Investigating the Optimal Conditions for Rearing and Breeding Tenebrio molitor in Zimbabwe

INTRODUCTION

Zimbabwe’s soya bean production has been reported to have declined by 46% due to the harsh economic conditions, the use of retained seed by 90% of the farmers, lack of adherence to recommended agronomic practices, low technical skills, and limited access to capital and inputs [1, 2].  Generally, soya bean is used as the main source of protein for humankind and livestock in Zimbabwe. Therefore, the decrease in productivity means that the gap has to be met by imports from neighboring countries like Malawi, South Africa, and Zambia. Local production is just enough to meet 5% of the country’s oil needs with the soya crude oil import bill hovering around US$119 million in 2016 [3]. However, researchers have decided to look for other sources of protein, particularly insects as a sustainable alternative to traditional livestock, as recommended by the Food and Agriculture Organization (FAO) of the United Nations [4-7].

Mealworms (Tenebrio molitor), giant mealworms (Zophobas atratus), lesser mealworms (Alphitobius diaperinus), greater wax moths (Galleria mellonella), silkworm (Bombyx mori), house cricket (Acheta domesticus), and African migratory locust (Locusta migratoria migratorioides) are amongst the species with the greatest potential as food and feed within the European Union [8]. This current study focuses on the rearing and breeding of T. molitor (mealworms) larvae as a sustainable alternative source of protein for livestock in Zimbabwe. T. molitor has been commercially produced in the US for over 70 years and is a favorable candidate for insect rearing due to its high protein content, well-balanced amino acid profile, potential health benefits, efficient feed conversion rate, low greenhouse gas emissions, low water footprint reduced land usage, ability to live on organic by-products, and available mass production technology [9]. T. molitor mealworms have also been reported to biodegrade organic waste into proteins [10].

Mealworm breeding has been reported to be cheap and fast from an economical point of view as its life cycle is very short [11]. However, the growth rate, size of individuals, and development rate are affected by several factors such as temperature, air circulation, humidity, light intensity, the composition of the feed, and density of specimens within the breeding tanks [12, 13]. Thus, this study focuses on how several factors such as the composition of feed, temperature, light intensity, humidity, and air circulation affect the growth of T. molitor mealworms.

MATERIALS AND METHODS

For the determination of each factor that affects growth, mealworms were reared with an ad libitum supply of feed for four weeks. The feed was replaced weekly to get rid of the insect waste in the environment and uneaten feed.

Determination of the effects of feed composition

 Mealworms were divided into five different dietary groups and each group consisted of 10 g (approximately 100 mealworms). The different dietary groups were as follows:

Group 1: 50 g of wheat bran only

Group 2: 50 g of wheat bran supplemented with 20 g of fresh carrots per day,

Group 3: 50 g of wheat bran supplemented with 20 g of fresh apple per day,

 Group 4: 50 g of wheat bran supplemented with 20 g of fresh cabbage per day,

Group 5: 50 g of polystyrene only.

The feed which showed the highest growth rate and development rate was selected and fed to mealworms during the investigation of the effects of humidity, light preference, air circulation, and temperature on their growth performance. 

Determination of the effects of relative humidity

Mealworms were divided into 2 groups and each group consisted of 10 g (approximately 100 mealworms).

Group 1: 50 g of wheat bran and 20 g of carrots in the absence of humidity,

Group 2: 50 g of wheat bran and carrots at 80% humidity.

Determination of the effects of temperature

The effect of temperature was investigated from the three groups of mealworms exposed to different temperatures using thermostatically controlled incubators. Each group consisted of 10 g of mealworm larvae (approximately 100 mealworms).

Group 1: 50 g of wheat bran and 20 g of carrots at 28 °C,

Group 2: 50 g of wheat bran and 20 g of carrots at 20 °C,

Group 3: 50 g of wheat bran and 20 g of carrots at 35 °C.

Determination of the effects of air circulation (Oxygen)

The effect of air circulation was observed from two groups of mealworms. Each group consisted of 10 g of mealworm larvae (approximately 100 mealworms).

Group 1: 50 g of wheat bran and 20 g of carrots in a tightly closed container with no air circulation,

Group 2: 50 g of wheat bran and 20 g of carrots in a well-ventilated container.

Determination of the effects of light

The effect of light preference was observed in two groups of mealworms. Each group consisted of 10 g of mealworm larvae (approximately 100 mealworms).

Group 1: 50 g of wheat bran and 20 g of carrots in a container in a cupboard,

Group 2: 50 g of wheat bran and 20 g of carrots in a container exposed to daylight.

Determination of mortality rate, growth performance, and development rate

For all the groups under investigation, mortality rate, growth performance, and development rate were determined as follows:

1. Mealworm pupae and dead larvae were removed daily from each group, and the numbers were recorded.
2. The time taken for larvae to develop into pupae was also recorded for each of the mealworms in all the groups.
3. The separated pupae were observed daily for enclosed beetles.
4. The average weight of the mealworm larvae was recorded each week on 40–50 randomly selected mealworms within a group.
5. The length of 20 randomly selected mealworms was measured and the average length of mealworm larvae in each group was recorded.

The feeding rate of mealworms from the two groups (absence of light and presence of light) was also determined by measuring the feed consumed every week.

