https://orcid.org/0000-0001-9556-7840
Abstract
Scirtothrips dorsalis (Hood), the chilli thrips, is reported for the first time as a pest of celery, Apium graveolens L. Infested celery plants in a polyhouse at Kalahalli village (13°05′46.0′′N, 77°47′12.2′′E) in Hoskote taluk of Bengaluru Rural district, Karnataka, India, had light to dark brownish scars on various parts besides discolored and distorted leaves. Association of fungal or bacterial disease was dismissed based on microscopic analysis of tissue sections. Eggs, juveniles, and adults of thrips were abundant, especially on the leaves. Diagnostic characters of the adult S. dorsalis collected on celery are presented here along with photomicrographs. Juvenile and adult stages of S. dorsalis were also recorded on 27 other plant species in 18 families during the surveys conducted in 18 districts of Karnataka from 2015 to 2021. The possible role of S. dorsalis as a vector of tospoviruses in celery needs to be investigated.
Scirtothrips dorsalis (Hood) (Thysanoptera: Thripidae), commonly referred to as the chilli thrips, is a polyphagous crop pest described by Hood (1919) based on the female specimens collected on castor and chilli in India. Native to tropical Asia, it spread gradually to Europe, the Middle East, Central Africa, and the New World (Minaei et al. 2015, OEPP/EPPO 2020, Atakan and Pehlivan 2021, CABI 2021). The New World introduction resulted in broadening of its host range to more than 225 plant species in 72 families (GPDD 2011). It is adept at colonizing new areas possibly because of the absence of natural enemies or the lack of defenses in its host that could limit its infestation. In India, S. dorsalis causes economic damage to cassava (Manihot esculenta Crantz), chilli (Capsicum annuum L.), groundnut (Arachis hypogaea L.), rose (Rosa sp.), taro (Colocasia esculenta [L.] Schott), and tea (Camellia sinensis [L.] Kuntze) among others (Amin et al. 1981, Chakraborti 2004, Duraimurugan and Jagadish 2004, Rajamma et al. 2004, Deka et al. 2020). In chilli, yield loss due to S. dorsalis alone can go up to 74% (Patel et al. 2009).
Scirtothips dorsalis feeds on the host epidermis and palisade parenchyma in tender leaves and apical tissues of tender fruits (OEPP/EPPO 2020). Through continuous feeding, S. dorsalis causes scars on various plant parts. Discoloration and distortion of leaves and young fruits are commonly associated with this species. Although the pest prefers young plant tissues, on severely infested plants, it also tends to feed on the upper leaf surface (Kumar et al. 2013). Incidentally, S. dorsalis is a vector of tospoviruses (Jones 2005) that can cause extensive yield losses (Ebratt et al. 2013). Exploratory surveys by the second author (R.R.R.) in 18 districts of Karnataka, viz. Bagalkote, Belagavi, Ballari, Bengaluru Rural, Bengaluru Urban, Bidar, Chikkaballapura, Chikkamangaluru, Chitradurga, Davanagere, Dharwad, Haveri, Kolar, Mandya, Raichur, Shivamogga, Tumakuru, and Vijayapura, from 2015 to 2021 ascertained the widespread and polyphagous nature of S. dorsalis.
The purposes of this article are to document S. dorsalis as a pest of celery, Apium graveolens L. (Apiales: Apiaceae), and to illustrate the important diagnostic characters of the species collected on that host.
On 26 March 2021, the first author (P.S.K.) came across a severe thrips infestation in celery (cv. ‘Istar’ from HM.CLAUSE India Private Limited) (Fig. 1) in a polyhouse belonging to Padma Agro Products at Kalahalli village (13°05′46.0′′N, 77°47′12.2′′E) in Hoskote taluk of Bengaluru Rural district, Karnataka, India. The affected plants exhibited light to dark brownish scars on various parts. The leaves were discolored and distorted (Fig. 2). No such symptoms were observed on leaf lettuce (Lactuca sativa L.) (unknown cultivar) or sweet basil (Ocimum basilicum L.) (cv. ‘Emily’ from Enza Zaden India Private Limited), the other two crops being grown simultaneously in the same polyhouse. Eggs, juveniles, and adults of thrips were abundant on each of the 20 randomly examined celery plants, especially on the leaves. Thrips samples were collected by slightly tapping infested leaves on a white plastic tray. The dislodged thrips were transferred to Eppendorf tubes containing AGA medium (9 parts of 10% ethyl alcohol; 1 part of glacial acetic acid; 1 ml of Triton X-100 in 1,000 ml of the mixture). Leaf samples were also collected for further laboratory studies. Microscopic analysis (Olympus BX41) of tissue sections indicated that there were no fungal structures nor was there any bacterial ooze, leading to the dismissal of disease association.
Damage symptoms on Apium graveolens plants due to Scirtothrips dorsalis infestation.
Close-up of damage symptoms on Apium graveolens leaves due to Scirtothrips dorsalis infestation.
Leaf tissue teasing exposed the embedded eggs, which were oval to bean-shaped. Hatching took place in 2–7 d. The two larval stages were yellow without wing pads unlike the prepupa, which had developing wing pads. Pupa had backward-curved antennae above the head. Neither eggs nor any other later stages of thrips could be found on leaf lettuce or sweet basil leaf samples.
