Spatial patterns of western flower thrips (Thysanoptera: Thripidae) in apple orchards and associated fruit damage

2007·4.6 MB·English

by  Eugene Miliczky

J.Entomol. Soc.Brit.Columbia 104,December2007 25
Spatial patterns ofwestern flower thrips (Thysanoptera:
Thripidae) in apple orchards and associated fruit damage
EUGENE MILICZKY\ STEPHEN D. COCKFIELD^
ELIZABETH H. BEERS^ and DAVID R. HORTON^^
ABSTRACT
Western flower thrips, Frankliniella occidentalis (Pergande), is an economic pest of
apples in orchards of North America. Western flower thrips causes damage ("pansy
spot") to apples by its egglaying activities during the bloom and immediate post-bloom
periods. Difficulties in monitoring this pest and incomplete understanding ofits biology
during thebloomperiodhave complicated control efforts in apple orchards. Densities of
western flower thrips were monitored in seven (2003) or eight (2004) apple orchards at
each offourbloom stages; in each orchard, thrips counts in blossom clusters were esti-
mated at four to six distances into the orchard from an orchard edge that abutted native
sagebrush-steppe habitat. We hypothesized that numbers of thrips in blossoms would
decline with increasing distance along transects into orchards ifthe native habitat acted
as a source ofthrips. Thrips numbers in blossom clusters peaked at full bloom andpetal
fall. Densities showed a linear drop with increasing distance into the orchard, which we
interpreted as evidence that the native habitat adjacent to each orchard did indeed act as
a source ofthrips moving into the orchards. Pansy spot incidence declinedwith increas-
ing distance into the orchard. The major drop in damage occurred between the border
row trees and samples taken at the adjacent distance (nine m away), suggesting thatbor-
der rows adjacent to native habitats should be monitored with particular care. Regres-
sion analyses showed that damage and thrips density were positively correlated, albeit
with substantial levels ofunexplainedvariation in levels ofdamage.
KeyWords: Frankliniella occidentalis, apple orchards, pansy spot, sampling, damage
INTRODUCTION
Western flower thrips, Frankliniella which damage is most noticeable are the
occidentalis (Pergande) (Thysanoptera: lighter-skinned cultivars such as Granny
Thripidae), is a pest of stone and pome Smith, Mcintosh, or Rome Beauty (Madsen
fruits throughout North America (Terry and Jack 1966, Terry 1991). It is not com-
1991, Beers etal 1993, Pearsall and Myers pletely clear at what stage of blossom or
2000). In stone fruits, damage is caused by fiiiit development that damage occurs, al-
the feeding activities ofthe adult or imma- though several studies have suggested that
ture thrips, whereas in apple orchards dam- most ofthe egglaying in tissues where dam-
age occurs as females deposit eggs into the age is of concern (i.e., on the developing
developing fruit. The egglaying causes a fiaiitlet) occurs at late full bloom through
whitish discoloration of the skin surround- petal fall or just following petal fall
ing the oviposition scar, a condition known (Venables 1925, Childs 1927, Madsen and
as pansy spot (Venables 1925, Childs 1927, Jack 1966, Terry 1991). Densities of aduh
Madsen and Jack 1966). Apple varieties in thrips in blossoms appear to peak at full
' USDA-ARS, 5230KonnowacPassRoad,Wapato, WA 98951 USA
n^aWtacshehei,ngWtAonS9t8a8t0e1UnUivSeArsity,TreeFruitResearchandExtensionCenter, 11GON. WesternAvenue,We-
^Correspondingauthor.Tel.: [phone].E-mail: [email protected]
26 J.Entomol. Soc.Brit.Columbia 104,December2007
bloom and petal fall (Terry 1991; see also fruit were affected by distance into the or-
below). chard, particularly for orchards that abut
The use ofinsecticides to control west- native sagebrush-steppe rangeland. Studies
em flower thrips in apple orchards requires done in nectarine orchards suggested that
care to avoid harming pollinators and to edges oforchards adjacent to native habitat
minimize disruption ofintegrated mite con- experienced higher densities of western
trol (Bush 2000, Smith etal. 2006). Thus, it flower thrips than areas abutting other or-
would be beneficial to growers ifchemical chards (Pearsall and Myers 2000, 2001).
