Species of Oligonychus infesting date palm cultivars in the Southern Arava Valley of Israel
ABSTRACT In a study of date fruit damage caused byOligonychus spp., we investigated whether the cultivar affects phenology, and on what hosts the mites over-winter. Samples were taken
from ‘Deglet Noor’, ‘Barhi’ and ‘Medjool’ trees from mid-April through mid-September during the years 1999–2002. In the ground-cover
mites were monitored by collecting Bermuda grass (Cynodon dactylon) under each sampling tree. Over 99% of the mites collected on Deglet Noor and Barhi fruit were identified asO. afrasiaticus. Mean population levels ofO. afrasiaticus reached ten mites or more (initiation of infestation) on Medjool in the second half of May, whereas on Deglet Noor this did
not occur before the first week of July. On Barhi the initiation of infestation varied between plots and years, ranging from
the second half of May to the beginning of July, but always occurred earlier than Deglet Noor. Mite populations on the pinnae
remained low from June through October, not exceeding seven mites per pinna, whereas on fruit strands they reached peak populations
of approximately 4000 mites per strand. The sex ratio (proportion of females) ofO. afrasiaticus on fruit of all three cultivars was highly female-biased, usually above 0.85. During winter,O. afrasiaticus was found on Bermuda grass in the orchard ground-cover as well as on fronds of all three cultivars.
- SourceAvailable from: Denise Navia[Show abstract] [Hide abstract]
ABSTRACT: Several mite species commonly attack cultivated citrus around the world. Up to 104 phytophagous species have been reported causing damage to leaves, buds and fruits, but only a dozen can be considered major pests requiring control measures. In recent years, several species have expanded their geographical range primarily due to the great increase in trade and travel worldwide, representing a threat to agriculture in many countries. Three spider mite species (Acari: Tetranychidae) have recently invaded the citrus-growing areas in the Mediterranean region and Latin America. The Oriental red mite, Eutetranychus orientalis (Klein), presumably from the Near East, was detected in southern Spain in 2001. The Texas citrus mite, Eutetranychus banksi (McGregor), is widely distributed in North, Central and South America. It was first reported in Europe in 1999 on citrus in Portugal; afterwards the mite invaded the citrus orchards in southern Spain. In Latin America, the Hindustan citrus mite, Schizotetranychus hindustanicus (Hirst), previously known only from citrus and other host plants in India, was reported causing significant damage to citrus leaves and fruits in Zulia, northwest Venezuela, in the late 1990s. Later, this mite species spread to the southeast being detected on lemon trees in the state of Roraima in northern Brazil in 2008. Whereas damage levels, population dynamics and control measures are relatively well know in the case of Oriental red mite and Texas citrus mite, our knowledge of S. hindustanicus is noticeably scant. In the present paper, information on pest status, seasonal trends and natural enemies in invaded areas is provided for these species, together with morphological data useful for identification. Because invasive species may evolve during the invasion process, comparison of behavior, damage and management options between native and invaded areas for these species will be useful for understanding the invader's success and their ability to colonize new regions.Experimental and Applied Acarology 11/2012; · 1.85 Impact Factor
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ABSTRACT: Many species are shifting their distributions due to climate change and to increasing international trade that allows dispersal of individuals across the globe. In the case of agricultural pests, such range shifts may heavily impact agriculture. Species distribution modelling may help to predict potential changes in pest distributions. However, these modelling strategies are subject to large uncertainties coming from different sources. Here we used the case of the tomato red spider mite (Tetranychus evansi), an invasive pest that affects some of the most important agricultural crops worldwide, to show how uncertainty may affect forecasts of the potential range of the species. We explored three aspects of uncertainty: (1) species prevalence; (2) modelling method; and (3) variability in environmental responses between mites belonging to two invasive clades of T. evansi. Consensus techniques were used to forecast the potential range of the species under current and two different climate change scenarios for 2080, and variance between model projections were mapped to identify regions of high uncertainty. We revealed large predictive variations