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In the fight against arthropod crop pests using plant secondary metabolites, most
research has focussed on the identification of bioactive molecules. Several hundred
candidate plant species and compounds are now known to have pesticidal properties
against a range of arthropod pest species. Despite this growing body of research,
few natural products are commercialized for pest management whilst on-farm use of
existing botanically-based pesticides remains a small, but growing, component of crop
protection practice. Uptake of natural pesticides is at least partly constrained by limited
data on the trade-offs of their use on farm. The research presented here assessed
the potential trade-offs of using pesticidal plant extracts on legume crop yields and
the regulating ecosystem services of natural pests enemies. The application of six
established pesticidal plants (Bidens pilosa, Lantana camara, Lippia javanica, Tephrosia
vogelii, Tithonia diversifolia, and Vernonia amygdalina) were compared to positive and
negative controls for their impact on yields of bean (Phaseolus vulgaris), cowpea (Vigna
unguiculata), and pigeon pea (Cajanus cajan) crops and the abundance of key indicator
pest and predatory arthropod species. Analysis of field trials showed that pesticidal plant
treatments often resulted in crop yields that were comparable to the use of a synthetic
pesticide (lambda-cyhalothrin). The best-performing plant species were T. vogelii, T.
diversifolia, and L. javanica. The abundance of pests was very low when using the
synthetic pesticide, whilst the plant extracts generally had a higher number of pests
than the synthetic but lower numbers than observed on the negative controls. Beneficial
arthropod numbers were low with synthetic treated crops, whereas the pesticidal plant
treatments appeared to have little effect on beneficials when compared to the negative
controls. The outcomes of this research suggest that using extracts of pesticidal plants
to control pests can be as effective as synthetic insecticides in terms of crop yields
while tritrophic effects were reduced, conserving the non-target arthropods that provide
important ecosystem services such as pollination and pest regulation. Thus managing
crop pests using plant secondary metabolites can be more easily integrated in to
agro-ecologically sustainable crop production systems. |
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