applications were made only ifdensities of The native habitat apparently was a source
thrips were high enough to warrant spray- of thrips moving into the nectarine or-
ing, and if applications were made only in chards. We tested whether thrips numbers
areas ofthe orchard actually requiring treat- and incidence of pansy spot within apple
ment. These needs have led to efforts to orchards declined with increasing distance
develop monitoring tools forwestern flower into the orchards, and assessed whether
thrips in orchards (Terry and DeGrandi- bloom stage affected this relationship. We
Hoffman 1988, Bradley and Mayer 1994), also analyzed the relationship between
as well as to studies that have defined the thrips densities and incidence ofpansy spot
relationship between bloom phenology and in apple fruit, as the quantitative relation-
timing ofdamage (Terry 1991). Objectives ship between thrips counts and damage to
of the current study were to determine apples has yet to be clearly established
whether counts of western flower thrips in (Beers a/. 1993).
apple blossoms and damage to developing
MATERIALS AND METHODS
Study sites and sampling methods. the major drop in thrips numbers occurred
Studies were done in April-May, 2003 and between 0 and 30 m, so two additional dis-
2004 at orchards located in northcentral and tances (9 and 18 m) were added in the 2004
southcentral Washington State. Three study. In all but two orchards, tree rows
(2003) or four (2004) orchards were moni- were parallel with the orchard borderbeing
tored near Brewster (Douglas County) in monitored, thus transects in most orchards
northcentral Washington, and an additional ranperpendicularto thetree rows.
four orchards (both years) were monitored Western flower thrips were sampled at
in western Yakima (Yakima County) lo- each offour bloom stages: pink, open king
cated in southcentral Washington State. In bloom, full bloom (80% of flowers open),
2003, cultivars were Granny Smith (four and petal fall (80% offlowers without pet-
orchards) and Red Delicious (three or- als). At each bloom stage and at each or-
chards); in 2004, cultivars were Granny chard, 25 flower clusters (each with five to
Smith (two orchards), Fuji (one orchard). six flowers) were clipped from 15-25 trees
Cameo (two orchards), and Red Delicious per distance; trees and flower clusters were
(three orchards). All orchards had at least chosen haphazardly. Height ofsamples was
one edge adjacent to native sagebrush- 1.5 to 2 m. The 25 clusters for each sample
steppe habitat. Insecticides were not applied were placed in a self-sealing plastic bag,
during the studies. stored in an ice chest, and taken to the labo-
Thrips densities and fruit damage were ratory for examination. At the laboratory,
monitored along transects at four (2003) or thrips were extracted by immersing and
six (2004) distances into each orchard, from agitating the blossoms in solutions ofwater
an edge in each orchard that abutted native and dishwashing soap. The tips of un-
habitat. In 2003, the four distances were 0 opened flowers in pink and king bloom
m
(border row oftrees), 30, 60, and 90 into samples were cut awaybefore immersion to
the orchard. The 2003 data suggested that allow the solution to freely circulate
.
J.Entomol. Soc. Brit.Columbia 104,December2007 27
through the flower. Extracted thrips were among distances and among bloom stages
stored in 70% ethanol until they were iden- using a doubly repeated measures analysis
tified and counted beneath a dissecting mi- of variance (due to repeated sampling in
croscope. For each sample of 25 clusters, space [=distance] and time [=bloom stage]
we recorded numbers of adult female F. within each orchard). The data were trans-
occidentalis formed by ln(x+l) (Zar 1974) and analyzed
Thrips damage was assessed in each using PROC GLM (SAS Institute 2002).
