man and Faeth 1988). Field Sampling Methods. We conducted our studies during the 1986 grape growing season in 3. Thompson Seedless vineyards located ...
This article is the copyright property of the Entomological Society of America and may not be used for any commercial or other private purpose without specific written permission of the Entomological Society of America.
POPULATION E c o ~ o c ~
Spatial and Temporal Dynamics of Spider Mites (Acari: Tetranychidae) in ‘Thompson Seedless’ Vineyards RACHID HANNA, LLOYD T. WILSON,’ FRANK G. ZALOM, DONALD L. FLAHERTYF AN D GEORGE M . LEAVITT3 Department of Entomology, University of California, Davis, CA 95616
Environ. Entomol. 25(2). 3 7 0 3 8 2 (1996)
ABSTRACT Quantitative knowledge of beneficial and pest arthropod distribution patterns in relation to plant vigor and habitat structure is essential for the development of reliable and
cost-effective methods for the assessment and management of arthropod populations in agroecosystems. The spatial and temporal distributions of the spider mites, TetrunycAus pucijii~7~5 McCregor, Pacific spider mite, and Eotetrunychus willarwttei (McGregor) were determined in 3 ‘Thompson Seedless’ grape, Vitk uin$era L,., vineyards located in the central San Joaquin Valley of California. I: pacifim.~was the dominant species in 2 of the vineyards, whereas E . willanwttei was dominant in the 3rd vineyard. Z pacijifi~~s infestations were most severe in vineyard areas with low vine vigor compared with area.. with high vigor. In contrast, E. wilZumttei appeared to be less sensitive to differences in vine vigor in the vineyards where the effect of vine vigor could be tested. Spatial variations in between- and within-vine distributions of spider mites were significant during most of the growing season in all vineyards. Z p ~ ~ i j i f i c u s and E. willumettei distributions overlapped considerably on the south and north zones of vine canopy but differed between top and interior zones. The majority of T. pacificus and E. willamettei were found on leaves near the base of shoots during spring, arid on midshoot leaves during summer, Aggregation indices indicated that both spider mite species were aggregated during most of the season, with ‘I: puc~fi~vus being more aggregated between vines and within vines. The biological sipificance of T ~ U C @ C U S arid E. willarrLettei spatial dynamics are discussed along with implications for population assessment. KEY WORDS Tetrarychus pac$ficus, Eotetrurychus wilhnwtiei, Vitis vinfern, plant vigor,
spatial distribution, aggregation
TWOSPECIES OF spider mites, Tetranychus PU:$cus McGregor, Pacific spider mite, and Eotetranychus willamettei (McGregor) are of major economic concem in California vineyards. Although feeding damage on leaves of grape, Vitis vinifera L., by both species similarly affects leaf photosynthetic rates and stomatal conductance (Welter et al. 198Ya), T. pacificus has a greater potential for reducing yield and quality of most grape cultivars (Kinn et al. 1974, Welter e t d. 1989b). The pest status of E. willumettei is more dependent on the grape cultivar and does not normally affect yield and quality of ‘Thompson Seedless’ grapes (Flaherty and Huffaker 1970); however, high levels of this species will reduce yield and quality of ‘Chenin blanc’ arid ‘Zinfandel’ grapes in vineyards located in the Sierra Nevada foothills (McNally and Famham 1985, Welter et al. 1989b). ‘Current address: Department of Entomology, Tcxas ACtM University, College Station, TX 77843. 2University of C:aliforiiia Cooperative Extension, Visalia, CA 9329 1. 3University of California Coopwative Extension, Madttrs, CA 93637.
In addition to their impact on grapevines, ?: pacifzcus and E. willumettei may interact negatively 011 Thorripson Seedless vines (Hanna 1992).E . willawtetlei is generally found on the grapevines earlier in the season and persists longer into the fall compared with T. pac$cu.s (Flaherty and Huffaker 1970).When present in spring, E . willamettei can be beneficial as alternate prey for the predatory mite Metaseiulus ( =Typhlodronzus =Gubndromnus) occiclentalis (Nesbitt) (Acari: Phytoseiidae), enhancing the suppression of T pac$cus later in the season (Flaherty and Huffaker 1970, Hanna 1992). Other evidence also indicates that on Zinfandel grapevines, damage by E. willamettei in early spring may result in lower ?: pac$ms densities during the summer (Karban and b!nglish-Loeb 1990); however, this effect was not observed in Thompson Seedless vineyards (IIanna 1992). Despite the importance of spider mites in vineyards, quantitative information on their spatial dynamics throughout the grape growing season is lacking. Flaherty and Huffaker (1970) studied the distribution of spider mites on vines but did not describe seasonal changes in distribution patterns, or possible factors affecting these patterns. Several
0046-225X/96/0370-OG82$02.O(MO0 1996 Entomological Society of America
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HANNA ET AL..: SPATIAL A N D T EMPORAL D YNAMICS OF SPIDER MITES
biotic and abiotic fiactors can caiise large variations in vine vigor which may affect spider mite distributions within vineyards. In addition, vine canopy structure affects the amount of sunlight intercepted by leaves and can alter within-vine physical factors such as temperature, humidity, and air movement (Smart 1985, Grimes and Williams 1990). Variations in these physical factors can affect the abundance of spider mites greatly (Iloltzer et al. 1988). Quantitative knowledge of spider mite distribution patterns in relation to vine vigor arid vine canopy structure is essential for the development of reliable and cost-effective methods for the assessinerit and management of spider mite populations in vineyards. This study presents quantitative information on within-vineyard and within-vine distributions of I: paci$c~cs arid E. willarnettei in 3 commercial Thompson Seedless vineyards. This information is iised in developing a general approach to spider mite sampling in vineyards.
