Among phytophagous insects, the Cicadomorpha are important economically because they damage crops by sucking plant sap and by transmitting plant pathogens, such as Spiroplasma citri and Xylella fastidiosa to citrus. In Argentina little knowledge exists about this subject. The aim of this work was to study the diversity of Cicadomorpha associated with citrus orchards in Entre Ríos province, and their seasonal fluctuation in relation with climatic and phenological conditions. A total of 1,554 specimens belonging to 28 species of Cicadomorpha were collected with yellow sticky traps in sweet orange (Citrus × sinensis (L.) Osbeck) and tangerine (Citrus unshiu Marc) orchards. The Shannon index and the Simpson index suggested a similar trend in the distribution of the dominant species in both crops. In the orange orchard, Cicadomorpha populations increased in the summer coincidently with temperature increases. On the other hand, a significant increase in abundance during the winter months was coincident with increase of early sprouts of the citrus plants. Entre Ríos province represents a new distribution record for 13 species. Tangerine is a newly recorded host-plant for 16 species studied, and eight species are reported for the first time on ‘Valencia Late’ orange.
The citrus (Sapindales: Rutaceae) industry is the most economically important activity in the world fruit market. The major producers of citrus are Brazil, China and the United States. Argentina ranks ninth and its geographical position has allowed the country to become a supplier of fresh citrus fruit during the boreal summer. Citrus orchards are located in 2 regions: Northwest (NOA) (Jujuy, Salta, Catamarca and Tucumán provinces) and Northeast (NEA) (Entre Ríos, Corrientes and Misiones provinces). The species grown mainly in NEA are sweet orange (Citrus × sinensis (L.) Osbeck) and tangerine (Citrus unshiu Marc) trees, which constitute 90% of agricultural production in this region (Segovia 2003; FEDERCITRUS 2011).
Among the most biodiverse lineages of phytophagous insects, the hemipteran infraorder Cicadomorpha comprises the superfamilies Cicadoidea (cicadas), Cercopoidea (spittlebugs and froghoppers) and Membracoidea (leafhoppers and treehoppers). To date, approximately 30,000 cicadomorphan species have been described world-wide (Dietrich 2002; Cryan 2005).
These insects have received much attention because of their economic importance. They can damage crops by sucking plant sap and transmitting plant disease organisms such as viruses, mollicutes and bacteria (Nielson 1968; Purcell 1985).
The causal agent of Citrus Stubborn Disease (CSD) is Spiroplasma citri, a phloem-limited mollicute that infects most citrus species as well as a wide range of non-rutaceous plant species. It is distributed in the United States, Northern Africa, the Mediterranean countries and Southeast Asia. CSD is naturally transmitted by leafhopper vectors such as Circulifer tenellus Baker and species of genus Scaphytopius Ball in the United States (Nejat et al. 2011).
Xylella fastidiosa Wells et al. 1987, is a xylemlimited bacterium that cause diseases such as ‘Pierce's Disease’ in grape (Vitis vinifera L.), ‘Phony Peach Disease’, ‘Citrus Variegated Chlorosis’ (CVC), and ‘Leaf Scorch’ in coffee, oleander, mulberry, oak, and maple, among others (Hopkins & Purcell 2002). It has a wider distribution in the Americas, from the United States to Argentina. Insect dissemination of X. fastidiosa is possible by vector insects belonging to the families Cicadellidae (leafhoppers) and Cercopidae (spittlebugs) (Redak et al. 2004). Recently species of Membracidae (treehoppers) have also tested DNA positive for X. fastidiosa in oaks (Zhang et al. 2011).
In citrus orchards, X. fastidiosa causes ‘Citrus Variegated Chlorosis’ (CVC), which was detected in northeastern Argentina in 1984 and in Brazil 3 yr later (Hopkins & Purcell 2002). This disease is considered a potential threat to the production of oranges, if a suitable vector is present (Damsteegt et al. 2006). In total, 11 species of leafhoppers have been shown to be vectors of CVC in Brazil (Lopes 1996; Gravena et al. 1998; Parra et al. 2003).
