Evaluation of different substrates on the life history parameters of African palm weevil, Rhynchophorous phoenicis (Coleoptera: Curculionidae) in Ghana

Abstract

African palm weevil (Rhynchophorus phoenicis) larvae are widely consumed in Ghana and most countries in Africa because of their high-quality protein and low greenhouse gas emissions than conventional livestock. However, the consumption of the larvae is hindered by the identification of viable agricultural by-products or artificial diets for production. To address this challenge, Rhynchophorus phoenicis was evaluated on three substrates, namely cassava, sugarcane, and palm yolk, to determine the fecundity and hatchability of eggs. Completely Randomized Design was used to assign substrates with four replications each. Overall, adults reared on palm yolk laid average of 76.2 eggs while the cassava and the sugarcane substrate recorded averages of 73.0 eggs and 63.3 eggs, respectively. This result exhibited a significant difference between the substrates. The egg hatchability of adults reared on cassava was comparable to those fed on a natural host (palm yolk). Cassava can be used as substitute for the production of adult African palm weevil.

Keywords:

• African palm weevil larvae
• substrates
• fecundity
• egg hatchability

1. Introduction

The world’s population is expanding quickly, and it is predicted that by 2050, there will be nine (9) billion people on the planet, with a 60% rise in the global demand for food (FAO, 2017; United Nations, 2017). This situation may have a significant impact on the sustainability of current agricultural production systems as well as food production in sub-Saharan Africa and other regions of the world (Debrah et al., 2022).

According to a report by the United Nations (2020), the COVID-19 pandemic caused a drastic increase in the world’s hunger in 2020, indicating that there were about 800 million people who were undernourished in 2020. This gives an indication that the world will struggle to meet its target on food security by 2030. This undoubtedly demands innovative strategies to make sure that sufficient, high-quality, safe, and nutritious foods are always accessible to everyone.

Insects could significantly contribute to human nutrition and alleviate the food shortage (Debrah et al., 2022). Edible insects are known to be promising alternative food sources to combat food insecurity in the world (Halloran et al., 2016).

Notably, insects can contribute to shaping the future sustainable food systems by providing high-quality animal protein with lower environmental impacts and greenhouse gas emissions than conventional livestock (Alexander et al., 2017).

The African palm weevil, Rhynchophorus phoenicis (Fabricius) (Coleoptera: Curculionidae) known locally as “Akokono” in Ghana, is one of the most economically important tissue-boring pests of many palm species globally (Haran et al., 2020; Malumphy & Moran, 2009). Nevertheless, despite their devastating nature in most palm species, attention is also given to the high nutrient contents, especially in the larvae state (Gourou, 1947).

According to Anankware et al. (2015), the larvae of the African palm weevil contain significant amounts of fatty acids, zinc, iron, and vitamins (thiamine and riboflavin), as well as crude protein. Moreover, Ekpo and Onigbinde (2005) confirmed critical mineral elements such as iron, zinc, manganese, potassium, magnesium, copper, sodium, and calcium are present in extremely high quantity in palm weevil larvae.

However, to maintain high production levels of the palm weevil larvae, there should be adequate substrates to meet the daily feeding demands of the insects. Otherwise, people will resort to felling down more palm trees, which are the natural habitat/feed for the insect. Historically, people harvested palm weevil larvae from palm trees in Ghana (Anankware et al., 2016). Ouattara et al. (2015) opined the drastic decline of palm species in Ghana as a result of human activities which is a serious ecological threat to other beneficial organisms in the ecosystem. As technology evolves, the palm weevil is now reared at home from their natural habitat (Fungo et al., 2016). To avoid the indiscriminate felling down of palm trees, there should be adequate alternative feed substrates which can serve the same purpose as the palm yolk. Rearing palm weevil larvae requires a lot of care and maintenance to get the best yield. There is the need to evaluate alternative feed substrates for the rearing of palm weevil larvae locally. The aim of our study is to evaluate different substrates on the life history parameters of African palm weevil, Rhynchophorous phoenicis (Coleoptera: Curculionidae) in Ghana.

2. Materials and methods

2.1. Preparation of substrates

Three local substrates, namely palm yolk, sugarcane and cassava, were used for the study. Palm yolk was carefully harvested using the earth chisel (Plate 1). Harvested palm yolk was chopped into sizes of 10 cm × 10 cm and soaked in water for 24 hours to make them soft for easy feeding and oviposition by the adults (Plate 2). The yolk was allowed to stand for about 1–2 hours to drain-off excess water before inoculation. About 500 g of palm yolk was weighed into plastic containers.

Plate 1. Harvesting of palm yolk from felled palm tree.

Plate 2. Soaked palm yolk .

Fresh sugarcane sticks were cut into node-length sizes measuring about 10 cm. The sample was then beaten and soaked for a day to soften it for egg laying (Plate 3). Since sugarcane is known to have high sugar content, no sugar was added before the inoculation of the adults.

