Abstract
Synthetic chemical pesticides can enhance crop yields but also have undesired effects. Alternative ‘botanical insecticides’ may also have non-target effects on pollinators and biocontrol services. Employing action thresholds (ATs) can reduce pesticide (whether synthetic or botanical) use compared to fixed-interval applications. Here the azadirachtin-based botanical formulation NeemAzal and a neem seed extract (NSE) were evaluated in field spraying trials alongside commonly-used synthetics (Voliam Flexi [chlorentraniliprole plus thiamethoxam] and imidacloprid) in developing ATs for the regular and cosmopolitan cauliflower pests Brevicoryne brassicae, Plutella xylostella and Spodoptera litura. We considered the size of the S. litura larvae infesting the crop in order to derive ATs. ATs per plant were higher for NeemAzal (0.55 larvae for P. xylostella and 3 larvae for large-sized S. litura) than for Voliam Flexi (0.30 larvae for P. xylostella and 0.80 larvae for S. litura) but were similar for B. brassicae (50 individuals). Higher ATs when using azadirachtin were associated with the diverse modes of action of botanicals, for instance NeemAzal and NSE deterred oviposition of S. litura. Although the exact values of ATs are likely to have regional limits, our approach can be applied for determining ATs against common lepidopteran and aphid pests in many other vegetable crop agro-ecosystems.
Highlights
Organic insecticides were introduced into agricultural production systems around eighty years ago
Spodoptera litura was the most abundant lepidopteran followed by P. xylostella but, when present, the numbers of aphids exceeded the numbers of lepidopterans by at least one order of magnitude (Supplementary Table S1)
With no insecticide applied, S. litura was recorded between early-August until the end of November, with peak abundancde around the end of September each year (Fig. 1a), while S. exigua, H. armigera and T. orichalcea were recorded for a shorter periods in November 2015 and October 2016 (Fig. 1a)
Summary
Organic insecticides were introduced into agricultural production systems around eighty years ago. Action thresholds can be developed by trialing a range of candidate ATs and subsequently adopting those that perform best and by using prior experience of the crop-pest relationship[6]. Due to their relative ease of derivation, ATs are adaptable and can be adjusted for planting dates[7], varieties, environmental conditions[6] and biocontrol services[9]. ATs have been used successfully for the management of many agricultural and horticultural pests[8,10] resulting in reduced use of agrochemicals[11] Their use is likely to be well suited to vegetable production systems as these often have unpredictable future crop prices, in developing countries. There has been almost no prior development of ATs using non-synthetic alternatives such as biopesticides based on plant-based products (botanicals)[11]
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