![]() ![]() |
![]() |
UFDC Home | Federal Depository Libraries of Florida & the Caribbean | UF Government Documents Collection | Internet Archive | | Help |
Material Information
Record Information
|
Full Text |
E-546 August 941 DEPARTMENT190 1 AGRICULTURE BUREAU OF ENTOMOLOGY AN D rLANT QUARANTINE AN EXPERIMENTAL COOPERATIVE COMMUNITY PROGRAM FOR TI-.'E CUU7 -T CN~O OF BUGS OF THE GENUS LYGUS ON ALFALFA SEED CROPS IN iE ,iA, E O ARIZONA IN 1939 and 1940 l/ By Loyd L, titt, Division of Cereal and Forage Insect Investigations INTRODUCTION Severe reduction of the alfalfa seed crop caused by plant bi~s of the genus Lypus in the Mohawk area of Yuma County, Ariz., in 1938, a cm- pared with the yields obtained in 1936 and 1937, resulted in an appeal by seed growers for aid in controlling these pests. Investigations during 1938 and preceding years showed a noticeable lack of uniformity in the time the seed crop was started in different fields, and that this irregularity, together with failure to clean up weed and alfalfa growth that harbore. the bugs during critical periods of the year, was favorable to their interfeld migration, survival, and multiplication. These investigations also indi- cated that thorough winter clean-up of host plants and the observance of much closer restrictions on the time of starting the seed crop in all fields within a locality were not only effective measures for the control of L_ uj, but also were beneficial in reducing some of the losses of alfalfa seed caused by the seed chalcid Bruchorhagus gibbus Bch, and thu penta toid Chlorochroa savi St~l. As a result of these observations an experirinal cooperative community program for the cultural control of Lyus huB "as inaugurated in the Mohawk area and followed during the years 1939 a39 940. During the period the control program was in effect, Lyus piopuictions became much reduced and increased seed yields were obtained. l/ The experimental program for the cultural control of LX!us in te Mohawk area was cooperative between the alfalfa seed growers of the district, the Agricultural Extension Service of the University of Arizona, worLng through G. E. Blackledge, County Agent for Yuma County, Ariz., a the Tempe, Ariz., Laboratory of Cereal and Forage Insect Investigations of the Bureau of Entomology and Plant Quarantine of the United States Depart~ent of Agriculture. The author expresses his gratitude to V. L. Wildermuth, Sciior Entomologist in charge of the Tempe, Ariz., Laboratory, under whose immediate supervision this investigation was conducted. -2- Two important features in the success of this program were the whole- hearted-,)operation of the alfalfa seed growers in following the cultural prs~ctces suggested and the fact that these practices did not involve '~:~i- variations from those already prevailing in the area. The apparent success of the Lygus control program, as conducted in this area, indicates that similar programs may be practicable in other alfalfa seed-growing districts, under different environmental conditions. The fo!!oiing discussion of the procedure and results of the community control experiment in the Mohawk area is therefore given. REVIEW OF LYGUS INVESTIGATIONS CONDUCTED IN ARIZONA AND SOUTHERN CALIFORNIA g/ In the alfalfa seed-producing areas of Arizona and southern Cali- fornia three species of Lygus have been present, namely, L. hesperus Knight, L elisu~' Vvn Duzee, and L. pratensis oblineatus (Say). 3/ Investigations for 1- y -s 1935 through .938 showed that these bugs caused severe damage to the alfalfa buds, flowers, and seeds. High Lygus populations caused an incre--se in flower fall and in the proportion of brown seeds and a decrease in seed i ild. Nine positive correlation coefficients computed from data sh~',in the relationship between the bug population and the proportion of ,ower fill and brown seed were statistically significant at the 5-percent level cnd six of these were significant at the 1-percent level. Field observations during May and June 1937 showed that Lygs hesperus constituted over 85 percent of the total number of Lygus collected in z'alfaf, L. elisus showed preference for weeds, and the indications were theft they remained