Population dynamics of aphids ( Aphididae ) on orange ( Citrus sinensis ‘ Thomson Navel ’ ) and mandarin ( Citrus reticulata ‘ Blanco ’ )

Citrus fruits represent one of the most important fruit productions worldwide. However, they suffer from a numerous constraints. Aphids are among the causes of the decline in the production of citrus. In this study, the diversity of citrus aphids and their seasonal occurrence were explored on orange and mandarin, during 2012 and 2013, in an orchard located in Skikda province (Algeria). In total, six different aphid species were found during two years. The most common species was Aphis spiraecola Patch, 1914. Climatic conditions had an important role in the infestation level by aphids. There were changes of aphid dynamics between the two years of the investigation. No aphids was recorded in six months in 2012 (January, June, July, August, September and December) and in three months in 2013 (January, February and August). Besides, the number of identified aphid species increased from two to five. On the other hand, the orange trees seemed to be the most infested host species.


INTRODUCTION
Citrus is an important fruit crop worldwide (Altaha et al., 2012).The citrus are cultivated from about 15° N to 35° S, between sea level and 1000 m, and are susceptible to frost unless the tree is dormant (Hill, 2008).They are grown in more than 100 countries all over the world, mainly in tropical and subtropical areas, where favorable soil and climatic conditions prevail.Citrus fruits are marketed mainly as fresh fruit or as processed juice (Peña et al., 2007).The citrus industry is one of the main components of Mediterranean agriculture, helping to guarantee incomes in underprivileged rural zones.Furthermore, citrus fruits provide the main source of vitamin C in the Mediterranean Basin, contributing to the general nutritional supply (Dambier et al., 2011).In Algeria, the citriculture has a strategic importance because it is a source to supply fresh fruits (Biche, 2012).However, it suffers since a few years from a considerable decline of the production and the quality of fruits.Among the causes of this decline, the pests such as the aphids keep a predominant place (Boulfekhar-Ramdani, 1998).The latter are serious pests of many agricultural crops.Therefore, a good understanding of their population dynamics is vitally important for crop protection (Kindlmann and Dixon, 2010).Little quantitative data are available on the population dynamics of citrus aphids because of sampling difficulties (Lapchin et al., 1994).Although aphids are dangerous pests, little is known about the aphid fauna of Algeria (Laamari et al., 2010).
In this study, we established an inventory of the aphid species present on different citrus trees in the Northeastern Algerian region of Skikda, based on the prospection work carried out during two years (2012 and 2013).This constitute a step towards exploring the diversity of the Algerian aphid fauna on citrus and their seasonal occurrence, and comparing the aphids associated with different citrus species in order to elaborate an appropriate control plan against these pests, and thus contributing to increase the production of citrus fruits.

MATERIALS AND METHODS
The orchard used in this study is located in the northeast of Algeria (Emjez Djich: Skikda) (36° 42' N, 6° 47' E), planted with approximately 15year-old citrus trees.The experimental area consisted of different citrus species and varieties, among them, we studied two species: 'Thomson Navel' sweet orange (Citrus sinensis L.) and 'Carval Hal' mandarin (C.reticulata Blanco), all grafted on Troyer citrange (C.sinensis (L.) Osb.× Poncirus trifoliata Raf.) rootstock.The trees were planted in rows with 5 m between rows.Intrarowtree spacing was also 5 m.From January 2012 to December 2013, samples were taken every month.
Orange and mandarin trees were almost under the same management regime.There were no other crops under the trees.In general, the orchard was cultivated with low pesticide application, and weeding was executed mechanically.
In 2012, five young citrus shoots from each variety were randomly collected.Lapchin et al. (1994) mentioned that sampling methods used to study the population dynamics of citrus aphids are generally based on counting infested shoots.Several authors (Yokomi and Tang, 1996;Kavallieratos et al., 2002;Boukhris-Bouhachem, 2011) have used young citrus shoots to study citrus aphid in different countries (Puerto Rico, Greece and Tunisia respectively).
In 2013, four leaves from each tree, and four trees from each variety were sampled.Thus, totally 32 leaves were examined for each sampling date.Fadamiro et al. (2008) and Yoldaş et al. (2011) also sampled leaves of citrus to study aphids in the United States and Turkey respectively.
On each sampling date, all the aphids (nymphs and adults regardless of species) and aphid mummies were counted on one shoot (in 2012) and on one leaf (in 2013).Individuals of dipteran predators were counted visually in the field only as larvae.Aphids were transferred in tubes to be conserved in ethanol and then identified in the laboratory using identification key of Stoetzel (1994) and those of Blackman and Eastop (2000).
Statistical analysis was done by using aphid densities obtained during study (24 sampling months).Data were analyzed by one-way ANOVA.All the statistical procedures were performed using SPSS for Windows 10.0.5 (SPSS, Inc.).Figures were drawn using Microsoft Excel 2007.

