet fields may be influenced more by weather conditions during a few critical days in the spring than by the populations in the nearby breeding grounds. The temperature distribution noted at Coalinga, where the air flowing down from the hills was much warmer than that in the valley to the east, also explains why leafhoppers congregate at the mouths of canyons in the spring, as noted by Sever in (29) and other investigators. Such concentrations probably occur when leafhoppers traveling on warm down-canyon winds are stopped by the cold air in the valley. 50 TECHNICAL BULLETIN 1030, U. S. DEPT. OF AGRICULTURE

TABLE 18.—Mean daily temperatures and time wind blew off the breeding grounds near Mendota, April 2U to May 7, 1987

Average Duration of temperature favorable wind at Date 6 a. m. to Time favorable wind blew time of morning 10 p.m.^ and evening flights

°F. Hours Minutes Apr. 24.. 77.3 6:05 to 8:30 p. m 2 25 25.. 68.8 8:25 to 8:30 a.m., 3 to 4 p. m 0 5 26.. 56.3 9:15 to 10:45 a.m., 11 a.m. to 1:30 p. m 0 0 27.. 52.4 0 0 0 28.. 57.2 0 0 0 29.. 64.0 10:15 to 10:30 a.m., 12:45 to 4:30 p.m 0 0 30.. 69.8 8:30 to 9 a.m., 11:45 a.m. to 12:05 p.m., 12:45 to 1:30 p. m., 7 to 7:30 p. m 1 0 May 1.. 75.2 12 m. to 12:15 p.m., 6:45 to 9:20 p.m 2 15 2.. 82.1 7:10 to 7:15 a.m., 3 to 3:45 p.m., 5:45 to 6 p.m., 6:30 to 12 p. m 2 35 3.. 76.1 9:45 to 10 a. m 0 0 4.. 68.4 11:30 a.m. to m., 5:45 to 6 p. m., 7 to 7:30 p. m 0 30 5.. 75.1 7:30 to 8:15 a.m., 4 to 4:15 p. m., 7 to 10 p. m 2 45 6.. 71.1 0 0 0 !.. 64.9 0 0 0 ^ Temperatures taken at 30-minute intervals from hygrothermograph at camp about 10 miles away. In brief, the most common route of movement of leafhoppers that bred in the central portion of the western San Joaquin Val- ley was toward the east and northeast, because the peculiar com- binations of temperature and light most conducive to flight were associated with west winds. However, the striking nature of these disseminations should not obscure the fact that movements in other directions also take place. Traps at Mendota and Fresno also caught leafhoppers on the prevailing wind, although not so many at any one time as when winds were directly off the breeding grounds. When warm westerly winds carry large numbers of leafhoppers into the valley those which land in unfavorable situations and some of them from host plants may move again. These leafhoppers may be carried farther eastward if westerly winds continue or they may drift up the valley in a southeasterly direction on the prevailing wind. Although no movements to the northwest were observed, the circumstances under which such disseminations might take place are easy to understand. If heavy movements into the valley on down-canyon winds were followed by warm southerly winds, such as sometimes precede a storm, large numbers of leafhoppers would be carried northward toward the lower San Joaquin Valley and the Sacramento Valley. td

> o

o >

h-H O

FIGURE 12.—Beet leafhopper flights of May 3, 1935. en 52 TECHNICAL BULLETIN 1030, U. S. DEPT. OF AGRICULTURE