Materials and Methods Growth and Development of Thompson Seedless Vines. Many grape cultivars are grown commercially throughout California, all belonging to V. wint$eru LA. Although most of these cultivars have similar growth patterns, differences in prunin training of canes, and differences in uses o harand vested grapes (i.e., table, wine, and raisin grapes), have led to cultivar-specific structure and manageinent of vine canopy (Winklcr et d. 1975).We have restricted our investigations to Thompson Seedless grape, which is the most widely grown cultivar in the San Joacpin Valley of California. Thompson Seedless vines used in raisin production are cane-pruned during the winter months, =5 canes per vine and 15 biids per cane l(Win ea?$ er et al. 1975). Hudbreak (or the initiation of vegetative growth) of Thompson Seedless grapes riorrnally occiirs after the accumulation of 4 6 DD (degree-days; >10"C) from 20 Fcbniary (Williams et al. 1985). After the accumulation of -200 DD from bud break in the absence of severe water stress, vine leaf area increases linearly reaching a maximum of =23 in2 per vine at 1,000 DD (Williams 198721). Total leaf area decreases during the remainder of the season because of shoot trimming and leaf senescence. Thompson Seedless vines are normally planted in an east-west orientation and trained on a single wire trellis or a cross arm with 2.5 m between vines (within a row) and 3.5 m between rows. On mature vines with fully developed canopies, both training systems result in 4 canopy zones which receive different amounts of sunlight (Mullins et al. 1992). The top zone of the canopy is generally fully exposed to siinlight throughout the day, whereas the north and soiith zones receive varying levels of sunlight depending on time of day arid season (Mullins et 4. 1992). By contrast, the interior of
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the canopy is generally fiilly shaded from siinlight. Differences in light intensity may influencc the distribution and abundance of arthropods (ßultman and Faeth 1988). Field Sampling Methods. We conducted our studies during the 1986 grape growing season in 3 Thompson Seedless vineyards located in the San Joayuin Valley near Fresno (Fresno County), Madera (Madera County), and Dinuba (Tiilare County). The 3 vineyards were selected on the basis of their history of spider mite outbreaks, spider mite species composition, and the distribiition of vine vigor. In this study, vine vigor is defined qualitatively; the majority of vines in the inoderate to high-vigor areas had dense foliage and long shoots, whereas those in low-vigor areas had relatively sparse foliage and short shoots. The Dinuba arid Fresno vineyards had areas with relatively high and low vine vigor, whereas the Madera vineyard had vines with only moderate-to-high vigor. Vines were trained on single trellis wire with canes of adjacent vines in contact at their terminal ends and forming a continuum within a row; however, their foliage rarely touched vines of' adjacent rows. Two 0.30-ha sampling plots were established within each of the 3 vineyards, 1 plot in each of the high- and low-vigor areas of the Diniiba and Fresno vineyards, arid 2 plots (which were similar in vine vigor) in the Madera vineyard. The sampling plots were at least 30 m from row ends and were separated by 70, 60, and 40 1711in the Madera, Dinuba, and Fresiio vineyards, respectively. Within each plot, we designated 9-15 experimental units (consisting of 3 adjacent vines per m i t ) from which we sampled leaves to determine between- arid within-vine distribution of I: pacijicms and E . willamettei. The 3-vine experimental unit was selected to minimize the effect of repeated leaf removal on mite densities. We sampled all vineyards at intervals of 7-14 d beginning on 15 April ("4 wk after bud brcak) in the Fresno vineyard, 6 May in the Dinuba vineyard, and 15 April in 1 plot in the Madera vincyard. We initiated sampling in the 2nd plot in the Madera vineyard on 16 August. We terminated sampling foIlowing propargite (Omite) treatments on 9 July in the Fresno vineyard and 19 July in the Dinuba vineyard. Miticides were not used in the Madera vineyard, allowing sampling to continue until 13 September, when numbers of 2: pacijims and E. willumttei populations declined to near 0 levels. On each sampling date in the Dinuba and Fresno vineyards, we sampled fifteen .?-vine experimental units from each of the 2 sampling areas. In the Madera vineyard, we sampled 15, 3-vine experimental units from only 1 area until 6 August. On 16 August, we initiated sampling from a 2nd area in the Madera vineyard, but we reduced to 9 the number of experimental units sampled from each of 2 sampling areas in this vineyard. We restricted sampling to primary shoot leaves which