RESULTS
The aim of this work was to obtain knowledge on population dynamics of aphids on two citrus species in Algeria (Mediterranean region).Thus, samples (shoots or leaves) were taken every month during two years to assess and identify aphids.

Aphids and population dynamics
Six species of aphids in total were found on two citrus varieties in this study (Tables 1 and 2) including Aphis spiraecola, A. gossypii, A. nerii, A. craccivora, Myzus persicae and Macrosiphum euphorbiae.We identified four species on orange and four on mandarin.

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The infestation of orange and mandarin trees by aphids varied markedly among sampling dates (Figures 1 and 2).In 2012, densities of aphids decreased to zero in six months (January, June, July, August, September and December).However, no aphids was noted in 2013 in three months only (January, February and August).We remark also that the highest levels of infestation were concentrated in autumn and spring.About 35 % and 43 % of the total infestations was recorded on April and November 2012 respectively.In 2013, the biggest proportion of infestation was in April with about 36 % of the total infestation, and in September with approximately 42 %.

Difference of infestation between citrus species
During the first year of the study, ANOVA showed significant difference among the aphids that infested citrus species in April (231,6 aphids/shoot on orange 15,6 aphids/shoot on mandarin, F = 17,469, p = 0.003) and in November (240,8 and 65,8 aphids/shoot on orange and mandarin respectively, F = 5,907, p = 0.041).During the second year, ANOVA revealed significant differences of the infestation degree between the examined citrus species in June (11,06 aphids/leaf on orange and 0,75 aphid/leaf on mandarin, F = 4,395, p = 0.045), September (107,31 and 19,88 aphids/leaf on orange and mandarin respectively, F = 5,280, p = 0.029) and October (11,06 on orange and 0,75 on mandarin, F = 8,901, p = 0.006).No significant difference was recorded between aphids found on different citrus species during the other months in both years of the study.
'Thomson Navel' orange trees seemed to be the most infested cultivar with a peak of approximately 241 aphids/shoot observed on November 2012 (Figure 1), and 107 aphids/leaf noted on September 2013 (Figure 2).On the other hand, 'Carval Hal' mandarin appears the least infested with maximal value of about 66 aphids/shoot on November 2012 and 33 aphids/leaf on April 2013.Besides, no aphid was recorded on mandarin in twelve months among 24 sampling dates.In general, on the two hosts, we noted a progressive increasing in the mean number of aphids until reaching a first peak in spring, and then decreasing until 0 in some months of summer.
The infestation level increased once again to arrive to a second peak in the same year in autumn, and it reduced another time in winter.

