Biology and life-table of Typhlodromus (Anthoseius) athenas (Acari: Phytoseiidae) fed with the Old World Date Mite, Oligonychus afrasiaticus (Acari: Tetranychidae)

The old world date mite Oligonychus afrasiaticus is an important spider mite pest of the date palms Phoenix dactylifera L. mostly in North Africa and the Middle East. A population of the predaceous mite Typhlodromus (Anthoseius) athenas has been recently found in Tunisia in association with a decrease of O. afrasiaticus densities. The objective of this paper was to assess the development and reproduction abilities of T. (A.) athenas on O. afrasiaticus under laboratory conditions at two temperatures: 27 and 32 °C. The results obtained show that females of T. (A.) athenas develop in 5 days at 27 °C and 4.1 days at 32 °C. The mean fecundity of T. (A.) athenas was 32.1 and 23.2 eggs per female at 27 and 32 °C, respectively. Life table parameters were estimated: the net reproductive rate (Ro) 27.9 and 17.9 eggs/female, the intrinsic rate of increase (rm) 0.322 and 0,344 female/female/day and the mean generation time (T) 10.3 and 8.4 days at 27 and 32 °C, respectively. At both temperatures tested, T. (A.) athenas intrinsic rate of increase was greater than that of O. afrasiaticus (rm = 0.213 at 32 °C, against rm = 0.166 day−1 at 27 °C). Typhlodromus (A.) athenas would be able to develop at a wide range of temperatures feeding on O. afrasiaticus and seems to be able to potentially control it.


Introduction
The tetranychid mite Oligonychus afrasiaticus (McGregor) (Acari: Tetranychidae) is one of the most important pests of date palm fruits around the world (Hussain 1969;Coudinand and Galvez 1976;Saleh and and Hosny 1979;Guessoum 1986;Edongali et al. 1988;Dhouibi 1991;Talhouk 1991;Baankoud and Basahih 2000;Elwan 2000;Palevsky et al. 2003Palevsky et al. , 2004. In several areas, O. afrasiaticusis controlled using sulphur (Coudinand and Galvez 1976;Guessoum 1986;Dhouibi 1991;Djerbi 1993;Palevsky et al. 2004). Similar programs have been conducted in California for many years for the control of the new world date mite Oligonychus pratensis (Banks) (Carpenter and Elmer 1978). Recently, the efficacy of sulphur treatments on O. pratensis and O. afrasiaticus has decreased dramatically (Gispert et al. 2001;Palevsky et al. 2004). Several other miticides were found to be effective (Al-Doghairi 2004;Palevsky et al. 2004). However, after application of pesticides, rates of residues exceed the minimum level recommended. In Saudi Arabia and other neighbouring countries, natives consume many dates; such high date consumption could lead to a high risk of exposure to pesticides, especially for children and other vulnerable population (Kamel et al. 2007). Considerable research efforts have been undertaken for finding an environmentally friendly way to control O. afrasiaticus, e.g., biological control. A limited number of species of enemies were reported to prey on O. afrasiaticus and O. pratensis. In California, the biological control of O.pratensis on date palms has been associated with the indigenous phytoseiid mite, Metaseiulus (Metaseiulus) flumenis (Chant), whose presence coincides with that of the pest during the summer months (Gispert et al. 2001). These authors suggested that inundative releases early in the season could delay pest population increase and reduce damage. In Algerian oases, native ladybird predators Cybocephalus palmarum Peyerimhoffand Exochomus flavipes (L.) were found feeding on O. afrasiaticus. In Algeria, the ladybird, Stethorus punctillum (Weise) was evaluated to control O. afrasiaticus in field and lab tests; the studies showed some promising results (Guessoum 1986;Idder and Pintureau 2008). Results obtained by Negm et al. (2014) Chaaban et al. (2011) mentioned that occurrence of phytoseiids on date palms was sporadic. Very few T. (A.) athenas were collected from pinnae of the Deglet Noor variety, but never founded in association with O. afrasiaticus on fruits of the same cultivar (Kreiter et al. 2008). Our field observations showed that in 2004 individuals of the predatory mite T. (A.) athenas were found to feed on O. afrasiaticus inhabiting fruits of Alig cultivar (Kreiter et al. 2008).
