May 1942 E-567 THE BEET LEAFHOPPER AND ITS CONTROL, ON BEETS GROWN FOR SEED IN ARIZONA AND NEW MEXICO By Van E. Romney, Division of Truck Crop and Garden Insect Investigations INTRODUCTION The commercial production of sugar beet seed on a large scale is a relatively new agricultural industry in the western part of the United States. The more important areas in which the seed is now produced include the Salt River Valley in Arizona, Mesilla Valley in New Mexico, valleys near Hemet and Perris in southern California and Shasta Valley in northern California, Virgin Valley in southern Utah and several mountain valleys in north-central Utah, an area near Klamath Falls and the Rogue River and Willamette Valleys in Oregon, and the Puget Sound area in Washington. Damage from curly top is a limitation of importance to the production of sugar beet seed in some districts. Curly top is a virus disease trans- mitted by the beet leafhopper. Eutettix teneJLlus (Bak.). The crop in Arizona and New Mexico is subject to beet leafhopper infestations in the fall from the time the seed germinates until the foliage practically covers the ground. Certain of the varieties grown in these areas are resistant to curly top, and on these varieties the leafhopper has not been a serious problem. However, many of the varieties are very susceptible to this disease, and it is on these susceptible varieties that leafhopper control may be needed. SOURCE AND TIME OF THE FALL INFESTATIONS Arizona The beet leafhoppers which infest the beets grown for seed in the Salt River Valley come from desert areas completely surrounding the valley, as shown in figure 1. The leafhopper breeds on extensive stands of chinch- weed (Pectis pappqsa Gray), and on Tidestromia languinosa (Nutt.) Standi., beginning in July and continuing until late fall. Figure 2 shows a 62-percent coverage of chinch-weed and a 1-percent coverage of Tidestromia as they occurred 12 miles northwest of Phoenix, October 16, 1939. Such stands are common over the desert areas, although usually more plants occur along the washes. The numbers of leafhoppers produced in the desert areas are governed largely by the abundance of the summer plants, which in turn depends upon the amount of rainfall during July, August, and September. Two generations of the leafhopper usually occur over desert areas by mid-September, but when heavy rains occur early in September, as they did in 1939 and 1940, addi- tional reproduction occurs in the breeding area. October and November rains, which germinate winter annuals in the desert, are important in reducing the number of leafhoppers entering the seed-beet fields. Beets planted after August 15 in the Salt River Valley of Arizona have been found to be promptly infested to a small extent by the beet leafhopper, and the number of insects increased gradually until mid-Septem- ber. Such early infestations did not result in serious damage except in poor stands of beets. From mid-September to mid-October, leafhopper numbers in the fields usually have not increased, although an unusual condition arose ?ctober 3-7, 1941, when a moderately light influx of leafhoppers infested the fields from Atriplex elegans (Moq.) Dietr. which grew abundantly in basins southeast of Phoenix. The largest infestations occurred about October 25. In one season out of six studied, additional influxes occurred late in November. The desert foothills around Safford, Ariz. . also contain stands of chinch-weed which are responsible for beet leafhopper infestations that occur in beets grown for seed in that valley. Very little is known about infestations in that area. In 1940 and 1941. moderate numbers of leafhoppers had infested the beets by late September, and later increases did not occur. New Mexico The foothill areas on both sides of the Mesilla Valley, N. Mex. , are known to be important as a source of beet leafhoppers which infest sugar beets grown for seed in that valley. Four annual host plants are important in this general area, which include chinch-weed (Pectis sp. ) , Tidestromia sp. , Trianthema portulacastrum L. , and Acanthochiton wrightii Torr. In addition to these four summer plants a perennial peppergrass (Lepidium alyssoides A. Gray) also grows throughout the summer breeding areas in New Mexico and Texas (fig. 1). and the abundance of this plant greatly influences the numbers which infest the beets in the fall. Rather large infestations in seed-beet fields in the Mesilla Valley have occurred by late September, although the more severe infestations have occurred after late October. NUMBERS REQUIRED FOR INJURY The density of the beet stands and the rate at which the soil is covered by beet leaves are important factors in determining the amount of curly top injury which will result from a given number of beet leafhoppers. - 3 - A stand of 700 to 1,000 beets per 3.00 feet of row in the Salt River Valley of Arizona has required a fall infestation