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We used the Taylor (Taylor 1961) and Iwao cornprisc =70% of total leaf area of Thompson Seedless vines (Williams 1987a). Ihring April, (Iwao 1968) regression iriodels to estimate the deMay, and the 1st half of June (the period of rapid gree of between- and within-vine aggregation of ?: vegetative growth of vines), wr sampled pririiary pac$cus and E. willnnzettei. The merits and applishoots originating from vine tnniks, and from l-yr- cability of these techniques for the estirnation of old wood iii the iiorth and south vine zones. TWO species aggregation were reviewed by Taylor leaves werca selected from the basal and middle (1984) and Kiino (1991). The 2 models provide portions of each shoot. As the vine canopy became essentially the same information about the spatial fully developed (at approximately the middle of patterns of organisms biit differ in the interpretation of their parameters. In the Ttylor model, samTuiie), 2 atlditional slioots were included in the sainpling, 1 shoot from the interior and 1 shoot ple variance ( S 2 ) is relatcd to sample mean density From the top canopy zones, for a total of 4 shoots (f)by the relationship s2. = a . f!3, This equation is (zones) from each experimental unit. We initiated typically linearized witlt logaritlirnic transforinaleaf sampling from the terminal region of each tion: 1n(,s2)= In(n) + b ‘ In@).The b coefficient is shoot by mrd-June. The 3 locations on a shoot rep- species-specific and is a measure of the density deresented different age classes of leaves. All S ~ I W pendence of aggregation (Taylor 1961, Taylor et al. pled leaves were classified according to sarnpling 1978). Both CI and b determine thc degree of agarea, vine, vine zone, and position on a shoot (leaf gregation of a species (Banerjee 1876, Wilson age). In addition, all l r m w were classified as sun- 1985, Ilanna and Wilson 1991). The Iwao linear regression model m = OL + p . exposed or shaded at the time of sampling, which R ( m is the Loyd [I9671 inean crowding [m = f + typically occurred between 0900 arid 1500 hours. All leaves were inimediately chilled arid brought to s 2 E - 13 arid f is sample mean density) provides tlae laboratory, where they were stored at 3°C iiiitil information on 2 aspects of a species aggregation. they wcre exainined under a binocular microscope. The y intercept a of the model estimates the size We counted all life stages of spider mites arid of the smallest unit of the population (number of predatory mites, arid immatiires and adnlts of the individuals), whereas the dope ( p ) estirnates the sixspotted thrips, Scolothrips sexmuculnlzrs (Per- rate of change in the aggregation of the. srnallcst population iini t as population density increases. gartde) (Thysnnoptera: Thripidae). used a 2-step process to estirnate betwecn-vine We Statistical Analyses. We used a random effect, parameters of the Taylor and Iwao regression. In 4-way nested analysis of variance (ANOVA) (PROC the 1st step, we calculated 7: pac$c:us and E. zuilGLM, SAS Institiite 1989), stratjfied by date, to Zamttei mean densities per leaf for each vine, coinpare iriean densities of 7: pnczjLicirs and E. wilstratified by vineyard, sampling (late, aiid vineyard Zairwttei between the 2 sampling areas of each area. In the 2nd step, we calciilated between-vine mneyard, aniong vines ( I st experirnental iinit, nestmeans and variances for each sampling date and cd within sampling area), among zoiies (2nd ex- vineyard area. In total, 35 and 33 data points for perirrieiital unit, nestcd within vine), and within 2: pac$ficx~s and E. zoillarnettei, respectively, were zones ( i t a . , k i f age effect was the 3rd experimental used in the regression analyses. Dates when spider unit, nested within zone, and was tlie residual mites were absent were not incliided in tlie analterm). ANOVAs were condiicted separately for yses. To estimate within-vine aggregation indices, each vineyard. All reported densities were calcii- we calculated within-vine density nieans and varilatcd on a per-leaf basis. We used the Ronferroni ances for each mite species, stratified by vineyard, procedure (Milliken arid Johnson 1984) to adjust sampling date, vineyard area and vine. In total, 290 probability vahics becai ~ s the e aiialyses were strat- and 200 data points for ?: pacificus and E. zoillnificd by sampling datc. Rcpeated-rrieasures analy- mettei, respectively, were used in tlae regressions. ses ;across dates were not appropriate because we Data were analyzed with PROC REG (SAS Instidid not sample all zones and leaves within zones tute 1989). on each sainpling datc. Stratification by sampliiig date allowed the estiiriation of teinporal changes Results in the size and proportion of the varihility of mite densities explainrd by variatioii between vineyard Distribution within Vineyards, Largc betwecnarcus, vines, zones, and leaves within zones (PROC and within-vineyard differences were observed in VABCOMP, SAS Institiite 1989). We used Border- the abuntlance of the 2 spider mite species during roiu-adjiisted linear contrasts (Milliken and John- the 1986 study. T paczjLiicl~~ was the domiiiant speson 1084) to coinpare mite densities in the top arid cies in the 13iniiba and Fresno vineyards, and E south zones witti densities in thc north and interior willarnettei was the doniinant species in the Mazoncs oi the vines. In addition, we used a 1-way dera vineyard (Fig. l ) . For brevity, we present ANOVA (stratified by date) to compare mean mite analyses of data only from the Dinuba and Madera densities on shaded and sun-exposed leaves. We vineyards (Tables I and 2) because of the similarity log-transforaied all data to correct for heteroge- in the distribution and abuiidance of T paci$c~i~ neity of variance before conducting the statistical and E . zoiZlnrn&-tei in the Dinuba and Fresno vineyards.
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HANNA ET
120- A.
AL.: SPATIAT. A N D
Dinuba
120-
TEMPORAI. DYNAMICS
B.
