Introduction
The oriental fruit fly, Bactrocera dorsalis (Hendel) (Diptera: Tephritidae), is one of the most important horticultural pests comprising of nearly 100 morphologically similar taxa present in at least 75 countries (Drew and Romig 2013; Zeng et al. 2019). They are characteristically invasive and polyphagous, with a broad host range and therefore widely distributed (USDA-APHIS 2016; CABI 2020). Originally,B. dorsalis was endemic to the tropical and subtropical regions of Asia from where it is believed to have spread to other parts of the world (Wan et al. 2012). In sub-Saharan Africa (SSA), it was first detected in Kenya in 2003, rapidly spreading into East and West Africa, as well as central southern parts of Africa within a short period (Lux et al. 2003; Mwatawala et al. 2004; Goergen et al. 2011 ; Magagula et al. 2015; Manrakhan et al. 2015). The high global dispersal capacity, colonization and adaption of B. dorsalis to new habitats is largely attributed to several factors incuding increased international trade, change in land use and strong climate tolerance (Qin et al. 2019; Zeng et al. 2019; Zhao et al. 2020). Colonization of various regions byB. dorsalis has a significant impact on the production of fruits and vegetables, as well as the socio-economic aspects of the whole value chain in the affected countries (Ekesi et al. 2016; EPPO 2020).
Bactrocera dorsalis has a long and intricate taxonomic history which has significantly impacted pest management strategies and market access (Schutze et al. 2015a). Previously, the world’s most important horticultural pests namely the Asian Papaya fruit fly, Bactrocera papayae Drew & Hancock, the Philippine fruit fly, Bactrocera philippinensis Drew & Hancock and the invasive fruit fly,Bactrocera invadens Drew, Tsuruta & White, were considered separate and independent Bactrocera species from B. dorsalis . However, through various reliable approaches such as molecular genetics (Boykin et al. 2014), sexual compatibility (Schutze et al. 2013; Bo et al. 2014) chemoecology (Tan et al. 2010) Cytogenetics (Augustinos et al. 2014), and morphology and morphometric analysis (Krosch et al. 2013; Schutze et al. 2012; 2015b), it was determined thatB. papayae , B. philippinensis and B. invadens are morphologically and genetically same biological species as B. dorsalis hence its junior synonyms (De Meyer et al. 2015; Schutze et al. 2015a). This synonymization has occasioned a broad intraspecific morphological variation between members of the B. dorsaliscomplex, including the existence of colour polymorphism.Bactrocera dorsalis has a high degree of intraspecific variation in their scutum colour that varies from entirely black pale to entirely red-brown, with the existence of variable lanceolate-patterned intermediates (Leblanc et al. 2013; Schutze et al. 2015a).
Melanism is one of the discernible phenotypic variations in conspecific and heterospecific insects (Ma et al. 2008). It commonly occurs through the existence of morphs that are incompletely or completely dark in pigmentation. This gradient of pigmentation can appear on the different parts of an insect body including thorax (e.g. Gryllus firmus(Roff and Fairbairn 2013), abdomen (e.g. Drosophila polymorpha(Brisson et al. 2005), and wings (e.g. Harmonia axyridis (Chen et al. 2019)). Melanin plays physiological and ecological roles including species recognition and communication, mimicry, warning, courtship/mate selection, resistance to temperature and prey-predator/parasite interactions (Wittkopp and Beldade 2009). Its synthesis which involves a complex of biochemical reactions (Futahashi et al. 2008; Arakane et al. 2009) can favour or detriment insect life-history traits in the function of its environmental conditions. Yin et al. (2016) found that in hot environments and under long photoperiods, Saccharosydne procerus(Matsummura), melanic morphs have greater fitness parameters (longevity, mating rate, fecundity, and egg hatchability); whereas non-melanic morphs can adapt more successfully to low temperatures and short photoperiods. In Spodoptera littoralis (Biosduval), dark larvae and pupae are heavier, and take a shorter time to emerge as adults compared to their pale counterparts (Cotter et al. 2008). To the best of our knowledge, no comprehensive studies have investigated the impact of melanisation on the life-history traits, wing size and shape of tephritids. Here, using the four melanic morphs of B. dorsalisreared at the International Centre of Insect Physiology and Ecology (icipe ), Nairobi, Kenya, we monitored the development of their preimaginal stages by recording their pupation time, emergence time, larval weight and pupal weight. Also, using the landmark-based geometric morphometric technique, we measured the wing size and shape of the adult flies.