Citrus Thrips: Biology, Ecology, and Control by Lynell K

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.' MICROCOPY RESOLUTION TEST CHART MICROCOPY RESOLUTION TEST CHART NATIONAL BUREAU Of STANDARDS·1963·A NATIONAL BUREAU OF STANDARDS-1963·A USE ONLY e,t ~ IU.nitedStates . ~ '"Department of STACKS 'I. ~culture/ Citrus Thrips.: Agricultural Research Seil/ice BiologY,Ecology, .-Iechnical ·-Bulletin Number :t6S'8 andContro·1

f UNIVE,iBITY OF C/;\UFORNlr\ I nl.\\fl~:; I SEP 7 1982 I ~r~iFT ~'fl~~C _ L''8tH!!')V L..~: '..J ~ • :Ju .11'. t fh'l.i:_ Abstract Contents Tanigoshi, Lynell K., and JoyceY. Nishio-Wong. Citrus Thrips: Page Biology, Ecology, and Controt. U.S. Department of Agriculture, Introduction ...... , ...... •....•. Technical Bulletin No. 1668, 17 p., iIIus. 1982. Economic impact ...... , ...... Distribution and host range ...... ,...... 1 This technical bulletin reviews .and summarizes our current Greenhouse mass rearing ...... •... 2 knowledge and research accomplishments to (1) mass rear Life history ...... •...... •...•...... 3 Scirtothrips citd under greenhouse conditions on California Developmental biology ...... 5 sumac, Rhus laurina; (2) define the developmental growth Temparature-dependent developmental rate models •... 7 parameters of S. citrf under different temperature regimes; (3) Monitoring and sampling ...... 7 develop an inexpensive and effective emergence-dispersal Insecticidal control ...... 10 monitoring trap; (4) associate thermal heat accumulation, Ground cover tactic ...... 10 conceived as degree-days, to the concept of thrip-days accu Conventional foliar sprays ...... 13 mulation; (5) valicate a nearly, reaHime, temperature-driven Central Valley ...... •...... •...... 14 simulation model, SIMDEV, to predict cohort field life stage Southern coast ...... 14 distribution; (6) evaluate the insecticidal tactic of controlling California-Arizona desert ...... 14 S. citd metamorphosis with ground cover, thripicidal spray Inland southern California ...... 14 applir;ations; (7) evaluate the impact of various weed ecosys Effects.of weed ecosystems on citrus thrips ..•...... 15 tems and both furrow and sprinkler irrigation systems on S. citri Biological control of citrus thrips ...... 16 population levels and their effects on Valencia orange fruit Literature cited ...... 17 quality; and (8) determine the biological control role of the predaceous phytoseiid Amblyseius hibisGi ::md its potential for early season population suppression of S. cltri.

Keywords: Scirtothrips citrf, developmental rate, SIMDEV, degree-days, critical period, citrus thrips-days, cover crops, ground cover application, biological control, predaceous mite, sampling, monitoring, fruit damage index, phenology.

".IIUIIIIII.IU.II"-""l

This publication contains the results of research only. Mention of pesticides does not constitute a recommendation for use, nor does it imply that the pesticides are registered under the Federal Insecticide, Fungicide, and Rodenticide Act as amended. The use of trade names in this publication does not constitute a guarantee, warranty, or endorsement of the products by the U.S. Department of Agriculture.

Issued July 1982 Citrus Thrips: Biology, Ecology, and Control By Lynell K. Tanigoshi and Joyce Y. Nishio-Wongl Introduction protocols; (3) chemical control tactics within the context of integrating pest management strategies; (4) cultural control, The citrus thrips, Scirtothrips citri (Moulton), is one of three such as ground cover management; and (5) biological control major arthropod pests of citrus, especially navel orange and strategies through the introduction and/or augmentation of na lemon, found in California. The other two key pests are the tural enemies were either lacking or incomplete and, therefore, California red scale, Aonidiella aurantii (Mask.), and citrus red required further research. mite, Panonychus citri (McGregor). If not properly controlled, irreparable economic and phYSiological damage from larval and We view this technical bulletin as a companion to the earlier aduit life stages results in stem-end ring scarring,. streaking, and bulletins of Horton (1918) and McGregor (1944). In contrast splashing patterns on the shoulder and stylar end of developing to the importance of S. citrito California's citriculture, only a fruit lets and stunting of the foliage. Due to its minute size, less modest literature exists about its life style, mode of daily subsis than 1 mm in length, the citrus thrips remained undiscovered for tence, and methods to constrain its biotic potential within a many years. S. citri injury to the foliage and fruit in the early days monoculturally conceived agrobusiness. Hopefully, we can help of California's citriculture was attributed to physical causes such focus on, conceptualize, and resolve the many innovative ap as freezing and wind scarring injury. proaches to S. citri control for today's citriculturists.

In 1908, Dudley Moulton was assigned by the U.S. Department Economic Impact of Agriculture's (USDA) Bureau of Entomology to study the citrus thrips and its injury. In a publication entitled "The Orange Estimates of crop damage and loss by the California Depart Thrips," Moulton (1909) described the citrus thrips as a new ment of Food and Agriculture (CDFA) indicated that overall species along with remarks on its life history, nature of injury, losses to the California citrus industry had increased by 35 pupation site, and a tobacco extract remedy. He remarked that percent during the 1972througll1975 growing seasons. During there were two broods a year. Moulton rightfully surmised that tl1is interval, assessable yield losses remained nearly constant, the first brood appeared before bloom in February and March, in contrast to a threefold increase in control costs by 1975. This but was in error when he stated that the second brood appears increase can be attributed to inflating operational and mana in July through October and that this brood feeds on the matur gerial costs and, in part, to the increased number of postpetalfall ing oranges and the third and fourth foliage flushes. There are sprays required to maintain S. citri populations at 1972 levels. between 8 and 12 generations of citrus thrips per year in Califor nia. The second brood normally occurs during May. In 1918, The 1978 CDFA estimate of citriculturallosses to insects and another Bureau entomologist, J. R. Horton, published Bulletin mites was nearly $41 rnillion. Of this value, nearly $10.5 million 616, entitled, "The Citrus Thrips," which even today contains or 25 percent loss was attributed to citrus thrips, whereas, valuable field data and accurate descriptions of the life history California red scale and citrus red mite losses were assessed at and phenology of S. cirri. about 51 percent. These large yearly losses suggest current chemical control strategy for S. citri requires fUither scrutiny, In spite of the acknowledged importance of S. citrito both especially if multiple treatments are required by mid-May of California and Arizona citriculture (Tanigoshi et al. 1980), the each year. last publication that discussed the relationships between the biology of S. citri, its chemical control, and control effects on Distribution and Host Range other citrus pests was McGregor's (1944) USDA Circular 708. Locality records (fig. 1) and the foHowing host plant associations This cirCUlar provided a comprehensive and timely review of the indicate that the citrus thrips is Nean::tic and is native to the citrus thrips especially written for growers, pest control per southwestern United States and northwestern Mexico. sonnel, and entomologists, Other than the discussion within Ebeling's (19S9) publication, "Subtropical Fruit Pests," a Cultivated host plants Noncultivated host plants paucity of literature exists since McGregor's publication on Navel orange California sumac either basic or applied research on S. citri, including chemical Valencia orange Liveoak and/or biological control, treatment thresholds, and population Lemon Mesquite management within the context of contemporary citrus pest Lime Willow management. Grapefruit California pepper tree Tangerine Buckthorn The phenological features of S. citri have been known for oVer Avocado Creosote bush 50 years. Further study, however, has revealed that the bio Grape Chamise logical parameters needed to conceptualize and conceive Deciduous fruit trees Fir innovative (1) population models; (2) sampling and monitoring Magnolia tree California laurel 1 Research entomologist, U.S. Department of Agriculture (USDA). Bailey (1964) believed S. citri to be a native species that has Agricultural Research Service, Boyden FrlJit and Vegetable Entomo, logical Laboratory; and staff research associate, Department of Ento found Citrus spp. to possess the biological prerequisites for mology, University of California, Riverside, 92521. near optimum growth and development. This host favorability 2 The year in italic, when it follows the author's name, refers to Literature and associated climatic features will potentially result in numer Cited, p. 17. ical increases of S. citri if left unchecked. He also felt that the RANGE MAP OF CITRUS T H RIP S INCA 1I FO R N I A