Natural enemies of citrus aphids
Predators of citrus aphids found in this study were mainly larvae of syrphids.Its number was slightly higher than parasitoids, although it remains very limited (Tables 3 and 4).The distribution and abundance of predators varied among citrus species.The highest number was found on orange.
Concerning parasitoids, their number was much reduced.There were only two mummified individuals on mandarin (on February 2012) and three mummified aphids on orange (on May 2013).Grissa, 2010).However, in our study, it was not found.
We found four aphid species on 'Thomson Navel' sweet orange and four on 'Carvalhal' mandarin.Two among the identified aphids (Aphis spiraecola and Aphis gossypii) were present on the two citrus varieties.While Aphis craccivora and Macrosiphum euphorbiae were determined on orange only, Myzus persicae and Aphis nerii were observed on mandarin.Similarly, Ben Halima- Kamel and Ben Hamouda (2005) and Kavallieratos et al. (2007) mentioned differences between the species of aphids that infested orange, lemon, sour orange and clementine in Tunisia, and those attacking orange and mandarin in Greece respectively.
In this study, the aphids identified on orange were Aphis spiraecola, A. gossypii, A. craccivora and Macrosiphum euphorbiae.Four aphid species are found frequently on orange trees in the Mediterranean region, which are Aphis spiraecola, A. gossypii, Toxoptera aurantii and Myzus persicae (Loussert, 1989).Ben Halima- Kamel and Ben Hamouda (2005) found besides these species, Aphis fabae Scopoli, 1763 and A. craccivora Koch, 1854 on 'Maltaise' and 'Valencia' oranges in Tunisia.Nevertheless, Kamel (2010) identified only A. gossypii on both 'Baladi' and 'Navel' orange in Egypt; and Lopes et al. (2006) showed that seven species of aphids were found on orange in Portugal, among them A. hederae Kaltenbach, 1843, A. solanella Theobald, 1914and Anoecia haupti Börner, 1950.Differences among these studies are due to environmental conditions and the effect of the host variety.
The most common species observed in this survey was Aphis spiraecola, and with lower importance Aphis gossypii.The hierarchy of the species is highly variable from country to country. A. gossypii is generally not the most abundant species (Lapchin et al., 1994).Several studies showed that A. spiraecola and A. gossypii were among the most abundant species on citrus trees in Algeria (Franco et al., 2006), in Morocco (Belati and Belabed, 2014), in Tunisia (Ben Halima-Kamel and Ben Hamouda, 2005;Lebda Grissa, 2010), in Italy (Yahiaoui et al., 2009), in Spain (Marroquín et al., 2004) and in the United States (Powel et al., 2006;Fadamiro et al., 2008).In contrast, Toxoptera aurantii and T. citricida were the major aphids of orange, mandarin, and other Citrus spp. in the southeast of Asia (Bayhan et al., 2006).
On the other hand, the peak of infestation and number of auxiliaries were observed mainly in spring and in autumn.This coincides with moderate temperature and the production of new shoots appropriate for the reproduction of aphids and consequently the occurrence of predators and parasitoids.Many researches mentioned the peak of infestation by some aphids on citrus trees in spring (Saharaoui and Hemptinne, 2009;Kamel, 2010;Yoldaş et al., 2011;Mostefaoui et al., 2014).The within-year dynamics of aphids are largely determined by seasonal changes in host quality.Aphids do best when amino acids are actively translocated in the phloem.Thus on trees, the leaves are most suitable for aphids in spring and autumn (Kindlmann and Dixon, 2010).
The fluctuations of infestation during two years of study seem to be influenced by the changes of temperatures.Many authors mentioned the importance of temperature for the biology of aphids (Bayhan et al., 2006;Dixon and Hopkins, 2010;Harrington and Clark, 2010;Gao et al., 2013).Several generations follow each other during campaign in favorable conditions, elevated humidity and temperature between 20 and 25 °C; during big heats of summer, the infestations are less numerous (Bellabas, 2011).In addition, aphid reproduction and survival could be significantly reduced in summer if there are longer periods when temperature remains above the optimal threshold for aphid growth (Qureshi, 2010).Planet Earth has experienced many significant climatic changes.The increase in the concentration of greenhouse gases in the atmosphere induces an increase in temperature, which influences other climatic parameters (Ameixa, 2010).Benhamiche et al. (2014) cited some indicators of climate change in Algeria.Recent changes in climate, particularly warmer temperatures, have already begun to impact on biodiversity and ecosystems.Changes in species distributions, population sizes, the timing of reproduction and migration events, and in the frequency of pest and disease outbreaks have all been documented and linked to elevated temperatures (Bergant et al., 2005;Roy and Majerus, 2010).
In this study, the number of identified aphid species increased from two in 2012 to five in 2013.Hullé et al. (2010) demonstrated that temperature changes had repercussions for aphid diversity and population dynamics.At a pan-European scale, the EXAMINE observation network has provided evidence for an increase in the number of aphid species present over the last 30 years and for earlier spring flights.