orchard at all distances to determine if a Greenhouse-Geisser adjusted P-statistics
correlation existed between damage and are presented throughout. To assess the
distance into the orchard. We limited fruit relationship between thrips density and
examination to the 2004 study. Developing distance into the orchard, we examined
apples varied from 1.9 to 3.2 cm in diame- linear and quadratic effects using the Poly-
ter at the time samples were taken, and nomial command in the Repeated statement
pansy spot was readily visible on all culti- of PROC GLM (SAS Institute 2002). The
vars (pansy spot disappears in red varieties effect of distance on proportion of fruit
as the fruit color intensifies later in the showing pansy spot was assessed using
ANOVA
growing season). At each of the six dis- repeated measures including only
tances in each orchard, 250 fruitlets were distance as the repeated factor. Proportions
haphazardly selected from 15-25 trees (10- were arcsine transformed before analysis
15 apples per tree) and examined for pansy (Zar 1974). Linear regression was used to
spot. Height ofsamples was again 1.5 to 2 test whetherthe percentage offruit showing
m. Two people did all ofthe fruit examina- damage depended upon thrips densities in
tion (each person sampling four orchards) the bloom samples. Separate regressions
to minimize variation among observers in were fitted for the four bloom stages, and
assessing forthepresence ofpansy spot. for thrips counts summed over the four
Data Analysis. Mean number ofthrips bloom stages.
per 25 blossom clusters was compared
RESULTS
We observed substantial variation spot declined with distance (F= 3.8; df= 5,
among orchards in counts of thrips both 35; P = 0.027; Fig. 3). Linear and quadratic
years (Figs. 1-2). Thrips densities were sig- effects were examined, and suggested that
nificantly affected by bloom stage both damage declined as a quadratic frinction of
years (Table 1). Densities were higher dur- distance into the orchard (linear: 5.1; df
ing the frill bloom period (mean =13.6 and = 1, 7; P= 0.048; quadratic: F= 6.0; df= 1,
14.4 thrips per 25 clusters in 2003 and 1; P^ 0.04). A second analysis was done in
2004, respectively; sample distances which 5 profile contrasts were extracted to
pooled) andpetal fall period(14.4 and 16.3) compare damage at consecutive distances
than in the pink (1.3 and 2.4) and king (i.e., 0 vs 9 m, 9 vs 18 m, 18 vs 30 m, 30 vs
bloom (4.6 and 7.0) stages. The interaction 60 m, 60 vs 90 m). The analysis suggested
between distance and bloom stage was not that the primary decline in damage occurred
significant either year (Table 1), so it is between 0 and 9 m (F= 14.8; df= 1, 7; P=
appropriate to examine the main effects of 0.006; the other 4 contrasts were non-
distance to determine whetherthrips density significant).
changed with location in the orchard (Figs. Percentage damage increased with in-
1E-2E). Distance effects were highly sig- creasing thrips density, but with substantial
nificantbothyears (Table 1). Thrips density scatter in the data (Fig. 4). Data for one of
declined as a linear function of distance the eight orchards (labeled "Orchard 3" in
fromthenativehabitatbothyears (Table 1). Fig. 4) fell consistently out of the scatter
Mean percentage offioiit showing pansy shown in the other data. Regression models
»
28 J.Entomol. Soc.Brit.Columbia 104,December2007
16 -
-(—D
<n 0 -
o
Full bloom C
0 60
g 40 O o
^
20
1
0
%75 Petalfall p
H
50
25
0
Mean
E,
36
24
12 -
0
0 15 30 45 60 75 90
Distance (meters)
Figure 1. Number ofadult female western flower thrips per 25 flower clusters in four bloom
stages as a function ofdistance into the orchard (panels A-D); 2003 data. Panel E shows thrips
counts averaged over bloom stage. Open circles show results for each orchard (N = 7 orchards;
some data points overlap), while filled symbols depict numbers averaged over orchard. See
ANOVA
Table 1 for results.
