Status, Importance, and Management of the White Mango Scale (Aulacaspis Tubercularis Newstead) In Ethiopia: A Review

INTRODUCTION

Mango (Mangifera indica L.) is one of the most flexible and widely grown evergreen fruit crops in tropical and subtropical regions [1]. It is known as the king of all fruits because it has excellent flavor, delicious taste, and an attractive fragrance with almost all the known vitamins, and many essential minerals [2]. In addition, it has a high nutritional value and is rich in vitamins such as vitamins A, C, B1, B2, and D [3, 4], making it valuable for both food and nutritional security, especially in developing countries like Ethiopia where achieving food and nutritional security is difficult.

Even though mango is predominantly grown as its desert fruit, it has also been cultivated for various uses such as making juices, jams, and other preserves [3, 5]. Likewise, it is used as a leafy vegetable and medicinal crop for many treatments. In addition, mango is used as fuel wood, fodder for animals, a rich source of nectar for honey bees, for canoe and furniture construction, as fence and windbreak, as an embellishment for villagers and home gardens, livestock, birds, and other living things from strong sun and rain [3]. The mango kernel contains 8-10% high-quality fat which can be used for soap and also as a substitute for cocoa butter in confectionery.

Mango is well known in developed nations for its high nutritional value and as a source of foreign currency for many developing nations. Mango production represents 51% of total global tropical fruit production [6]. In recent years, mangoes have become famous as fresh fruit and processed products on the global market and are commercially grown in more than 80 countries. World production of mangoes is around 46 million tons per annum and India is the world's biggest grower of mangoes, with a 40% share of total world production [7], though, the production and productivity of the crops are highly challenged by white mango scale these days. 

The white mango scale (Aulacaspis tubercularis Newstead) belongs to the order Hemiptera and is feuatered by posseing a piercing and sucking mouth. The worldwide dispersal of this insect pest may have came to existence by the movement of planting materials. Because of the absence of a strict internal quarantine system for the geographical exchange of planting materials.

The white mango scale emerged as a devastating insect pest that destroys mango production, causing 50 to 100% economic losses, and killing the plant in most mango-growing areas around the world [8-10]. Based on assessment reports, the white mango scale has been identified as a new, fast-spreading, and devastating insect pest of mango, which causes premature leaf drop, twigs and branches die back fruit stunt, distortion, and premature fruit drop that seriously affects the quality and quantity of mango yield [11].

The insect is a serious insect pest of mango across the mango-producing parts of the world. Almost all mango-growing tropical and sub-tropical regions are affected by the white mango scale. Due to the spreading nature of the insect and the less successfulness of managing the white mango scale across all growing regions, mango production and productivity is declining at an alarming rate [9].

Similarly, the introduction, establishment, distribution, and control practices of the white mango scale in Ethiopia’s mango growing areas have not been sufficiently studied yet. Ethiopia is not listed in a list of white mango scale distribution maps of the globe reported [10]. This indicates that there is an information gap in white mango scale occurrence, distribution, and severity status in Ethiopia with the other world. Even though not sufficiently studied, very few reports indicated that the prevalence and pest infestation becomes gotten worse and worse over time. The problem expands due to a lack of internal quarantine problems that can limit the geographical expansion of the pest into new areas [12]. Hence, putting efforts into the occurrence, distribution, and management of the pest is very essential to manage properly crop damage from the white mango scale. As Abate and Dechassa [9] reported that white mango scale population dynamics are varied along with agroecology. Therefore, the agroecological-based study has not been sufficiently done yet so far in all regions. Wale and Melis, [13] also reported that the population dynamics white mango scale and its natural enemies are varied in time and space, influenced by factors such as temperature, rainfall, wind speed, relative humidity, and sunshine hours. 

Although not verified very well, numerous types of research indicated that integrated pest management options are the best solutions for controlling option of white mango scale [9, 14, 15]. In general, this review aimed to compile the current status of the white mango scale, its effect on mango production, and management methods in Ethiopia with research findings that have been done in the country these days. 

Status of white mango scale

The white mango scale was initially seen in Asia and and subsequently was well known all over the globe. Noawadays the production of white mango remains a great challenge [9]. The white mango scale is among the most damaging pests of mango trees in most mango-growing areas. This insect pest injures the shoots, twigs, leaves, branches, and fruits by sucking the plant sap with the mouthparts, leading to its deformations, defoliation, drying up of young twigs, dieback, poor blossoming, death of twigs through the action of toxic saliva and so upsetting the commercial value of fruits and their export potential, particularly to late cultivars where it causes visible pink blemishes around the feeding sites [16, 17]. In nurseries, early severe infestation slows growth. Young mango trees are especially prone to leaf drop and dead branches in hot, dry weather [18]. Infested immature fruit dropped, mature fruit became smaller, less juicy, rotted and unfit for commercial use, and fruit quality and quantity declined [19, 20].