on these hosts as long as the condition of the plants w>. sui~t2le. L. praensis oblineatus appeared to be more abundant during the fll, winter, and spring than during the summer, Plant-feeding mirids, of hich Lus is one genus, normally seek a satisfactory growth of a pre- ferred host plant for oviposition. Lygu spp. prefer the succulent, tender terminal parts of their host plants. ,'igrations of the bugs within the cultivated area were found to be vory iportant in causing high Lygus infestations in the alfalfa seed fields. The cutting of alfalfa for hay caused the adult bugs to migrate by fight to neighboring earlier out fields, where they found optimum feeding conitio s on the new growth. Lack of uniformity in the time of cutting the first hay crop and in starting the seed crop, in neighboring fields, p% provided a continuous supply of suitable food and enabled the bugs to survive cid multiply, with resultant extremely high Lygus populations and severe cmage to the alfalfa seed crop. 2/ For detailed information on the Lygua investigations see U. S. ep r .eiit of Agriculture Technical Bulletin 741, Three Species of the (Ceuin Ly-us and Their Relation to Alfalfa Seed Production in Southern Arizona sad California. 3/ Order Hemiptera, family Miridae. - 3- Field studies and laboratory [hlot tests hve shown that high mortality of the nym1nhal stages is caused wy the removal o. sacculet food, as in lhe cutting of the alfa hay crop .nd subsequ et e r to th e oli. :tic conditions of southern [rizcna. When air tvmje( ies reached 9O' to 1000 F an es imated mortality of g0 t 95 percent of tho nymphs occur ed in test plots. Field observations durin the r vel'~n t of the see6 crop in Arizona have shown that the La.> prpu>aiow- (1duItv and nymphs) were reduced in alfalfa fields by clean cutting during Ppril, and in many cases the populations never again reached the precutting Jevel. Except in felds into Nx.hich a migratory influ.- occurred, from 2 to 3 weeks elapsed after the hay crop was cut before even a small increase in the population was detected in the seed crop immediately following. LOCATION AND ORGANIZATION OF THE COMMUNITY EXPERIMENT The cooperative community experiment in the cultural control of Lyiius bugs on alfalfa was conducted in the Mohawk area of Yuma County, Ariz., because of the interest in and desire for it shown by the local seed growers, G. E. Blackledge, Agricultural Agent for Yuma County, supported the in- auguration of this movement and gave of both his time and facilities. The Mohawk area includes the Antelope and Mrohawk Valleys, within which are situated the towns of Weilton and Roll. The district is in the Lower Sonoran Life Zone of the mesquite and creosote bush associations Rainfall is low and temperatures high, and this results in a prevailing low relative humidity. The temperature range between day and night is great. The cropped land consists of about 6,500 acres, which is all irrigated by water pumped from wells. According to the official estimate, 5,430 acres (approximately 83 percent of the cropped land) produced alfalfa seed during 1939. The University of Arizona report 4/ on the vwater condition and crop production in the Mohawk district indicated that alfalfa seed was the only profitable crop. The first step in the organization of the control program was the distribution of a questionnaire to the alfalfa seed growers to obtain infor- mation essential in formulating a plan of action. One of the purposes of the questionnaire was to ascertain the prevailing cultural practices used in seed growing. The outstanding information obtained by this means was as follows: (1) The preferred dates for turning to seed were from April 15 to May 15; (2) the majority of the seed growers irrigated the seed crop before it bloomed; (3) most of the growers required a period of 6 to 10 days to harvest their hay crop before the seed crop was started; and (4) over one-half the seed growers in the Mohawk area indicated that they were able to recognize Ly gus bugs and, in many cases, Vnew their general habits and the type of injury they cause to the growing alfajfa seed crop. The replies to the questionnaire indicated that a community control experiment could be arranged without expense, special operations, or too 4/ University of Arizona. Report of Septemer 16, 1.936, on the Moha,.