linked to all factors, although prevalence had a greater influence than the statistical model once the best modelling strategies were selected. The major areas threatened under current conditions include tropical countries in South America and Africa, and temperate regions in North America, the Mediterranean basin and Australia. Under future scenarios, the threat shifts towards northern Europe and some other temperate regions in the Americas, whereas tropical regions in Africa present a reduced risk. Analysis of niche overlap suggests that the current differential distribution of mites of the two clades of T. evansi can be partially attributed to environmental niche differentiation. Overall this study shows how consensus strategies and analysis of niche overlap can be used jointly to draw conclusions on invasive threat considering different sources of uncertainty in species distribution modelling.PLoS ONE 01/2013; 8(6):e66445. · 3.73 Impact Factor
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ABSTRACT: The spider mite Tetranychus evansi is an emerging pest of solanaceous crops worldwide. Like many other emerging pests, its small size, confusing taxonomy, complex history of associations with humans, and propensity to start new populations from small inocula, make the study of its invasion biology difficult. Here, we use recent developments in Approximate Bayesian Computation (ABC) and variation in multi-locus genetic markers to reconstruct the complex historical demography of this cryptic invasive pest. By distinguishing among multiple pathways and timing of introductions, we find evidence for the "bridgehead effect", in which one invasion serves as source for subsequent invasions. Tetranychus evansi populations in Europe and Africa resulted from at least three independent introductions from South America and involved mites from two distinct sources in Brazil, corresponding to highly divergent mitochondrial DNA lineages. Mites from southwest Brazil (BR-SW) colonized the African continent, and from there Europe through two pathways in a "bridgehead" type pattern. One pathway resulted in a widespread invasion, not only to Europe, but also to other regions in Africa, southern Europe and eastern Asia. The second pathway involved the mixture with a second introduction from BR-SW leading to an admixed population in southern Spain. Admixture was also detected between invasive populations in Portugal. A third introduction from the Brazilian Atlantic region resulted in only a limited invasion in Europe. This study illustrates that ABC methods can provide insights into, and distinguish among, complex invasion scenarios. These processes are critical not only in understanding the biology of invasions, but also in refining management strategies for invasive species. For example, while reported observations of the mite and outbreaks in the invaded areas were largely consistent with estimates of geographical expansion from the ABC approach, historical observations failed to recognize the complex pathways involved and the corresponding effects on genetic diversity.PLoS ONE 01/2012; 7(4):e35601. · 3.73 Impact Factor
E. Palevsky et al. (2003) Phytoparasitica 31(2):xx-xx
Species of Oligonychus Infesting Date Palm Cultivars in
the Southern Arava Valley of Israel
?and U. Gerson
In a study of date fruit damage caused by Oligonychus spp., we investigated whether the
cultivar affects phenology, and on what hosts the mites over-winter. Samples were taken from
‘Deglet Noor’, ‘Barhi’ and ‘Medjool’ trees from mid-April through mid-September during
the years 1999–2002. In the ground-cover mites were monitored by collecting Bermuda
grass (Cynodon dactylon) under each sampling tree. Over 99% of the mites collected on
Deglet Noor and Barhi fruit were identified as O. afrasiaticus. Mean population levels of O.
afrasiaticus reached ten mites or more (initiation of infestation) on Medjool in the second
half of May, whereas on Deglet Noor this did not occur before the first week of July. On
Barhi the initiation of infestation varied between plots and years, ranging from the second
half of May to the beginning of July, but always occurred earlier than Deglet Noor. Mite
populations on the pinnae remained low from June through October, not exceeding seven
mites per pinna, whereas on fruit strands they reached peak populations of approximately
4000 mites per strand. The sex ratio (proportion of females) of O. afrasiaticus on fruit of all
three cultivars was highly female-biased, usually above 0.85. During winter, O. afrasiaticus
was found on Bermuda grass in the orchard ground-cover as well as on fronds of all three
KEY WORDS: Date palm; Phoenix dactylifera; date mites; Oligonychus afrasiaticus;
Oligonychus senegalensis; Medjool; Barhi; Deglet Noor; Arava Valley, Israel.