RESULTS AND DISCUSSION

The effects of feed composition on the growth, development, and survival rate of mealworms

The average weight and the size of the mealworms from each dietary group were measured every week to determine the effects of different feed compositions, and results are reported in Figure 1. The results show the highest increase in both the average weight and the length of mealworms from the group that was fed with wheat bran and carrots and the lowest increase from the group that was fed with polystyrene. The wheat bran and carrots group also showed the highest development rate as well as the survival rate as 28% of the mealworms developed into pupae. 

The larvae of T. molitor, also known as mealworms, which belong to the species of edible insects have long been used as food worldwide and are commonly bred [14, 15]. The efficiency of rearing is influenced by many factors and these include temperature, relative humidity, feed, light preference, oxygen, and the region of rearing. The composition of the feed is one of the key factors in the whole rearing such that improper choice of feed results in slowing down the larvae development or death of the mealworms [16, 17]. In this particular study, to guarantee the quality of feed as well as to increase the efficiency of rearing and production of insects, the wheat bran was further enriched and supplemented with fruits and vegetables. Polystyrene foam was added to the experiment as feed to investigate the sustainability of the state of nature and waste disposal.

Figure 1 shows that mealworms fed with polystyrene had the lowest % survival rate and this was caused by the fact that there is low nutritional value in polystyrene. The groups fed with wheat bran and apple, wheat bran and cabbage, wheat bran and carrots, and wheat bran only had the lowest mortality rates. It was observed that wheat bran supplemented with either a fruit or vegetable had a positive effect on life development as mealworms in these groups increased in weight and length and also had the most accelerated rate of development into pupae and then adults. The choice of feed is crucial as it results in rearing gain; a short life cycle of insects is considered in determining if one is a good farmer or not [18]. It has also been reviewed that feed also affects the nutritional value of edible insects [19]. 

The effects of temperature on the growth, development, and survival rate of mealworms

Mealworms were reared at different temperatures of 20 °C, 28 °C, and 35 °C. Figure 2 shows the results that were obtained. There was an increase in the development rate of the mealworms from larvae stage to pupae and finally beetles at 35 °C (Figure 2c). However, at 28 °C mealworms had the highest average weight and length as well as the survival rate. The lowest survival rate, average weight, and length were observed from the group of mealworms reared at 20 °C.

Figure 2 shows that the optimum temperature for rearing and breeding of T. molitor in Zimbabwe in the range tested is 28 °C. The results obtained were in line with Xu et al. [20], who also investigated the optimum temperature for breeding mealworms (T. molitor). At 28 °C, mealworms had the highest survival rate of 98%, the highest average weight of 117.5 mg, and the longest average length of 33 mm. Generally, insects have different adaptations to survive when exposed to low temperatures. One of the ways is water loss [21] and in this present study, the group of mealworms bred at 20 °C delayed developing into pupae, which could be a result of water loss.

The effects of air circulation on the growth, development, and survival rate of mealworms

Mealworms were reared in the presence and absence of oxygen and the results are shown in Figure 3. The results show that mealworms grow, develop, and survive better in the presence of light than in the absence of light.

Results from this study showed that oxygen is an important factor in the efficient breeding and rearing of T. molitor mealworms (Figure 3). There was a high mortality rate in the group of mealworms reared in an air-tight container compared to the group of mealworms that were reared in a well-ventilated space.

The effects of humidity on the growth, development, and survival rate of mealworms

Mealworms were reared in the absence of humidity and at 80% humidity, and results are shown in Figure 4. At 80% humidity mealworms grew and developed better than in the absence of humidity.

Relative humidity has also been shown to have an impact on the lifecycle, growth, and development rate of T. molitor mealworms as the group of mealworms with 80% humidity had the higher average mass and average length, the number of pupae collected as well as the survival rate compared to the group of mealworms with 0% humidity as shown in Figure 4.  This indicated that mealworms require the presence of humid conditions for efficient rearing and breeding. Another important factor in rearing mealworms is to know their growth rate in the presence or absence of light.

The effects of light preference on the growth, development, and survival rate of mealworms

The mealworms were reared in the presence and absence of light and the results are shown in Figure 5. Results show that mealworms favor darkness more than light as an increase in the average weight and length of the mealworms from the absence of light group was observed. The increase in development and survival rate was also observed in this group.  The feeding rate of these mealworms was also investigated and results show that mealworms feed at a greater rate in the dark than in the presence of light as explained by a gradual decrease of mass in the feed (Figure 5e).

Figure 5 confirms that mealworms develop more efficiently in the absence of light as the figure shows a higher survival rate, average mass, and length of mealworms reared in darkness as compared to those reared in the presence of light. The feeding rate of mealworms in the dark was also found to be more than that of mealworms in the presence of light. This feeding behavior of mealworms is similar to the adults which are also called the darkling beetles as they are fond of darkness. They are often found underneath rocks and logs because the leaf litters offer proper covering and is a high resource for nutrients [22].

CONCLUSION

Mealworms showed effective growth, development rate, and high survival rate when fed with wheat bran and carrots, at 28 °C, in the absence of light, in the presence of air (oxygen), and 80% humidity.

ACKNOWLEDGMENTS: University of Zimbabwe Research Board (Harare, Zimbabwe) is acknowledged.

CONFLICT OF INTEREST: None

FINANCIAL SUPPORT: Reagents used in this study were provided by the Biotechnology and Biochemistry Department at the University of Zimbabwe, Zimbabwe.

ETHICS STATEMENT: None