-k Municipal Water Conservation District. -4- a derrrture from current farm practices used by the majority of the seel groers. Mr. Elackledge therefore, called a meeting of the growers i e ':'k area in Januiry 1939, at which 4he insect, its habits, the k'ind cf i.~ ry it causes, ani certain farm practices which appeared bene- -rucing L Ygus d S -;e were discussed. Folloing this, the seed _e e ,duted their own orgDnization for carrying out a program, which cc. in setting definie ates to start the seed crop, the inauguration of cle.-u.p measures, and he selection of a comh-tee to handle the plan. The :ru zation was completed approximately 3 months prior to the dates se f: a arng the seed ciop, The periods agreed upon during which the !a crop preceding the seed crop was to be cut were between April 20 S93 mcd bet een April 16 and 26 in 1940. All alfalfa fields in aitrict were to e cut cleanly without leaving uncu plants, and re to be cleaned of jl alfalfa and weeds. All fall and winter o be eliminated by FeruIry 1,. The comitte deemed it advisable to iicc: orate these requiio lents in an agreement which the seed growers e 3e d to sign, pledging themselves to ccrry out the program to com- pleto. Each grower was ftirnished a copy of this agreement so there would he no a'stion as to the cities he was to perform Cormit.ee meetings were o lhe <. different times Io discuss Iny problems rising and to review the progre.s of the work. Any suggestions agreed upon by the committee that ar~e red o be beneficial to the program here miieographed and sent to each seed grower. FARMING PRACTICES) U' IN THE MOHAWK A?,EA EXFEIMENT ocordiuo to the v rogram adopted, the fnil and winter growth of .... I na. reecds were to .e cleaned up by psturing "ith ctte or sheep, oi ,, -in December od nuary. The spring crop of alfalfa was to be cut or rel off in reh in time to allow approximately 40 days for the SmnL of the lst Ir y crop and the toree of food i the roots essen- i he gorous ee y cvovth of the seed c, cir cre was to he U' i: cutting the f hfa closely and cle. ny a r'der to reduce the ii t coo supply and ;-i( La the st2~rvn ) the bubs. In Goth 1939 d <40 ,he committee .:cive in secur'ir os' ce by ii the growers o ne de limits set fcr Lhe c uting of the 1as hny crop. o I'ears a ehe as cn e on the proper ica of the acreage on i I i s hay cut i completed a the iep cified in the agree- I'i ,x oha Valleys) was ,v t, it ws a it''d Ithat vppro i:: .ey 5,00 acres (92 percent) 0 ;acres c' e ole oc jcc rding the agree- rent in V4' tis *, T, 5u 'cres, or ) ucon, in the Mohawk Valley ... ... .. or 75 pierce i he P~ulcove Valle y tv,,ing to the greater the c wt }arries in .he n sioee Valley, there -v iovei e ii K e h' fields. The com- S 1 cen.d i~hfuilly ,'or he program, and the per- s o" alln-up shoAu i hese reourds3 cl, ry illustrate the results -5- Regulation of irrigations, so as to prevent the production of too heavy and succulent a growth of the alfalfa plants in a 8eed field, aided in checking the Lygus increases, as the resultant plant growth was not optimum for the best Lygus development. Unusually light irrigations during the winter of 1939-40 prevented abundant growth of alfalfa and, owing to the shortage of pasture, resulted in an excellent winter clean-up by grazing stock. The consequent reduction in winter carry-over of Lygus bugs appeared beneficial in that the Lygus numbers were low in April at the time the seed crop was started. METHODS USED IN EVALUATING THE EFFECTIVENESS OF THE COMMUNITY CULTURAL CONTROL EXPERIMENT The effectiveness of this control experiment in 1939 was determined by comparison of records of L~us populations, flowers fallen, proportion of brown seeds, and seed yields made in the area that year with similar records made in the same fields the previous year. For the year 1940, in addition to the comparisons with the records of the two preceding years in the Mohawk area, a comparison was also made with data obtained in the South Gila area, where no community control program was conducted. This seed district