SPRING MOVEMENTS WESTWARD THROUGH THE COAST RANGE Under certain circumstances leafhoppers from the San Joaquin Valley also move westv^ard through the coast range. Härtung (20) and Sever in (29) indicate that the Salinas and other coastal valleys are infested by leafhoppers from the San Joaquin breeding grounds. The work of the writers near Coalinga in the spring of 1935 shows how such movements take place. The data can best be seen when plotted on maps (figures 12 and 13). Each arrow represents a run of 5 miles with the car trap. The solid dots in the head of the arrow show the number of leafhoppers caught, each dot representing one-half leafhopper. Blank circles show zero collections. The direction of the arrow indicates wind direction, and the number of vanes the velocity (Beaufort Scale). The numerals give time of day, morning hours being underlined. When there was no detectable wind the arrow was omitted. Each map shows all the observations made during 1 day when the temperature was above 65° F. Figure 12, which records the flights of May 3, 1935, shows that leafhoppers originating in the vicinity of Coalinga may move southeast up the valley on the prevailing wind and then turn toward the west and move up the Cottonwood Pass toward the upper end of the Salinas Valley. In the morning of this day leafhoppers were caught near Coalinga, going south on a moderate wind. Farther south in Cottonwood Pass this wind swerved westward, and in the afternoon was blowing southwest up the pass. Leafhoppers were caught here in considerable numbers traveling in the general direction of Paso Robles. In the evening leafhoppers were still moving south and southeast near Coalinga. Counts made at the writers' request, by W. Suttie of the Spreckles Sugar Company, showed increases in the vicinity of Shandon on the next day, indicating that a real movement took place. The leafhoppers that were moving out of the Coalinga district in the morning may not have been the same lot that moved westward through Cottonwood Pass in the afternoon, but the possibility of a circuitous movement through this pass into the coastal valleys is clearly indicated. On May 4, leafhoppers were caught moving west toward the coast up Waltham Canyon, just west of Coalinga (fig. 13). They were taken here on westerly winds in both morning and afternoon. They were also caught moving west beyond the mountains at 6:30 p. m., although 15 to 30 minutes later near San Lucas in the Salinas Valley they were moving east. This reverse movement, however, could have been of little significance, since it could have lasted only a very short time compared with the day- long westward flights. On this same day there was also a movement south from Coalinga in the morning and there were traces of a movement up the Cottonwood Pass, as on May 3, but the principal westward flight was from the southeast up the Antelope Valley through Cholame and then toward the coast. The most significant thing about this flight was the manner in which it stopped. Shortly after 2 o'clock in the afternoon a cool wind M/H[f4^SS5

O to

Oí FIGURE 13.—Beet leafhopper flights of May 4, 1935. CO 54 TECHNICAL BULLETIN 1030, U. S. DEPT. OF AGRICULTURE began to blow from the direction of the coast. After a short period of confusion, when leaf hopper s were caught on winds going first one direction and then another, all movement stopped, although the temperature was still above the threshold. In other words, the difference in the temperature of east and west winds resulted in a one-way flight. The observations illustrate how and why leafhoppers move westward through the coast range. The sea breezes enter the San Joaquin Valley through major gaps in the mountains, such as Cottonwood Pass, as they do farther north in the vicinity of Suisun Bay. The air circulation through the passes is part of the system of prevailing winds, but is much less strongly developed because the passes are narrower and higher than the broad gap at the northern end of the valley. These winds are cold in the spring. They come more directly off the ocean than the winds which enter at the north end of the valley and traverse most of its length before reaching the vicinity of Cottonwood Pass. Under certain conditions the normal circulation through the passes is reversed and warm air from the San Joaquin Valley flows out westward toward the coast. When this happens leafhoppers are carried with it through the Cottonwood Pass, Waltham Canyon, and probably other such gaps. This happened on May 3 and on the morning of May 4. But when the warm air currents from the interior are pushed back by cold wind from the sea, as they were on May 4, all leafhopper movement stops. As a result there is a definite tendency for them to move in only one direction. From ground collections alone it might be inferred that such movements were the result of some directional instinct when in reality they merely reflect leafhopper reactions to local wind and temperature.