Fresno
120-
%loo-
100-
100-
3 80:
80 -
80 -
60 -
6o
L
8 60: 3
O
:-
40 --CI-
2 *G 4 0 8 20-
3 73
0 1 ' SPII3)KH MITES
c.
Madera
Low vigor Medium vigor High vigor
5
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2
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Date of Sampling Fig. 1 . Seasonal patterns of 'I: p c ~ i j k i ~(A-C:) s and .E. i.oihnettei (1.1-47) abmdaiice in 3 vineyards (A and D, Dinuba; 13 and E, Fresno; C iind F,Madera). L3at.a points are rneim mite de bars are standard errors. Data are plotted separately for vines with high (G), B I I ~low (J) vigor at the 13irii11>it ;uitl Fresno vineyards (1 high and low vigor plot iit em$ vineyard), and for vines with nietliurn vigor (13) at the Madera vineyarcl ( 2 mediurn-vigor plots were saiiip1c:d).
.
In the Dinuba and Fresno vineyards, T ?iucijkus occurred earlier, increased more rapidly, and it occiirred at significantly higher deiisitics on low-vigor vines than on high-vigor vines (Fig. 1 A and R; Table 1).Differences in vine vigor explained 5 8 and 48% of the variation in T pacijims densities during the increasing phase and at peak density, respectively. In the Madera vineyard, where vines were of similar vigor in the 2 sainpling areas, 7: pncijicus densities and their seasonal patterns of growth and decline also were similar (Fig. IC; Table 2 ) . Diffcrences between the 2 sampling areas explained 5 4 % of tlie spatial variation in T pacificus densities (Table 2). Populations of E. willnnwttei did not appear to be irifliiertced by vine vigor. E . zuillnnu?tteidensities were veiy low in the Dinuba and Fresno vineyards, with equal densities occurring on high- arid lowvigor vines (Fig. 1 D and E; Table 1). The area effect explained 112% of the spatial variation in E . willnmttei densities in the Dinuba vineyard (Table 1). E . tuillnmettui wa.. substantially ~riore abundant in both areas in the Madera vineyard (88.5 rt 12.6 [mean t SEM] initcs per leaf at pcxik abundance) compared with its abundance in the
Dinuba and Fresno vineyards (2.8 3. 1.13 and 2.8 3- 0.97 mites per leaf at peak abiindance, respectively). The area effect expl;dned 5 2 % of t h r h tial variation in E. wille,, ei aliiindancc~ i i i the. Madera vineyard (Fig. 1 I'; Table 2). Distribution Among Vines. Diffcrtwc distribution of mites airlong vines (within ;a siiinpling area) were gc~~ierally siiililar in all 3 viiieyards (Tables 1 and 2). Mean 'f puc@x~sdensitirc in tlic Diiiubn vineyard were significantly dificrviit among vincs, diiririg t 1 1 ~ rapid incrcasca (27 Jiiiic,) and peak (9 JliIy) ph21Ses of d)Illldilnce. Uiffbrences among vines also expl&ied 2 4 4 5 % of the spatial variation in incan 'I: pm:ijiiCris d n i n ~ l a ~ (Table I), except during thc initial incrcwsc pli, (10% of spatial variation) when T p~ciflcusdeiisities were low. Similar patterns were observed in the Madera vineyard ( T d ~ 2). k T l i ~grcatcxt effect occurred during thc lapid iiicreasc a i d peak phases of mite abiindance (29-66% of spatial variation). But the magnitiicle of tlie vine effect was negligible (54%) during tlie declining phase. Among-vine distribution of E. willanief~eifollowed ternporal patterns similar to T pczcificus, but the significance and rnagriitudca of' the cffi%ctw ( w
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Table 1. Results front nested ANOVA of T. pacificus and E . wiUamttei densities in relntion to siunpIlin# area, vine, canopy zone, and leaf age in the Dinuba vineyard n
.
uare.-I,
P valiie for 2'. padficus (do
ßetween area& Among vines
Among zones
P values for E. willunwttei (df) Leaf
agec
Between areas
Among vines
-.
._ 0.024 ( 2 8 )
Ainong zones
1,t:af
0 . I ) l l (60) 0.089 (89)
- (90) - (237) - (210)
W('
Effect tests 8 June I8 June
27 June 9 July 1.9 July 8 June
18 June 27 June 9 July 19 July
0.1.52 (1) 0.101. (1.) 0.070 (1) 0.000" ( I ) 0.080 (I) 0.02 0.06 0.08 0.48 0.11
0.089 (28) 0.000 (28) 0.000* (28) 0.000* (28) 0.000 (28) 0.10 0.34 0.45 0.24 0.34
0.000* (60) 0.037 (60) 0.000 (89) 0.001 (90) 0.008 (90)
-
(90)
(90) - (237) - (240) - (240)
0.158 (1) 0.841 (1) 0.079 (1) 0.016 (I)
0.001 (28) 0.000 (28) 0.000 (28)
Proportion of variation explained 0.68 0.1.6 0.12 0.47 0.01 0.10 0.36 0.02 0.06 0.23 0.05 0.15 0.40 0.12
-
-
0.051 ($10)
0.000 (90)
0.10 0.14 0.08
0.10 0.1.4 0.I4 0.21
0.15
-- (240)
0.79 0.70 0.73 0.52
*, P < 0.05 after ßonferroni adjustment. "
Effect tests for 6 May and 23 May were not included because of the absence or near-0 abundance of spider Initc!s. Area effect represents high versus low vine vigor in this vineyard. This is the residual term in this nested model, so P value for the leaf-age effect could not be calculated.
