Efficacy of indigenous pesticidal plants for tomato and brassica pest management in Malawi

The cultivated tomato, Lycopersicon esculentum, and rape (Brassica napus) are the most important horticultural vegetable crops grown in southern Africa. However, their production is highly constrained by insect pests including red spider mites Tetranychus evansi and the aphids (Brevicoryne brassicae and Myzus persicae). The results on field surveys revealed that majority of smallholder farmers have considerable knowledge about pesticidal plants and reported Tephrosia vogelii, Vernonia amygdalina, Tithonia diversifolia and Azadirachta indica as particularly important. T. vogelii was the most popular species in Zambia (60.7%) and Malawi (53.4%) while brassicas constituted 34% and 44% of all vegetables grown in Malawi and Zambia respectively. Although insect pest infestation during the field trials was low but nonetheless, significant differences were observed between pesticidal plant extracts treated plots and the control. Use of pesticidal plants resulted in significantly lower damage to vegetables. The results further revealed that pesticidal plant products as compared to untreated control decreased the incidence of red mites, aphids and diamond back moth significantly. Although synthetic insecticides were most effective against these pests, the pesticidal plant extracts examined offered valuable contribution to pest management efforts. T. diversifolia was the most effective at reducing numbers of both red spider mites and aphids followed by T. vogelii. Crude extracts applied at a concentration of 10% w/v reduced effect onthe abundance of red spider mites and aphids significantly (P<0.001) when compared to control treatment. All the plant extracts tested had some level of toxic effect at 4% w/v. Yields of tomato treated with pesticidal plants ranged from 24,414 to 38,320 Kg ha-1 compared to control plots (21,590 Kgha-1). Vegetable damage due to levels of insect pest (aphids, red mites and diamondback moth) infestation varied from 6 to 45%. T. vogelii was a promising species in field and laboratory trials and analysis of the extracts identified several rotenoids as the biologically active components. Laboratory studies showed for this first time that the rotenoids tephrosin and deguelin were toxic against red mite and aphids. However, field experimentation with T vogelii showed that compounds were not present on the leaf surface when applied on bean leaves indicating total degradation of the compounds under the sun after three days. The study has also shown that there is substantial temporal and spatial variation in the occurrence of these rotenoids, which might affect harvesting protocols. Pesticidal plant extracts can improve vegetable production for resource poor farmers at controlling vegetable pests at much reduced costs as claimed by farmers since most plant materials are found locally or can easily be cultivated.

Name dc:type.qualificationname

phd

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of Greenwich

Year dc:date.issued

2015

Author dc:creator

• Nyirenda, Stephen Pearson Maxon

Advisors dc:contributor.advisor

• Belmain, Steven R.
• Stevenson, Philip

• SB Plant culture

Language dc:language

en