In Argentina, studies conducted by De Coll et al. (1996, 2000) in Misiones province revealed that many species of cicadomorphs were able to acquire X. fastidiosa, including leafhoppers: Bucephalogonia xanthophis (Berg), Diedrocephala bimaculata (Gmelin), Hortensia similis (Walker) Macugonalia cavifrons (Stål), Sibovia sagata (Signoret), Sonesimia grossa (Signoret), Frequenamia spiniventris (Linnavuori), Scaphytopius bolivianus (Oman), Curtara samera De Long & Freytag; and treehoppers: Ceresa ustulata Fairmaire, Entylia carinata (Forster). CVC was also observed in Corrientes province, where species associated with citrus were studied (Beltrán et al. 2004). In the last decade, CVC has been recorded in Concordia department, Entre Ríos province (Costa et al. 2009) in sweet orange and tangerine, but the insects associated with the affected orchards have not yet been studied.
Considering the importance of these insects for the transmission of X. fastidiosa and other plant pathogens, the aim of this work was to study the diversity of Cicadomorpha associated with citrus orchards in Entre Ríos province, Argentina, and their seasonal fluctuation in relation with climatic and phenological conditions.
The knowledge of cicadomorph species composition in citrus orchards and their abundance in the community are essential for integrated pest management, and can be used to predict situations that favor population peaks of these insects.
MATERIALS AND METHODS
Study Sites and Record of Climatic and Phenological Conditions
The study was conducted at the Agricultural Experimental Station of INTA, in Concordia department, Entre Ríos, Argentina (S 31° 22′27″ W 58° 06′ 59″; 46 m asl). Two orchards were chosen as study sites: sweet orange, variety ‘Valencia Late’, grafted onto trifoliate orange (Poncirus trifoliate (L.) Raf.), planted at 7 × 3.5 m row spacing, which were 25 yr-old at the beginning of the experiment (779 trees); and tangerine orchard, variety ‘Satsuma Okitsu’ (Citrus unshiu Marc), grafted onto trifoliate orange, planted at 5 × 2 m row spacing, 12 yr-old at the beginning of the experiment (200 trees).
Between 2006 and 2009, the phenological stages of sweet orange plants were recorded monthly as follows: Fl (early sprouts), F2 (elongating shoots, small leaves), F3 (elongated shoots, leaves completing expansion), F4 (leaves reaching final size), and F5 (no new shoots, twigs and leaves predominantly adult). Meteorological data were collected by the weather station located in the Agricultural Experimental Station of INTA, in Concordia, Entre Ríos.
Sampling Method and Specimen Identification
The specimens were collected with yellow sticky traps, measuring 12.5 × 10 cm, installed at 1.5 m height. Four traps were installed in the orange orchard, and in the tangerine orchard 2 traps (1 trap per 100 plants, approximately) (Felippe et al. 2006); traps were replaced every 2 wk between Oct 2006 and Jan 2009. The specimens collected from sticky traps were removed using benzene to dissolve the glue and separately placed in plastic tubes with 70% alcohol.
ABUNDANCE (N) AND SPECIES RICHNESS (S) OF CICADOMORPHA FAMILIES, ON SWEET ORANGE AND TANGERINE ORCHARDS IN ENTRE RÍOS.
Taxonomic terminology for the Cicadomorpha at the family and subfamily levels followed Rentes Lenicov et al. (1999) and Dietrich (2005). Species identifications were based on literature available for identifying leafhoppers: Lawson (1931), Christensen (1942), Linnavuori (1959), Nielson (1968), De Long & Freytag (1976), and Young (1952, 1968, 1977); treehoppers: Barreira & Sakakibara (2001) and Andrade (2004a, 2004b), and spittlebugs: Torres (1950) and Costa & Sakakibara (2002). The collected specimens were deposited in the Entomological Collection of Museo de La Plata, Argentina (MLP). Material that was damaged by removal from the traps could not be identified.