Plate 3. Soaked Sugarcane.

Cassava was peeled with a knife and chopped into 10 cm × 5 cm sizes (Debrah et al., 2022). The cassava was soaked in water for a day and removed to stand for 2–3 hours for water to drain off (Plate 4).

Plate 4. Soaked cassava.

3. Adult sourcing, sexing and culturing

Adult palm weevils were obtained from the HEALTHYNSECT Project Insectary. Setae on the male weevil’s rostrum, lacking on the females, were used to determine the gender of newly emerged weevils (Figure 1). The experiment was subjected to completely randomize design with four replications each for the three treatments. A pair (male and female) of newly emerged (virgin) adults were put into each of the 12 bowls containing the substrates and kept under in a laboratory condition with an average temperature of 29.3°C and average relative humidity of 73.9% RH. About 100 g of sugar was added to each substrate except the sugarcane substrate to promote egg laying by the adults.

Figure 1. Differences between the male and female adult weevil based on their mouthpart.

Afterwards, two halves of coconut choir were put in each substrate to serve as hiding places (crevices) for the adult during mating. Wire mesh and thin cloth were cut into sizes that could fit well on the lid of containers (Plate 5) to prevent adults from escaping and ensure flying insects and other unwanted materials do not enter the experimental setups. Afterwards, the experimental setups were shelved for mating and data collection (Plate 6).

Plate 5. Cutting of thin cloth.

Plate 6. Experimental set up.

4. Data collection and analysis

Data collected on growth parameters were number of eggs, egg hatchability, and percentage hatchability. The number of eggs were carefully counted by recording the number of eggs laid by each pair of adults every two (2) days for thirty(30) days with the help of a handheld magnifying glass. After counting, eggs were transferred onto new/fresh substrates using a camel hairbrush to monitor progeny emergence. Those eggs that were successfully hatched (1–24 hours old) were recorded. Percentage hatchability of eggs was determined by finding the proportion between eggs hatched and total number of eggs laid expressed as a percentage.

Data obtained were arranged with Microsoft excel and Analysis of Variance (ANOVA) was carried out using MiniTab (18^th Edition) software. Tukey’s Honest Significant Difference test was used to separate the means at 95% confidence level (p < 0.05).

5. Results and discussion

5.1. Number of eggs laid (fecundity)

The effect of different substrates on the number of eggs laid by the palm weevil adults during the experimental period is presented in Figure 2. Generally, there was significant difference (p < 0.05) among the various feed substrates used in rearing the palm weevil adults for all the days of the experimental period.

Figure 2. InInfluence of different feed substrates on the number of eggs laid by female adults (the bars represent the significant difference among the treatment at 5% probability level)

Note: Bars represent the significant difference among the treatment at 5% probability level

At the second day, palm weevil adult fed with palm yolk (PY), cassava (CS), and sugarcane (SG) recorded 2.0, 1.5, and 1.0 mean number of eggs, respectively. From Figure 2, there was a significant difference (p < 0.05) between the palm yolk and the sugarcane substrate. However, there was no significant difference (p > 0.05) between the palm yolk substrate and cassava substrate in terms of number of eggs laid.

On the 10th day, cassava recorded a mean number of eggs of 19.8, which was higher than what was recorded on sugarcane (15.0). Similar interaction was also observed between palm yolk (20.0) and sugarcane (15.0).

At 30 days, the palm weevil adults reared on the palm yolk significantly laid the highest number of eggs with an overall average of 76.2 compared to the other two substrates with sugarcane recording an average of 63.3 and 73.0 on cassava.

The estimation of the number of eggs laid by each pair of palm weevil adults is very important, especially during domestication of palm weevil larvae on different artificial feed substrates. This helps farmers to choose from a range of feed substrates suitable for palm weevil rearing and understand the life cycle. Other researchers (Ebenebe and Okpoko, 2016; Quaye et al., 2018) investigated egg-laying behaviour of R. phoenicis on different feed substrates under a controlled environment in the laboratory with 27 ± 2°C and 75 ± 2% RH average temperature and relative humidity, respectively. They opined that there was a significant (p < 0.05) effect of different feed substrates on the egg-laying behavior. The type of feed substrate available (Awmack & Leather, 2002) determines the female’s choice of oviposition site and egg safety due to the egg-laying behavior of the female where eggs are mostly laid in a cluster (Murphy & Briscoe, 1999).

Nevertheless, this study demonstrated that feeding adult African palm weevil with palm yolk results in higher number of eggs laid.

This could be that there were enough food materials in the phloem and xylem of the palm yolk that supported egg laying of the adult palm weevil, as noted by Miguens et al. (2011). Shahina et al. (2009) also concluded that female adult palm weevil prefers to feed and lay eggs on fresh cut palm yolk pieces. This implies that the palm yolk is favorable for the culturing of the African palm weevil to lay/produce more eggs. This could be attributed to their natural habitat of growth in the palm trees. However, it would help in the multiplication of the population of the African palm weevil for continuous production cycle, which would serve as food for consumption.