Figure 1.-Distribution of Scirtothrips citri (Moulton) (modified from Horton 1918). liveoak (Quercus spp.) is probably the native host. Ewart (per greenhouses. Both greenhouses, shown in figure 2, measure sonal communication) believes California sumac, Rhus laurina 5.5 by 8.2 m with the house on the right divided into four rooms, (Nutt.), to be an equally good candidate for the native host each about 2.1 by 4 m. All the interior walls ofthe compart designation. R. laurina is commonly found in the coastal and mentalized greenhouse consist of 150-mesh/inch stainless inland valle~'S of southern California. Stands of sumac can be steel screen. The.environment of each greenhouse, precluding found adjacent to citrus orchards throughout its range. Since the watering system, is controlled and programed by an en 1918, when Horton published a map showing the known distri vironmental control system. Heat is provided by a gas-fired, low bution of S. citri, this insect has established populations within static pressure propeller fan heater. Cooling is accomplished the coastal areas of Santa Barbara, Ventura, and San Diego with a 75,000-Btu evaporative cooler fitted with particulate and Counties, and the desert valley areas of Coachella, Imperial, smog filters and four motorized shutters placed near the corners and Yuma in Imperial County and Yuma County, Ariz. (fig. 1). of the greenhouse. High-volume air circulation at 6,000 Wlmin The natural habitats of the citrus thrips are generally the tem is provided by turbulalor jet fans. Amelioration of intense solar perate Grassland Biome of the Central Valley, Coastal Chapar radiation, between April to October, is.accomplished with a ral Biome, and the irrigated areas of the Sonoran Desert Biome. whitewash spray formulated with 3.8 L of water, 0.45 kg of Citrus thrips is also known from Maricopa County, Ariz., and the calcimine, plus 20 ml of white glue. Every potted plant is States of Baja California Norte and Sonora, Mexico These watered with a drip emitter through which essential nutrients areas are either in the Chaparral or irrigated Sonoran Desert are injected at the rate of 1 part nutrient to 200 parts water Biomes. (Hoagland and Amon 1950). Temperature .extremes in the greenhouses can range from about 21 ° to more than 40°C. Greenhouse Mass Rearing Hundreds of R. laurina shrubs can be propagated in redwood A system was developed by Tanigoshi and Nishio-Wong (1981) nursery flats filled with the standard University of California, ~o mass rear S. citr; within environmentally controlled green Riverside (UCR), soil-sand-peatmoss mix (Matl

2 o

A B

Figure 2.-Citrus thrips rearing greenhouses: (A) Open greenhouse; (8) partitioned greenhouse; (C) nutrients; (0) cooling unit; and (E) heating unit. was enhanced when the seeds were autoclaved with steam host for S. citri under greenhouse conditions. Relative growth heat at 0.7 kglcm2 for 10 s. This fire climax shrub is tolerant to rates for this infested group were compared with those of 10 wide fluctuations in greenhouse temperatures (18° to 49°C) and control shrubs from weekly height measurements (fig. 3). By the ambient relative humidity between 10 and 80 percent; however, second week, tile citrus thrips populations underwenta 25-fold the onset of irreparable wilting of terminal shoots will occur if the immature and a 3-fold adult numerical increase (fig. 3). Terminal elongation of the test plants demonstrated a suppression effect, soil is allowed to dry to the tOUGh. whereas incremental growth amorlg the control plants was When grown under greenhouse conditions, R. laurina will linear. Foliage estimates for each treated plant produced an produce numerous terminal shoots throughout the year and average of 285 leaves pershrub. If we conservatively estimate respond vigorously to frequent pruning. Unlike the citrus vari that 40 percent of the leaves from.a mature, pruned R. laurina eties, the foliage of sumac shrubs remains suitable for S. citri shrub are suitable for citrus thrips growth and development, feeding and ovipositing activity for longer periods. then the average production of immatures and adults is 2,500 and 500, respectively. Based upon available greenhouse Our production schedule consists of four groups of uniformly space, especially to propagate and maintain vigorous pest-free sized, 3.8 L laurina shrubs, 45 to 55 cm high, at various S. R. host plants, the mass production of S. citri by the system de infestation phases. By infesting a group of plants every citri scribed above can be easily expanded or reduced by controlling week, each group will potentially support two generations of the number of cohort R. tC:JUrina shrubs infested per week. citrus thrips for 4 weeks. This staggered schedule provides us with a continuous cycle of the active life stages of S. citri. Because this species pupates in soil and organic debris, the Life History containers are placed on trays filled with 2 to 3 cm of vermiculite The citrus thrips overwinters in the tissues of leaves, stems, and and UCR soil mix. twigs inlhe egg stage. The majority of these eggs are laid during Tan/goshi and Nishio-Wong (1981) conducted a study to mea the fall, generally concurrent w:,h the fall growth flush of citrus. sure the potential carrying capacity of R. laurina as a suitable These overwintering eggs are not in diapause as one can read i

3 24] ~.citri • lMMATURES 20 . o ADULTS ~. u.. • <{ W .-1 16 " -~I u en, 12 d z 8 Z <{ w ~ 4 0_ • -o~ ~. 0 0 ,# I 65 R·laur.ina /~ • TEST .. to CONTROL 60 ~ u /' z 55 l- I ,/' _. • 0 w I 50 to/.~·- z ~ ./ w ~ 45

40 o 5 14 21 27 TIME IN DAYS

Figure 3.-Population trends of Scirtothrips citri and the relative effect of colonization on Rhus laurina shrubs (from Tanigoshi and Nishio-Wong 1981). ly induce hatching by incubating late tall or winter terminal The first instar is spindle shaped and nearly colorless with bright toilage at room temperature. Looking like a translucent .wrig red eyes. As this instar feeds on citrus, it turns more yellowish glin9 worm, the newly hatched larva emerges to the leaf surface and grows to about 0.4 mm in length. When the first instar has throl.Jgh the incision made by the female's ovipositor when the grown to about twice its length, it will seek refugia, such as leaf egg was laid. From our life history study (Tanigoshi et al. 1980) veins and subaxillary pits, to undergo molt. This behavioral and empirical laboratory observations of terebrantian eclosion, pattern precedes each molting phase and is similar for all of the a considerable mortality occurs during emergence as often immature, active life stages. Duration of this life stage is about evidenced by the presence of partially emerged dead larvae on 4 days. fruit and leaf surfaces. Most of these larvae appeared to have experienced difficulty freeing their terminal abdominal segment The second instar is more robust and usually more yellow than from the expended .chorion. During the warmest months, the the first instar. Second instars measure about 0.9 mm in length. duration of the egg stage is 6 to 8 days. When fully grown, these larvae usually seek pupation sites by randomly dispersing to the ground (Reed and Rich 1975). There are four instars between the egg and adult stages (fig. 4). The first two are feeding instars called larva, and the latter two Prepupa differ in appearance from the second instar mainly by are nonfeeding ones called prepupa and pupa. possessing two pairs of external wing pads, which extend to

4 mal unit model capable of evaluating and updating an array of early season S. citri thresholds.