White mango scale was primarily noted in Ethiopia at Loko, Guto Gidda district, East Wollega zone of the Oromia region in August 2010 at Green Focus Ethiopia Ltd [21]. The scale infested all growth stages of the crop at Green Focus Ethiopia. Since the pest is rapidly spreading and less quarantine regulation leads to invasion of the insect pest across almost all mango-growing regions of the country. 

About 1,666,040 householders contributed to mango production on 16,363.5 hectares of land in the 2019/20 cropping season which shared 12.5% of the fruit production of the nation [22]. But, it was manufactured on 19,497.92 hectares in the 2018/19 cropping year; mango yield is reduced by 16.08% and 6.09%, respectively on or after the 2018/19 to 2019/20 cropping years.

According to reports, the white mango scale can cause mango production to cease. Despite different alternative management options, quarantine regulation remains the most effective option currently available to limit the spread of the white mango scale [23]. Numerous research studies have indicated that spraying insecticides with other integrated pest management options is the most effective way to control the white mango scale. With the current status of the insect, mango farms in Ethiopia will be out of production due to this insect unless the insect is managed. According to existing reports, mango production in Ethiopia is under serious threat, possibly leading to the total loss of mango production in the future.

Distribution of white mango scale

The white mango scale is a tropical species which got its root in Asia. Now, it is developed in many tropical and subtropical regions of the globe [9, 24]. Widely distributed in several countries in Africa, Asia, Oceania, North America, South America and the Caribbean. Its distribution was limited to Europe (Figure 1).

In Ethiopia, beginning from the pest establishment in 2010, the pest has been distributed speedily throughout the country [25] due to bio climatically favorable conditions, fewer quarantine regulations, easy transferability through planting materials, carried with birds and other flying animals across locations due to its very small size and flying of the male adult white mango scale itself with the aid of wind for a long distance [26]. In addition, inappropriate agronomic practices facilitate the occurrence and severity of the white mango scale. At present, the white mango scale is found in almost all regions of Ethiopia that grow mangoes, including Oromia, Amhara, Benishangule Gumuze, Gambella, Tigray, southern people's nation, and rift valleys [27]. (Figure 2).

Biology and host plants of white mango scale

The white mango scale had five to six generations per year [10, 30, 31] (Figure 3). Its regeneration was highly influenced by temperature, making it a thermophilic insect. It is stated that the female white mango scale lays 80-200 eggs depending on the temperature at which it hatches nymphs after a week. This insect pest can be reproduced from five to six generations per year, at a maximum daytime temperature of 26°C and a night-time minimum temperature of 13°C [31]. The nymphs feed on plant tissues and reproduce on them [23, 32].

The white mango scale had sexual dimorphism and its post-embryonic growth comprises four male instars (nymph I, nymph II, pre pupa, and pupa) and two female instars (nymph I and nymph II) [28, 33] (Figure 3). Crawlers and male adults are the only stages that can move [34]. The crawlers are movable and move over the host plant to find a proper place to settle. Once established, it inserts the stylet into the plant tissue and begins feeding  [35]. Female crawlers are randomly established on leaves, stems, or on fruits where they feed [33]. They habitually travel away from their mother whereas male crawlers form groups of ten-eighty individuals near the adult female [28, 36]. In most cases, about 80% of the hatched crawlers are usually male [10]. The armored scale insect has also long thread-like mouthparts (stylets) six to eight times longer than its body [37].

The white mango scale is polyphagous, feeding on plants in above thirty-seven genera in twenty-three families including stone fruits, citrus, papaya, avocado, guava, coconut, ginger, cinnamon, melon, pumpkins, and cucumbers [10, 28, 38, 39]. The pest injures the shoots, twigs, leaves, branches, and fruits of mango by sucking plant fluid with the mouthparts, causing deformation, defoliation, drying up of young twigs, dieback, poor blooming, loss of twigs perhaps through toxic substances activity [17, 19] (Figure 4). Fruits that are heavily infested drop too early and mature fruits are smaller in size, less juicy, rotten, and unfit for marketable use [19, 24, 40] (Figure 4).