of 2,900 acres is located 35 miles west of the Mohawk area and in past years has been similar to it in alfalfa culture and seed yields. Both Yuma County and State yields of alfalfa seed have also been used in comparisons with the yields of the controlled Mohawk area for 1938, 1939, and 1940. Lygus Population Surveys Alfalfa fields were selected at random, in which systematic weekly surveys were made during the development of the seed crop. Ljyus popula- tions, stage of plant growth, Lyus damage, and abundance of other insects were recorded for each field surveyed. Field surveys were made before the seed crop was started, and the data showed that fields exhibiting the higher Lygu populations, in the hay crop prior to the seed crop, usually contained the higher populations in the seed crop. In most cases these higher populations seemed attributable to the fact that the alfalfa hay crop had grown for more than 40 days, which had permitted greater numbers of Lygus bugs to develop. In making the survey records of Lygua population the same system was followed in all fields. Six fields in the Mohawk area were studied in detail in 1938, prior to the inauguration of the community program. Eleven fields were studied in that area in 1939, and in 1940 records were kept on 13 fields in the Mohawk area and 7 in the South Gila area, and were available for comparisons. The population records in 1940 for field No. 7 are given in detail in table 1 and show the type of records taken during the development of the seed crop. -6- Table l.--Lyvgus sweeping records on alfalfa in field No. 7, Mohawk area, 1940 Date of Type of Numbers captured per 100 sweeps record plant growth Adults Nymphs Total 1940 Mar. 29 New growth 7 8 15 Apr. 5 New growth 6-8" tall 31 4 35 Apr. 12 New growth 3-10" tall 21 1 22 Apr. 24 Pasture stubble 4 4 8 Seed crop May 2 New growth 8 20 28 May 9 New growth 17 16 33 May 15 New growth 12-18" tall 41 10 51 May 22 Bud 48 28 68 May 29 Blooming 46 106 152 June 6 Blooming 50 198 248 June 13 Full bloom 12 14 26 June 20 Full bloom 18 2 20 June 24 Late bloom 44 4 48 July 2 Late bloom and regrowth 84 52 136 July 17 Late bloom and regrowth 24 16 40 July 23 Maturing 0 4 4 854 The populations used in the following comparisons were based on the total number of Lygus captured during the development of the seed crop. The estimated number of LyZus captured in field No. 6 was as follows: 5,192 in 1938, 1,302 in 1939, and 1,040 in 1940. In the Mohawk area the average number of Lygus captured per 100 sweeps during the seed-crop development was 275.8 in 1938, 75,1 in 1939, and 83.2 in 1940. In the South Gila area, in 1940, 437.9 Lyg2s were captured per 100 sweeps. The important information derived from these population records was that high populations were present where no community program was functioning, that is, for the years 1938 in the Mohawk area and 1940 in the South Gila area, and that low Lyg popu- lations prevailed where the community program was in operation, that is, for the years 1939 and 1940 in the Mohawk area. Field observations made during these years also showed that where high Lvgus populations occurred, severe Lyg2s damage was present. and where the Lygus numbers were low, the seed crop usually was satisfactory. The predators, mainly bugs of the genus Geocoris, probably affected the populations somewhat in 1940, although the numbers of Lyus bugs per 100 s .eeps were lower in 1939 when few predators were observed. Many obser- vations in 1940 indicated the apparent absence of the first and second instars of Lygus, which is not the normal condition, and it was considered th"t the predators had devoured the nymphs because predators had been obser'ed feeding on the nyitihs. Nevertheless, the main reason for the lower -7- Lygus population in 1939 than in 1940 appeared to be the more zealous clean cutting of the alfalfa in 1939., and past observations indicate that the predators cannot be depended upon to serve as a means of controlling the Lygus bugs. Raceme Sampling To check the field observations., raceme samples (150 to 200 racemes) were collected from every field in which the systematic sweeping records were obtained. These raceme samples were