This study was conducted on newly hardened excised 'Eureka' lemon leaves. Individual leaves were placed in modified Tashiro acrylic cages (Tashiro 1967). Unlike his original cage, the 160-mesh plastic screen was replaced with a dialysis mem brane, which facilitated the observation of developmental ,. events (fig. 5). The Tashiro cage was deemed "thrips proof" after the original rubberband gaskets were replaced with either neoprene or latex gaskets 0.5 cm wide and 1 mm thick. This type of gasket and another identical one were bonded around the upper and lower surfaces of the middle plate. Unlike the rubberbands, these gaskets never retracted away from their original position. The tight interface surfaces eliminated crev ices and escape routes through which citrus thrips could seek protection and remain undetected during routine examination of the arena (fig. 6). Newly hardened lemon leaves were easily maintained ·for 2 weeks with water supplied by dental wicks placed through holes drilled into the lids of museum jars. The cages and jars were then positioned on portable watering plat forms (fig. 7). Figure 4.-The generalized life history of the citrus thrips. Scirtothrips citri. The constant temperature experiments were each conducted in modified 1.8-m3 freezers. Temperatures were controlled to with in 1 and a 16-h photophase was provided by a fluorescent about their third abdominal segment. The four segmented an ac, lamp yielding 85 foot-candles. A constant circulation of air was tennae are directed forward. The prepupa is about 0.6 or 0.7 mm long atthis life stage, which lasts about 2 days.

The pupa is pale, yellowish-green, and measures about 0.7 mm in length. The wing pads extend beyond the sixth abdominal segment, and the antennae lie backward over the head. After molting to the adult stage, the wings will exceed the tip of the abdomen. The duration of the pupal stage is about 5 days.

Adult female citrus thrips are about 0.7 to 0.9 mm in length, males being somewhat smaller. The average life of an adult ranges from 15 to 50 days during the cooler periods of the year. As many as 250 eggs may be laid by a single female, depending upon diel abiotic factors and quality of available food impinging

upon her. F~om July through early October, the citrus thrips can BN·4913= complete egg to adult development in about 13 to 16 days. Bl'lth Figure 5.-Unassembledcomponents of the modified Tashiro adult life stages readily feed upon and injure developing fruitlets acrylic cage. (to about 4 cm in diameter) and tender, terminal foliage. By late .Iuly, several ovipositional scars may appear on navel orange rind. These scars are light green and about 3 to 5 mmin diam eter. They usually show a small brownish, corky center where the sawlike ovipositor was inserted. When the fruit colors at harvesttime, this ovipositional scar is virtually imperceptible.

Developmental Biology

The phenology of the citrus thrips in southwestern U.S. citrus orchards is well known (McGregor 1944, Ebeling 1959); how ever, the effects of a wide range of temperatures on S. citri life stage development, longevity, and survivorship are poorly known. Because of a paucity of data on the relative duration of S. citri development over a wide range of constant temperature regimes, the following study (Tanigoshi et al. 1980) was con ducted to determine the effect of thermal perturbation on citrus thrips temperature-dependent growth rate on citrus foliage. Such empirical laboratory information could provide the factors BN·49134 necessary to drive and couple both a developmental and ther Figure 5.-Assembled Tashiro acrylic cage.

5 larval eclosion (fig. 8). It appeared thatthe hatching larvae were unable to pull their terminal abdominal segment free of their chorion. Except for about 6 days for development at 21.1°, developmental times for the first stage larva averaged about 2 days at temperatures between 23.9° to 37.8°.

At a constant 37.SoC, no second stage larval development occurred. Apparently, this temperature exceeds t'1e upper crit ical (lethal) limit for active, immature stages when maintained at a constant temperature. The prepupal stage was the shortest developmental stage at each temperature. At 21.2°, total devel opmental time for incubation and active stages was more than threefold that at 35°, and no imaginal eclosion occurred at either 18.3° or 37.8°. The application of least square, polynomial re gression analysis to the developmental data revealed that the egg, second instar, prepupa, and total developmental curves are cubic; whereas, first and pupal instar developmental curves BN·49133 are more quadratic in form (table 1). Except for the first instar, Figure 7.-Portable watering platform supporting 12 acrylic the R2 for the regression of developmental period (P) on tem i~>olation cages. perature averaged 0.866. The low R2. of 0.438 for first stage larvae was largely attributed to the wide range ofdevelopmental variation observed for each constant temperature regime. provided at all times, and the ambient relative humidity alter Percent survival for the two feeding instars indicated that the two nated between 25 and 45 percent. extreme constant temperatures were unfavorable for survival and development (table Except for the first instar .at Incubation to first instar eclosion was derived by isolating indi 2). 32.9°C, survival was moderately favorable between 23. go to 35° for both vidual females in cages for 24 h. Life stage developmental times immature stages. Survival of the nonfeeding, quiescent pre and survival rates for the second instar, prepupa, pupa, and pupal stage was significantly higher than for any of the earlier adults were determined by randomly selecting first instars from developmental stages. Perhaps the shorter development of the our greenhouse colony and isolating them individually in the cages. Each cell was observed three times daily at 0800, 1600, prepupa and its propensity to seek shelter in soil and litter debris under the tree canopy may reduce the harmful effects of tem and .2000 h until either preeclosion mortality or imaginal eclo sion occurred. perature fluctuations. For the same basic reason, the percent survival and maturation of the pupa were higher than those Eggs of S. citri developed and hatched at temperatures of 18.3° observed for larval instars reared under identical environments. to 37.8°C; however, at this temperature all larvae died during Table 1.-Regression coefficients of developmental period (P = hours to develop) for each life stage of S. citri (Tanigoshi et a/. 1980) [,o=days and T=temperature in DC)

. ." --- -, --~-- ~~~ ~--- ..,..~-~--~ ~- -~'-'~--- -.~-.--..--. -'--"--~-' .. -.---- __ -" .... . ---...... ---....,----....- --. Polynomial equation: P;=B.+B1T+B2T- 2 +B3y3

Life stage Bo B1 B2 B3 R2

0 Egg 5.97594.103 -5.621860102 1.80969.101 -0.19464·10 0.815 1st instar 4.61017.102 -2.70322.101 .43768·10" C) .227 0 0 2a instar 1.86920.103 -1.66342.102 5.22251·10 - .05597·10 .798 Prepupa 6.39257.102 -5.34157.10'1 1.55476.10° - .01532.10° .889 0 Pupa 7.22534.102 -4.23151.101 .65749·10 (1) .881

4 1 .. Total 1.74416.10 -1.67865.103 5.50307.101 - .60285.10 .997 1Both 1st and pupal instar equations are quadratic, that is. B. + B,X + B2X, and, therefore. do not contain a B3 coefficient.

Table 2.-Percent survival of immature life stages of S. citri at data. The utility of SIMDEV is under current investigation as a several constant temperatures (Tanigoshi et al. 1980) tool to forecast first generation eclosion by overwintering eggs in the field. This estimation, when coupled to biofix activity and Constant Percent surviving meteorological information, should provide commercial inter Temperature ests with the signal to initiate monitoring and sampling before (0G) 1st 2d Prepupa Pupa the situation becomes critical. 18.3 '0(0:3) 48(14:29) 57(8:14) 12(1:8) Another more general growth model being evaluated is the 21.1 18(2:11) 33(13:39) 92(12:13) 42(5:12) 23.9 11(2:18) 55(29:53) 97(28:29) 80(24:30) application of the concept of the heat unit summation or degree 26.7 64(18:28) 65(19:29) 100(29:29) 71 (12:17) days to predict the pattern of early season emergence from 29.4 62(10:16) 51(43:85) 93(38:41) 82(31:38) overwintering eggs, subsequent life stage distribution, and 32.2 41(7:17) 59(33:56) 88(29:33) 72(21:29) generations of the citrus thrips (Sevacherian et al. 1977). The 35.0 39(14:36) 57(24:42) 92(22:24) 73(16:22) life stage development studies we have conducted corroborate 37.8 19(4:21) 0(21:21) 0 0 the opinion of Lewis (1935) that 21°C apprOXimates the lower developmental temperature threshold for S. citri. By superim 1Ratio of alive : n individuals observed. posing periodic thrips adult and immature counts on to degree day accumulations from daily maximal and minimal deviations Temperature-Dependent Developmental Rate Models from 17°, .beginning on 1 January, we may corre!ate the .cyclic patterns of S. citri population peaks with specific accumulated By simply inputting hourly temperature data for the specific degree-ciays. quadratic and cubic regression equations describing each S. citri life stage developmental rate (11,0 X 100), one can estimate Monitoring and Sampling the time period (,0) required to complete a specific stage or generation. A generalized variable temperature developmental Central to the implementation of citrus insect pest management model ofTanigoshi and Browne (1978), designated SIMDEV, is (IPM) programs in the Southwest is the standardization of a modified here (fig. 9) to simulate life stage development for S. sampling method for the citrus thrips. The current sampling citri. Several adult female citrus thrips were isolated in an acrylic technique used by most field workers and growers consists of cage for 24 h. These females were removed, and the 36 acrylic early monitoring for intuitive treatment levels with field experi isolation cages were then placed on three watering platforms. ences stored within their "head" computers. These mental These platforms were placed inside of a ventilated, glass computers can rapidly output control decisions from input de topped, 76- by 46- by 41-cm isolation cage and placed in a rived from periodic visual scans of .both citrus thrips levels and shaded, outdoor .location. Observations were made three times fruiting phenology. This intuitional-visual system will generally daily at 0800, 1600, and 2000 h until either preeclosion mortality reveal the presence of S. citr; in the orchard, but not their life or imaginal eclosion occurred. stage distribution or population density.