Considering that nothing is known on the behaviour and the relative importance of natural enemies of O.afrasiaticus, the objectives of the present study were: (i) to determine the population dynamicsof O. afrasiaticus and T. (A.) 'Kentichi','Bessr' and 'Deglet Noor'. This area is located in Segdoud, in the region of Tozeur (Djerid), in southwest Tunisia. Tozeur belongs to the superior bioclimatic Saharan upstairs with a temperate winter. The annual mean temperature is about 21.6°C, while 44°C in August and 2.5°C in January are the absolute maximum and minimum temperatures, respectively. The annual average rainfall is about 96 mm.
The oases in this area are continental with a mean planting space of 10 x 10 m. The principal vegetation was date palm trees with a predominance of the Deglet Noor variety and very few market-gardening or fodder practices for farmer consumption. The plot had historical outbreaks of O. afrasiaticus and in 2005 received two sprays of wettable sulphur on July 3 rd and August 15 th . In 2006, three wettable sulphur were applied on July 2 nd , July 24 th and August 1 st . Every year, the concentration of sulphur applied was about 100 -150 g per palm tree.
Sampling of mites on fruits. Date fruits of Alig variety were used to assess seasonal abundance of O. afrasiaticus and T. (A.) athenas.
Ten trees were selected for subsequent sampling. These trees were chosen at random to ensure coverage of the orchard, 2 trees located in the North, South, West, and East and in the middle of oases. Samples were taken at random of 10 date fruits per palm tree from bunches in the four locations (N, S, W and E). Samples were collected weekly from 30 April to 25 October in 2005 and from 15 April to 4 November in 2006. Mite populations were monitored twice a month until harvest during the rest of the two years of this study.
Date fruits were placed in plastic bags, kept in an insulated cooler with ice, brought to the laboratory and stored at a temperature of 4°C. Motile forms of O. afrasiaticus and phytoseiid mites were counted under a stereomicroscope. The phytoseiid mites were mounted on slides in Hoyer medium and identified at Montpellier SupAgro (France) as T. (A.) athenas.

Life history and life table studies
Stock culture. Date palm fruits (cv. Deglet Noor) that were well-infested with spider mites (O. afrasiaticus) were collected from the Segdoud oases in July 2007. With several hundred of spider mites of all stages from these dates, a stock colony was started on sorghum plants (Sorghum sp.) (Ben . Cultures were kept in a climatic chamber at 25 ± 1°C, 60 ± 10 % RH, 16:8 L: D. The initial population of T. (A.) athenas was collected from date fruits ofAlig cultivar in 2007. The stock culture of the predatory mite was maintained on O. afrasiaticus on sorghum leaves in a rearing chamber (25 ± 1°C, 60 ± 10 % RH, 16:8 L: D). The species was identified and morphologically characterized at Montpellier SupAgro, France.
The rearing units of T. (A.) athenas were similar to those described by Overmeer (1985): a piece of black hard plastic, on a water-saturated sponge in a plastic tray with water, with the borders covered with tissue paper, to ensure a constant water supply to the phytoseiids and to prevent escape. Twice a week, O. afrasiaticus infested sorghum leaves were added to each rearing unit (25 ± 1°C, 60 ± 10 % RH, 16:8 L:D).The colonies were started more than 3 months before the beginning of experiments, so that various generations had past under the rearing conditions. Experimental unit. All experiments were carried outon non-infested sorghum leavesused in the O. afrasiaticus rearing. The experimental unit was a modified Huffaker cell (Overmeer 1985). Fifty O. afrasiaticus females were placed in each cell and they were allowed to feed, oviposit and produce web for 5 days prior to the introduction of phytoseiids. Thus, a phytoseiids placed inside the cell had different O. afrasiaticus stages to prey upon and some webbing as oviposition site.
