Quail (Coturnix coturnix) farming is currently gaining global recognition as a source of high-quality animal protein in the form of meat and eggs [1]. These products can improve human nutrition and contribute towards achieving food and nutritional security as well as achieve the sustainable development goals set by the United Nations [2]. Quail have high adaptability, fast growth rates, strong resistance to avian diseases, and high feed efficiency [3]. They require minimal space for their production, meaning a large flock can be reared even under landless production systems [1,3]. These birds have very short generation intervals, can reach sexual maturity by six weeks of age, and the hens can produce over 100 eggs in their first production cycle [1,2,3,4]. For optimum quail production, high-quality feeds containing high levels of energy and protein should be provided to the birds daily. Unfortunately, these conventional feedstuffs have become unsustainable due to their high market prices and high demand by livestock, humans, and the biofuel sector [2]. Indeed, fish and soybean meals have been strongly criticized for their high market prices [5].

Moreover, soybean production generates high land-use competition and incurs high variable costs due to the use of pesticides, fuel, chemical fertilizers, and machinery at the farm level, which, in turn, contributes to the emission of greenhouse gases that have detrimental effects on the environment [2,5]. These aspects have prompted researchers to look for alternative protein sources, whose production does not affect the environment [6]. The use of already-known insect protein sources (Musca domestica, Hermetia illucens, Bombyx mori, and Tenibro molitor), as well as new and lesser-known alternatives such as the mopane worm (MW) (Gonimbrasia belina) can be a worthy solution. Gahukar [7] pointed out that edible insects are renewable natural feed sources that provide nutritional, economic, and ecological benefits. Indeed, several authors confirmed that insects are essential sources of proteins, amino acids (AA), carbohydrates, lipids, vitamins, and some minerals [8,9]. For instance, caterpillars, to which MW belongs, contain high levels (50–60% dry weight) of highly digestible (77–98%) crude protein [10]. Thus, the utilisation of MW in quail diets can complement both crop and animal food production systems because their production does not rely on the use of arable land and gallons of water [11].

Moreover, the use of insect meals in quail diets is desirable due to better feed conversion efficiency, low greenhouse gas emission, low risk of transmitting zoonotic infections, and low water requirements with little or no animal welfares issues [12]. Due to the high cost of conventional protein sources (fish and soybean meal), several scholars have reported the potential of insect (worm) meals as an alternative protein source in animal feeds [13,14,15]. Notably, the MW has been used as a protein source in Jumbo quail [2], indigenous chicken hens [16], and broilers [17] with good results. However, there is a paucity of information on the use of dietary mopane worm meal (MWM) in various quail breeds.

Mopane woodlands are found in semi-arid regions of Southern Africa and host one of the most valuable larvae, G. belina [27]. Mopane trees grow on nutrient-rich clay soil at a preferred altitude of 300 to 900 m, where they receive an average annual rainfall of 550 mm [28]. Mopane worm is a species of the emperor moth, native to the tropical parts of Southern Africa. Its sizeable and edible caterpillar feeds primarily on mopane tree leaves and, to a lesser extent, on other tree leaves within the mopane woodland [26,28]. The worm is consumed in significant quantities as part of family diets and as a food source in rural areas [28]. However, MW consumption has religious restrictions on a large part of the population in South Africa, making it an ideal protein source for quail. On average, a MW life cycle takes about 4 to 6 weeks and is divided into five growing stages, known as instars [29].