of from 125 to 150 leafhopper adults or nymphs per 100 feet of row to induce about 20 percent total curly top by the following April or May. This amount of disease or more is appar- ently required before the yield of seed is measurably reduced in stands of this density, but thin stands cannot tolerate this amount without a loss in yield. Observations made during two seasons in the Safford, Ariz., district indicate that somewhat similar numbers would be required to induce injury in that area. A s'tand of 600 to 800 beets per 100 feet of row in the Mesilla Valley, N. Mex.. has required a fall infestation of from 75 to 100 beet leafhoppers per 100 feet of row to warrant insecticidal control measures. More growth takes place in the fall and winter in Arizona than in New Mexico, and the average stands in New Mexico are thinner. The resulting difference in soil coverage in the two areas is probably largely responsible for the difference in numbers of beet leafhoppers required to cause conspicuous injury. With thinner stands in both districts, smaller numbers of leafhoppers may induce pronounced injury. CONTROL OF THE BEET LEAFHOPPER Cultural Methods Beet leafhopper control by cultural methods has been found effective in reducing curly top injury in nonresistant varieties. Early planting (late August to early September), good stands, and care conducive to rapid growtn until the leaves almost completely shade the soil have greatly reduced curly top in seed-beet fields of the Salt River Valley, Ariz. Under favor- able conditions early-planted beets there may almost completely cover the soil by late October when large leafhopper influxes occur. The field shown in figure 3 was first irrigated (planted a few days earlier in dry soil) September 2, 1940. On October 25. or 53 days later, the beet leaves shaded about 98 percent of the soil surface. Such fields as this have been found to be unfavorable environments for the beet leafhopper. By the time the foliage covers about 80 percent of the soil in the fields, beet leafhopper adults begin to leave them, and fields with 90 to 98 percent of the soil covered with leaves have invariably been found to contain small numbers of leafhoppers. Furthermore, leafhopper reproduction does not occur in fields where the foliage is dense. Fields with poor stands or poor growth (fig. 4). or both, are subject to October and November infestations, and there may be late fall reproduction in them during milder seasons, such as 1939-40. Such fields may be severely damaged. Good cultural practices will also reduce losses from curly top in nonresistant varieties in the Mesilla Valley of New Mexico, but they have not been so effective in reducing beet-field leafhopper populations there as in the Salt River Valley of Arizona. Moderate leafhopper infestations have occurred in the Mesilla Valley as early as mid-September, when the beets were still small, and under such conditions insecticidal control measures are needed. Owing to a colder climate in New Mexico than that in Arizona. it is more difficult to obtain the rapid growth of the beets necessary for early soil coverage. Some seasons the seed beet fields in the Mesilla Valley are subject to large leafhopper inrestations during late October and at later intervals; therefore every effort should be made to obtain a dense leaf growth by that time in order to avoid the necessity of insecticidal control . Data from the Safford, Ariz., district are limited, jut the indica- tions are that control of curly top by cultural practices in that district will be a little more difficult than in the Salt "River Valley, since beets in that area make slower fall growth and are subject to damaging beet leafhopper infestations in September while the beets are still small. Chemical Control During seasons when unusually large or early leafhopper infestations occur, cultural practices should be supplemented by chemical control. This holds true generally for the Mesilla Valley and Safford districts and for fields with poor coverage in the Salt River Valley. Pyrethrum-in-oil 1/ spray applied at the rate of 6 to 9 gallons per acre was found to be very effective in controlling the beet leafhopper in seed-beet fields. The machine used is illustrated in figure 5. Six gallons per acre proved sufficient for a reduction in leafhopper populations of 90 percent or greater, when there was little wind and temperatures were between 50° and 75° F. It was found necessary to increase the quantity of spray as the temperature rose above 75° F. , until at temperatures above 90° F. as much as 9 gallons per acre were required to kill 90 percent or more of the leaf- hoppers. An increase in wind velocity decreased the efficiency of the spray, but winds up to 8 to 10 miles per hour 4 feet above the ground were not too great to permit spraying. 