The geographic distribution for each identified species was taken from the literature and summarized in a table, along with the new information provided by this work.
The Cicadomorpha samples were identified to the subfamily and species level and counted under a stereo microscope. Total numbers of collected individuals (N) and species (S) were recorded monthly.
The relative importance (RI) of a species takes into account not only its abundance but also its occurrence or frequency. Thus, species poorly represented in terms of individual numbers but frequently recovered over a long period can be balanced with abundant species with sporadic occurrence (Rentes Lenicov & Virla 1993; Murúa et al. 2006). For each sampled site the relative importance of species was determined using the formula: RI = (ni/nt) ×(mi/mt) ×100, where ni = number of individuals of species “i”, nt = number of individuals of all species, mi = number of samples containing species “i”, and mt = total number of samples. A “very frequent” species is defined as having a RI equal to or higher than 1% the RI of “frequent” species lies between 0.02% and 0.99%; and “rare or occasional” species have a RI equal to or lower than 0.019% (Paradell et al. 2001).
ABUNDANCE (N) AND RELATIVE IMPORTANCE (RI) OF CICADOMORPHA SPECIES COLLECTED FROM SWEET ORANGE AND TANGERINE ORCHARDS IN ENTRE RÍOS.
DIVERSITY INDEXES OF CICADOMORPHA ON SWEET ORANGE AND TANGERINE ORCHARDS IN ENTRE RÍOS.
Community or habitat diversity, can be measured by methods that consider only the number of species (Margalef's index), or methods that highlight the structure of the community considering both the number of species and their relative importance. Such methods may highlight the dominance of a few species (Simpson's index), or else the degree of evenness among the whole set of species (Shannon-Wiener's index). The specific richness index of Margalef (DMg ), the Shannon-Wiener index (H'), and the dominance index of Simpson (D) were calculated as follows: H' = -Σpi. × log2pi; D = 1-Σ (pi)2; DMg = S-1/LnN; where pi = ni/N, ni = number of individuals of species “i”, N = number of individuals of all species, and S = number of species (Moreno 2001).
Seasonal fluctuation of Cicadomorpha in sweet orange orchards was evaluated through monthly values of collected individuals and analyzed with respect to climatic and phenological variables.
RESULTS AND DISCUSSION
A total of 1,554 specimens were collected (1,068 on sweet orange and 486 in tangerine plants) belonging to 4 families of Cicadomorpha, of which Cicadellidae was the most abundant in both orchards, followed by Membracidae. Other studies in citrus have shown similar results in terms of higher abundance of Cicadellidae (De Coll et al. 2006; Giustolin et al. 2009). Of all collected specimens, 95.56% (1,485 specimens) were identified and used in further analysis (Table 1).
The number of collected specimens varied between samples (Fig. 1). The highest number was obtained in Jan 2007 (N = 149) and the lowest number in Jun 2007 (N = 3).
In total, 28 species of Cicadomorpha were collected. Of these, 24 species were found in the orange orchard, while 20 species were collected in tangerine trees, and 16 were shared by both study sites. Cicadellidae presented the highest species richness in both orchards (Table 1).
GEOGRAPHIC DISTRIBUTION OF CICADOMORPHA COLLECTED SPECIES AND THEIR ASSOCIATION WITH CITRUS ORCHARDS IN ENTRE RÍOS.
Species composition also varied between different months. The highest value of species richness was recorded in Jan and Dec 2007, and Apr 2008 for the orange orchard (S = 8) and only in Dec 2007 for tangerines (S = 10). The lowest species richness was recorded in May 2007 in both orchards (S = 1) (Fig. 1).