FALL MOVEMENTS The difficulties of drawing conclusions from intermittent ground collections is well illustrated by the fall dissemination. Popula- tion counts on the plains and foothills of the western San Joaquin Valley in the fall show a few leafhoppers wherever green plants are found, but the highest numbers will be found in the lower portions of the canyon bottoms, with decreasing numbers as the distance above the mouth increases. This information has been interpreted to mean that leafhoppers come in through the en- trance and gradually ascend the canyon. The data from Hospital Canyon in 1935 indicate, however, that so simple an explanation is erroneous. This canyon runs roughly northeast by southwest. The trap and instrument set-up here in 1935 is shown in figure 3. Most of the catch in the trap on the north rim of the canyon was taken when the winds blew from the northwest or east (table 17). Insects flying on east winds would be entering the hills but those traveling on northwest winds would be going paral- lel to the range. Another trap in the bottom of the canyon caught the highest numbers on winds from generally southwest and southeast. Leafhoppers flying on southwest winds would be leav- BEET LEAFHOPPER IN CALIFORNIA 55 ing the canyon. Often the wind blew from the north or the east on top of the rim and from the southwest in the bottom of the valley at the same time. Thus, leafhoppers entering the canyon over the ridge from the north might be blown out again immediately on the down-canyon wind. It is difficult to reconcile this information with the concept of a gradual movement up the canyon. A more probable hypothesis is that leafhoppers may enter the canyon either through the mouth or over the ridges and may move farther up the canyon or out again. The concentration in the canyon is probably due to the presence of better host plants there than elsewhere. In other words, leafhoppers move whenever conditions are favorable and in any direction the wind happens to be blowing. We have already noted that population counts made in the fall do not show regular accumulations as they do in the spring on host plants but, on the contrary, fluctuate erratically. This fluctuation supports the concept of irregular and more or less random movement in the fall. There is, however, a tendency toward westward movement because in the fall east winds tend to be warmer than west winds. This tendency is reversed in the spring. For instance, in the spring of 1936 for 8 days of record the morning rise in temperature averaged 7° F. in the hour preceding the wind change when it blew out of the hills, but only 3.6° in the next hour when the prevailing wind was blowing. The evening drop for 7 days of record averaged 4.9° on the valley wind and 3.4° on a down-canyon wind. That is, in the spring the temperature rose faster in the morning and dropped more slowly in the evening when the down- canyon wind was blowing. On the other hand, in the fall the opposite was true. The average morning rise on a down-canyon wind for 11 days was 3.3° ; on a valley wind it was 6.7°. The average evening drop for 15 days was 6.0° on a valley wind and 8.4° on a down-canyon wind. The result is that warm winds tend to carry leafhoppers out into the valley in the spring but in the opposite direction in the fall. This tendency is accentuated by the fact that the warm weather that often precedes a storm is usually accompanied by south and east winds. The detailed data from Ora Loma (fig. 9) illustrate this point. The heaviest catches were obtained on southeast winds preceding cold stormy weather. A large area of Russian-thistle in that direction was undoubtedly the reason and equally heavy catches might have been taken on the prevailing wind if the trap had been located on the other side of the Russian-thistle patch, but the data demonstrate that heavy flights counter to the prevailing winds do occur in the fall when conditions are favorable. In brief, the routes followed by the leafhopper in dissemination are determined by the winds that blow at the times most favorable for flight. They may be the prevailing winds, but more often are atypical air currents, which blow in the morning and evening during warm weather. There is no evidence that populations are disseminated by any directional instinct or other form of guidance, but recurring patterns of wind and temperature may result in movements that simulate guided movements. It is 56 TECHNICAL BULLETIN 1030, U. S. DEPT. OF AGRICULTURE

often possible to map fairly definite routes, but any consistency in such routes is the result of correlations between weather factors and only indirectly a product of biological causes.

SUMMARY Studies to obtain a better understanding of the movements of the beet leafhopper (Circulifer tenellus (Baker)) were made in the San Joaquin Valley of California during the years 1935 to 1937. In this valley the beet leafhopper utilizes two sets of seasonal host plants. The summer host plants are usually located in the valley. Winter and spring hosts grow on the foothills of the west side of the valley or on the adjacent plains. Major disseminations from one set of hosts to the other occur in the spring and again in the fall, with minor movements at other times of the year. Disseminations are made by night. Major nights are associated with the maturity of the insects and the drying of host plants over large areas. Disseminating leafhoppers consist largely of unparasitized and nongravid individuals. Adults make flights when the temperature is above a somewhat variable minimum, usually 60°-64° F. When the temperature is above this threshold, large numbers of leafhoppers take to the air in the crepuscular period near sunrise and sunset and most flights occur at these times of day. However, if low temperatures during these favorable periods prevent flights substantial movements may occur during warmer parts of the day. Evening flights are far more common than morning flights because temperatures are more often favorable in the evening. There is a difference between the reactions of males and females during the twilight period of activity, in that, if wind velocities are low, males are attracted to conspicuous objects and can be seen to swarm around them. At higher wind velocities both males and females are carried along in the wind stream. Other factors being equal, the numbers of leafhoppers that move on any particular day are proportional to the temperature. Leafhoppers in flight come to the ground whenever temperature or any other condition becomes unfavorable. Some of them land on host plants, but most of them probably come down on nonhosts or even on bare ground. Such insects again take to the air when the weather is favorable, and continue movement until they find host plants or die. Both males and females migrate but females are known to cover longer distances. In the spring females tend to stop over and accumulate on breeding host plants but males continue flights. In the fall both sexes move more or less continuously. This seasonal difference in behavior of females may be associated with egg development in the spring. The number of leafhoppers in the air decreases with the height above the ground, and major disseminations occur close to the surface. Leafhoppers in flight move with the wind and at about the same speed. Thus, disseminations having a directional char- acter result from a passive drifting on more or less constant winds. BEET LEAFHOPPER IN CALIFORNIA 57

Because of the restrictions imposed on flight by light and temperature, and the tendency for favorable conditions in the San Joaquin Valley to coincide with atypical weather, the routes followed by major disseminations are likely to be across or counter to the prevailing winds. Thus, leafhoppers bred in the spring on the west side of the valley move in large numbers eastward across the valley and at right angles to the prevailing winds, because strong down-canyon winds blow in the evening and morning on warm days. Westward movements toward the coast take place when warm air masses from the valley move through the passes in the coast range. Movements from the coast to the east are largely prevented because winds from the coast are cold. In the fall leafhoppers from summer host plants in the valley move toward the hills to the west because winds from the valley toward the hills are warmer than those blowing in the opposite direction.