Comparisons of observed and model estimates for both indi Two monitoring traps that provide life stage counts ofwithin-tree vidual life stage development periods and total instar summa populations of the citrus thrips have been described by tions indicated a nonsignificant, underestimation of these McGregor (1926) and Reed and Rich (1975). The McGregor parameters by SIMDEV (fig. 10). The model underestimated pest gage is a periodic monitoring device that measures foliage female development by 18.7 h or 5 percent that observed for five populations of citrus thrips by beating citrus terminals across a female S. citri. Because SIMDEV simulates growth chamber wire screen lying over a Tanglefoot coated (sticky) sheet of studies that exclude density dependent processes of intra paper. Thistrap was the prototype for the one conceived by specific and interspecific competition, caution should be exer Reed and Rich for continuous citrus thrips surveillance. The trap cised when one attempts to compare model estimates with field is a dispersal-emergence one designed to capture mature

7 larvae and prepupae dropping to the ground cover to pupate PVC-acetate trap (fig. 12) best met the standards for an S. cirri and those emerging adults seeking food, shelter, and oviposi dispersal-emergence ground trap. Moreover, this trap was im tional sites in the tree canopy. The Plexiglas trapping surfaces pervious to 48-h exposures to various irrigation systems, very were each 0.31 rifand were covered with a thin layer ofTangle stable to gusting winds, and inexpensive. The traps are 10 cm foot (fig. 11). The Plexiglas plates were periodically removed high and were obtained by cutting 3-m lengths of 2.5-mm thick and taken to the laboratory and examined under a stereoscopic PVC pipe with an 10 of 20.3 cm. A clear acetate plate (439 cm2) microscope for the preserlce of S. cirri life stages. was evenly coated (fig. 13) on both sides with Tanglefoot and placed over the top opening of the PVC disk. Each plate was Tanigoshi and Moreno (1981) conducted further field trials with individually covered with clear vinyl folders 645 cm2 when they additional trap designs to reduce bulk and weight of processed were collected in the field. This technique facilitated handling of material and to reduce preparation, count, and cleanup times. the sticky acetate plates for transport, detailed examination After two seasons of field evaluation, we determined that a under a stereoscopic microscope, and storage in the refriger

INPUT hourly temperature data

< 100%

SUBROUTINE

begins

< 100% 1ST INSTAR LARVA

=100%

< 100% 2ND INSTAR LARVA

< 1 PREPUPA

:100\:

<100% PUPA

OUTPUT hours to develop per stage

Figure g.-Flow chart for the Scirtothrips citri-SIMDEV developmental mode/.

8 ator (fig. 14). The difference in surface area between the stan Comparative field evaluation of both traps (table 3) indicated no dard Reed-Rich trap and that of the PVC-acetate trap is 65 significant differences for larval and adult counts between traps percent; citrus thrips counts for the standard trap were adjusted per equivalent surface area. From this comparative analysis of downward accordingly. two weekly sample means, Tanigoshi and Moreno (1981) con cluded that the reduction of the survey plate area by 65 percent did not disproportionately reduce the trapping efficiency of the smaller PVC trap. The approximately fOurfold time-labor reduc 16 OBSERVED tion attained by the PVC-acetate trap can be related to an '" increase in sampling resolution per equivalent area sampled or ALT. TEMP. ESTIMATE to a threefold increase in the number of equivalent citrus or 1.4 chards per equivalent time. .~ Figure 15 is an example of the resolution of the PVC-acetate 0... Iw- ~ trap under commercial field conditions. This graph represents ~': 0 12 pretreatment and posttreatment accumulated citrus thrips ....J days for immature and adult life stages caught on two traps W placed In a 4-ha navel orange orchard in Orange Cove, Calif. ~~~:. > 10 ~;,¥ Since citrus thrips injury is related to both the number ofS. citri W W- present and IAngth .of time the)l.feed, the concept of thrips-days e is proposed, rather than weekly means, to represent population 8 levels and a conceptual model of their irreparable injury to fruit 0 and foliage. Citrus thrips-days is derived by adding the number t- V) > < .4 C

EGG 1ST 2ND PUPA PUPA LIFE STAGE

Figure 10.-Scirtothrips citri life stage development at alter nating temperatures. Length of each bar represent develop mental period for designated life stage. BN-49131 Figure 12.-PVC-acetate Scirtothrips citri emergence trap.

BN·49132 BN-4S'130 Figure 11.-Scirtothrips citrl emergence box trap (after Reed Hgure 13.-Machineused to apply an even coat of Tangleioot and Rich 1975). to both sides of clear acetate plates.

9 of S. citri present at the beginning of the sample periocl to the Table 3. -Mean weekly catches of S. citri captured on Reed number present at the end of the period, then, dividing the sum Rich and PVC-acetate traps placed under 'Washington' navel by 2 and multiplying the result by the number of elaps&d days orange, Riverside, Calif., 1979 (Tanigoshi and Moreno 1981) between samples. ~~._.______.-.lL~_larv_Cle;_,t>,_ ==~~9u Its_l .._.. ______...... __....~ ~!~_1..6_ _July 23 _ . _~..!X..3Q.... .,' Au~~_.. Trap~ __. _~ ...l::....~.,~__ _. ~.., __ P, .. ____'=---~ ft.__ .. _L_ A

Reed-Richl 13.1 3.4 17.3 29.3 141 12.2 28.7 17.5 PVC-acetate 10.1 2.8 20.6 12.0 17.4 7.2 27.2 21.6 - -..-~..~--- --_. .. ----~-. ~- - - . -" -

1Counts were reduced by a factor of 65 percent.

20enotes significant difference. P c_ 0.05. using Student's t-test.

Insecticidal Control

Ground Cover Tactic

Recent studies in South Africa by Milne and de Villiers (1977) have shown the feasibility of using soil applications of systemic pesticides for control of the South African citrus thrips, Scirto thrips aurantii Faure, on citrus. Dimethoate 40 percent emulsifi able concentrate (Ee) was applied at a rate of 20 kg AI/ha in irrigation basins at 20 percent petalfall. Percent mean cullage at BN·49129 harvest was 3.5 percent as compared with a foliar application of Figure 14.-Clear vinyl folder facilitates the job of assessing SClrtothrips citn population levels on Tanglefoot coated parathion and Abate which, when coming off the packing belt, acetate plates. packed out at 0.9 percent. These percent cullage figures were not significantly different at the 5-percent probability level.