taken at random, when the seed crop was mature, from the same parts of the field as were the sweeping records. From the raceme samples the numbers and percentages of flowers forming pods or failing to do so and the numbers and percentages of good or injured seeds were determined. Flower Fall and Pod Set To ascertain the pod set and flower fall, 50 racemes, taken at random from the sample, were examined under the binocular microscope. In this way a determination could be made of the flowers which produced pods and the flowers which failed to do so, or "flower fall." A bract on the raceme indicates the location of every bloom. When a seed pod is produced a definite enlargement of the basal attachment of the pedicel occurs. The presence or absence of this characteristic pedicel development permits an accurate count of the number of flowers producing seed pods, and the flower fall. Detailed studies previously published 5/ showed that an average of 52.50 percent of the alfalfa flowers failed in fields of very low Lygus infestation, whereas in fields of high population severe damage occurred to the 47.50 percent of the flowers expected to set seed. Carlson 6/ has investigated the effect of Lygus feeding upon alfalfa and has shown the occurrence of many types of plant reaction. Seed Classification Seed examinations were made by taking one seed pod from each raceme in a sample until 250 seeds were obtained. All the seeds in each pod were examined under a binocular microscope to determine the number of good seeds, and the numbers damaged by chalcids, pentatomids, and LYg-us bugs, respec- tively. Lgs damage is shown as brown seeds. Experimental work has shown that the feeding of the Lygus bugs is responsible for a very high percentage, though not all, of the brown seeds. A complete classification of all seeds examined was thus recorded. Detailed records of raceme and seed examinations were completed for every field surveyed, as shown for field No. 5 (table 2). 5/ See footnote 2. 6/ Carlson, John W. Lygus Damage to Alfalfa in Relation to Seed Production. Jour. Agr. Res. 61: 791-815, illus. 1940. STAT P -8- Table 2.--Results of the examination of the raceme sample from field No. 5, Mohawk area, collected July 4, 1940 Raceme examination for pod set and flower fall Pod set per raceme Flower fall per raceme Pod set per raceme Flower fall per raceme Number 15 18 6 5 19 12 9 11 15 17 12 21 14 13 1 18 15 11 18 8 12 6 16 19 14 Number 16 11 21 17 9 3 26 15 8 4 17 7 15 17 15 9 6 8 13 16 22 16 10 6 3 Totals for 50 racemes 655 Percent 52.E Seed examination of raceme sample Classification of seeds Number of seeds Percent of seeds in each classification Good seeds B. gibbus-infested Pentatomid-damaged Brown seeds Number 8 11 19 12 13 19 17 14 12 6 9 9 23 17 15 3 14 12 14 19 12 14 13 7 18 Number 12 9 4 15 10 5 5 7 11 26 10 11 20 23 13 13 4 7 6 2 25 3 6 13 13 583 47.09 236 0 1 13 250 94.40 0.CO 0.40 5.20 100.00 1 -9- These records show that when the Lygjs populations were high a greater flower fall and more brown seeds occurred, which fact is in agree- ment with the positive correlations obtained in previous years. There was a noticeable variation in the different samples. For example, the sample from field No. 40, South Gila area, showed 84.89 percent flower fall, 26.4 percent brown seeds, and 70.0 percent good seeds with a Lygys population record of 5,731, whereas the sample from field No. 36, in the Mohawk area, under control, showed 56.71 percent flower fall, 2.4 percent brown seeds, and 96.0 percent good seeds with a Lygus population record of 475. Indi- vidual field records were summarized by years for the comparisons used in the community program, and the data are presented in table 3.< Table 3.