Comparative study of biological activity of fluorinated 5-aminopyrazoles on Spodoptera frugiperda

The new sowing methods have increased the incidence of at least two species of lepidopteran in corn crops: the fall armyworm, Spodoptera frugiperda, and the corn earworm, Heliothis zea. The fall armyworm (FAW) Spodoptera frugiperda (J. E. Smith, 1797) (Lepidoptera: Noctuidae) is a widespread plague that attacks maize (Z. mays L.), which is grown in summer and winter crops (Campos, Ferreira, Costa, & Lasmar, 2014). The fall armyworm is a highly migratory polyphagous lepidopteran. It can colonize over 80 different plant species including grasses and crops such as alfalfa, soybean, sorghum, and corn. It is a harmful insect for sweet corn and vegetable crops (Bohnenblust & Tooker, 2012). It can be found in Southern USA, México, Central and South America (Murúa, Molina-ochoa, & Coviella, 2006), (Farías et al., 2008). The fall armyworm affects the quality of plants because the larvae feed on leaf margins and reduces the plant vigour. The greatest damage, though, is caused when they bite the crown of the plants, which causes weakness, poor development and, eventually, the death of the plant (Kumar & Mihm, 2002). Larval densities are often reduced to one or two per plant in heavy infestation as larvae can exhibit cannibalistic behaviour (Bohnenblust & Tooker, 2012).

It is well known that this lepidopteran is very useful for preliminary studies on new target compounds for insecticidal activity. Particularly, Spodoptera frugiperda has been used for many bioassays with N-phenylpyrazoles (Jensen-Korte et al., 1990) (Leslie et al., 1999),(Wang et al., 2013). As part of our research project in sustainable agrochemistry in a previous work, a series of 5-amino-1-aryl-1H-pyrazole-4-carbonitrile fluorinated were synthesized from aryl hydrazine and (ethoxymethylene)malononitrile (Plem, Müller, & Murguía, 2015) using a simple methodology. As excellent stereoselectivity and yields were obtained, we tested some of the compounds containing fluorinated atoms in the aryl part of molecule. These compounds are considered to be precursor analogues to Fipronil. Fipronil was the first phenylpyrazole introduced in the market for pest control (Jiang, Zheng, Shao, Ling, & Xu, 2014). It is noteworthy that substances belonging to the phenylpyrazole family have mainly herbicidal effects, whereas Fipronil is a powerful insecticide (Simon-Delso et al., 2015).

 In this work, the 5-amino-1-aryl-1H-pyrazole-4-carbonitrile synthesis was tested on the fall armyworm Spodoptera frugiperda.