less than that of I: pacijicus. In the Dinuba vineyard, where E. willarnettei density was low compared with densities in the Madera vineyard, the vine effect explained 1O-21% of the spatial variation in E . willamettei abundance. In the Madera vineyard, where E. willarnettei density was very high, among-vine differences were greater (but still not significant) during the increasing and peak phases of E. willarnettei population growth (27-
33% of spatial variation), becoming negligible during the declining phase (Table 2). Distribution Among Vine Zones. I: pnc$cus and E. willarnettei showed dynamic distribiition patterns within vines or among zones in all 3 vineyards. Complete analyses are presented for both species in the Diniiba and Madera vineyards (Tables l and 2). For brevity, we present graphs (as mean density per leaf for each species on 4 vine
Table 2. Results from nested ANOVA of T. pacificus and E . willamttei densities in relation to sumpling area, leaf age in the Madera vineyard
vine, canopy zone, and
P value for T.p a o i j h s (dt)
Date('
27 June 09 July 19 July 29 July 08 Aug. 16 A L I ~
23 Aug. 29 Aug. 06 Sept. 13 Sept. 27 June 09 July
19 July 29 July 08 Aug. 16 Aug. 23 Aug. 29 Arrg. 06 Sept. 13 Sept.
Between are&
Among vines
-
0.000' (14) 0.000* (14) 0.000* (14) 0.000 (1.4) 0.774 (14) 0.156 (16) 0.476 (16)
-
-
0.386 ( I ) 0.575 (I) 0.064 ( 1 ) 0.342 (1) 0339 (1)
-
0.01 0.01 0.04 0.01.
0.00
0.662 (16) 0.589 (15) 0.033 (1.6)
0.66
0.43 0.57 0.29 0.05 0.06 0.05 0.04 0.02 0.08
Among . mnes
P
Leaf age':
Between
areas
Effect tests 0.017 (45) - (120) 0.061 (45) - (120) 0.419 (45) - (120) 0.340 (45) - (120) 0.000 (45) - (120) - (144) 0.811 (1) 0.000 (54) - (1.42) 0.985 (1.) 0.000 (52) 0.000 (54) - (144) 0.608 (1) - (136) 0.360 (1) 0.000 (51) - ( 144) 0.529 ( I ) 0.170 (54) Proportion of variation explained 0.28 0.06 0.49 0.08 0.43 0.01 0.69 0.02 0.43 0.52 0.55 0.01 0.38 0.59 0.02 0.35 0.35 0.59 0.02 0.63 0.00 0.34 0.00 0.85 0.07 I
value for
E.
wihrwltd
(df)
Among vines
Among zollc?s
1,t:d age
0.000 (14) 0.000 (14) 0.001 (14) 0.000 (14) 0.026 (14) 0.436 (16) 0.052 (16) 0.015 (16) 0.622 (1.5) 0.372 (1.6)
0.000 (45)
- ( I 20)
0.007 (45) 0.000 (45) 0.316 (15) 0.014 (45) 0.000 (54) 0.002 (52) 0.010 (54) 0.000 (51) 0.000 (54)
- ( 120)
0.33
0.29 0.16 0.28 0.03 0.16 0.38 0.I9
0.27 0.27 0.29 0.12 0.01. 0.10 0.I 2
0.03 0.02
- (120)
-- ( 1 20)
- (120) - (144) - (142) -- (144) - (136) - (144)
0.38
0.38 0.57 0.45 0.68 0.72 0.60 0.69 0.7 I 0.59
0.30
0.68
0.15
*, P < 0.05 after ßonferroni adjustment. Effect tests for dates 15 April, 6 May, 23 May, 8 June, and 18 June were not included because-: of the absence or near-0 numhcrs of spider mites. Only 1. area was sampled before 16 August: both areas sampled in this vineyard had vines of medium vigor. This is the residual term in this nested model, so P value for the Id-age effect could not bc! c.dcnlated.
April 1996
H ANNA ET AL.: SPATIAL A N D T EMPORAL D YNAMICS OF SPI13ER Ml'mS
B. 27June
375
/ 60
45 30 15
0
h'
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/t
'
f
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b
4
0
0
Fig. 2. Population distribution of T pacijiws on 3 leaf positions in 4 vine canopy zones in the Dinuba vineyard. (A) Initial increase phase, 8 June. (B) Rapid increase phase, 27 June. (C) Peak phase, 9 July. ( 0 )Declining phase, 19 Jdy.
