Optimizing Western Flower Thrips Management on French Beans by Combined Use of Beneficials and Imidacloprid

Abstract

Western flower thrips (WFT), Frankliniella occidentalis (Pergande), is an important pest of vegetable crops worldwide and has developed resistance to many insecticides. The predatory mites Neoseiulus (=Amblyseius) cucumeris (Oudemans), the entomopathogenic fungus Metarhizium anisopliae (Metsch.), and an insecticide (imidacloprid) were tested for their efficacy to reduce WFT population density and damage to French bean (Phaseolus vulgaris L.) pods under field conditions in two planting periods. Metarhizium anisopliae was applied as a foliar spray weekly at a rate of one litre spray volume per plot while imidacloprid was applied as a soil drench every two weeks at a rate of two litres of a mixture of water and imidacloprid per m². Neoseiulus cucumeris was released every two weeks on plant foliage at a rate of three mites per plant. Single and combined treatment applications reduced WFT population density by at least three times and WFT damage to French bean pods by at least 1.7 times compared with untreated plots. The benefit-cost ratios in management of WFT were profitable with highest returns realized on imidacloprid treated plots. The results indicate that M. anisopliae, N. cucumeris, and imidacloprid have the potential for use in developing an integrated pest management program against WFT on French beans.

Keywords: Amblyseius; Frankliniella occidentalis; Phaseolus vulgaris; benefit-cost ratio; entomopathogenic fungus; neonicotinoid; predatory mite.

Figure 1

Mean (± SE) number of adult western flower thrips per plant recorded on field-grown French beans in the various treatments over time by plant tapping in March to June (A) and June to September (B) 2011 at KALRO-Embu, Kenya. Where = application of imidacloprid in respective treatments; = release of Neoseiulus cucumeris in respective treatments; and = application of Metarhizium anisopliae in respective treatments.

Figure 2

Mean (± SE) number of larval western flower thrips recorded on field-grown French beans in the various treatments over time by plant tapping in March to June (A) and June to September (B) 2011 at KALRO-Embu, Kenya. Where = application of imidacloprid in respective treatments; = release of Neoseiulus cucumeris in respective treatments; and = application of Metarhizium anisopliae in respective treatments.

Figure 3

Mean (± SE) number of larval and adult western flower thrips recorded in flowers of field-grown French beans in various treatments in March–June (A) and June–September (B) 2011 at KALRO-Embu, Kenya. Data are for 21 days after treatment application. Means followed by the same letter in each category indicate no significant differences in the population density of thrips between treatments by post hoc comparisons using Tukey’s HSD test: p < 0.05.