Difference of infestation between citrus species
A difference in infestation between the tested citrus species was noted, with the biggest number on orange.In the same way, Marroquín et al. (2004) found, in their study in Spain, that clementine was the most attacked host species, followed by lemon, sweet orange, grapefruit, and satsuma.Additionally, Kavallieratos et al. (2002) mentioned differences between the number of aphids that infested orange and tangerine trees in Greece.
Winged aphids visit many plants and the selection is realized based on many physical and chemical factors.This latter include stimulating or inhibiting substances such as essential nutrients (amino acids and sugars) which launch usually a behavior effect (Herrbach, 1985); and plant secondary metabolites that are involved in insect-host interactions mainly by chemical derived substances and volatile substances (Bhatia et al., 2011).

Natural enemies of citrus aphids
Very limited number of predators and parasitoids were observed in this investigation.The predators found consisted of syrphid larvae.Some syrphids have larvae devour aphids (Sarthou and Speight, 2005;Biche, 2012).They feed on the aphid by piercing and sucking out the body contents, while holding the prey aloft (Sullivan, 2008).
Climate change can have diverse effects on natural enemies of pest species.The fitness of natural enemies can be altered in response to changes in herbivore quality and size induced by temperature and CO 2 effects on plants (Thomson et al., 2010).Majority insect life history traits are linked to temperature.All trophic levels stand to be affected by the increase in average global temperature: the herbivores, their natural enemies (parasitoids, predators and pathogens), and hyperparasitoids and tertiary predators (van Baaren et al., 2010).Hotter, longer summers would imply extended periods of prey scarcity for aphid natural enemies and could further impede their ability to survive this difficult season (Qureshi, 2010).
In conclusion, this study showed differences between infestation of orange and mandarin trees by aphids in northeastern Algeria (Skikda).The 'Thomson Navel' orange was more infested than 'Carval Hal' mandarin.Toxoptera citricida, which is an effective vector of tristeza virus, was not identified; but four among the six aphid species identified are reported to transmit this quarantine virus.Comparing same periods of the two years of study, the degree of infestation changed.Thus, more concentration should be given to orange trees during all seasons, especially in spring and autumn, for best controlling aphid pests.On the other hand, a reduced level of parasitoids and predators were remarked, that cannot limit the proliferation of aphids.Therefore, new adapted auxiliaries should be researched and used, to contribute to the increasing of citrus production.

ACKNOWLEDGMENTS
I would like to thank Slimani and Innal from the technical institute of fruit trees (ITAF Emjez Djich), for their assistance, as well as all persons who had helped me to realize this study.

Figure 1 :
Figure 1: Seasonal abundances of aphids in the studied citrus orchard from January to December 2012.Values indicate mean number of aphids (nymphs + adults) of 5 shoots.

Figure 2 :
Figure 2: Seasonal abundances of aphids in the studied citrus orchard from January to December 2013.Values indicate mean number of aphids (nymphs + adults) of 16 leaves.

Table 3 :
Number of predators of citrus aphids found on orange and mandarin trees in 2012.Values indicate total number of predators on 5 shoots.

Table 4 :
Number of predators of citrus aphids found on orange and mandarin trees in 2013.Values indicate total number of predators on 16 leaves.