Developmental time of T. (A.) athenas. Thirty to forty female deutonymphs were placed individually in the arenas described above, together with two young adult males. Males that died or escaped were replaced with new ones. Every 8 hours the arenas were checked under a binocular microscope to see whether females had oviposited. Eggs were placed individually in each arena. This procedure was repeated 100 times (= 100 replicates). Every 8 hours the arenas with single eggs were checked, total development from egg until adulthood and survival of immature stages were recorded.
Reproductive parameters. Once females reached the adult stage, two males were added to each arena. Mites were observed every 8 hours until first oviposition. After that, females were transferred to new arenas and every 24 hours the number of eggs laid per female/arena/day was counted. The progeny was fed with O. afrasiaticus until they reached the adult stage. Mean preoviposition, oviposition and postoviposition periods, fertility, and sex ratio were calculated for each female. Experiments concerning reproductive parameters were conducted with 30 replicates in a room at 27 ± 1°C and 32 ± 1°C, RH was 60 ± 10 % and the photoperiod was 16:8 (L: D).
Data analyses. The life table was constructed considering the female cohort studied in this experiment. The net reproductive rate (Ro, mean number of female progeny produced by a single female during its mean lifetime, expressed in ♀/♀); gross reproductive rate (GRR, in ♀/♀), generation time (T, mean period between birth of the parents and that of the offspring, measured in days), intrinsic rate of increase (r m , in ♀/♀/day), finite rate of increase (λ, in ♀/♀/day) and doubling time (Dt, time for population to double, measured in days) were calculated using the method recommended by Birch (1948).
Data on developmental time, duration of female reproductive periods, and fecundity were analysed using one-way ANOVA. SPSS 10 (Statistical Package for the Social Sciences, version 10) was used for statistical analyses.

Life history and life table parameters
Developmental time of immature stages. The development times of T. (A.) athenas are given in Table 1. Development time decreased as temperature increased from 27 to 32°C for both females (F (1, 117) = 140.2, P<0.000) and males (F (1, 43) = 17.6, P<0.0001). The overall development time was shorter for males than for females. The immature survival rate was more  important at 27°C (87 %) than at 32°C (77 %) (Table1).
The sex ratio of T. (A.) athenas was biased towards females and varied with temperature from 85.5 % to 77.2 % of females at 27 and 32°C (  Table 3. The net reproductive rate (Ro) decreased from 27.9 females/female at 27°C to 17.9 females/female at 32°C. The intrinsic rate of natural increase (r m ) of T. (A.) athenas increased with temperature from 0.322 to 0.344 female/female/day. Population doubling times (D t ) was 2.15 and 2.01 days at 27°C and 32°C, respectively. The mean generation time (T) was 10.3 days at 27°C and 8.4 days at 32°C.

Discussion and conclusions
Field surveys. In this study, we have observed that fruit dates infestation of Alig cultivar was initiated between the third week of July and the first week of August, and increased rapidly during August. Densities remained relatively high until early September. It seems that excessive dry and heat conditions favour O. afrasiaticus populations, despite sulphur use  (Moraes et al. 2004;Kolokytha et al. 2011a, b). In Greece, this species was recorded on different plants and trees including citrus, olive, forest, rosaceous fruit trees, stone fruit trees, grapes, roses, carnations and oregano (Papadoulis et al. 2008). Typhlodromus (A.) athenas was not found on Alig fruits when O. afrasiaticus populations peaked. However, we observed its presence from mid-September to December. Palevsky et al. (2004) founded that phytoseiids were very scarce on untreated 'Medjool', 'Barhi' and 'Deglet Noor' date palms. Oligonychus afrasiaticus has been associated with the indigenous phytoseiid Neoseiulus bicaudus (Wainstein). The highest densities of the N. bicaudus were recorded in mid-June (ca. 2 mites/strand) but none was found from mid-July until the end of August, when pest populations peaked at ca. 15,000 mites/strand. Several hypotheses can explain the cause of the seasonal asynchrony between tetranychid pests and their natural enemies. First the dry conditions prevailing in the summer in