--Populations of Lygus and percentages of brown seeds, good seeds, and flower fall in control and noncontrol areas Lygus Pedicel per 100 bases Flower Seeds Brown Good Area Sweeps sweeps examined fall examined seeds seeds Mohawk Number Number Number Percent Number Percent Percent 1938 8,100 275.8 6,228 62.1 1,735 21.6 72.22 (6 fields) 1939 1/ 12,200 75.1 11,995 60.5 2,750 8.15 88.95 (11 fields) 1940 l/ 13,500 83.2 14,698 57.8 3,250 7.38 91.08 (13 fields) South Gila 6,200 437.5 7,616 73.73 1,750 30.86 62 66 (7 fields) I/ Community control program functioning. Comparisons between the data for 1940 from the control area (Mohawk) with those from the noncontrol area (South Gila) indicate that the LYus populations were more than 400 percent higher in the South Gila than in the Mohawk area; losses due to brown seed were more than 300 percent greater in the South Gila than in the Mohawk area; the proportion of good seeds was approximately 30 percent less in the South Gila than in the Mohawk area; and the flower fall was more than 25 percent greater in the South Gila than in the Mohawk area. It may also be seen from table 3 that high Lygus populations occurred when no community program was functioning, and resulted in a greater flower fall, less good seed, and more brown seed. The summaries for the Mohawk area in 1938 and the South Gila area in 1940 show general similarity during a time when no community program was oper- ating, and in the following discussion on seed yields mention is made of the comparable seed production. - 10 - Alfalfa Seed Crop Yields The alfalfa seed growers appraise the results of the program by increases in yields. In 1940 the yield of thresher-run alfalfa seed per acre for the Mohawk experimental area was 314 pounds, whereas for the South Gila area (with no control program) it was only 171 pounds. In the Mohawk area the fiel s with good thick stands of alfalfa and free from salt spots produced from 400 to 600 pounds of seed per acre. Thick stands produced the maximum yields of seed this year in the Mohawk area where the Lygus populations were low, whereas in the noncontrol area (South Gila) in 1940 the thick stands had extremely high Ly&us populations and the seed yields were practically failures. About 25 percent of the 2,895 acres of alfalfa on which the production of seed was attempted in the South Gila area was not threshed because the Lygus damage was so severe. This condi- tion is similar to that found by J. W. Carlson 7/ in his investigations. In 1938 the seed yield in the Mohawk area was 174 pounds per acre when no uniform control program was followed. It is interesting to note the simi- larity of the yields for the Mohawk area (174 pounds in 1938) and the South Gila area (171 pounds in 1940) in years in which neither district conducted a cultural control program. The yields for 1938, 1939, and 1940 in the Mohawk area were 174, 186, and 314 pounds of seed per acre, respectively, which showed a, 12-pound or 7-percent increase in lZ97) and a 140-pound or 80-percent increase in 1940 above the yield of 1933. These results certainly appear to indicate that the cultural control program was effective in reducing Lygus damage and increasing alfalfa seed production. The estimate for Yuma County showed a 23-pound or 13-percent decrease in seed yield per acre in 1940 from that of 1939, and a 63-pound or 29- percent decrease in 1940 from the 1938 yield. The estimate for the Yuma Valley area of Yuma County for 1940 was 105 pounds, or one-third as much as that produced in the Mohawk area, The estimates of alfalfa seed produc- tion for 1940 in the South Gila and Mohawk areas of Yuma County were ob- tained from the seed warehouses. These estimates are in agreement with those used by the agricultural statistician from whom the estimate for the Arizona yield was obtained. The estimated yield for Arizona in 1940 was 18 pounds or approximately 9 percent less than the estimated yield in 1939. The yields for the Mo awk area, other sections of Yuma County, and the State of Arizona are presented in table 4. These figures show that the yields for Arizona and Yuma County have been decreasing since 1938, whereas the yields in the community cultural control area (Mohawk) have increased during the two years in which the program has operated. 7/ Carlson, John W. Alfalfa-Seed Investigations in Utah. Utah Agr. Expt. Sta. Bul. 258, 47 pp., illus. 1935. - 11 - Table 4.