Organic Tomato Production in Alabama: Host Preference of the Tomato Hornworm (Manduca quinquemaculata) and Performance of Selected Biopesticides

Abstract

Organic farming largely excludes the use of chemical fertilizers, synthetic pesticides, genetically modified organisms, antibiotics, and growth hormones. Organic food production in the Southeastern United States is low and not reflective of the national trend. Warm temperatures and high rainfall patterns in this region cause a rapid decomposition of soil organic matter and high insect pest populations; both conditions do not augur well for vegetable production. The specific objectives of this study were to (1) conduct insect host-preference assessments using three popular tomato cultivars and 2) assess efficacy and cost effectiveness of selected biopesticides against tomato hornworm. Field trials involving three tomato cultivars: Celebrity, Mountain magic and Rocky top were conducted at the George Washington Carver Agricultural Experiment Station Organic Research Farm, Tuskegee University Alabama in 2018 and 2019. The experiments were set up as a Completely Randomized Design (CRD) with 3x4 factorial treatment arrangement (i.e., 3 tomato varieties and 4 spray treatments) replicated 4 times. An assessment of relative performance and cost-effectiveness of the biopesticide active ingredients: Azadirachtin, Spinosad, and Pyrethrin against hornworms on tomato was done. An improvised Economic threshold (ET) of one adult hornworm per 10 foot-row of tomatoes was used. Biopesticides were sprayed on designated plots when visual sampling revealed the attainment of ET populations. The hornworm counts at different sampling dates were analyzed using SAS statistical software. Tomato hornworms showed equal preference for Celebrity, Mountain magic and Rocky top tomato cultivars. Plots treated with the candidate biopesticides recorded similar hornworm populations as untreated control plots in 2018 whereas in 2019, Spinosad and Azadirachtin performed better than the control. Based on the total volume of biopesticide used, per unit cost of each biopesticide, and reduction of hornworms in treated plots, none of the biopesticides was cost-effective in 2018. This is because none of them was effective (i.e., performed better than untreated controls) against the hornworm. In 2019, however, the use of Spinosad and Azadirachtin resulted in hornworm counts that were significantly lower than those recorded in the control study. However, these significant differences in hornworm populations did not translate into differences in tomato yields. Except for a significantly lower hornworm population observed approximately 57 DAT, pyrethrin treatments resulted in hornworm populations that were comparable to those recorded on control plots. Insignificant effects on tomato yield renders moot, any computations of cost-effectiveness. Pyrethrin is clearly the least expensive option but cannot be described as the most cost-effective.