Sugarcane, which was the least performed substrate in the current study, has been noted by Resh and Cardé (2009) to be one of the best feed substrates for the rearing of palm weevil adult under laboratory conditions. It may be that sugarcane is more attracted to palm weevil larva than the adult palm weevil, which may be a key factor on their development and performance. They further emphasized that sugarcane is more resistant to rotting which gives it the capacity to last longer before depleting; as observed in the present study.

Nonetheless, Wai et al. (2015), using ratio and proportion, recorded similar average number of eggs, where they recorded 270 eggs within 9 weeks and 70 eggs within 4 weeks in her study. Hopkins and Ekbom (1999) also noted that egg laying continues in most species of Coleoptera until they die which normally takes place within 10–11 weeks.

5.1.1. Number of hatched eggs (hatchability)

Results in Figure 3 shows a significant difference (p < 0.05) among the three feed substrates on the number of eggs hatched throughout the experimental period. The palm yolk substrate exhibited superiority over the other two substrates (sugarcane and cassava).

Figure 3. Influence of different feed substrates on the number of eggs hatched.

Note: Bars represent the significant difference among the treatment at 5% probability level

For instance, at Day 1, there was a significant difference between the sugarcane (p < 0.05) substrate (1.0 eggs) and palm yolk substrate (2.0 eggs). However, there was no significant difference between the cassava substrate (1.3 eggs) and the two other substrates (sugarcane = 1.0 eggs and palm yolk = 2.0 eggs). This may be that female adults laid either large number of poor-quality eggs or few good quality eggs (El-Shafie et al., 2018; Rossiter, 1991; Shashina et al., 2009).

Lee and Lee (2004) also reported that the palm weevil adult fecundity, hatchability, development, survival, and longevity were much dependent on the type of feed substrate given to them. Palm yolk exhibited superiority over the other substrates, which is an indication that palm yolk substrate gives the best conducive environment and nutrient for the insect growth.

At the 11th day, sugarcane substrate recorded 5.3 eggs mean hatchability which was significantly different (p < 0.05) from cassava (8.3 eggs). Sugarcane has been noted to be the best medium to enhance egg laying and egg hatchability of the African Palm Weevil adult (Shashina et al., 2009). However, the sugarcane substrate was the least performed substrate, which recorded lowest egg hatchability. The cassava substrate performed better than all the other substrates in our present study but to the best of our knowledge it has not been reported in the literature as a feed substrate for the palm weevil adult. Generally, in a hierarchical manner, palm yolk substrate performed best followed by cassava and then sugarcane.

5.1.2. Percentage hatchability

From Figure 4, it was observed that there were no significant differences between the substrates (p > 0.05) for the percentage hatchability. For that matter, all substrates performed equally. The palm weevil adult fed with palm yolk substrate had 96.06% egg hatchability, cassava substrate had 96.59% egg hatchability, and sugarcane substrate produced 94.68% hatchability. Furthermore, this shows that the differences in feed do not have any effect on the percentage of eggs hatched. This implies that the percentage egg hatchability of palm weevil eggs is independent on the type of feed substrates. Kaakeh et al. (2001) recorded 77.7% of hatchability when the female palm weevils were fed with sugarcane substrate, which was lower than that of our present study. This observed discrepancy may be due to the variety of sugarcane used or may be attributable to other biotic and abiotic factors in the two different locations.

Figure 4. Influence of different feed substrates on the percentage of eggs hatched

Note: Bars followed by the same letter do not differ significantly by Tukey test (p ≤ 0.05).

Sharaby and Al-Dhafar (2013) in a research on how to rear palm weevil adult on feed substrates (date palm tree and sugarcane stem) under laboratory conditions observed that the percentage egg hatchability was dependent on the amount of moisture the substrate could hold. This finding is in tandem with this study where all the substrates were observed to have high moisture content and hence recorded almost the same percentage hatchability. The three substrates used in our present study had high water retention properties and homogenous environmental conditions which provided enhanced egg hatchability.

6. Conclusion

There were significant differences on the performance of the palm yolk, sugarcane, and cassava substrates regarding the number of eggs laid but percentage hatchability was not affected by the type of substrates fed to the palm weevil adult. Overall, it was evident from the results that the palm yolk substrate was the best performing substrate among the three substrates used in this study followed by the cassava substrate. Thus, cassava substrate could be the best alternative feed for the rearing of palm weevil adult in the absence of palm yolk. The use of cassava as a substrate for the rearing of Palm weevil should be promoted to reduce the indiscriminate felling down of palm trees in Ghana.

Acknowledgments

The authors would like to thank HEALTHYNSECT Project for sponsoring this research through the provision of logistics needed for conducting the study. Again, we would like to thank Alexander von Humboldt Foundation Project-UENR for allowing this research to be carried out at their insect laboratory.

Disclosure statement

No potential conflict of interest was reported by the author(s).