were fitted both with and without the Or- which the Orchard 3 data had been ex-
chard 3 data (Table 2). In all models, the cluded (Table 2); r^ values were low in all
intercept term differed significantly from models (<0.26), reflecting the large scatter
zero, indicating that the models predicted in the data. Regression models fitted to
some level ofdamage at a measured density counts summed over bloom stage were sig-
of zero thrips (see also the scatter plots in nificant (Fig. 5), albeit again with substan-
Fig. 4), Slope coefficients were signifi- tial scatteraroundthe fittedlines.
cantly different from zero in all models for
J.Entomol.Soc.Brit.Columbia 104,December2007 29
18 2004 Pink A.
o
12
o
6
§9 8
0 - 8
King bloom B.
30
20
10
{fi 0
o
90
O
60
qz
lO
CM 30
i—
0)
Q. 0
Q. 75 Petal fall
50
25
0
0 15 30 45 60 75 90
Distance (meters)
Figure 2. Number ofadult female western flower thrips per 25 flower clusters in four bloom
stages as a function ofdistance into the orchard (panels A-D); 2004 data. Panel E shows thrips
counts averaged over bloom stage. Open circles show results for each orchard (N = 8 orchards;
some data points overlap), while filled symbols depict numbers averaged over orchard. See
ANOVA
Table 1 for results.
DISCUSSION
Densities of western flower thrips in with increasing distance into orchards, from
apple blossoms peaked between the full transects beginning at the edge of orchard
bloom and petal fall stages (Figs. 1-2), as that abutted native habitat. Other studies
shown also in other sampling studies done done in crop or commercial forest systems
in apple orchards ofwesternNorth America have shown that non-agricultural habitats
(Madsen and Jack 1966, Terry 1991). We adjacent to crops may be a source of pest
observed a linear decline in thrips densities thrips colonizing the crop, leading often to
30 J.Entomol. Soc.Brit.Columbia 104,December2007
Table 1.
Results fromrepeatedmeasures ANOVAassessingeffects ofbloom stageanddistanceintothe
orchard on thrips densities in bloom clusters (Figs. 1-2). Greenhouse-Geisser adjusted P-
statistics shown for main effects and interaction terms. Linear and quadratic contrasts are ex-
tractedforthe distance effects.
2003 2004
df F P df F P
Bloom stage 3,18 5.3 0.042 3,21 9.3 0.003
Distance 3,18 8.2 0.009 5,35 5.7 0.007
Linear 1,6 15.4 0.008 1,7 10.0 0.02
Quadratic 1,6 0.7 0.45 1,7 3.9 0.09
StageX distance 9,54 1.1 0.36 15,105 1.0 0.42
15 30 45 60 75 90
Distance (meters)
Figure 3. Percentage offruit showing pansy spot as a function ofdistance into orchard. Open
symbols show results for each orchard (N = 8 orchards; some data points overlap), while filled
symbols show damage averaged over orchard. 250 fruit examined at each distance for each or-
chard.
high densities of thrips at field edges suggest that apple growers may in some
(Lewis 1973). Native habitat adjacent to circumstances be able to limit spray appli-
nectarine orchards was a source ofwestern cations toborderrows.
flowerthrips colonizing orchards in western A major impediment to developing con-
Canada (Pearsall and Myers 2000, 2001). trol protocols for western flower thrips in
Orchards surrounded by other orchards apple orchards has been our lack of infor-
tendedto have lowerdensities ofthripsthan mation regarding the quantitative relation-
orchards adjacent to native habitat (Pearsall ship between adult thrips density and egg
and Myers 2000). These studies failed to density in blossoms, orbetween density and
describe quantitatively how thrips densities actual damage (Terry 1991, Beers et al
or damage to fiiiit declined with increasing 1993). We showed that percentage damage
distance into orchards. In our study, pat- was positively correlated with thrips counts
terns in damage paralleledpatterns in densi- inblossoms (Figs. 4-5). However, there was
ties of thrips. Incidence of pansy spot de- substantial unexplained variation in the
clined with increasing distance into or- data, at all bloom stages (Figs. 4-5). More-
chards, with the major drop in damage lev- over, fruit damage often occurred even in
m
els occurring between 0 and 9 into the the absence ofthrips in samples (see scatter
orchard. The density and damage results plots in Fig. 4 and regression models in
J.Entomol. Soc.Brit.Columbia 104,December2007 31
16
• WithOrchard 3
12
T WithoutOrchard 3
o
8
o 4
Q.