1935-56 Experiments. — Tests of the effectiveness of commercial appli- cations of the pyrethrum-in-oil spray were arranged by having unsprayed strip plots left in six separate commercially sprayed fields and comparing these with sprayed strips of similar size in the same fields. The results are give:* in table 1. Fields 1, 2, and 3 were sugar beets in the Mesilla Valley of New Mexico, and fields 4. 5, and 6 were table beets southeast of El Paso, Tex. The sugar-beet fields were sprayed once, fields 2 and 3 in late November and field 1 in mid-December. The table beets were sprayed in early December and again in late February. The plot sprayed once in field 6 1/ Formula: 10 parts white oil — a highly refined petroleum oil having a viscocity of 90 to 100 seconds- Saybolt at 100° F. . and an un- sulfonated residue of 94 percent. 20 parts kerosene — a good grade of commercial kero- sene complying with Federal Specification VV-K-211. 1 part pyrethrum extract — an extract of pyrethrum flowers in petroleum oil having a pyrethrin content of not less than 2.0 percent (or 2.0 grams of pyrethrins per 100 cc). - 5 was ruined by root rot, and therefore two strips sprayed twice were compared with the unsprayed in this field. Beet leafhopper infestations in the fields were large by late October, and no marked increase occurred until late December. Even though the appli- cations were not timely, the yield of seed from plots sprayed once was increased in all cases, with an average of 33 percent, over the unsprayed plots, and there was an average increase of 60 percent in the plots sprayed twice. Curly top injury was reduced indirectly by reducing the numbers of leaf hoppers, and this was responsible for the increases in yield in the sprayed plots. There was considerable curly top damage even in the sprayed plots, and the yields were not large. The viability of the seed was very poor in all the plots, but slightly higher in the sprayed plots. Populations of plant bugs (Lygus spp.) were high in these fields in the spring, and the results of later investigations^/ indicate that the low percentage of viable seed was at least in part due to these insects. 1937-38 Experiment. — This experiment was conducted just northwest of Buckeye, Ariz., and consisted of 16 plots, approximately one-half acre in size, arranged as a Latin square, with 4 treatments. However, owing to the fact that leafhopper numbers were so reduced by rapid leaf growth by late November that no additional treatments seemed worthwhile, the experiment was left with 8 plots sprayed October 30, 1937, to be compared with 8 un- treated plots. The rate of application was 10 gallons per acre because the temperature during the application ranged from 88° to 89° F. and the wind velocity 4 feet above the ground was 7 to 8 miles per hour. Leafhopper counts 24 hours later showed a reduction in populations of 94 percent for adults and 99 percent for nymphs. Results from the single application of October 30, 1937, are given in table 2. The beet plants averaged 1,031 per 100 feet of row in the plots. Owing primarily to dense leaf growth, the numbers of leafhoppers in the unsprayed plots decreased from 401 per hundred feet of row on November 1 to 96 on December 21 and to 77 by January 21. Leafhopper numbers in the sprayed plots increased slightly during this same period. On November 1 the beet leaves covered about 40 percent of the soil surface, on November 30 they covered about 80 percent, and by December 21 approximately 95 percent of the soil surface was covered with beet foliage. Curly top counts made May 23-25, 1938, showed a 34.2-percent decrease in the total curly top and a 58-percent decrease of severely diseased plants in the sprayed plots as compared with the unsprayed plots. The average yield of clean seed from the sprayed plots exceeded that produced by the unsprayed plots by 55.1 percent. The seed stalks averaged 230 per 100 feet of row in the sprayed plots, and 192.9 per 100 feet of row in the unsprayed plots. There was n'o significant difference in the percentage of viable seed from the sprayed and the un- sprayed plots. 2/ Hills, Orin A., and Romney, V. E. A Progress Report on Hemip- terous Insects Affecting Sugar Beets Grown for Seed in Arizona and New Mexico. U. S. Dept . Agr., Bur. Ent . and Plant Quar., E-552. October 1941. 6 - The average increase in seed from the sprayed plots over the unsprayed plots amounted to 359 pounds per acre, which was worth $30. The cost of spraying at the 10-gallon rate was approximately $5 per acre, which leaves a net gain of $25 from the spray application. 