Approximately 1,700 hectares of date palms, Phoenix dactylifera L., are cultivated in
Israel along the Syrian-African rift, from Lake Kinneret (Sea of Galilee) in the north –
where the relative humidity is high, to the Red Sea in the south (the Southern Arava Valley,
or SAV) – which is extremely arid. Nine cultivars are grown, the most important being
‘Medjool’, ‘Hayany’ (primarily in the North), ‘Deglet Noor’ and ‘Barhi’ comprising 49%,
12%, 11% and 7% of the area, respectively (2).
Spider mite outbreaks on date palms first took place in Israel during the early 1980s
in the SAV, primarily on cv.Deglet Noor.
satisfactory control was achieved with one acaricidal treatment applied in late July.
Oligonychus senegalensis Gutierrez and Etienne (Acari: Tetranychidae) (then misidentified
Mites were observed during June, and
Received May 1, 2002; received in final form Dec. 2, 2002; http://www.phytoparasitica.org posting Jan. 30, 2003.
Dept. of Entomology, ARO, The Volcani Center, Bet Dagan 50250, Israel. *Corresponding author [e-mail:
Crop Protection Dept., Extension Service, Ministry of Agriculture and Rural Development, Be’er Sheva 84100,
Southern Arava R&D, M.P. Elot, 88820, Israel.
Dept. of Entomology, The Hebrew University of Jerusalem, Faculty of Agricultural, Food and Environmental
Quality Sciences, Rehovot 76100, Israel.
Phytoparasitica 31:2, 20031
as Oligonychus tylus Baker and Pritchard) occurred on the damaged fruit, whereas O.
afrasiaticus (McGregor) was found only in the plant ground-cover (5).
Senegal, O. senegalensis is a pest primarily of rice and sorghum (10). It has been recorded
as a pest on date palms in Israel only. Based on the pest status of O. afrasiaticus in
neighboring countries (3,8), Gerson et al. (5) suggested that this mite could become a
serious pest of date palms in Israel. Mite populations on date palms later dropped and
remained undetected in the SAV until 1992, when mite damage was again observed on date
fruit. Infestation levels increased during the 1990s despite prophylactic applications of
sulfur and dicofol. Mite outbreaks spread to all date palm plantations in the SAV, causing
substantial economic damage to the cvs. Deglet Noor, Barhi, ‘Khadrawy’ and Medjool.
Depending on duration of infestation and mite population level, fruit damage ranged from
minimal scarring of the cuticle, typically seen surrounding the points of contact between
fruits, to fruit surfaces that were completely scarred and sometimes cracked. Such damage
has not been observed in the northern, more humid areas of Israel.
Our goal in this study was to obtain answers to the following: what species of
Oligonychus were causing date fruit damage in Israel, and on which cultivars? Are the
species that occur on the trees the same as those found in the ground-cover? Is the
phenology of the mites affected by the cultivar and on what hosts do mites overwinter?
In India and
MATERIALS AND METHODS
Samples were taken twice a month from mid-April through mid-September, from
Deglet Noor, Barhi and Medjool trees (plots 1, 8-10 and 12; Table 1). All sampling trees
were kept pesticide-free throughout the study. At least five trees were sampled in each
plot, four fruit strands (each from a different fruit bunch) per tree. Ten pinnae from young
fronds of Deglet Noor and ten extra pinnae from one-year-old fronds were sampled in plots
1 and 9 during 2001. On Medjool and Barhi (also in 2001) pinnae were sampled similarly
from plots 10 and 12 early in the season, until mites were detected on the fruit. In plot 8,
mites were monitored by collecting
herbicides applied), under each sampling tree. Four additional trees that had no under-story
growth were also monitored in this plot during 1999 and 2000. For species identification
infestedfruit werecollectedorobtained from anadditional 11SAV plots, two in theCentral
Arava Valley and one located on the northwestern shore of the Dead Sea (Table 1). Over-
wintering populations in 2002 were monitored on 90-cm segments of date fronds (6) from
Deglet Noor, Barhi and Medjool trees (plots 9, 10 and 12, Table 1) bearing ca 60 pinnae
and on Bermuda grass (sample was standardized to ca 400 g, the average weight of a 90-cm
date frond segment).
