Phytochemical Characterization and Biopesticidal Activity of Aqueous Plant Extracts Against French Bean Thrips and Aphids

Abstract

In Kenya, French bean (Phaseolus vulgaris L.) is grown by small scale farmers mainly for exports markets. Production is majorly affected by due to moisture stress content, low soil fertility, insect pests such as bean flower thrips and black bean aphids. The use of synthetic pesticides in the management of agricultural pests over the years has been associated with many challenges such as health risks to humans and the environment. This has led to a growing interest on development of alternative strategies of plant pests’ management that possesses less, to no harmful effects to the ecosystems and human health. Some of these alternative strategies include the use of plant based biopesticides such as which can be either commercial or homemade plant extracts. The plant based biopesticides have an advantage over synthetic pesticides, as they are biodegradable, and have minimal harmful effects to beneficial insects and are safer to the environment. Plants such as such as Azadirachta indica, Capsicum frutescens, Allium sativum, Lantana camara and Tagetes minuta have been used previously in the management of various pest by farmers individually and in mixture forms. However, information from the literature, on the efficacy of the mixture from the five plants above on aphids and thrips affecting French beans is lacking. This gap in knowledge necessitated the need to validate the efficacy of those plant extracts that organic farmers based in Kangari in Murang’a county were using on pests affecting their Frech beans, garden peas and other vegetables like cabbage in other study areas. This study evaluated the efficacy of homemade plant extracts comprising of Capsicum frutescens, Allium sativum, Lantana camara Tagetes minuta and Azadirachta indica. The plant materials were sourced from different locations as follow; leaves of Lantana camara leaves, Tagetes minuta, were collected from ICIPE, fruits of Capsicum frutescens and bulbs of Allium sativum were purchased from local grocery stores and neem was purchased from the OACK in Kangari, Murang’a. During the efficacy trials, the plant extracts were used individually and in a mixture form where one mixture of plant extracts was fermented while the other two were unfermented. The efficacy of the above plants above were tested against bean flower thrip- Megalurothrips sjostedtiis (Trybom) and black bean aphid - Aphis fabae. The efficacy of homemade plant extracts was carried out against a commercially available plant based biopesticides namely Pyeneem® (Pyrethrin 10g/L + Azadirachtin 10g/L) as a positive control. The efficacy trials were done in controlled, semi controlled, and open field environments. The results from efficacy trials revealed that there was significant difference among various concentrations across different treatments against bean flower thrips and bean black aphid. In the laboratory, the mean percent mortality induced by unfermented plant extract from Tagetes minuta, Lantana camara, Capsicum frutescens, Allium sativum combined with Azadirachta indica (UPE + N) against thrips was 77.3% which was comparable to that of the positive control Pyeneem® at 87.5 % A similar trends was observed in the screenhouse, In open field trials, at Chuka and Kalro Kandara, the effect of the unfermented plant extract from Tagetes minuta, Lantana camara, Capsicum frutescens, Allium sativum combined with Azadirachta indica on the mean number of bean flower thrips and that of black bean aphid was comparable to that of Pyeneem at 3.2 and 4.9 (bean flower thrips) and at 14.4 and 20.3 (black bean aphid) respectively. The phytochemical screening of individual and mixed plant extracts was carried out using qualitative methods. However, profiling of bioactive compounds in the plant extracts associated with insecticidal activities was done using liquid chromatography mass spectrometry. Polyphenols such as flavonoids, phenolic acids, and terpenes were detected in the mixture in varies masses. Moreover, mixing the various plant extracts such as T. minuta, L. camara, C. frutescens, A. sativum combined with A. indica in tap water had a multiplicative rather than additive synergistic of the bioactive compounds as compared to the use of single plant species. The study showed the potential of using mixed plant extracts from different plant species in the management of French beans pests. Thus, the use of plant extracts can be a useful tool in the management of pests as they can drive sustainable agriculture by promoting productivity, while preserving biodiversity which is in line with Sustainable Development Goals.