5000 ORANGE COVECHASE 1979 - NAVEL * + ~... • 4000 • .....-..

f/) > 4: 0 3000 •/. f/) Q.. c.:: :r:: ~

f/) 2000 ::> c.:: ~ • U • II ./._. 1000 )/ • 18 26 4 11 17 28 5 APRil MAY JUNE

Figure 1S.-Charting the use of the PVC-acetate trap to monitor early season citrus thrips populations and to evaluate the impact of a 1.1 kg Al/ha application of dimethoate on May 17. I c immatures, A - adults, • '" 100 percent bloom drop.

10 The significance of the South African study was to eliminate biota, e$pecially arthropods; and (4) residual activity of the foliar application of pesticides potentially destructive to benefi various \!ompounds under orchard conditions. cial parasitoids, predaceous insects, and mites. S. citri is known to pupate in soil and organic debris found mostly under the tree We have chosen to detail the third year's results concerning canopy. Studies conducted by Tanigoshi et aJ. (1982) on control of first and second generation S. citrion navel orange. A 'Washington' navel, from 1978 to 1980, to evaluate the notion 2.5-ha block of 12-year-old 'WaShington' navel orange located that this propensity to pupate in soil and ground cover accumu in Woodcrest, Calif., was selected for the purposes outlined lation may be the "weak link" in the citrus thrips life cycle toward above. Tree spacing was 4 by 6m; water was provided with which control tactics should be focused. The following granular sprinkler irrigation. All 64 trees in each of the plots received and liquid formulations of organophosphate and carbamate dosage rates equivalent to: plot 1 = 6.7 kg AI/ha carbofuran 4F, type pesticides (table) were selected based on their toxicity plot 2 = 6.7 kg Allha FMC 35001 4EC, plot 3 = 4.5 kg Altha fonofos 4EC, and plot 4 = 4.1 kg Allha chlorpyrifos 4EC. Plot 5 (LOso )' pests controlled, phytotoxicity, and residual persistence parameters. is the untreated control. These formulations were applied with a high-pressure sprayer specially b:'passed to produce 18 psi Active Active pressure through a.,handwand delivering 2.5 L of water per ingredient ingredient minute. Each plot was sampled weekly to 9 September 1980. Chemical (kg/ha) Chemical (kg/ha) Within-tree population levels were monitored with the PVC-acetate ground trap. Samples were taken from the corner Chforpyrifo$ 15G 6.7 Carbofuran 4F 3.4 trees and central four trees in each plot (figs. 16 and 17). Chlorpyrifos 4EC 6.7 FMC 35001 4EC 6.7 Do. 4.1 Oxamyl10G 5.6 Petalfall was essentially completed between 22 and 27 April, Do. 3.4 Oxamyl2L 5.6 and the critical period was considered terminated by about 7 Fonofos4EC 4.5 Do. 4.5 July. By the end of July, population levels from all five plots Carbofuran 10G 5.6 Do. 3.4 demonstrated a marked S. citri numerical increase. Obviously, Carbofuran 4F 6.7 Weed oil (50 pct) 16.1 the effective residual persistence of the pesticides was negli Do. 4.1 gible 3 weeks beyond the terminus of the critical period; how 1Liters per hectare. ever, within the 8-week "critical period" (fig. 18), emergence PVC-acetate traps revealed reductions in accumulated citrus The purpose of our monitoring and sampling procedure was to thrips-days for both life forms by 1 July of 2.2-,4.8-, 6.9-, and demonstrate: (1) control of first and second generation citrus 11.2-fold for carbofuran, chlorpyrifos, FMC 35001, and fonofos, thrips; (2) duration of effective control; (3) effects on soil micro respectively, when compared with the untreated control plot.

V) CON TROl > - IMMATURES <{ --- ADUlTS 0 3

V) a..

0::: 2 :::c l

.-""",. """,. V) ..",..-.--- :::J lO'x 1 ...,...... "'" .,,' 0::: I- * "",.,.- " U .'~.--.--. ---,-_ ..__ .,..···-···"Li" ______I __ e. ___ .-_1-._1-- 0 / l' 25 8 22 6 20 3 17 15 29 M A M J J

Figure. 16.-Accumulatlons of S. cilri·days on PVC-acetate traps placed under navel orange trees at Woodcrest, Calif., in 1980. Star indicates dates for 100 percent bloom drop; vertical line denotes end of critical period.

11 ~ 7 N 7 ././.".,. FMC 35001 AEC, 6.7kg AI/he CARBOFURAN -iF, 6.7 kg AI he 6 6 -IMMATURES 5 ---ADULTS ...-'. 5 /./ 4 V) 4 > 3 3 < 2 ..

C I .,.,,~ 2 + • ...... t y' " 10'.1 * ..' ",' " lO'xl y V) oJ._._._'_'r.'-'-'-'-'~, , , , , ..._._...... _._.::-'-'-'-'-:-., •••••••• -:-_ ..... " .... a.. 0" ,._._._, , ,• .,:::0:'_."-'-":-'-'-'-' , .-'-'-' ...... "'::::...... I .".,, iii iii I" ~ , , I 12 ~ 7 CHLORPYRIFOS AEC, 5.0kg AI/he :x: .",.,.' ;/./._. FONOFOS 4E, -i.5kg AI/he 10 t- 6 /.

5 V) ,j' B ::J ex: 4 t 6 3 ,.'.'." " u 4, . 2 I " I ", ,. 1O'x2~ ..~",...",. lO'xl + y' "",." I 1 " * ,. .... I ,., " t * I ...... ' oJ·--·--·--·--·..•.. ·~·--I.. Ir.=:::::::~::::~~~ QJ._._._._._...... _...... RI,_--·· iii i , iii iii i • I 25 B 22 6 20 3 17 iii 15 29iii 12 26 9 2'5 ' B ' 2'2 ' (, '2'0 3 1'7 I 15 2'9 12 2'6 9 M A M J J A S M A M J J A S

Figure 17.-Accumulations of S. citri-days on PVC-acetate traps placed under navel orange tree':; at Woodcrest, Calif., in 1980. Stars indicate dates for 100 percent bloom drop; vertical lines denote ends of critical period; and arrows indicate treatment days.

~ .. ~ )...... 250 RIVERSIDE, CA. 1974 IMMATURES I < ADULTS ~ 200 "-.. In ~ 150 I'" I '" 100 ::I l-'" V 50

Z-

• OVERWINTER• OVERWINTER EGGS .. I 6 8 GENERATIONS .. 2ND- GEN LARVAE •1ST GEN ADULTS•

1ST GEN LARVAE I I I I I '• D I J • F M • A M • J J A S • 0 • N I

----••SHOOT ELONGATION • r FULL BLOOM • - • CRITICAL PERIOD---- SUMMER FLUSH DORMANCY PHENOLOGY OF 'NAVEL' ORANGE & CITRUS THRIPS

Rgure 18.-Phenograph of navel orange and Scirtothrips citri showing the critical period for developing fruitlets (Tamgoshiet al. 1981).