Samples were collected in residential gardens on St. Augustine grass, Stenotaphrum
secundatum (Walt.) Kuntze, and on the palmate leaves of Washingtonia filifera (Linden)
Wendl. Fruit strands, pinnae, fronds and foliage were washed in 70% ethanol, the plant
matter was discarded, and the ethanol stored in plastic containers (1). The larger debris
was sieved out in the laboratory, mites were separated with a 150-mesh sieve and then
back-washed with 96% ethanol into a petri dish with a grid for counting. In the winter
of 2002, additional samples of date palm fronds and grasses were collected and the over-
wintering mites were extracted with Tullgren funnels. For the results presented in this
study, only the counts of the mature females and males of Oligonychus sp. were included,
the latter also being mounted. Males of O. afrasiaticus were distinguished from those of
? 100 g of Bermuda grass, Cynodon dactylon L. (no
2E. Palevsky et al.
TABLE 1. Sampling plots in the Southern and Central Arava Valley and the northwestern coast of
the Dead Sea (plots listed from south to north).
Farm Cultivar Tree age
Yair Exp. Stn.
?Plots 1-16, Southern Arava Valley; plots 17-18 Central Arava Valley; plot 19, northwestern coast of the Dead
?Young trees (less than 5 years old) did not bear commercial fruit.
O. senegalensis by their aedeagal structure (10). The dynamics of another tetranychid,
Eutetranychus palmatus, and a few species of phytoseiid predators found in the ethanol
wash, will be presented in a future manuscript.
To assess the variability of population levels of O. afrasiaticus between trees, the
coefficientof variation(standarddeviation/averagein %) wascalculatedfor thepopulations
found on the pinnae and strands of the sampling trees within each plot.
hypothesis that O. afrasiaticus was more dominant than O. senegalensis, 99% upper
confidence limits (99% CL) for the annual percentages of O. senegalensis were calculated
for each cultivar. To determine whether the timing of fruit infestation by this mite differed
among the cultivars, a variable was defined as the Julian date when pest populations
reached ten or more individuals. The 95% confidence intervals for this variable were
calculated to determine the cultivar and season variation. ANOVA was used to compare the
two extraction methods (Tullgren funnel vs ethanol wash) and to determine whether pest
populations were higher on any one of the hosts monitored, namely, the three date palm
cultivars and the ground-cover (done only for Tullgren funnel data, within each month).
To test the
Ondatefruit O. senegalensiswas foundonly in 1999, on onesamplingdateout of seven
on only one of the 12 trees sampled in each cultivar. On this sampling date, on Deglet Noor
and Barhi only two and three out of a sample of 118 and 102 males, respectively, were
identified as O. senegalensis. Thus, even for this sampling date the relative percentage and
upper 99% CL for O. senegalensis were, respectively, 1.7% and 7.0% for Deglet Noor and
2.9% and 9.6% for Barhi (Table 2). On the Bermuda grass ground-cover, O. afrasiaticus
was the dominant species in 2000 but not in 1999, possibly due to the small sample size
Phytoparasitica 31:2, 20033
TABLE 2. Percent of Oligonychus species and upper confidence limits (99% CL) of Oligonychus
senegalensis (Os) infesting fruits of two date cultivars in the Southern Arava Valley, Israel, in 1999
and 2000. Males of O. afrasiaticus (Oa) were distinguished from those of Os by their aedeagal
structure (n=number of males mounted).