zones and 3 leaf ages) for selected phases of pop- 0.75; df = 90, 240; P < 0.05). Densities were genulation growth: initial, rapid, peak, and declinin erally not significantly different on top compared phases; for 2: pac$cus in the Dinuba vineyar with south zones, and on north cornpared with in(Fig. 2) and the Madera vineyard (Fig. 3); and for terior zones ( P > 0.05). E. willarnettei in the Madera vineyard (Fig. 4). Tetranychus pacificus distributions in the MaTetranychus pacijiws densities in the Dinuba dera vineyard with respect to vine zone di€€ered vineyard differed significantly among 3 vine zones slightly from the Dinuba vineyard (Fig. 3). In the on 8 June (Fig. 2A; Table 1).The greatest propor- Madera vineyard, the zone effect explained a nontion of explained variation (68%) in mite densities significant and negligible proportion of the variaoccurred during the initial phase of population tion in T. pacijicus abundance during the initial growth, declining to 515% of the total variation in and rapid increase phases of growth (Table 2). mite abundance during the rapid increase, peak, During the initial increase phase, 65% of ?: paciand declining phases (Table 1).During the initial ficus were found on top and south zones and 35% increase phase, T. pacijicus occurred in greater 011 north and interior zones (Fig. 3A). During the numbers on top and south zones compared with rapid increase phase, 60% of Z: pac+us werc the north zone (Fig. 2A); ( B = 0.03, df = 90, P < found on top arid south zones and 40% were found 0.05) ( B is the Bonferroni least significant differ- on north and interior zones (Fig. 3B). These difence for comparing zones; interior shoots were not ferences in T. pacijicus distributions during the inisampled in the Dinuba vineyard during this tial and rapid increase phases were not statistically phase). During the rapid increase, peak, and de- significant ( B = 0.20-0.95; df = 120, 144; Y > clining phases of population growth, ?: pacijicus 0.05). The zone effect became more pronounced displayed a strong preference for both south and later in the season when it ex lained 3543% of top shoots which harbored 71-88% of 7: pac$cus spatial variation in 2: pacijicus Iensities during the densities, compared with north and interior shoots (extended) peak phase of growth (Fig. 1C). The which harbored 12-29% (Fig. 2 B-D); (B = 0.30- contribution of the zone effect became negligible
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Ea,
h
Fig. 3. Population distribution of T. pncijijiczis on 3 leaf positions in 4 vine canopy zones in the Madera vineyard. ( A ) initial increase phase, 27 June. (13) Hapid increase phase, 19 July. ((2) Peak phase, 16 August. (1)) I.)eclining
phase, 1.3 September.
(7%) on 13 September during the declining phase (Fig. 3D; Table 2). T pacijicus occurred in greater niimbers on top and south zones during the peak phase of growth compared with north and interior zones (Fig. 3 C ) ; ( B = 0.18, df = 144, P < 0.05). The top zono siipported greater numbers of T pac$ficu~ coinpar(2d with the south zone ( P < 0.05), and the north zone supported higher numbers conipar~dwith the interior zone ( P < 0.05). J h r ing the declining phase of the popiilation cycle (Fig. 3D), 2: p a c ~ f i ~ unumbers s remained higher on top and south zones (70% of T pacijkus numbers) compared w'th north and interior shoots ( B = 0.82, df = 144, P < 0.05). The top zone supported similar mite densities compared with the south zone ( P > 0.05), biit the north zone supported significantly greater mite densities compared with the interior zone ( P < 0.05). Eotetranychus willamettei also showed differences in abiindance among vine zones in the Madera vineyard (Fig. 4). In the Dinuba and Fresno vineyards, where E. willnmettei densities were low, its abundance was slightly higher on the north and intcrior zones compared with the top arid south zones (data not shown). This preference was not
significant at any time during the season, and the zone effect did not accoiint for > 15% of the spatial variation in mite abundance (Table 1; analysis not shown for the Fresno vineyard). However, E . willamettei abundance was not detectably different among the 4 vine zones in the Madera vineyard after the Bonferroni adjiistinent on any of the 10 sampling dates (Table 2). Durin the initial increase phase, vine zone explainecf 29% of spatial variation in E. toillanwttci abundance, with greater abundance occurring on the south zone (Table 2; Fig. 4A); ( B = 0.18, df = 120, P < 0.05). During the rapid increase phase, E. willairmttei was more abundant on interior arid north zones (75% of E. willarnettei niiinbers) compared with south and top zones (Fig. 4B); ( B = 2.06, df = 120, P < 0.05). Despite this preference for north and interior zones, the zone effect explained only 16% of the spatial variation in E. willninettei abiindance during the same period. Differences among zones declined during the peak and declining phases; however, E. willamettei remained in greater abiindance on the north and interior zones compared with the top and south zones (Fig. 4 C and D; L3 = 0.731.01; df = 142, 144; P < 0.05).
April 1996
HANNA ET AL,.: SPATIAL, AND TEMPORAL D YNAMICS 01.' SPIDER MI'I'ES
377
B. 8August 10 8 6 4 2 0
125 100 75 50 25 0
125 100 75 50 25 0
B .