After 3 years of field trials, we conclude that the aforementioned om mended in the UCRtreatment guide, they are seldom the pesticide formulations and 50 percent weed oil offer good pos material of first choice. These materials just do not possess the sibilities as topical soil treatments for S. citricontrol on navel and 3- to 5-week toxic residual to kill S. citri emerging from eggs or Valencia orange. This tactic is highlypredicated on intensive pupae as provided by dimethoate. early season sampling and monitoring of new growth for the emergence of larva! instars from overwintering eggs and pres Unlike the citrus belts of the Lower Rio Grande Valley of Texas ence of P1 adults. Failure to control larvae and adults, which and Florida, one must carefully preface discussions of control exceed an accumulated 50 to 100 thrips-days or two to four practices in California's citriculture with one of four different fruitlets infested per 20 random fruit examined (Tanigoshi et al. growing areas. These four areas are generally known as the: (1) 1981) during the critical period, can cause irreparable .economic Central Valley; (2) southern coast; (3) inland southern Califor injury at harvest. nia; and (4) California-Arizona desert. As one would surmise, each area differs from the other based on physiographic and A fruit is counted as infested if it hasone or more citrus thrips on climatic features. Rather than attempting to define each area it. The number of random 20-fruit samples taken should be with a multitude ofphysical parameters, a more direct reflection adjusted according to the orchard size and the experience of the of these parameters is mirrored from the proportion of each sampler. Always remove the sepal (button) from ynungfruitlets. citrus cultivar grown in the aforementioned areas. This will minimize the sampler's chances of overlooking larvae hidden from sight between the sepal and the developing frujtle!. Estimates of California's fruit and nut acreage in 1978 (table 4) The unseasonable cool, damp winters of 1977-78 and 1979-80. indicate that the navel orange comprises 70 percent of the citrus resulted in a later-than-normal emergence and growth of over grown in the Central Valley as compared with 23 percent for wintering S. citri populations in California. Thus, our ground Valencia. The southern coastal counties (Santa Barbara. Ven cover treatments were applied after rather than (normally) be tura, Los Angeles, Orange, and San Diego) are nicely balanced fore pelalfal!. while conventional ground and air applications between the Valencia and lemon cultivars; whereas, in the were delayed by2 to 3 weeks after petalfall by those individuals inland southern California counties (western Riverside, San utilizing the sampling techniques described earlier. Bernardino) and desert counties (south-central Riverside, Im perial), the proportions of orange, lemon, and grapefruit are Conventional Foliar Sprays nearly equal. The main features separating the last two areas The standard chemical for citrus thrips control for the past are that the warmer and drier desert season commences about several years continues to be dimethoate. Even though para 3 weeks sooner and irrigation is provided by flood basin tech thion, azinphosmethyl, dioxathion, and phosphamidon are rec- nique rather than by sprinkler. drip. or furrow irrigation.

13 Table 4.-Percentage of citrus cultivars grown in each of the the end of September. During this period, as many as three S. growing areas in California' citri applications may be needed. Preferred materials are the botanicals (for example, ryania, sabadilla) and dimethoate. The Central Southern Inland short-lived botanicals are preferred over the organophos 2 Cultivar Valley coast southern California phates, especially if there is a need to conserve resident and ------·--Percent------·-······ introduced complexes of parasitoids and predators of the Cali Navel 70 4 28 fornia red scale, mealybugs, and citrus red mite. Both dilute and Valencia 23 46 23 concentrate treatments providing good outside coverage are Lemon 6 45 20 common with fair to good suppression reported from air appli· Grapefruit 5 29 cation of dimethoate at 1.1 to 2.2 kg Allha in 140 to 187 L. 1Synthesized from the 1978 California Fruit and Nut Acreage Report, Aircraft treatments on lemon, grapefruit, Valencia, and navel Statistical Bureau, California Department of Food and Agriculture. have given good results, especially when a grower encounters 21ncludes the desert area, which comprises about 2 percent of the total problems with irrigation schedules and escalating citrus thrips acreage for lemon and grapefruit. populations.

California-Arizona Desert. Two chemical programs for the Recommendations for chemical control of the citrus thrips may citrus thrips are used in this area: (1) a sulfur program, if mite be found in the current "Treatment Guide for California Citrus problems are anticipated; or (2) a single (rarely two) postpetal Crops," University of California Cooperative Extension Leaflet fall treatment with dimethoate. A properly timed prebloom appli 2903. cation of wettable or dusting grade sulfur will reduce a potential population increase of the first generation citrus thripB. Central Valley. The UCR treatment guide recommends a mist spray coverage by airblast equipment of 1.1 kg Al/ha of di This treatment should provide seasonal control of Yuma spider methoate 4EC in 935 to 2805 Llha. Overthe last 3 years, many mite, Eotetranychus yumensis (McG.), and citrus flat mite, pest control advisors in the San Joaquin Valley have increased Brevipalpus lewisi McG. Our studies show that sulfur dust, their reGommendations to 2.2 kg Al/ha. Apparently, poorly applied at a rate of 90 to 112 kg/ha, provides good outside understood combinations of population tolerance and meteor coverage; wettable sulfur is applied at 67 to 84 kg/ha. Avoid ological factors are rendering dimethoate less effective at the application of sulfur when temperature of 38°C or higher are 1.1 kg rate. Most of the orchards in the Central Valley are expected to occur within a week of application. A second appli sprayed with ground airblast equipment or mechanically oscil cation of sulfur for S. citri suppression is usually made after lated boom sprayers at 2805 to 4675 L/ha or low volume airblast petalfall. If citrus thrips are present, all bearing citrus cultivars in sprayers applying 468 to 935 Llha. The higher volume is recom the desert should be treated at this time or irreparable damage mended for dilute spraying to insure outside, peripheral cover to developing fruitlets will rapidly occur from this stage of groltVth age. Orchard speeds range between 2.4 and 4.8 km/h. The through development to walnut-sized fruit. Orchards should be remaining acreage is equally treated with either helicopter or monitored soon after this second application to determine if a fixed-wing aircraft at 4.7 Llha in 75.7 L of water; some acreage third application is needed within 2 to 3 weeks. is as low as 38 to 57 L. Chief factors affecting the trend toward GiOund application of citri pesticides with conventional air concentrate spraying are time, energy, and labor costs. S. blast equipment should be applied in 935 to 3740 L of waterIha; Mature citrus plants receive one or two citrus thrips treatments with low volume concentrate spray equipment, 468 to 935 L of per season; occasionally a third spray is required. Multiple water are adequate. Aerial application should be made with a treatments are often appli-ad to newly planted trees and applied minimum of 140 to 187 Llha. To insure good coverage in the at 0.23 L of dimethoate 2.67EC in 327.4 Llha. desert, apply pesticides early in the morning while temperatures are still moderate. Common rates for applying dimethoate 4EC Southern Coast. The southern coastal area is primarily com by ground are 2.3 Llha and 4.6 L by air in as low as 93 to 187 L posed of lemon growing districts located in several coastal of water. In 1980, 187 L by concentrate application of di valleys from San Luis Obispo County southward to San Diego methoate gave good control probably due to correct timing and County. Because lemon varieties bloom and set fruit continu favorable weather. ously the year round, special care must be taken to avoid using nonoil pesticides harmful to beneficial natural enemies and The lemon's growth cycle here, unlike lemons grown in the other pollinizers. Also, lemons are harvested over an extended period areas, is treated like other citrus varieties in that only one crop so that the chemical pesticide selected must meet the legal per year is harvested; thus, one can speak of a specific petatfal! restrictions and requirements relating to residue on fruit and period reference for S. citritreatments on desert lernon. worker reentry intervals. Inland Southern Caliiornia. The interior citrus growing area of Over the past decade, the citrus ti .dps has become increasingly southern California is mainly centered around the western in more pestiferous in the coastal influenced lemon growing area. land valleys of Riverside and San Bernardino Counties. Because of their multiple fruit setting characteristic, sampling When commercially available, the botanicals are generally pre and monitoring will facilitate r::henological timing of preventive ferred over the organophosphates. In 1980, as many as 80 treatments traditionally considered necessary to protect as percent of the citrus acreage managed bya prominent pest much of this young fruit as possible from citrus thrips injury. control advisor in the Corona-Riverside districts received no The citrus thrips season generally extends from mid-April to the citrus thrips treatments. This program, however, is an excep· end of November. The critical period occurs from early May to tion, as most ofthe commercial groves in the same districts