Oligonychus species (%)
1999 Deglet Noor
TABLE 3. Percent of Oligonychus species and upper confidence limits (99% CL) of Oligonychus
senegalensis (Os) infesting date fruits of cv. Barhi and Bermuda grass in the ground cover in the
Southern Arava Valley, Israel, in 1999 and 2000. Males of O. afrasiaticus (Oa) were distinguished
from those of Os by their aedeagal structure (n= number of males mounted).
Oligonychus species (%)
collected in that year (99% CL, Table 3). On the Barhi trees above the patches of Bermuda
grass no specimens of O. senegalensis were recovered from the fruit samples in either year
(plot 8, Tables 1 and 3). All mites collected from fruit taken from 11 additional plots in the
SAV and two plots from the central Arava Valley were O. afrasiaticus. North of the Arava
Valley, mite damage typical of Oligonychus spp. was recorded from one plot located on the
northwestern shore of the Dead Sea, where only O. senegalensis was found.
During the years 1999–2001, O. afrasiaticus on Barhi fruit was consistently first
observed (in plot 8) in May or early June (Fig. 1a). Populations increased during June
and July, their levels exceeding 1700 mites per fruit strand in 1999. On Deglet Noor (plot
1), mites were first observed between mid-June and the beginning of July, but populations
did not increase until mid-July to early August (Fig. 2), peaking at levels
fruit strand in 2000.
In 2001, mites were monitored on Deglet Noor, Barhi and Medjool on the same farm in
adjacent plots (9, 10 and 12). The pests were found on Medjool from late April to the third
weekof August(ca16 weeks), onBarhifrom mid-Mayuntilthefirst weekinAugust(ca12
?2600 mites per
TABLE 4. Mean number of Oligonychus afrasiaticus females on frond segments (90 cm, ca 400 g)
of three date cultivars and on Bermuda grass (ca 400 g) in the Southern Arava Valley, winter 2002,
using two extraction methods.
Extraction methodMonth Number on frond segments/Bermuda grass
Barhi Deglet NoorMedjoolBermuda
Tullgren funnels5.2 a
?Within month and extraction method, figures followed by a common letter do not differ significantly at P
4E. Palevsky et al.
Fig. 1. Mean number of female Oligonychus afrasiaticus: (a) on date strands of cv. Barhi and (b)
on Bermuda grass in the ground cover (Table 1, Plot 8). For 1999, 2000 and 2001, coefficients of
variation (%) on strands ranged from 125–200, 48–200, and 31–224, and on Bermuda grass ranged
from 22–182, 18–182, and 64–140, respectively.
weeks) , and on Deglet Noor from the third week of June until the third week of September
(ca 13 weeks) (Fig. 3). High mite populations were seen on Medjool and Deglet Noor fruit
for 6 weeks and on Barhi for 8 weeks. No mites were detected on the pinnae, whether new
or one year old, before being found on fruit. On Deglet Noor, mite populations on pinnae
remained low from June through October, not exceeding seven per pinnae, whereas on fruit
strands populations peaked at approximately 4000 mites per strand (Fig. 4). The sex ratio
(proportion of females) of O. afrasiaticus on all three cultivars was highly female-biased,
usually being above 0.85 (Fig. 5).
Sporadic infestations were seen on young Medjool trees in the past, but damage to
maturetreeswasobservedforthefirsttimein2000. In2001, infestationsonyoungMedjool
trees occurred very early in the season (mid-May) throughout the SAV. During the same
period heavy infestations were also seen on young Barhi trees (plot 11) that were bearing
fruit for the first time.