--Alfalfa seed production and acreage for the Mohawk area, with comparisons Mohawk area 1938 1939 1/ 1940 l/ South Gila area 1940 Total area (acres) 5,240 5,340 4,618 2,895 Total seed Production (thresher-run pounds) 910,531 1,007,281 1,448,751 494,185 Yield per acre (pounds) 174 186 314 171 Increase in yields over 1938 for the Mohawk area Pounds Percent 12 140 Yields in pounds per acre? Arizona 2/ Yuma County a/ Mohawk area 4/ South Gila 4/ Yuma Valley 1938 222 218 174 1939 204 178 186 1940 186 3/ 156 314 171 105 Acres in 1940 43,000 3/ 25,000 4,618 2,895 17,500 1/ Community control program operated. 2/ Estimates obtained from U. S. Dept. Agr. Agricultural Statis- tician. 3/ Preliminary estimates of yield and acreage in Arizona for 1940 by U. S. Dept. Agr. Agricultural Statistician. 4/ Actual production and acreage records for the Mohawk and South Gila areas were obtained with the cooperation of Mr. G. E. Blackledge, Yuma County Agricultural Agent, and the seed dealers of Yuma County. DISCUSSION Field observations have shown that even when conditions are other- wise optimum for seed production, high Lyus populations may cause a complete failure of the alfalfa seed crop, while low populations normally cause little damage. Fortunately, however, conditions favorable to seed produc- tion also tend to reduce Lygus populations. The observations in Yuma County indicate that early development of the seed crop secured by starting it - 12 - s ox-able for both Lygus control and seed production, for (1 s ) The crop develops before Lygus populations have I ijurious numbers. (2) Less excessive heat early .!t ihe transpiration and thus reduces the water require- s oi > salfa plant. (3) Advantage is taken of the known tendency r igh poportDon of flowers to produce pods during the latter part of C, he Ihe range between day and night temperatures is at a maximum oe i.s and high temperatures occur. (4) The crop has an oppor- u t uare U oeloe the advent of the violent storms of midsummer which ',ig of seed or other losses before the crop can be threshed. The res.ts n the cultural control area and the noncontrol area ....~ ~s .eIiat the observations made in previous years, indicating po'n c of a year-round program. Clean winter culture, with retarded ..... of he alfls, ts very important for full success of the program, e t s timing of he April cultural and cutting practices is handi- ca i large Lygus populations are allowed to develop during January and february ierzuse of heavy irrigations and long periods of growth between cuttib or pasturing of the alfalfa, The practice of cutting or pasturing j0 c s. during the winter, is suggested as a means of reducing the ubr A Lsz~: ae to complete a generation during the period prior to .i rtig the s d erop. The potential life span of adults maturing in February oul permit them to lay eggs in the crop which produces seed, 1u if iost of ih~ can be prevented from developing in January and February the pote, l populations to infest the seed crop in May and June are S edud Whe excellent clean-culture methods were followed in :ry a.8 3e u, ry it as evident that the eggs and nymphs were largely destroyed an] te mitgant adults were mostly spent. The crop imivately preceding the seed crop should be harvested as o pstured, because gradual harvesting of the crop by pasturing pro- s VC uniformi cty in growth which is undesirable from the stand- po~it o" L~i's control. It is important that this crop be completely ca up in nl! fieds ;ithin the 10-day period, so that little available oth Lgus bugs is left anywhere in the area. Cutting this crop .or .ay also leaves t bugs less shelter from the heat than does pasturing. The Soil 2oisture should be low when the last crop prior to the ,' i of the se crop is removed, so that new growth may be retarded perod of the nymphs thus be prolonged. Irrigations can o o- to bring about this condition, but Lh timing varies with the soil ;I a .d mut v wo kd out by the individual grower. On o prreuisites for a successful community control program he ihole-herted willingness of the growers to cooperate ani ca ryo fix plan providing for the more effective utilization in ao~t ref a.. LMu o' the cultural practices prevailing in the area. ~'m or .small groups of farms probably can be con- mii areus and shouo c n notieable benefits 1 :,otces so as to reduce I.Yo bu:s. - 13- Fundamental ecological knowledge of Lygus bugs is essential i determining the cultural