LITERATURE CITED

(1) ANNAND, P. N., CHAMBERLIN, J. C, HENDERSON, G. F., and WATERS, H. A. 1932. MOVEMENTS OF THE BEET LEAFHOPPER IN 1930 IN SOUTHERN IDAHO. U. S. Dept. Agr. Cir. 244, 24 pp., illus. (2) ANONYMOUS. 1939. CONTROL OF INSECT PESTS. Sci. Digest 6 (2) : 96. (3) BALL, E. D. 1917. THE BEET LEAFHOPPER AND THE CURLY-LEAF DISEASE THAT IT TRANSMITS. Utah Agr. Col. Expt. Sta. Bui. 155, 56 pp., illus. (4) BARNES, DWIGHT F., FISHER, CHARLES K., and KALOOSTIAN, GEORGE H. 1939. FLIGHT HABITS OF THE RAISIN MOTH AND OTHER INSECTS AS INDICATED BY THE USE OF A ROTARY NET. Jour. Econ. Ent. 32: 859-863, illus. (5) BONNET, A. 1911. RECHERCHES SUR LES CAUSES DES VARIATIONS DE LA FAUNULE ENTOMOLOGIQUE AERIENNE. Acad. Sci. Compt. Rend. (Paris) 152: 336-339. (6) CARSNER, E. 1919. SUSCEPTIBILITY OF VARIOUS PLANTS TO CURLY-TOP. Phytopathol- ogy 15: 745-758. (7) — 1926. SUSCEPTIBILITY OF THE BEAN TO THE VIRUS OF SUGAR BEET CURLY- TOP. Jour. Agr. Res. 33: 345-348, illus. (8) CARTER, WALTER. 1927. EXTENSIONS OF THE KNOWN RANGE OF EUTETTIX TENELLUS BAKER AND cuRLY-TOP OF SUGAR BEETS. Jour. Econ. Ent. 20: 714-717. (9) '— 1930. ECOLOGICAL STUDIES OF THE BEET LEAFHOPPER. U. S. Dept. Agr. Tech. Bui. 206, 114 pp., illus. (10) CHAMBERLIN, JOSEPH C, and LAWSON, F. R. 1940. A MECHANICAL TRAP FOR THE SAMPLING OF AERIAL INSECT POP- ULATIONS. U. S. Bur. Ent. and Plant Quar. ET-163, 6 pp., illus. [Processed.] (11) CODY, CHAS. E. 1939. ROTARY TRAP AIDS IN COLLECTING FIELD DATA ON INSECTS. Market Growers' Journal for the Commercial Vegetable Producer 65, 512 pp., illus. (12) DANA, B. F. 1936. OCCURRENCE OF CURLY-TOP IN THE PACIFIC NORTHWEST IN 1935. U. S. Dept. Agr. Bur. Plant Indus. Plant Disease Reporter 20 (4): 72-76. 58 TECHNICAL BULLETIN 1030, U. S. DEPT. OF AGRICULTURE