Frequenamia spiniventris was the most abundant species in all samples with 773 specimens, followed by the leafhoppers S. bolivianus (251 specimens), Molomea lineiceps Young (136), Tapajosa rubromarginata Signoret (74), and the tree-hoppers Cyphonia sp. with 66 specimens. All other species comprised less than 60 collected specimens (Table 2). Other studies conducted in Argentina have shown similar results, although F. spiniventris was more abundant in weeds associated with citrus (Beltrán et al. 2004; De Coll et al. 2006).
Frequenamia spiniventris was also the most frequent species in all samples, followed by S. bolivianus and M. lineiceps. The species T. rubromarginata had not been previously reported in citrus, whereas in this study it was very frequent in the tangerine orchard and frequent in oranges, similarly to Cyphonia sp. (Table 2).
Other frequent species in the sweet orange orchard were Osbornellus infuscatus Linnavuori, Curtara samera (De Long & Freytag), Ceresa sp., Agalliana ensigera Oman, Protalebrella brasiliensis Young, and Xerophloea viridis (Fabricius). The first 3 were also frequent in tangerine, along with Cephisus siccifolius (Walker), B. xanthophis and D. bimaculata (Table 2). The remaining species were rare or occasional in both orchards. Similar results have been obtained by studies in Brazil (Molina et al. 2006; Nunes et al. 2007; Menegatti et al. 2008; Miranda et al. 2009; Molina et al. 2010), where Dilobopterus costalimai Young was the dominant species; but other research has shown B. xanthophis as the most abundant and frequent species on citrus (Yamamoto et al. 2001; 2002; Coelho et al. 2008).
Both orchards presented high species richness with DMg = 3.312 for sweet orange, and DMg = 3.113 for tangerine. For the diversity analysis we used the Shannon-Wiener index and the Simpson dominance index. The results obtained for the Shannon index in the orange (H' = 1.6) and tangerine orchards (H' = 1.8) suggested a similar trend in the distribution of dominant species. This was also reflected by the Simpson index with D = 0.342 for orange and D = 0.261 for tangerine) (Table 3).
In sweet orange orchards, Cicadomorpha presented 2 population increases throughout the sample: 1 in the summer, between the months of Dec and Feb; and another in the winter (Jul–Aug). Results also showed that temperature increases (spring and summer) were coincident with increases in abundance of specimens. On the other hand, a significant increase in the number of specimens during winter months was coincident with the increases of early sprouts (phenological stage Fl) of the citrus plants (Fig. 2).
Most Cicadomorpha tend to be extremely polyphagous and range widely within a number of agricultural and native plant communities (Redak et al. 2004; Giustolin et al. 2009). Citrus are not primary but occasional hosts for them, while other alternative host plants are also important for the development of the different cicadomorphan species (Milanez et al. 2001). This may explain why the number of collected insects was so low in some months during the sampling. These studies require further analysis, since species with a wide range of host plants will exhibit low seasonality (Novotny 1998).
Following a literature review, all known information on geographic distribution of the species found in this study and their associations with citrus was summarized in Table 4. Entre Ríos province was cited as a new distribution record for the following species: A. ensigera, Atanus serricauda (Linnavuori), B. xanthophis, C. siccifolius, C. samera, D. bimaculata, F. spiniventris, M. cavifrons, O. infuscatus, S. bolivianus, S. sagata, S. grossa, and X. viridis.
Tangerine represents a new record as hostplant for all species studied here, while the occurrence of A. ensigera, A. serricauda, Dalbulus maidis (De Long & Wolcott), O. infuscatus, P. brasiliensis, T. rubromarginata, Tretogonia notatifrons Melichar, and X. viridis in Valencia Late Orange is reported for the first time (Table 4).
The present work, along with research conducted by De Coll (1996, 2000, 2006), Remes Lenicov et al. (1999) and Paradell et al. (2000), gathers all information about diversity, frequency and seasonal fluctuation of Hemiptera Cicadomorpha associated with citrus orchards in Argentina. This knowledge is essential for the control of the vectors of one of the most important diseases of citrus in America.