the South of Tunisia prevented phytoseiids to establish themselves. Sabelis (1985) mentioned that among factors that determine the effectiveness of phytoseiids, humidity appears to be of major importance. The egg stage is especially sensitive to low humidity (Sabelis 1985;Ferrero et al. 2010). Differences in temperature thresholds between herbivorous pests and their natural enemies can allow pests to escape regulation early in the season and to reach damaging levels (Campbell et al. 1974;Hodek and Honek 1996;Obrycki and Tauber 1981). The second probable factor was the application of sulphur. Earlier findings revealed that sulphur can initially suppress pest mite populations (Croft 1990), but mite densities often increase after applications stopped, apparently because sulphur increases mortality and decreases fecundity of predators (Van de Vrie et al. 1972;Hanna et al. 1997;James et al. 2002;Costello 2007). Another reason for the scarcity of phytoseiids on fruits might be the presence of other food sources on the date palm. We founded T. (A.) athenas motile forms and eggs under the female shield of Parlatoria blanchardi (Targioni & Tozzetti) scales on pinnae. Generalist predators can feed on various species of Tetranychidae, Tarsonemidae, pollen, fungi, and other mites (McMurtry and Croft 1997;Luh and Croft 2001). Typhlodromus (A.) athenas was able to survive and reproduce on pollens of almond, apple, apricot, cherry, pear, plum, olive, and pollen (Kolokytha et al. 2011a). The predators' ability to survive and develop when reared exclusively on pollen at a wide range of temperatures may also be advantageous for the increase and built up of its population before the tetranychid population development (Kolokytha et al. 2011b).
Biological parameters of T. (A.) athenas. This study was the first to evaluate the biology and predatory efficiency of the phytoseiid predator, T. (A.) athenas on O. afrasiaticus. Under laboratory conditions, this species is able to complete development when fed exclusively on O. afrasiaticus at 27°C and 32°C.
The results of this study clearly show the effects of temperature on development time, longevity and fecundity on T. (A.) athenas feeding on O. afrasiaticus. Raising the temperature from 27°C to 32°C causes a significant reduction of 0.9 day on the total development time from egg to adult female. Many other studies performed on several species of phytoseiid showed that the duration of development decreases with temperature increase (Stenseth 1979;Camporese and Duso 1995;Reuveny et al. 1996;Lee and Ahn 2000;Ferrero et al. 2007;Kolokytha et al. 2011b). When T. (A.) athenas fed on all stages of O. afrasiaticus, the total developmental time was found shorter than for other Phytoseiid a species feeding on the same prey, such as C. negevi (9 and 7.93 days, respectively at 25°C and 35°C) (Negm et al. 2014) and Neoseiulus barkeri Hughes (9.63 and 9 days, respectively at 25°C and 35°C).
The immature survival is higher at 27°C (87 %) than at 32°C. Similar results have also been reported by Kolokytha et al. (2011b)  The longevity of T. (A.) athenas females decreased from 14.7 to 9.2 days respectively at 27°C to 32°C. The oviposition period follows the same pattern. This trend has been observed in several species of phytoseiid (Lee and Ahn 2000;Grissa 2003;Sengonca et al. 2003;Gharbi 2006;Ferrero et al. 2007). When Euseius scutalis (Athias-Henriot) was evaluated against O. afrasiaticus at 26°C and 70 % RH, adult female longevity was 19.4 days (Al-Shammery 2010). Negm et al. (2014) estimated that female longevity of C. negevi and N. barkeri reared on O. afrasiaticus were respectively 31.8 and 20.1 days and 35.7 and 27.4 days at 25°C and 35°C (35 % RH), respectively.
The sex ratio of phytoseiid mites is female biased (Amano and Chant 1977;Tanigoshi 1982). This agrees with the present findings.
Increasing temperature from 27 to 32°C had a significant effect on the female fecundity. The number of eggs increased from 23.2 at 27°C to 32.1 at 32°C. Total fecundity of T. (A.) athenas fed on O. afrasiaticus at 27°C is higher than that of E. scutalis (16.2 eggs at 26°C) (Al-Shammery 2010). However, the total fecundity value of T. (A.) athenas fed on O. afrasiaticus was quite similar to that ofother Phytoseiidae species feeding on O. afrasiaticus such as C. negevi (21.6 and 38 eggs respectively at 25°C and 35°C) and N. barkeri (18.8 and 34.8 eggs respectively at 25°C and 35°C) (Negm et al. 2014).
