Management of Spoladea recurvalis (Lepidoptera: Crambidae) on amaranths using biopesticides

Abstract

Amaranths are African indigenous vegetables (AIVs) that are gaining popularity in several countries around the world due to their nutritional, medicinal and economic values. Insect pests are however a major challenge to optimum production of amaranths. The lepidopteran defoliator moth commonly known as Hawaiian beet webworm/amaranth leaf-webber, Spoladea recurvalis has been reported to be a major pest in amaranth fields, with the potential of causing complete defoliation under severe outbreaks. The most common management practice for S. recurvalis is the use of synthetic insecticides. However, resource-poor farmers cannot afford synthetic insecticides, application of insecticides poses health and environmental risks and indiscriminate use may lead to the development of insecticide resistance. Entomopathogenic fungi (EPF) and Bacillus thuringiesis (Bt) microbial pesticides have been suggested as the most promising alternatives to synthetic insecticides for management of various pests. This study, therefore evaluated the potential of fungus- and Bt-based products for the management of S. recurvalis. Twenty-four EPF isolates from three genera (14 Metarhizium anisopliae, 9 Beauveria bassiana and 1 lsaria fumosorosea) were screened in the laboratory to assess their pathogenicity against second instar larvae of S. recurvalis. Only M. anisopliae ICIPE 30 provided moderate control, causing 58.3% larval mortality. Eleven isolates tested against adult S. recurvalis, viz. 8 M. anisopliae, 2 B. bassiana and 1 l. fumosorosea, were pathogenic. Metarhizium anisopliae ICIPE 30 and B. bassiana ICIPE 725 caused the highest mortality of 92% and 83%, respectively. Metarhizium anisopliae ICIPE 30 had the shortest LT50 value of 4.8 days. Bacillus thuringiensis Subsp. kurstaki product Halt® caused between 40 and 50% mortality of S. recurvalis larvae. A consecutive application of M. anisopliae ICIPE 30 and Bt did not cause a significant increase in larval mortality compared to separate applications of both products. Compatibility of M. anisopliae ICIPE 30, the most effective fungal isolate against adult S. recurvalis and an attractant, Phenylacetaldehyde (PAA) was investigated under laboratory and field conditions. PAA completely inhibited germination of the conidia when put together in a desiccator in the laboratory. Effects of spatial separation of conidia and PAA in an autodissemination device were investigated in the field, and results showed that conidial germination was minimized by placing PAA at 5 and 10 cm cm from M. anisopliae ICIPE 30 conidia. Horizontal transmission of M. anisopliae ICIPE 30 between freshly emerged moths inoculated with fungal conidia (‗‗donors‖) and untreated freshly emerged moths (―recipients‖) maintained together for 24 hours was investigated in laboratory tests. Infected moths were able to transmit the infection to untreated moths resulting in 76.9% mortality with a LT50 value of 6.9 days. To improve the efficacy of B. thuringiensis Halt® against S. recurvalis larvae, 13 chemical additives (7 inorganic salts, 3 nitrogenous compounds, 2 protein solubilizing agents and 1 organic acid) were investigated. Boric acid was found to be the most effective additive and enhanced the potency of Bt by 2.9-fold. Boric acid and Bt could therefore be integrated in S. recurvalis IPM to reduce the use of synthetic insecticides in amaranth production systems.