Varietal Resistance and Use of Crude Plant Extracts in the Management of Field Pests in Green Gram (Vigna Radiata) in Machakos County, Kenya

Abstract

Breeding has significantly improved drought tolerance in green gram but marked yield losses continue to emanate from field pest’s damage. Important pests of green gram are pod borers, Maruca testutalis (Geyer), aphids, Aphis gossypii (Glover) and whiteflies, Bemisia tabaci (Gennadius). Management of these pests has partly been constrained by the limited understanding of crop traits that modulate their infestation. Continuous use of synthetic pesticides causes serious health and environmental consequences. To reduce these challenges, and lower the cost of production, farmers rely on indigenous knowledge to manage crop pests. Several plant species are known to have insecticidal properties against a range of insect pest. Despite growth on research, there is limited data on pesticidal plants and only a few of them have been commercialized for pest management hence remaining a small but growing component in crop protection. Field experiments were conducted in southeastern Kenya to evaluate a collection of old and new green gram varieties for tolerance to field pests, and to identify traits that confer resistance for objective one. The old varieties were KS20 and N26, both released in 1990s whereas the modern counterparts were Biashara, Karembo and Ndengu-Tosha. Evaluating the efficacy of four plant extracts of diverse species, including neem (Azadirachta indica), melia (Melia volkensii), tick berry (Lantana camara) and garlic (Allium sativum) in the management of green gram field pests was also studied. Respective plant extracts were prepared and applied at 10mL/20L of water at seven days interval and compared to lambda-cyhalothrin which was applied at the rate of 5mL/20L of water as a standard check and untreated control. Results showed significant differences amongthe varieties in maturity, leaf area, leaf hair density, leaf moisture content and pod wall thickness. Earliness significantly reduced pest infestation, whereby KS20 matured early while N26 was late,and the new varieties were intermediate. Leaf area (R2 = 0.52) and leaf moisture content (R2 = 0.71) positively correlated with pest infestation while leaf hair density (R2 = 0.47) and pod wall thickness (R2 = 0.58) showed a negative association with pod borer and aphid counts. Crops applied with tick berry and melia plant extracts showed significantly higher number of pod borers,aphids, whiteflies and natural enemies of about 70 -80% compared to neem and garlic which showed 40-50% of the pest numbers. Pod damage was highest in melia and tick berry ranging from 70-90% and lowest in neem and garlic with a range of 36 -63% accordingly. Crops sprayed with garlic extract out-yielded (2.8 t/ha) the other treatments while those applied with melia recorded the least grain yield (2.6 t/ha). However, objective one results did not reveal any particular traitsthat associated with either the old or new varieties, which implied that breeding of green gram in Kenya has not selected for tolerance to field pests. Nonetheless, green gram breeding programs could select for early maturity, open plant canopy, pubescent leaves and thicker pod walls to reduce field pests’ infestation. On objective two, the four plant extracts offered an effective control of keypest species, especially neem and garlic extracts, that was comparable in terms of yield harvested to the synthetic pesticide. Overall, the plant extracts had lower negative impact on the beneficial organisms compared with the synthetic insecticide. Results of this study imply that integrated field pest management practices in green gram could deploy varieties with morphological traits that impair pest infestation as well the incorporation of pesticidal crude plant extracts.