14 received at least one, possibly two, dimethoate 4EC sprays at Furrow irrigated-new annual Weeds-Continued 2.3 and 4.7 Uha by ground or aircraft, respectively. In the same Chenopodium murale L. - nettle leaf goosefoot year (1980), S. citri second generation in the three more arid Conyza bonariensis (L.) Cronq.- flax-leaved fleabane areas of California never occurred as generally expected dur/;ng Conyza canadensis (L.) Cronq. - mare's tail the latter days of April. Subsequent generations of citrus thrips Eragrcstis sp. - lovegrass and increasing population levels normally found on enlarging Malva parviflora L - cheeseweed fruit and flush growth were of no economic concern in late May Sonchus oleraceus L. - annual sowthistle and early June 1980. Lemon varieties are treated somewhat like those grown.in the coastal areas in that about three blooms and Sprinkler irrigated-established annual weeds fruit sets occur ysarly; surveillance for citrus thrips population Amaranthus retroflexus L. - red root pigweed levels is initiated in early June for summer set fruit. Baccharis glutinosa Pers. - seep-willow Brassica geniculata (Desf.) J. Ball-shortpod mustard Depending upon natural enemy-key pest complex at treatment Bromus mollis L. - soft chess time, aerial or ground sprays of dimethoate and phosphamidon Bromus rubens L. - red brome are commonly applied. Sabadilla and ryania are adequate Bromus tectorum L. - downy brome .. alternatives if minimal impact to parasitoid populations of Cali Bromus wildenowii Kunth - rescuegrass fornia red scale and/or black scale, Saissetia oleae (Olivier), *Cirsium lanceolatum (L.) HiII- bull .thistle is desired. Cyperus eragrostis Lane -giant sedge Gnaphalium leucocepnalum Gray - cudweed Effects of Weed Ecosystems on Citrus Thrips Hordeum leporinum Link. - wild barley Lactuca serricola L. - prickly lettuce Several single, 20-tree unreplicated plots were established in a Lolium multifJorum Lam. - Italian ryegrass 15-year-old Valencia orange grove on the Citrus Research Malva parviflora L. - cheeseweed Center, Agricultural Experiment Station, University of California, *Paspalum dilatatum Poir. - dallisgrass Riverside, to conduct research concerning: (1) numerical re ..Polygonum coccineum Muhl. - swamp smartweed sponse of S. citrito different orchard-weed ecosystems; and (2) Sonchus oleraceus L. - annual sowthistle the effects of citrus thrips on tree physiology, fruit yield, and Sorghum halepense L. Pers. - Johnsongrass quality. The grove is supplied with both furrow and sprinkler Tragopogon porrifolius L. - salsify irrigation. The furrow block is subdivided into new or established *Most common species. bermudagrass and annual weed plots, orbare soil; while the sprinkler block is subdivided into established bermudagrass Treatments with paraquat at labeled rates and mowing pro and annual weed plots and new bermudagrass plots. Each cedures were scheduled in relation to the size and density of weed complex is further subdivided for control by the contact weed growth on a commercially practical basis from April to herbicide, paraquat, or by periodic mowings. The list of weed mid-October. ~r:'lplexes and species is as follows: In the older (1977), established weed block under sprinkler Furrow irrigated-established annual weeds irrigation, the early season weed population was predominantly Amaranthus retroflexus L. - redroot pigweed a mixed stand of summer annuals. These, in descendt"~lg order Brassica geniculata (Desf.) J. Ball- shortpod mustard of their total popUlation, included mare's tail, Conyza canaden Bromus rubens L. - red brome sis; bull thistle, Dirsium lanceolatum; smartweed, Polygonum Bromus wildenowii Kunth - rescuegrass coccineum; and red root pigweed, Amaranthus retrof/exus. Two Chenopodium murale L. - nettleleaf goosefoot perennial weeds, dallisgrass, Paspalum dilatatum and johnson *Cirsium lanceo/atum (L) HiII- bull thistle grass, Sorghum halepense, were present e:ther as seedlings or *Conyza bonariensis(L.) Cronq. - flax-leaved fleabane in a repressed state of activity. By late summer, these two *Conyza canadensis(L.) Cronq. - mare's tail grasses developed to over 60 percent of the weed population in Cyperus eragrostis Lane - giant sedge the untreated plot. In the treated half of the block, successive Gnaphalium lel.lcocephalum Gray - cudweed applications of paraquat progressively eliminated most of the Hordeum leporinum Link. - wild barley annual weeds by late summer. Johnsongrass was reduced to a Lactuca serriol& L - prickly lettuce few scattered plants, while dallisgrass was only partially con Lolium multiflorum Lam. - Italian ryegrass trolled by each paraquat application. By late summer, the weed ., Malva parviflora L. - cheeseweed population in the treated plot had shifted to a monoculture of Paspalum dilatatum Poir. - dallisgrass dallisgrass, occurring as a scattered stand. A September treat Polygonum coccineum Muhl. - swamp smartweed ment and colder temperatures killed most of the dallisgrass Polygonum lapathifolium L. - willow weed stand. Smartweed was also relatively resistant to treatments Rumex crisp us L - curly dock with paraquat in the early part of the treatment period. Weed Salsola iberica Sennen & Pau - Russian thistle growth in the newly established (1979) weed plots was very Senecio vulgaris L. - common groundsel slow in the early season, and only two paraquat treatments were Sonchus oleraceus L. - annual sowthistle required during the summer and fall. • Sorghum halepense (L.) Pers. - Johnsongrass Furrow irrigated-new annual weeds In the furrow irrigated area of established annual weeds, the Amaranthus a/bus L. - tumble pigweed dominant species throughout the summer months were mare's Amaranthus blitoides S. Wats. - prostrate pigweed tail and flax-leafed fleabane, with a lesser population of giant Amaranthus retroflexus L. - red root pigweed sedge, Cyperus eragrostis, and bull thistle. Some dallisgrass