In 2001 and 2002, mean population levels reached ten mites or more (initiation of
infestation) on Medjool in the second half of May, whereas on Deglet Noor this did not
Phytoparasitica 31:2, 20035
Fig. 2. Mean number of female Oligonychus afrasiaticus on date strands of cv. Deglet Noor (Table
1, Plot 1). Coefficients of variation (%) for 1999, 2000 and 2001 ranged from 53–283, 58–278, and
Fig. 3. Mean number of female Oligonychus afrasiaticus on date strands of three cultivars in 2001
(Table 1, Plots 9, 10 and 12, respectively). Coefficients of variation (%) on strands of Medjool, Barhi
and Deglet Noor ranged from 36-188, 35-199, and 50-211, respectively.
occur before the first week of July, the differences between these two cultivars being very
substantial (no overlap between 95% confidence intervals, Fig. 6a). On Barhi, the initiation
of infestation varied between plots and years, ranging from the second half of May to the
beginning of July, but it always occurred before Deglet Noor; these differences, however,
were considerable only in 1999 and 2001 (Fig. 6).
In the spring (April–May) of 2000 and 2001, mites were found in the ground-cover on
Bermuda grass before being noted on the fruit of Barhi trees (plot 8, Fig. 1). From May
through August, the presence of mites in the ground-cover coincided with that on the fruit.
During the winter of 2002, extremely low populations of O. afrasiaticus were detected
on the fronds of Medjool, Barhi and Deglet Noor, the maximum values being 0.2, 2.4 and
0.2 mites per 90-cm frond segments, respectively (Table 4). No significant differences
were found between the two extraction methods used. In February higher populations were
6 E. Palevsky et al.
Fig. 4. Mean number of female Oligonychus afrasiaticus on pinnae from young fronds and from
one-year-old fronds, and on date strands of cv. Deglet Noor in 2001 (Table 1, Plot 9). Coefficients
of variation (%) on pinnae from young fronds, from one-year-old fronds, and on date strands ranged
from 65–164, 46–143, and 50–211, respectively.
Fig. 5. Sex ratio of Oligonychus afrasiaticus collected from three date palm cultivars in 2001 (Table
1, Plots 9, 10 and 12, respectively).
extracted from Bermuda grass and Barhi than on Deglet Noor and Medjool. Populations
were higher in March on Bermuda grass than on the three date palm cultivars. In general,
mite populations seemed to be more consistent on Bermuda grass than on the frond
segments. In residential gardens at a distance of at least 2 km from the nearest commercial
date palm plots, O. afrasiaticus was found over-wintering on St. Augustine grass and on
the palmate leaves of W. filifera.
Phytoparasitica 31:2, 20037
Fig. 6. Mean Julian date (plus 95% confidence intervals) of initiation of mite infestation (when
populations reached ten mites or more): (a) on cvs. Deglet Noor, Barhi and Medjool in 2001 and
2002 (Table 1, Plots 9, 10 and 12, respectively); and (b) on cvs. Deglet Noor and Barhi in 1999–2001
(Table 1, Plots 1 and 8, respectively).
In this studywehaveshownthat thedominant mitepestof datefruit in theSouthern and
Central Arava Valley is O. afrasiaticus, a species well suited to hot and dry environments
(7). It is not clear why O. afrasiaticus has displaced O. senegalensis, but one explanation
could be the timing of the initial colonization. O. afrasiaticus appears to be a ground-cover
resident, and thus in place when date fruits become suitable for colonization. It was always
the first species to be found on the fruit and in the ground-cover. O. senegalensis, on the
other hand, was found on the ground-cover plants only in mid- to late-summer, concurrent
with peak date fruit infestation, suggesting that this mite has to colonize the date palm
system each year anew. The only site where O. senegalensis retains sole pest status on date
fruit in Israel is along the northwestern shore of the Dead Sea. Neither acarine pest has
been observed on date fruit north of that region.
We found that the cultivar had a substantial effect on the phenology of O. afrasiaticus.
