Relationship between Spodoptera frugiperda (Lepidoptera: Noctuidae) damage and yield loss in maize
Fall armyworm (FAW), Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae) feeding on maize results in extensive foliar damage if plants are attacked during the pre-flowering stages. Infestation during late plant growth stages results in ear damage. Insecticide application is the most common method of FAW control and is extensively applied in some farming systems and often without any consideration of infestation levels or estimated yield losses. While some information on FAW infestation and yield loss are available from South American studies, little information is available on the relationship between infestation level, severity of damage and plant response to injury in Africa. Decisions to apply insecticides for FAW control should be based on infestation levels, expected yield losses and cost of control. The lack of data on infestation levels and yield loss, plant growth at time of infestation and insecticide application, and relationship between severity of damage and yield loss were addressed in this study. This study also investigated the efficacy of granular insecticide formulations, to address the poor levels of control of FAW larvae inside plant whorls that are generally reported for spray applications. Field trials were conducted at different localities and under either natural or artificial infestation. Treatments in the various trials consisted of spray applications at different plant growth stages to keep plants pest-free for certain periods of time, infestations at different plant growth stages, and studies on the relationship between the degree of leaf damage, determined on a 1 – 9 scale, and yield. A laboratory as well as a field trial were conducted to evaluate the efficacy of granular formulations applied into plant whorls. Granular insecticides used were: spinosad SC, beta-cyfluthrin GR, carbaryl GR and a diatomaceous formulation. Insecticide applications during the V7-growth stage, four to five weeks after seedling emergence, resulted in the highest yield gain. The degree of damage to plants did not correlate strongly with yield and yield loss but was strongly dependent on the plant’s growth stage. Yield losses increased with increased incidence of infested plants per plot. The largest yield gains were obtained in plots where protection was implemented when 30 – 60% of plants were infested. Plots that remained unprotected during the three field trials suffered yield losses of 41.9, 26.5 and 56.8%, respectively. Yield gains were not significantly higher in any treatments that received more than three insecticide applications. Yield was higher in plots that received protection during early vegetative stages compared to protection during later growth stages. Granular insecticides were not as effective as the foliar spray application and larval mortality ranged between 2.5% (carbaryl GR) and 87.5% (beta-cyfluthrin GR). Spinosad dust and beta-cyfluthrin were effective in controlling FAW larvae but only under laboratory conditions. Granular insecticide formulations can be used by small scale farmers as an alternative control measure against FAW.