Agromizid Leafminer infestation of different tomato (Lycopersicon lycopersicum) varieties and an assessment of the effectiveness of Azadirachta indica extracts as a management technique.

Abstract

The relative susceptibilities of 14 tomato varieties (“Rodade”, “Nemonetta”, “Rio Grande”, “Season Red”, “Little Wonder”, “Money Maker”, “RVF”, “Cal-J”, “RVFN”, “Star 9065”, “Floradade”, “Rodade Plus”, “Heinz 1370” and “Campbell”) to leafminer (Liriomyza spp.) (Diptera: Agromyzidae) infestation were studied during the months May-June (cold-dry season) and September-November (hot-dry season) in 2015 at the University of Zimbabwe, Harare. Tomato plants were transplanted into pots four weeks after sowing and placed in an open field in a Randomized Complete Block Design. Varietal susceptibility was assessed by recording the number of mined leaves and number of mines per plant. During the cold-dry season, the varieties “Nemonetta”, “RVF”, “Rio Grande”, “Star 9065”, “Rodade plus” and “Season Red” recorded overall low levels of infestation, with no significant differences being noted among them. On the other hand, “Little Wonder”, “Rodade”, “Campbell” and “Floradade” recorded high numbers of mines/plant. Results indicated that “Nemonetta” and “Rodade” were the most susceptible varieties during the hot-dry season, while “Little Wonder”, “Cal-J”, “RVFN”, “Heinz 1370”, “Season Red”, “RVF” and “Star 9065” had the lowest number of mines/plant. Results also indicated that leafminer activity and damage were highest during the hot-dry season. Field and laboratory trials were also carried out to assess the effectiveness of three rates (20, 30 and 40 ml per 10 litres of water) of a commercial neem (Azadirachta indica) oil formulation (Achta® 1% EC) and the insecticide abamectin in controlling leafminers. Leaf dip bioassays were later carried out in the laboratory in order to determine the toxicity of Achta 1% EC under more controlled conditions. Results from the field and laboratory studies showed that Achta 1% EC applied at 40 ml per 10 litres of water was just as effective as the insecticide abamectin. Achta 1% EC is therefore a suitable biopesticide for inclusion in the basket of chemicals which can be used in rotation by farmers so as to slow down insecticide resistance development by Liriomyza spp. Further studies are, however, recommended in order to determine if Achta 1% EC has any adverse effects on leafminer parasitoids. It is also important to assess for Achta 1% EC phytotoxic effects on tomato plants under a broad range of environmental conditions.