15 and johnsongrass were present in the furrow bottoms through injury. The data for adult emergence indicate thatthe presence out the summer. The weed stand in the unirrigated dry row or absence of various organic accumulations under the Valen middles became moisture stressed by midsummer, but there cia tree's canopy was of no significance to S. citri pupation. was some influx of Russian thistle, Sa/sola iberica, in August. The treatment program throughout the summer months was Collaboration with Lowell Jordan, UCR Weed Scientist, re more effective in the furrow irrigated plot than in the sprinkled vealed a continual shift of the vegetation in each grove area. In areas as evidenced by less regrowth between treatments. The situ visual evaluation of citrus thrips damaged fruit indicated no two Conyza species and the bull thistle were nearly eliminated significant difference among plots with established annual weed by the second paraquat treatment in the sprayed plots. Weed growth under both furrow and sprinkler irrigation and with a solid regrowth throughoutthe rest of the season consisted of dallis ground cover of berrnudagrass unrler sprinklers and equally grass and some redioot pigweed confined to the furrow bottoms divided into treated and untreated plots. and ridges. An additonal 2 years of monitoring weed ecosystems, tree Weed growth in the newly established (1979) weed plots under physiology and productivity, citrus thrips population and con fUiroW irrigation was very slow in the early season, with only two comitantfruit injury, and microarthropod complexes should pro paraquat treatments being required during the summer and fall vide us with a more complete picture of cover crop management months. Because of rapid regrowth of bermudagrass following in California's citriculture. each treatment in the established plots, five applications were required during the early treatment season in the treated plots. Two paraquat sprays maintained good suppression of berrnu Biological Control of Citrus Thrips dagrass in the treated, newly established (1979) plot. There is little knowledge of natural enemies of both S. citri and The first year evaluation (table 5) of the. indirect or direct influ the South African citrus thrips, Scirtothrips aurantii Faure, on ences of weed cover crops and paraquat on S. citri population citrus. The feeding behavior of both species is remarkably levels and their feeding activity on Valencia orange was incon similar, in contrast to dissimilar morphology and geographic clusive. With the exception of the immature S. citri-days accu distribution. Milne (1977) summarized reports that suggest mulations in the established bermudagrass, paraquat, and predaceous soil mites such as cunaxids, bdellids, anystids, sprinkle irrigation plot, by July 14 the populations of maturing and laelaptids may prey on citrus thrips pupating in the soil second instars were genBrally more abundant than those popu and organic debris lying beneath citrus trees. lations residing in clean cover, berrnudagrass, or new annual weed plots. With the exception of the sarne aforementioned Bravo-Mojica (1975) conducted predation studies on three established berrnudagrass plot, there was no significant differ citrus canopy inhabiting predators, Aysha decepta Banks ence among plots for adult citrus thrips levels. These prelimin (Aranea, Clubionidae), Anystis agilis (Banks) (Acarina, ary data suggest that the foliage-inhabiting immature instars are Anystidae), and Amblyseius hibisci (Chant) (Acarina, Phyto responding to the physiological effects of nutritional stress being seiidae). Further special attention is now being directed toward imposed on tree vigor by the different weed ecosystems. Collab the prey preferences of the facultative predator A. hibisci. This orative stUdies on tree growth, leaf nitrogen, water potential, phytoseiid has long been considered an important predator of and soluble solids hopefully will corroborate the numerous em several spider mite species in avocado and citrus groves pirical observations for associating foliage vigor, abundance, throughout California (McMurtry et al. 1970). Pollens are also an and color with S. citri population levels and potential economic important component of A. hibisci's diet that may induce a higher rate of oviposition than citrus red mite, Panonychus citri, prey. Kennett et al. (1979) have observed A. hibiscipredation Table 5.-Accumulated S. citri-days at postcritical periOd in a on immature instars of citrus thrips. furrow (F) and sprinkler (S) irrigated Valencia orange grove, Table 6 shows the results of isolating individual, mature female Riverside, Calif., 14 July 1980 A. hibisci in modified Munger cells with either ice plant pollen or at two different densities of S. citri larval instars. The original 1 2 2 Plot Program Immatures Adults number of S. citri larvae was maintained daily. All A. hibisci Established annuals (F) T 192.6ab 35.4 b females were preconditioned by isolation without food for 24 h. U 154.6 bc 52.3 b The resulls indicate that A. hibisci can readily capture and consume citrus thrips larvae (fig. 19) and that a minimum of 10 Established annuals (S) T 247.6a 95.7a U 183.8ab 32.1 b thrips per day will support a daily fecundity commensurate to that observed on an ice plant pollen diet. These data suggest New annuals (F) T 84.4 cd 42.0 b that in the absence of the cyclic occurrence of pollens and P. citri U 83.9 cd 27.2 b on citrus trees, A. hibisci dietary preferences will shift to Established bermudagrass (S) T 167.8abc 49.6 b the late-spring and early summer population increases of the U 57.3 d 38.3 b citrus thrips. This notion, regarding the feeding and behavioral New bermudagrass (S) T 86.7 cd 36.3 b responses of A. hibisci to numerical increases and reSUltant U 90.4 cd 37.1 b fruitlet injury by early season S. citri popUlations, is under Clean cover (F) TW 93.1 cd 51.4 b investigation. TE 116.6 bcd 26.3 b lT '" Paraquat, U '" mowed. W '" West, E ':' East. 2Means followed by the same letter are not significantly different at P '" 0.05 level, according to Duncan's multiple range lest.

16 Table 6.-0viposition data for Amblyseius hibisci females Matkin, O. A., and Chandler, P. A. 1957. The U.C.-type soil reared on different foods 1 mixes. p. 68-85. In Baker. K. F. (ed.). The U.C. system for producing healthy container-grown plants. California Agricul Days tural Experiment Station Manual No. 23. Food 2 3 McGregor, E. A 1926. A device for determining the relative Ice plant pollen2 0.67 1.47 1.82 degree of insect occurrence. Pan-Pacific Entomologist 3 :29-33. 10S.citrildal .72 1.10 1.76 ___ 1944. The citrus thrips, measures for its control, and 20 S. citri/day ...... 5:.:3=-- ____1...... _43:...... _____1_.77_ their effect on other citrus pests. U.S. Department of Agriculture. '27QC. Circular No. 708. 12 p. 2Malephora croce a (Jacq.). McMurtry, J. A., Huffaker. C. B .• and Vrie, M. van de. 1970. 3Larvallnstars. Ecology of tetranychid mites and their natural enemies: a review. I. Tetranychid enemies: their biological characters and the impact of spray practices. Hilgardia 40(11):331-390. Milne, D. L. 1977. Biological control of citrus thrips, Scirtothrips aurantii: What are the prospects? C:trus and Subtropical Fruit Journal 497:14,16. ___ and de Villiers, E. A. 1977. Soil application of systemic pesticides for control of thrips and nematodes on citrus. Citrus and Subtropical Fruit Journal 497:9. 18. Moulton, D. 1909. The orange thrips. U.S. Department of Agriculture. Bureau of Entomology. Technical Series No. 12. 4 p. Reed. D. K., and Rich, J.R. 1975. A new survey technique for citrus thrips. Journal of Economic Entomology 68:739-742. Sevacherian. W .• Stern. V. M.• and Mueller. A. J. 1977. Heat accumulation for timing Lygus control measures ina safflower cotton complex. Journal of Economic Entomology 70:399-402.

State of California. Department of Food and Agriculture. 1978.

BN·49136 Estimated damage and crop loss caused by insects and mite Figure 19.-Amblyseius hibisci seizing and devouring im pests 1978, 28 p. mature Scirtothrips citri. Tanigoshi, L. K.• and Browne. R. W. 1978. Influence of temperature on the life table parameters of Metaseiulus t occidentalis and Tetranychus mcdanieli (Acarina: Phyto Literature Cited seiidae, Tetranychidae). Annals of the Entomological Society of America 71 :313-316. Bailey. S. F. 1964. A revision of the genus Scirtothrips Shull ___. Nishio, J. Y .• Moreno, D. A., and Fargerlund, J. 1980. (Thysanoptera: Thripidae). Hilgardia 35(13):329-362. Effect of temperature on development and survival of Scirto Bravo-Mojica. H. 1975. Ecological studies on the citrus thrips. thrips citri on citrus foliage. Annals of the Entomological Society Scirtothrips citri (Moulton). (Thysanoptera. Thripidae) in of America 73:378-381. southern California. Ph.D. Dissertation. University of California. ___ and Moreno. D. S. 1981. Traps for monitoring Riverside. 159 p. populations of the citrus thrips, Scirtothrips citri (Thysanoptera: Thripidae). Canadian Entomologist 113:9-12. Ebeling, W. 1959. Subtropical fruit pests. University of Cali ____ and Nishio-Wong, J. Y. 1981. Greenhouse rearing of fornia, 436 p. dtrus thrips, Scirtothrips citri for experimental testing. Journal of Hoagland, D. R., and Arnon, D. W. 1950. The water culture Economic Entomology 74:213-214. method for growing plants without soil. California Agricultural .___• Bailey. J. B., and Moreno, D. S. 1981. Citrus thrips: Experimental Station, Circular No. 347, 32 p. a major pest of California citrus. University of California. Cooperative Extension Leaflet No. 21224. 4 p. Horton. J.R. 1918. The citrus thrips. U.S. Department of ~_~__. _, Moreno, D. S., Nishio-Wong, J. Y .. and Fargerlund. J. Agriculture. Bulletin No. 616. 42 p. 1982. Soil applications of insecticides for control of Scirtothrips citri on citrus. Journal of Applied Entomology (In press). Kennett, C. E .• Flaherty, D. L., and Hoffman, R. W. 1979. Effect of wind-borne pollens on the population dynamics of Tashiro. H. 1967. Self-watering acrylic cages for confining Amblyseius hibisci (Acarina: Phytoseiidae). Entomophaga insects and mites on detached leaves. Journal of Economic 24:83-98. Entomology 60:354-356.

Lewis, H. C. 1935. Factors influencing citrus thrips damage. University of California. 1980. 1980-1982 treatment guide for Journal of Economic Entomology 28:1 011-1 015. California citrus crops. Leaflet No. 2903, 97 p.

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