Termin,al\f
Interior
200
40
150
30
100
20
50
10
0
0
Fig. 4.. Population distrihtion of E . willanwttei on 3 leaf positions in 4 vine canopy zones in the Madela vineyard. (A) Initial increase phase, 1.9 July. ( U ) Rapid increase phase, 8 August. (C) Peak phase, 23 August. (1)) I)ecbhring
phase, 6 September. Distribution Among Leaves Within Zones. ?:
pac@us and E. willawttei occiirred in greater abundance 011 specific lcaves within zones. Similar spatial and temporal patterns of the within-zone effect were observed in id1 3 vineyards. Significance levels of the leaf' age effect (and interaction between zones arid leaves) could not be calculated because the leaf position (or age) effect was the residiid in the nested model. It was possible, however, to calculate the proportion of spatial variation in mite abundance explained by the leaf age cffect. Leaf age effect in the Dinuba vineyard explained a large proportion of the spatial variation in T. PUcijiccws abundance, except during the initial growth phase when tlie leaf' position effect explained only 16% of the spatial variation in T. pac$cus abundance. During the remaining growth phases, the leaf age effect explained 36, 23, and 40% of the spatial variation in T pac@x~s abundance during the rapid increase, wak, and declining phases, respectively (Table 1 . In general, T. pnc$cus was inore abundant on midshoot leaves, with basal and terminal leaves supporting approximately similar mite densities during most of the population cycle (Fig. 2). These differences became negligible after the rapid decline in ?: pnc$cu.s densities. ?: pnci-
f
fic1~sdistribution
patterns 011 leavcs witliin m i w s in the Madera vineyard were similar to those observed in the Dinuba vineyard (Fig. 3 ) . The lcaf age cffect explained a large percentage of thc s p a tial variation in ?: pir$cus abundance throiighout most of the population cycle (Tab1e 2). Eotetrmychus willnmetttei distributioii patterns 011 specific leaf positions within xones wew siniilw to those of 'I: pciajicus. In tlie Dimitxi virieyar-tl,wlic-rci E. tuillmwttei deiisities were very low, the lcaf p s i tion effect explained 52-79% of tlie sp~itidvariat'1011 in mite abuiidance (Table 1). E. zuillairwttei wit< found primarily on basal leavw during the initid increase phase of pop1 dation growth, and ill eqilal numbers on basal arid middle leaves diii-ing tlw r ~ inainder of the sampling period E. zoiZZr~~u~t/c~i wx~ found rarely on tc~~nirial leaves i n the Dinuba v i i w yard, where its abundance was very low (Fig. 1U). In the Madera vineyard, E. willnnettei was fbiirid in greater abundance on mid-shoot leaves, whereas basal and terrninal leaves siipported equivalent ptwxsiltages of E. wiZZaw&?ttei densities (Fig, 4). Tliesc. among-leaf differences were most proiioiiiicctl during the rapid increase and peak phxm of abundance (Fig. 4 l3 and C). Differences lxtween I c a v c ~ declined later in the season, when only slightly Iiighcr
378
3CJ
120. 'Z l o o r
A.
__e_
ENVIRONMENTAL ENTOMOLOGY
Dinuba Exposed Shaded
120
Vol. 25, no. 2
Madera
B.
100
80 60 40
20
:r
1
.
1
.
I
C. 100
80
4
3
0
T
60 40
20
0 Date of sampling Fig. 5. Popidation distributions o f T. p a c i j ~ u s(A and B) and E . will"& (C and U) on sun-exposed (E) and shaded (J) leaves in the Dinuba and the Madera vineyards. Data points are mean mite densities per leaf; error bars are standard errors.
densities of E. willurwttei occurred on middle leaves during the declining phase of abundance (Fig. 4D). Distribution in Relation to Sunlight. At the time oiir saim les were taken, all selected leaves wcre classifieias sim-exposed or shaded. The resiilts indicated that the frequency of leaf exposure to sunlight in the 4 vine zones changed through the season. The most consistent patterns were obtained for the top and interior zones which were, respectively, the most and the least likely to be sun-exposed ($1 = 3.0-79.3, P < 0.05). Leaves in the soiith and north zones received a variable amount of sunlight but were equally likely to be sun-exposed through mid-July ($1 = 0-1.8, P > 0.05). Later in the season, leaves in south zones were inore likely to be sun-exposed than those in = 5.9-17.3, P < 0.05). We the north zones
also tested for the association between leaf exposure to sunlight and spider mite densities across dates and by date for each of the 3 vineyards. i? pacijicus occurred in reater densities on exposed compared with shade leaves across dates in each of the 3 vineyards (t = 7.14-15.4; df = 1,6742,536; P < 0.05); however, this pattern was observed only during the peak and declining phases of abundance (Fig. 5 A arid B), (t = 3.77-7.81; df = 178-214; P € 0.05). In contrast, E . willamettei occiirred in greater abundance on shaded than on exposed leaves only in tho Madera vineyard (across dates: t = 7.82; df = 2,536, P < 0.05). These differences were caused primarily by the greater abundance on shaded than on exposed leaves during the declining phase (Fig. 5 6 ; t = 3.443.88; df = 202-214; P < 0.05).
i
H ANNA ET
April 1996
AL.: SPATIAL A N D
TEMPORAL DYNAMICS
OF SPIDER
MITES
:379
Table 3. Between- and within-vine aggregation indices of 2'. pacificus and E . wiUumctei 13ehveen vines
Species n
T.pncij5~ur.v
35
E. roillanmtttei
33
'I: ~Jacijiclls E. willorrwttsi
35 33
'1
Within vines
rs! n Taylor power law 1.57 rt 0.07 0.93 290 1.33 t 0.05 0.96 200 Iwao patchiness regression 1.72 t 0.21 0.68 290 1 .L2 -+ 0.04 0.97 200
Intercept t SEM" Slope t SEMb 0.64t 0.08 0.58 t 0.05 11.8 t 5.4 4.36 i: 1.1
Intercept t SEM'
Slope i: SEMb
9
~~~
1.59 t 0.05 1.70 t 0.07
12.7 t 0.1 1.2.9 rt 3.5
1..76 t 0.02
0.97
1..58 t 0.02
0.96
2.78 t 0.06 1.88 rt 0.08
0.86 0.71
Intercepts are In ri Li)r Taylor power law and a for Iwao patchiness regression. Slopes are b for Taylor power law and /3 for Iwao patchiness regression.