The intrinsic rate of natural increase (r m ) is an important parameter, describing the growth potential of a population under prevailing climatic and feeding conditions, as a reflection of the overall effects of temperature and food on the population's development, reproduction, and survival characteristics (Sabelis 1985;Krips et al. 1998;Roy et al. 2005).
In this study, the r m value of T.  (Tsolakis et al. 2016), Typhlodromus (Typhlodromus) pyri Scheuten on T. urticae (at 25°C, r m = 0.110, λ = 1.12, Ro = 11.5) (Puchalska and Kozak 2015), Typhlodromus (Anthoseius) bagdasarjani on T. urticae (at 25°C, r m = 0.129, λ = 1.14, Ro = 13.6) (Ganjisaffar et al. 2011), T. (A.) athenas on almond pollen (r m = 0.100, λ = 1.105, Ro = 15.71) (Kolokytha et al. 2011a,b). Negm et al. (2014) found that r m value of C.negevi and N. barkeri feeding on mobile life stages of O. afrasiaticus increased for both predators from 0.14 to 0.19 female/female/day for C. negevi and from 0.13 to 0.16 for N. barkeri from 25°C to 35°C. Al-Shammery (2010) reported that r m of E. scutalis fed onsame prey at 26°C and 70 % RH averaged 0.161. In biological control practices, r m value can be used for selecting promising biocontrol candidates on the basis of their reproductive potential and to predict the outcome of pest-natural enemy interactions (Jervis and Copland 1996). Theoretically, a predator that has a population growth rate equal to or greater than its prey should efficiently regulate the population of its prey (Sabelis 1992). Our laboratory results indicated that T. (A.) athenas would be efficient in biological control of O. afrasiaticus as Ben Chaaban et al. ( , 2011 demonstrated that feeding on 'Deglet Noor' dates, O .afrasiaticus presents the following demographic performances: r m = 0.213 at 32°C and r m = 0.166 at 27°C. The r m value of T. (A.) athenas in the present study was higher than that of its prey on dates at 27°C and 32°C. Nevertheless, T. (A.) athenas consistently not suppressed populations of O. afrasiaticus in fields. One hypothesis to explain that is that when the temperature exceeds 30°C, T. (A.) athenas could avoid the adverse effects of the high temperatures by relocating themselves on shaded sites at lower temperatures where their population could increase and when the preys are scarce. Kolokytha et al. (2011b) showed that, the optimal developmental of T. (A.) athenas fed on eggs and all stages of T. urticae was at around 31°C. Typhlodromus (A.) athenas showed seems to be a well-adapted organism to climatologic conditions of Greece and Mediterranean basin in general. When summer temperatures may rise above 35°C, biological control of spider mites with T. (A.) athenas may be applicable because spider mites can develop well under such high temperatures (Sabelis 1985).
In Tunisian climatic conditions, further experiments on the predation rate of T. (A.) athenas, in case of mass-releases, are needed in order to get a better understanding of its efficiency for a successful biological control of O. afrasiaticus. The predatory mite, Metaseiulus (Metaseiulus) flumenis, was mass released to control the Banks grass mite, the major pest of date palms in California, early in the season when there is a higher proportion of prey eggs which are the preferred life stages of the predator (Ganjisaffar and Perring 2015). Based on the negative impact of high temperatures on M. (M.) flumenis, its also recommend to make the releases during the cool morning hours and in the inner canopy of the trees or inside the date bunches with minimum exposure to the sun and heat (Ganjisaffar and Perring 2017).
Moreover, even if r m values are important, behavioural traits may be decisive in the success of a predator. For instance traits that allow predators to locate their prey fast, prey preference and alternative food occurrence may help understand the feeding behaviour of the phytoseiids traits like these remained unattended in our present study and need further investigations.