1939-<0 Experiment . — This experiment involved 20 plots measuring 100 by 123 feet (0.23 acre) and was conducted near Coldwater, Ariz. Ten randomized blocks with 2 treatments (sprayed and unsprayed) each were sprayed on November 6 and 7 with 6 gallons of pyrethrum-in-oil per acre. The temperature was 68° to 76° F., and there was no wind. Within 24 hours there was a 92-percent reduction in adult beet leafhoppers, and at this time nymphs were scarce. The beet leaves covered about 45 percent of the soil surface on November 8, just after spraying, and by December 20, 70 percent of the soil surface was covered by the foliage. The results of this experiment are given in table 3. The stand averaged 580 plants per 100 feet of row in the plots, which would be rated as only a fair stand. The leafhopper population 24 hours after treatment (Nov. 8) averaged 332 per 100 feet of row in the unsprayed plots and 28 in the sprayed. By December 20 the leafhopper numbers in the unsprayed plots decreased from 332 to 213 per 100 feet of row, while at the same time those in the sprayed plots increased from 28 to 72 per 100 feet of row, which was probably due to a shifting of leafhoppers from the unsprayed to the sprayed plots. Curly top counts made April 19-20, 1940, showed a 39.5-percent de- crease in total curly top and a 52.3-percent decrease of severely diseased plants in the sprayed plots as compared with the unsprayed plots. The average yield of clean seed from the sprayed plots exceeded that produced by the unsprayed plots by 22.1 percent. More seed stalks were produced in the sprayed plots than in the unsprayed plots. An average of 319 seed stalks per 100 feet of row was found April 19-20, 1940, in the sprayed plots, and 210.5 per 100 feet of row in the unsprayed plots. There was no differ- ence in the viability of the seed from the sprayed and unsprayed plots. The average seed increase of 219 pounds per acre due to the one spray application was worth $18.62. The cost of the treatment at the 6-gallon rate was approximately $3.50 per acre, or a net gain of $15.12. THE 1940 CROP IN ARIZONA AS AFFECTED BY CURIA TOP The winter of 1939-40 was abnormally warm in Arizona. The Weather Bureau at Phoenix recorded the following departures from the mean monthly normal temperatures in degrees Fahrenheit for November to March inclusive: +4.6°, +5.6°, +4.4°, +1.7°, and +4.1°. This extremely warm season . was favorable for leafhopper activity and reproduction, especially in fields with poor coverage. This was the first time in six seasons that beet leaf- hopper reproduction occurred in the Mesa and Tolleson districts during the winter months. On the basis of beet leafhopper populations in the seed-beet fields in the Mesa and Tolleson districts in late October 1939, it did not appear that serious curly top injury would occur, and very little commercial - 7 spraying was done. However, approximately two-thirds of the seed-beet crop planted in the Salt River Valley in the fall of 1939 did not have good leaf coverage at that time, owing mainly to interference with field operations by rains in early September. This situation and the mildness of the winter resulted in a greater amount of curly top damage than had been noted in other seasons. The conditions in a field southwest of Mesa were typical of a number of fields in the valley. It had 80 adult leafhoppers per 100 feet of row in late October in a stand which averaged only 271 plants per 100 feet of row. The field passed the winter with not more than 75 percent of the soil covered by foliage. By early March there was an average of 60 adults and 90 nymphs per 100 feet of row. There was 55 percent total curly top in this field by late April. It is important to note, however, that of 17 fields studied in the Mesa and Tolleson districts in the spring of 1940 only 5 had more than 20 percent curly top infection. Many observations had brought out the fact that in good stands there must be more than 20 percent total curly top by late April for the disease to be a major factor in reducing seed yields. Evidently other factors of importance besides curly top contributed to the abnor- mally low average yield and poor germination of the 1940 seed crop. Table 1. — Results obtained by commercial applications of pyrethrum-in-oil for control of the beet leafhopper, as indicated by strip-plot experiments in New Mexico and western Texas Field No. Adult leafhoppers per Plants Severely Yield Increase Seed Treatment 100 feet of row having curly diseased beets per of Shortly seed after first Feb. top 100 feet per application 1-2 of row acre in yield balls over with untreated viable checks seed Number Number Percent Number Pounds Percent Percent 1 . Sprayed 12-14-35 16 28 57.7 205 1,173 44.5 35.0 Unsprayed check 166 182 89.7 566 812 32.8 2. Sprayed 11-22-35 66 57.7 222 567 22.5 40.8 Unsprayed check 223 300 91.7 372 463 35.8 3. Sprayed 11-29-35 28 42 53.7 186 777 28.9 36.3 Unsprayed check 123 145 78.2 246 603 34.8 4. Sprayed 12-8-35 4 566 131 721 28.7 Sprayed 12-8-35 and 2-22- 36 122 1,022 82.5 35.0 Unsprayed check 297 825 261 560 31.0 5. Sprayed 12-9-35 10 280 74 583* 39.8 Sprayed 12-9-35 and 2-23- 36 69 607 45.6 27.0 Unsprayed check 150 440 136 417 15.0 6. Sprayed 12-10-35 and 2-22 -36 280 34.6 Sprayed 12-10-35 and 2-22 -36 26 550 92.1 186 372 78.8 20.8 Unsprayed check 391 750 98.5 441 208 17.5 Table 