Although variation in fruit set phenology seems to explain the discrepancy between
8 E. Palevsky et al.
Fig. 7. Timing of pre-infestation periods, from fruit set to initiation of infestation, and infestation
periods, on three date cultivars (Deglet Noor, Barhi and Medjool) in 2001 (Table 1, Plots 9, 10 and
Medjool and Barhi, it does not account for the extended pre-infestation period of Deglet
Noor (Fig. 7). Additional factors inherent in each cultivar, such as fruit chemistry and/or
morphology, could affect the timing of infestation. More research is required to determine
the role of these factors. By harvest time, populations decrease substantially on Medjool
and Deglet Noor but not on Barhi, since it is picked when the fruit are still yellow.
We were unable to find in the literature any quantitative studies on the phenology
and within-plot distribution of O. afrasiaticus on different date cultivars. Gispert et al.
(6), who studied the temporal and spatial distributions of O. pratensis on Deglet Noor,
showed fruit bunch infestation to be generally clumped, with heavily infested bunches
being adjacent to non-infested ones, and suggested that infestation resulted from random
arrivals of individual mites. Similar observations were made by Coudin and Galvez (4) in
Mauritania, who noted that only a few females were required for the colonization of O.
afrasiaticus. Our direct observations of clumped bunch infestation and high coefficients
of variation (similar to those found by Gispert et al., ref 6) on all three cultivars and in
the ground-cover (values listed in captions to Figs. 1–4) confirm that the distribution of O.
afrasiaticus is clumped.
During the winter months (November–March) O. pratensis was found on fronds and
on grasses in the ground-cover (3,6). Hussain (7) reported over-wintering O. afrasiaticus
on fibers and the frond bases, but no over-wintering mites on the pinnae, offshoots, or
the ground-cover. In two out of our three study years O. afrasiaticus was found in the
ground-cover in the spring (April–May) before being seen on the fruit. We also found
the pest without difficulty on different grasses during winter. On pinnae (sample size =
20 pinnae per tree), however, we did not find O. afrasiaticus during the spring before
they were observed on fruit. Using the Gispert et al. (6) methodology, which entailed
a threefold increase in the number of pinnae sampled per tree and the addition of the mid
vein, we detected populations on fronds of cvs. Barhi, Deglet Noor and Medjool, but their
levels were much lower than those reported by Gispert et al. (6). More quantitative studies
designed to monitor wind-borne movement of O. afrasiaticus from the ground-cover are
needed to determine the role that these mites play in the colonization of the date fruit.
The sex ratio of O. afrasiaticus found in this study was extremely female biased, being
Phytoparasitica 31:2, 20039
usually above 0.85, in contrast to a sex ratio of 0.3–0.83 (proportion of females) reported
by Coudin and Galvez (4), who cultured the mite on beans in the laboratory. Although it is
difficult to reconcile these rates with our data, we attribute them to three different reasons.
First, the rearing of the mite on an unnatural host plant under laboratory conditions. Then,
the high rate of sibmating that can be expected within the mites’ profuse webbing, which
is known to induce a strongly biased female sex ratio (9). Finally, females developing on
an exhausted food resource, caused by the dense mite populations, can shift their sex ratios
towards daughters (11,12). We hypothesize that the strongly female-biased sex ratio of
O. afrasiaticus allows the wind-borne mite to colonize fruit bunches efficiently, increase
rapidly and disperse to bunches of the same and different cultivars.
We would like to thank the three anonymous reviewers for their helpful comments. We are indebted to
Dr. Avraham Genizi for statistical guidance, Miri Zarhi for technical support, and Mr. Amnon Greenberg of
the Southern Arava R&D Center, and the date growers of the kibbutz farms Elot, Samar and Yotvata for their
invaluable assistance. This research was funded in part by a grant from the Chief Scientist of the Ministry of
Agriculture and Rural Development, the Fruit Board of Israel, and the Jewish National Fund. This manuscript is
contribution no. 501/03 of the Institute of Plant Protection, ARO, The Volcani Center, Israel.
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