The production of MW involves two generations each year, with outbreaks occurring first in early summer and again in late summer. The males follow chemical pheromones secreted by the females during mating, after which the mated female lays a cluster of 50–200 eggs around twigs and leaves of host plants. The eggs hatch after about 10 days to produce tiny black larvae (caterpillars) [29]. The larvae pass through five stages (instars) during their growth phases; each stage lasting for not more than a week. During instar stages I–III, the caterpillars cluster together in groups of 20–200, whereby they feed on leaves of mopane trees and other trees that grow close to the mopane woodlands [30]. When the larvae pass stage IV, they moult and displace from the unit. At this stage, they are now referred to as mopane worms (caterpillars) and can grow to approximately 80 mm long [29]. During this phase of growth, the MW ceases feeding and begins to descend the tree trunk to burrow through the soil and form pupae. This last stage of growth (instar V) before pupations is the most favourable period for harvesting because the gut does not contain any indigested material [29]. Mopane worms that are harvested undergo a series of separate processing stages. The first stage of processing is to degut the worms. This is done by pushing from the head towards the anus in-between two fingers to remove the undigested material in the gut [31]. The second stage involves boiling the worms in brine for 20–60 min, followed by sun-drying for 2–4 days. These two last steps of processing are essential for removing spines, which extends the shelf life of the worm up to a year [31].

Successful utilisation of MW as a potential protein source in quail diets would require mass production to maintain a sustainable and continuous supply of the worms [13]. Most insects utilized by man as food or feed exist naturally in the environment, where they are collected. However, to a lesser extent, insects with economic value have been domesticated for commercial production. The concept of insect farming is a relatively a new venture of diversification that encompasses rearing insects in a confined area (i.e., a farm) and controlling their rearing conditions, diet, and food quality. Insects farmed in captivity are isolated from their natural communities [32]. Indoor and/or semi-outdoor insect farming in a monitored or controlled ecosystem ensures successful insect mass production [33]. In addition, commercializing insects of the Lepidoptera genus, such as MW and mulberry silkworms, could be an economically viable business because these worms are prolific.

The intensive production of MW to ensure continuous availability has been investigated in Zimbabwe, South Africa, and Botswana [34]. Large-scale, industrial worm production coupled with sustainable worm breeding and processing technology can ease the challenges of worm availability and reduce the selling price of MW [35]. Indoor and semi-outdoor worm production aims to increase production and protect the worms from viral and parasitoid infection. Twigs or leaves that carry eggs should be covered with a white sleeve to protect eggs from parasitoid infections, or eggs can be stored in a white container until they hatch. Gardiner [35] demonstrated that a captive breeding population of MW could be established and sustained for over three years.

Overexploitation is one problem that could limit the efficient utilisation of MW in quail diets. Over-harvesting manifests when there is a food shortage or increased returns from the general sale of the worms [36]. The collection of MW has also increased due to lack of regulations. Consequently, this has increased competition between local people and outsiders at the expense of the lifecycle and sustainability of the worm. Some harvesting practices literally hinder the sustainability of MW production. Destruction of mopane trees by chopping off branches to make the worm reachable, incorrect timing, and a prolonged harvesting period are some practices that hinder the sustainability of MW production. Further, harvesting too many under-aged larvae and harvesting beyond the carrying capacity of the area pose serious threats to the sustainability of MW production. Trampling by harvesters, firewood collection, and litter also hinder MW production [31].

In addition, on-site processing of the worms with fire leads to patches of the veld being burnt, reducing the grazing capacity of the veld. With the existing challenges facing MW production, local and/or traditional regulations may need to be employed. The regulations would monitor caterpillar development and abundance, and the harvesting of giant, more mature worms rather than smaller, young ones. Furthermore, the increased demand for the worm calls for government legislation and permits to control harvesting [30]. The regulations in communal areas may involve managing the number of harvesters, the number and size of MW, and the number of days spent harvesting [37]. Non-harvest areas should also be established as nature reserves, which will serve as sacred and rotational harvesting sites. This is vital since the population of MW not picked in one period would lead to abundance in the next period [38]. It is also essential to recognize property rights, whereby locals will be allowed to manage and control their land, which will help them protect their resources from outside harvesters.