<n
c
12 16
16 B.
12
8 o
o o
4 —
cP^9^ o o
§ o King bloom
0
,
a 4
I 0 20 40 60 80 100
0 20 40 60 80
Thrips per25 flowerclusters
Figure 4. Scatter plot and linear regressions showing relationship between numbers of adult
female western flower thrips in blossom clusters and percentage of 250 fruit showing pansy
spot. Data from Orchard 3 are solid dots, other data are circles. Regressions fitted with (solid
lines) andwithout(dashed lines) data for Orchard 3 (a RedDelicious orchard located in western
Yakima). SeeTable2 forregression statistics.
Table 2). We observed a damage level of our samples to the four bloom stages be-
4.4% at one distance in an orchard for tween pink and petal fall means that we
which no thrips were collected in any ofthe failed to sample during other stages of ap-
four sampling periods (Fig. 5). These re- ple development in which the fruit is sus-
sults suggest one or more ofthe following: ceptible to thrips; (c) our method for ex-
(a) our sample sizes were too small ortaken tracting thrips from blossoms missed a sig-
too infrequently to adequately estimate nificantnumberofthrips.
thrips densities in the orchard; (b) limiting One Red Delicious orchard ("Orchard
32 J.Entomol. Soc.Brit.Columbia 104,December2007
Table2.
Results from linearregressions relating percentage offruithaving pansy spot to thrips density.
Separatemodels fittedwith andwithoutresults forOrchard3. Scatterplots andregression lines
shownin Figures4and5.
Bloom stage Include Orchard3 Intercept olope rD r
rmKvrlg. 1es 4.JO U.1j4 U.jU U.Uz
iNO U.ZUj Un.1Z11
KincThlnnm TFia 4R^ Yes 4.89 0.013 0.88 0.01
No 2.91 0.078 0.03 0.11
Full bloom(Fig. 4C) Yes 3.53 0.073 0.004 0.17
No 2.89 0.036 0.003 0.20
Petal fall (Fig. 4D) Yes 3.11 0.118 0.0002 0.26
No 2.87 0.049 0.003 0.20
Cumulative (Fig. 5) Yes 3.28 0.037 0.003 0.18
No 2.72 0.019 0.001 0.23
' All interceptterms significantly{P< 0.05) different fromzero.
0 50 100 150
Cumulative thrips numbers
Figure 5. Scatter plot and linear regressions showing relationship between numbers ofadult
female western flower thrips in blossom clusters (summed over the four bloom stages) and per-
centage of250 fruit showing pansy spot. Data from Orchard 3 are solid dots, other data are cir-
cles. Regressions fitted with (solid lines) and without (dashed lines) data for Orchard 3 (a Red
Delicious orchardlocatedinwesternYakima). SeeTable2 forregression statistics.
3" in Fig. 4) had damage estimates well orchards supported potentially large num-
above levels seen in the remaining seven bers of western flower thrips. Bush (2000)
orchards, for unknown reasons. This or- cautioned against destroying flowering dan-
chard had stands of feral alfalfa in the un- delions in orchard understory during the
derstory, and it is possible that the alfalfa bloomperiod in apples, as thrips occupying
acted as a source of western flower thrips dandelion flowers may then move into the
moving into the trees whenever the under- trees and cause damage to developing fruit.
story was mowed. Venables (1925) stated It is possible that the alfalfa in Orchard 3
that alfalfa cover crops in apples may affect acted as an important source of western
incidence ofpansy spot. Pearsall andMyers flower thrips moving onto developing fruit.