Adult thrips specimens were removed from the preservative medium, kept in 2% NaOH for 30 min, transferred to 60% ethyl alcohol and left for a day after which the specimens were dehydrated through a series of 70–100% ethyl alcohol washes. The specimens were cleared in clove oil for 5–10 min before mounting individually on microscope slides in Canada balsam. Finally, the slides were dried at 45°C for 30 min in an oven.
Microscope slide-mounted adults were observed under a Nikon Eclipse 80i microscope (4× and 10×) and photomicrographs of habitus, antennae, head, prothorax, pterothorax, forewing, and abdomen of the species were acquired using a Nikon DS-Vi1 camera mounted on this microscope. The plate was formed using Adobe Photoshop CS2 software. Measurements (µm ± SD) of the important diagnostic characters were taken for four female specimens using an ocular micrometer installed in an Olympus BX51 research microscope.
The species was identified as S. dorsalis using the keys provided by Chakraborty et al. (2019). The specimens were deposited in the National Insect Museum at the Indian Council of Agricultural Research – National Bureau of Agricultural Insect Resources in Bengaluru, India.
Female S. dorsalis was yellow with median brown shading on abdominal tergites III–VII, abdominal sternites devoid of brown markings but with brown antecostal ridges on tergites and sternites (872.50 ± 5.00 µm) (Fig. 3A); antennal segment I white, II slightly shaded, III–VIII dark (172.50 ± 0.00 µm) (Fig. 3C). Head wider than long (72.50 ± 0.00 µm), postocular and ocellar region striate; ocellar setae III (7.50 ± 0.00 µm) between hind ocelli, behind tangent between their anterior margins (Fig. 3D). Pronotal posteromarginal setae S2 (25.00 ± 0.00 µm) longer than S1 (Fig. 3I). Metanotal sculpture generally transversely arcuate anteriorly, with irregular longitudinal reticulations or striations posteriorly (Fig. 3K). Forewing (510.00 ± 0.00 µm) second vein with 2 setae; posteromarginal fringe cilia straight (Fig. 3G). Abdominal tergalmicrotrichial fields with 3 discal setae (Fig. 3H); VIII with anteromedian discal microtrichia, posteromarginal comb complete (Fig. 3J); IX with posteromedian discalmicrotrichia (Fig. 3E). Abdominal sternites with microtrichia extending across median area on posterior half. Male was similar to female in colour and sculpture, but smaller (Fig. 3B).
(A–L) Scirtothrips dorsalis. A, Female; B, Male; C, Antenna; D, Head; E, Female abdominal tergites IX–X; F, Mesonotum; G, Forewing; H, Female abdominal tergite VII; I, Pronotum; J, Female abdominal tergite VIII; K, Metanotum; L, Male abdominal tergites IX–X.
We recorded S. dorsalis for the first time on celery. CABI’s (2021) list of hosts of S. dorsalis across the world does not have celery. We also recorded S. dorsalis on 27 other plant species of agricultural importance or of ornamental value belonging to 18 families, all angiosperms: capsicum (Capsicum annuum L.), chenille plant (Acalypha hispida Burm. f.), chilli, chrysanthemum (Chrysanthemum sp.), coriander (Coriandrum sativum L.), cosmos (Cosmos caudatus Kunth), cotton (Gossypium sp.), curry tree (Murraya koenigii [L.] Sprengel), dahlia (Dahlia sp.), eggplant (Solanum melongena L.), garden cress (Lepidium sativum L.), gerbera daisy (Gerbera sp.), groundnut, guava (Psidium guajava L.), jasmine (Jasminum sambac [L.] Aiton), lemon (Citrus limon [L.] Osbeck), mango (Mangifera indica L.), mung bean (Vigna radiata [L.] R. Wilczek), okra (Abelmoschus esculentus [L.] Moench), pomegranate (Punica granatum L.), Rangoon creeper (Combretum indicum [L.] DeFilipps), rose, rose geranium (Pelargonium graveolens L’Hér.), spiny amaranth (Amaranthus spinosus L.), striped cucumber (Diplocyclos palmatus [L.] C. Jeffrey), tea, and tomato (Solanum lycopersicum L.). Juvenile stages, besides adults, were recorded on all the 27 plant species, which indicated that the thrips species could breed and complete its life cycle on them (Burckhardt et al. 2014). Coriander was the only other known host of the same family (Apiaceae) as celery to harbor S. dorsalis during our surveys in Karnataka. The host plants recorded were less diverse than CABI’s (2021), since S. dorsalis is known to infest almost 225 plant species belonging to 72 families (GPDD 2011). Incidentally, we also recorded S. dorsalis in a new crop of celery in the same polyhouse on 5 and 15 April 2021.
Our study reports celery as a new host of S. dorsalis. The detailed photomicrographs of diagnostic characters of S. dorsalis presented here would be beneficial to students and researchers for authentic identification of the species. The possible role of S. dorsalis as a vector of tospoviruses in celery needs to be investigated once the virus diseases of the crop are identified in India or elsewhere.
Acknowledgments
We thank G. Murali for drawing our attention to thrips damage in his celery crop and for permitting us to sample crops on various occasions in his polyhouse. We also appreciate S. Pandian’s assistance in sampling thrips in the polyhouse.