2. — Results obtained by one application of pyrethrum-in-oil for control of the beet leafhopper, as indicated by eight replications of two treatments made October 30, 1937, near Buckeye, Ariz. Treatment Beet leafhopper adults and nymphs per 100 feet of row Plants with curly top 1/ Nov. 1 Dec. 21 Jan. 21 Plants with severe curly top Yield of Seed clean balls seed with per viable acre seed Number Number Number Percent Percent Pounds Percenl Sprayed 9 26 34 41.1 10.8 1,011 78.0 Not sprayed 401 96 77 62.5 25.3 652 75.9 Difference required for significance at the 5-percent level 8.5 4.9 144 2/ 1/ Curly top counts by systematic samples made May 23-25, 1938 2/ Not significant. Table 3. — Results obtained by one application of pyrethrum-in-oil for control of the beet leafhopper, as indicated by 10 replications of 2 treatments made November 6-7, 1939, near Coldwater, Ariz. Treatment Plants Beet leafhopper adults and nymphs . with per 100 feet of row curly top 1/ Nov. Dec. 20 Plants with severe curly top Yield of Seed clean balls seed with per viable acre seed Sprayed Not sprayed Number 28 332 Number 72 213 Percent Percent Pounds Percen; 43.5 23.2 1.208 69.6 71.9 48.6 989 69.6 Difference required for significance at the 5-percent level 102 i/ Curly top counts by systematic samples made April 19-20, 1940. - 9 - SUMMARY In the fall, beet leafhoppers infest fields of beets grown for seed in Arizona and New Mexico and transmit curly top, a disease which reduces the seed yield. The insects come from surrounding desert breeding areas. Several summer annuals serve as host plants for the leafhopper, and in New Mexico a perennial peppergrass is also of importance. In the Salt River Valley of Arizona large infestations usually do not occur in good stands of beets until late October. In the Mesilla Valley, N. Mex . , and Safford, Ariz., districts rather large infestations often occur by late September. The number of leafhoppers required to induce injury is in general- greater in Arizona than New Mexico. Fields in the Salt River Valley of Arizona planted before September 10, with stands ranging from 700 to 1,000 beets per 100 feet of row, have required fall leafhopper infestations of from 125 to 150 per 100 feet of row to induce a conspicuous amount of curly top damage by the following April. Fields in the Safford, Ariz., district will probably require somewhat similar numbers. Fields in the Mesilla Valley of New Mexico, however, with stands ranging from 600 to 800 beets per 100 feet of row, have required only from 75 to 100 beet leafhoppers per 100 feet of row to warrant insecticidal control measures. Curly top injury can be prevented to a large extent in the Salt River Valley of Arizona by planting the beets early and giving adequate care to get the soil surface in the field practically covered with foliage by late October. Such fields become unfavorable environments for the beet leafhopper. In the Safford and Mesilla Valley districts the crop has been subjected, in some seasons, to moderate September infestations before good coverage was obtained. Owing to colder climates it is also more difficult to get enough fall growth in these districts to afford complete coverage. A pyrethrum-in-oil spray applied in the fall at the rate of 6 to 9 gallons per acre has given better than 90 percent reduction of beet leaf- hopper adults and nymphs within 24 hours. Such • reductions in leafhopper numbers have reduced curly top and resulted in increased yields of seed. The viability of the seed was not significantly affected by the spraying. Curly top was only partially responsible for the poor beet seed crop in Arizona in 1940. Hoq (D 8 & CO O Qj O o cl ci- te If 2S a. © p. ta IS » »1 LIBRARY OTATE PLANT BOARX> Figure 2. — Photograph showing a 62-percent coverage of chinch-weed (Pectis) and a 1-percent coverage of Tidestromia 12 miles northwest of Phoenix, Ariz., October 16, 1939, which is "typical of summer host-plant conditions in the breeding areas of the beet leafhopper, especially along the lowlands of washes. m^m Figure 3.— A field of beets in the Salt River Valley of Arizona photographed October 25, 1940, 53 days after plant- ing. The foliage covers about 98 percent of the soil sur- face, thus creating an environment which is unfavorable for the beet leafhopper. Figure 4- — A field of beets in the Salt River Valley of Arizona photographed October 25, 194-0, 32 days after plant- ing. The foliage covers about 40 percent of the soil" sur- face, which is not sufficient at this season of the year to repel the leafhopper. This "type of field is therefore sub- ject to large infestations of the beet leafhopper during outbreak years. Figure 5.^-Spray machine used to apply a pyrethrum-in-oil spray for control of the beet leafhopper. The machine is shown spraying a comparatively thin stand of beets in the Salt River Valley of Arizona in October 1940. UNIVERSITY OF FLORIDA mini nun i 3 1262 09224 7732