(2000) showed that alfalfa and other flow- In any case, data for Orchard 3 added sub-
ering plants in the understory of nectarine stantially to the scatter ofpoints surround-
.
J.Entomol. Soc.Brit.Columbia 104,December2007 33
ing the regression lines (Figs. 4-5, Table 2). tween damage and thrips densities in apple
These results again reinforce observations blossoms can be difficult to predict (Terry
made elsewhere that the relationship be- 1991).
ACKNOWLEDGEMENTS
We thank Merilee Bayer for field assis- tained from the Washington Tree Fruit Re-
tance. Mike Bush and Wee Yee provided search Commission and the Washington
reviews of an early version of this manu- State Commission on Pesticide Registra-
script. Funding for these studies was ob- tion.
REFERENCES
Beers, E.H.,J.F. Brunner, M.J. WillettandG.M. Warner(eds). 1993. Orchardpestmanagement: aresource
bookforthePacificNorthwest. GoodFruitGrower,Yakima,WA.
Bradley, S.J. andD.F. Mayer. 1994. Evaluationofmonitoringmethods forwesternflower thrips, Frank-
Uniella occidentalis (Thysanoptera: Thripidae), during the blossom period of 'Granny Smith' apples.
JournaloftheEntomological SocietyofBritishColumbia91: 63-68.
Bush,M.R. 2000. Thoseannoyinglittle flowerthrips. GoodFruitGrower51 (7): 62.
Childs, L. 1927. Two species ofthrips injurious to apples in the Pacific Northwest. Journal ofEconomic
Entomology20: 805-808.
Lewis,T. 1973.Thrips: theirbiology, ecologyandeconomic importance.AcademicPress, NY.
Madsen, H.F. and I.D. Jack. 1966. Therelation ofthrips to pansy spot on apples. The CanadianEntomolo-
gist98: 903-908.
Pearsall, I.A. and J.H. Myers. 2000. Population dynamics ofwestern flower thrips (Thysanoptera: Thripi-
dae)innectarineorchards inBritishColumbia.JournalofEconomicEntomology93: 264-275.
Pearsall, I.A. and J.H. Myers. 2001. Spatial and temporal patterns of dispersal ofwestern flower thrips
(Thysanoptera: Thripidae) in nectarine orchards in British Columbia. Journal ofEconomic Entomology
94: 831-843.
SASInstitute. 2002. SAS 9.1 forWindows. SAS InstituteInc.,Gary,NC.
Smith,T.J.,J.E. Dunley, E.H. Beers,J.F. Brunner,G.G. Grove, C.-I.Xiao,D. Elfving,F.J. Peryea, R
Parker, M. Bush, C. Daniels, T. Maxwell, S. Foss and S. Martin. 2006. 2006 Crop protection guide for
treefruits inWashington. WashingtonStateUniversityCooperativeExtension,Pullman, WA.
Terry,L.I. \99\.Frankliniellaoccidentalis(Thysanoptera: Thripidae)ovipositioninapple buds: role of
bloomstate,blossomphenology,andpopulationdensity. EnvironmentalEntomology20: 1568-1576.
Terry, L.I. andG. DeGrandi-Hoffinan. 1988. Monitoringwesternflowerthrips(Thysanoptera: Thripidae) in
"GrannySmith"appleblossomclusters.TheCanadianEntomologist 120: 1003-1016.
Venables, E.P. 1925. Pansy spots on apples (a peculiar form ofthrips injury). Proceedings ofthe Entomo-
logical SocietyofBritishColumbia22: 9-12.
Zar,J.H. 1974. Biostatisticalanalysis. Prentice-Hall,NJ.
J.Entomol. Soc.Brit.Columbia 104,December2007