Aggregation Indices. Aggregation indices based on the Taylor (1961) arid Iwao (1968) regression techniques were calculated for both I: pucifcus and E. willumettei. Data from all 3 vineyards were pooled in the analyses. The Taylor power law provided a highly significant fit of the relationship between variances and the means of between- and within-vine densities of both species (Table 3). Both a and b coefficients indicated that I: pacijk7~7 and E. willamettei were aggregated in their distributions between and within vines [ln(ap,, u ,, u , , , ~> ) 0 and hl,t,,b,,, b,,,,:,b,,, > 1, P < 0.0ff. The p and w subscripts denote the Coefficients for I: pucizfc~~s and E. willunwttei, respectively, and the e and g subscripts denote between- and withinvine coefficients. T pucijijicus was more aggregated (at the densities recorded in this study) than E. willumettei in both between- and within-vine distributions (hr,, > h,,, t = 2.75, df = 64, P < 0.01 and hi,, > b,L,g,t = 6.06, df = 486, P < 0.01; between-vine and within-vine u coefficients did not differ between species, P > 0.05). Both species were also significantly more aggregated within vines than between vines [In(ul,,) > ln(u!,,), t = 2.06, df = 331, 0.01 < P < O.OS;\n(n,,) > ln(uto,), t = 2.38, df 229, P < 0.01; hpg> b,,, t = 3.37, df = 331, P < 0.01; b,, > b,,,, t = 4.62, df = 229, P < 0.01). The Iwao regression model also indicated that both 2: pncijic7~sand E. willunzettei displayed aggregated distributions (Table 3). This model indicated that the basic unit of T pucijicus and E. willurnettei populations is based on groups of individiials [ln(ur.uIg: n,,,,, uw,) > 0, P < 0.051, and the size o f t lese asic pophation units did not change with regard to between- arid within-vine distributions [ln(a,,) = ln(ui>g), t = 0.18, df = 331, P > 0.05 and ln(a,,,,) = ln(a,), t = 1.16, df = 229, P > O.OS]. The basic-unit size of I: puciLficus was also similar to that of E. tvillumettei in both distribiitions [ln(up,) = ln(atos),t = 1.67, df = 64, P > 0.05; In(al,J = ln(u,,,J, t = 0.07, df = 486, P > 0.05). The Iwao regression also indicated that these basic units of I: pacifcus and E. willumettei populations were highly aggregated (b,,, b,, b,,, hlLg> 1, P < ().OS), and that both species were more aggregated within vines than between vines (b,,g > h),,,,t = 3.04, df = 331,P < 0.01 and b,Lg
> b,,, t = 2.97, df = 229, P < 0.05). T pacijicus was more aggregated than E. wil1nrncttec.i between > b,,, t = 2.95, df = 64, P < 0.05) and vines within vines (b,, > b, t = 7.95, df = 486, P < 0.05). Discussion
Our results indicated that the distributions of 1 ': puc$cus and E. willarnettei in Thompson Seedless vineyards followed predictable and dynamic spatial patterns. Differences in apparent vine vigor (associated with areas within vineyards) showed a substantial effect on within-vineyard distribution of 2: pucijic7~s.This species increased more rapidly and reached greater densities on low-vigor than on high-vigor vines in the Dinuba and Fresno vineyards (Fig. 1). In the Madera vineyard, where vines in both sainpling areas were of moderate to high vigor, differences in I: pucifcus densities between the 2 sampling areas were considerably less than we observed in the other 2 vineyards. Although these observations clearly suggested an association between vine vigor and relative size of 1T: pnc$cus infestations, the low number of replications does not allow 11s to make strong statistical inferences regarding the effect of vine vigor on mite abundance. In addition, we do not know the uncierlying factors causing these differences in mite abundance on vines of different vine vigor. It is known that reduced plant vigor can be the result of water and nutritional (and biotic) stresses which, at some levels, can lead to spider mite outbreaks (Jepson et al. 1975, Kliewer et al. 1983, Mattsori and Maack 1987, Holtier et al. 1988, Younginan et al. 1988). It is also possible that higher mite abundance on weak vines could be caused partly by mite concentration on the lower total leaf area on weak compared to vigorous vines. In contrast to the distribution of I: pacijic7~sin relation to apparent vine vigor, E. willnrrwttei densities were similar in both sampling areas of the Diniiba and Fresno vineyards (Fig. 1). In the Madera vineyard, E. willumnettei densities increased rapidly during the middle of tlie summer, but there was little difference between the 2 sampling areas where the vines were o f similar apparent vigor. Hecause of the low abundance of E. willurwttei i i i
380 the Dinuba and Fresno vineyards, it was not possible to siiggest R relatioiisliip between vine vigor and the abiindance of E. willnmettei Very little is I