Achieving a balanced diet for growth and productivity of quail remains one of the most significant challenges in the poultry industry. Therefore, the associated protein source must be characterized by proper AA profiles with high digestibility and good palatability [64]. Moyo et al. [17] reported higher body weight gain in broilers fed with a diet containing 12% MWM than those in the control group. Similarly, indigenous chicken hens fed with a diet containing 18% MWM had increased average daily gains (ADG) with a lower (better) feed conversion ratio (FCR) [16]. A lower FCR points to high feed efficiency [65], which could be attributed to better ingredient combination, digestibility, and absorption of nutrients [66]. Contrary to these results, the feed consumption, ADG, FCR, and dressing percentage of guinea fowls fed with 4.5% MWM did not differ from those fed the control diet [51]. These variations in performance could be attributed to the involvement of different species as well as the age and sex of the birds, which are known to produce different results even when birds are offered the same feed [66].

Increased average weight gain remains one of the tools vital to monitoring the nutritional value of a specific diet and animal growth. Positive results have been reported when insect-based protein meals were used in poultry diets. For example, Radulovic et al. [67] and Pretorius and Pieterse [68] reported an increase in bodyweight gain in chickens fed with M. domestica meal and maggot larvae meal, respectively. Similar results were noted by Moyo et al. [17], Moreki et al. [45], Hwangbo et al. [69], and Schiavone et al. [70], where feeding chickens with insect-meal-containing diets improved overall growth performance compared to the control diet. Loponte et al. [71] also observed better feed efficiency and increased body weight in barbary partridges fed with insect (T. molitor and H. illucens) larvae meals rather than dietary soybean meal. The excellent nutritional profiles of the insect meals could have prevented nutrient dilution by supplying more of the required nutrients at a time, resulting in moderate feed consumption and increased weight gain with better FCR. This was supported by Hwangbo et al. [69], who concluded that the availability of vital and readily digestible protein in insect meals may explain the reason for better feed utilization.

Moyo et al. [17] observed that inclusion of MWM into chicken diets positively impacts the colour of breast meat, and results in meat pH values between 5.80 and 5.91 at 24 h post-mortem. Meat pH is essential to consider because it determines meat acid accumulation, which affects the colour and water-holding capacity of the meat [72]. Previous studies have shown that the presence of chitin, a fibrous substance in insects, has intrinsic antioxidant properties, with enzymatic reactions that increase the conversion of glycogen into lactic acid, resulting in the decline of pH immediately after slaughter [73]. The ultimate pH is affected by the degree of glycogen reserves in the meat of birds before slaughter. Another vital meat quality trait is meat tenderness, which is affected by diet, sex, strain, age, and the environment [74]. Tenderness is usually increased by post-mortem protein proteolysis, which is followed by the degradation of the myofibrillar protein [75]. Gunya et al. [76] reported a positive influence on breast meat tenderness of birds fed with dietary Eisenia foetida worm meal. Likewise, Moyo et al. [17] reported an increase in tenderness (low shear force value) for breast meat of broilers fed with dietary MWM. The authors attributed the improvements in meat tenderness to rigour resolution due to the enzymatic breakdown of collagen that keeps meat fibres together.

Lautenschläger et al. [42] opined that such an effect might be due to the level of protein (53.7%) and fat (23.2%) content found in MWM. Teye et al. [77] reported improved appearance, juiciness, and tenderness of meat due to high dietary fat. Meat juiciness improves the meat texture, which is a function of the quality and composition of fat [78]. As such, MWM is an excellent dietary ingredient that can supply the needed composition and quality of fat. The total unsaturated fatty acid (FA) ratio to total saturated FA in MWM is 54:49, with increased α-linoleic and palmitic acids [79]. The α-linoleic and palmitic FAs are higher in dietary MWM and can be used to cure coronary heart disease and chronic ailments [59]. The ability of MWM to cure coronary heart diseases in humans is an indication that its proposed inclusion into the quail diet will further protect quail egg and meat consumers from such ailments, beyond the protein it offers them.