(13) DAVIES, W. MALDWYN. 1935. A WATER-POWER MECHANICAL INSECT TRAP. Bul. Ent. ReS. 26: 553-557, illus. (14) DORST, H. E., and DAVIS, E. W. 1937. TRACING LONG-DISTANCE MOVEMENTS OF BEET LEAFHOPPERS IN THE DESERT. Jour. Econ. Ent. 30: 948-954, illus. (15) FELT, EPHRIAM PORTER. 1928. DISPERSAL OF INSECTS BY AIR CURRENTS. New York State Museum Bul. 274 (April), pp. 59-129. (16) FULTON, ROBERT A. 1937. DETERMINATION OF CHLOROFORM EXTRACT OF BEET LEAFHOPPER. Indus, and Engin. Chem., Analyt. Ed. 9: 437-438, illus. (17) and CHAMBERLIN, JOSEPH C. 1931. A NEW AUTOMATIC INSECT TRAP FOR THE STUDY OF INSECT DIS- PERSION AND FLIGHT ASSOCIATIONS. Jour. Econ. Ent. 24: 757-761, illus. (18) and ROMNEY, VAN E. 1940. THE CHLOROFORM-SOLUBLE COMPONENTS OF BEET LEAFHOPPERS AS AN INDICATION OF THE DISTANCE THEY MOVE IN SPRING. Jour. Agr. Res. 61: 737-743, illus. (19) GLICK, P. A. 1939. THE DISTRIBUTION OF INSECTS, SPIDERS, AND MITES IN THE AIR. U. S. Dept. Agr. Tech. Bul. 673, 150 pp., illus. (20) HäRTUNG, WM. J. 1924. EVASION OF CURLY LEAF DISEASE OR "BLIGHT". Farm Bur. Monthly, Monterey County, Calif., 6: 13-16. (21) HILLS, ORíN A. 1937. THE BEET LEAFHOPPER IN THE CENTRAL COLUMBIA RIVER BREEDING AREA. Jour. Agr. Res. 55: 21-31, illus. (22) LAWSON, F. R., Fox, D. E., and COOK, W. C. 1941. THREE NEW DEVICES FOR MEASURING INSECT POPULATIONS. U. S. Bur. Ent. and Plant Quar. ET-183, 6 pp., illus. [Processed.] (23) and PIEMEISEL, R. L. 1943. THE ECOLOGY OF THE PRINCIPAL SUMMER WEED HOSTS OF THE BEET LEAFHOPPER IN THE SAN JOAQUÍN VALLEY, CALIFORNIA. U. S. Dept. Agr. Tech. Bul. 848, 37 pp., illus. (24) MCCLURE, H. ELLIOTT. 1938. INSECT AERIAL POPULATIONS. Ann. Ent. Soc. America 31: 504-513, illus. (25) MEUNIER, K. 1928. EXPERIMENTELLES üBER DEN SCHWARMTRIEB UND DAS PERIODISCHE AUFTRETEN VERSHIEDENER AKTIVITATSFORMEN BEIM MAIKäFER (MELOLONTHA MELOLONTHA L.) Z. angew. Ent. 14: 91-139, illus. (26) PIEMEISEL, R. L., and LAWSON, F. R. 1937. TYPES OF VEGETATION IN THE SAN JOAQUÍN VALLEY OF CALIFORNIA AND THEIR RELATION TO THE BEET LEAFHOPPER. U. S. Dept. Agr. Tech. Bul. 557, 28 pp., illus. (27) ROMNEY, V. E. 1939. BREEDING AREAS AND ECONOMIC DISTRIBUTION OF THE BEET LEAF- HOPPER IN NEW MEXICO, SOUTHERN COLORADO, AND WESTERN TEXAS. U. S. Dept. Agr. Cir. 518, 14 pp., illus. (28) SEVERIN, HENRY H. P. 1930. LIFE-HISTORY OF THE BEET LEAFHOPPER, EUTETTIX TENELLUS (BAKER), IN CALIFORNIA. Calif. Univ. Ent. Pub. 5: 37-88, illus. (29) 1933. FIELD OBSERVATIONS ON THE BEET LEAFHOPPER, EUTETTIX TENEL- LUS, IN CALIFORNIA. Hilgardia 7: 281-360, illus. (30) 1919. NOTES ON THE BEHAVIOR OF THE BEET LEAFHOPPER (EUTETTIX TENELLUS (BAKER) ). Jour. Econ. Ent. 12: 303-308. BEET LEAFHOPPER IN CALIFORNIA 59

(31) 1927. CROPS NATURALLY INFECTED WITH SUGAR BEET CURLY-TOP. Science 66: 137-138. (32) and HENDERSON, C. F. 1928. SOME HOST PLANTS OF CURLY-TOP. Hilgardia 3: 339-392. (33) STAGE, H. H., and CHAMBERLIN, J. C. 1945. ABUNDANCE AND FLIGHT HABITS OF CERTAIN ALASKAN MOSQUI- TOES, AS DETERMINED BY MEANS OF A ROTARY-TYPE TRAP. Mosquito News 5(1) : 8-16, illus. (34) THOMAS, I., and VEVAE, E. J. 1940. APHIS MIGRATION. AN ANALYSIS OF THE RESULTS OF FIVE SEASON'^ TRAPPING IN NORTH WALES. Bul. Ent. Res. 27: 393-405, illus. (35) TUTT, J. W. 1902. THE MIGRATION AND DISPERSAL OF INSECTS. Ent. Rec. 14: 292-295. (36) UvAROV, B. P. 1931. INSECTS AND CLIMATE. Trans. Ent. Soc, London, 79(1) : 1-247, illus. (See Part II, pp. 87-104.) (37) WILLIAMS, C. B., and MILNE, P. S. 1935. A MECHANICAL INSECT TRAP. Bul. Ent. Res. 36: 543-552, illus.

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