practices best suited for- their control in y district. However, it is known that the habits of .the respective specie are quite similar throughout large regions. The cultural methods of control found effective in the Mohawk area of Arizona, as described herein, iway therefore, with certain modifications, be found applicable to-other alfalfa, seed-growing areas in the Western States. SUMMARY A community experiment for the cultural control of bugs of the genus Lygus was conducted in 1939 and 1940 in the Mohawk area of Yuma County, Ariz., in cooperation with the Agricultural Extension Service of the Uni- versity of Arizona and the Agricultural Agent of Yuma County. Questionnaires were distributed to the alfalfa seed growers to obtain information on the prevailing cultural practices so that a plan of action could be formulated by which the Lygus damage might be reduced without excessive expense to the growers. At a meeting of the seed growers Lygus damage to the alfalfa seed crop and beneficial cultural practices were discussed, a program was agreed upon, and a committee of growers was selected to supervise its execution, According to this program a clean-up of alfalfa and weeds was to be con- ducted in January and repeated in March and April, seed crops in all fields were to be started within a definitely restricted period, and other pre- scribed cropping methods were to be followed. The experiment for 1939 and 1940 gave favorable results in reducing the populations of Lygis bugs, lowering the damage to the alfalfa seed crop caused by Lygus and other seed insects, and increasing the seed yields, whereas the noncontrol areas continued to have high LygEu populations, severe damage to the alfalfa seed crop, and low seed yields. The average LygLs populations per 100 sweeps during the development of the seed crop was 75.1 and 83.2 under control conditions, as compared with 275.8 in the Mohawk area in 1938 and 437.9 in the South Gila area in 1940, under noncontrol conditions. The losses due to brown seed for the control area were 8.15 and 7.38 percent, whereas in the noncontrol areas the losses were 21.6 and 30.86 percent. The comparisons for 1940 between the control area (Mohawk) and the noncontrol area (South Gila) indicate that Lygu popula- tions were more than 400 percent higher in the South Gila than in the Mohawk area; losses due to brown seed were more than 300 percent greater in the South Gila than in the Mohawk area; the percentage of good seeds was approximately 30 percent lower in the South Gila than in the Mohawk area; and the flower fall was more than 25 percent greater in the South Gilasthan in the Mohawk area. The alfalfa seed yields for 1938, 1939, and 1940 in the Mohawk area were 174, 186, and 314 pounds per acre, respectively, the control program being in effect during the last two of the three years. A 12-pound or 7- percent increase occurred in 1939, and a 140-pound or 80-percent increase resulted in 1940 above the yield of 1938. The estimates for Yuma County showed a 23-pound or 13-percent decrease in seed yield in 1940 from that UNIVERSITY OF FLORIDA 14l1111l 11 111111l 111111111I111111111111111 3 1262 09230 4038 of 1939, and a 63-pound or 29-percent decrease in 1940 from the 1938 yield. The yields for the different seed areas of Arizona show that the yields for the State of Arizona and Yuma County have been decreasing since 1938, whereas the yields in the community cultural control area (Mohawk) have increased since 1938 or for the two years during which the program has been functioning. Lygus damage has been reduced and the seed yields have been increased in the area where the community program was in operation. These facts strongly indicate that cultural practices known to be detrimental to the bug's development can, if executed in an efficient manner throughout a district, reduce the losses in seed production due to Lygus. This method of control does not cause the seed grower a large added expense, which is an important consideration in the control of insects attacking low-value "cash-return" crops, such as alfalfa seed, and it may be found more or less applicable in the other alfalfa seed-growing areas of the Western States. |