The effect of water volume and dosage rate on the efficacy of Break-Thru S240 for stem borer control
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The two most prominent stem borer species responsible for damage to maize in South Africa are the African maize stem borer, Busseola fusca (Fuller) (Lepidoptera: Noctuidae) and spotted stem borer, Chilo partellus (Swinhoe) (Lepidoptera: Crambidae). Chemical control of stem borers is particularly important because these pests may cause yield losses of up to 80 % on individual fields. Cryptic feeding, overlapping of generations and recurrence of infestation of the same planting at later crop growth stages are some of the factors hampering effective chemical control of these pests. In some situations C. partellus and B. fusca may occur as mixed populations in the same planting which complicates chemical control. Water volume and the correct insecticide dosage rate are two crucial aspects of chemical control. The addition of organo-silicones to insecticide spray applications may increase the efficacy of such sprays applied for stem borer control. The aim of this study was to determine the effect that the tank-mix organosilicone Break-Thru S240 has on the efficacy of insecticides applied for control of stem borers in maize and to evaluate the effect of this adjuvant on movement of spray applications into whorls of maize plants. Field trials were conducted between January 2007 and May 2008. Maize plants that were used were in the whorl stage approximately five weeks after emergence. All applications were done by means of a C0²-presurised knapsack sprayer and were directed into the whorls of plants. Prior to this study the feeding site of stem borer larvae inside plant whorls was described as "deep inside" the whorl or in the "yellow-green" area of whorl leaves. The position of larval feeding damage caused by B. fusca and C. partellus in whorl leaves was similar. Leaves 3 and 4 made up the largest proportion of damaged leaves and any application that moves further than the 80 and 70 % distances on leaves 3 and 4 respectively, can be considered to be successful. Break-Thru S240 and Agral was superior to other tank-mix adjuvants that were tested. The addition of Break-Thru S240 to different insecticides applied against C. partellus resulted in an increase in efficacy, measured in terms of larval mortality, of between 14 and 58 % for the insecticides. However, Break-Thru S240 did not result in increased efficacy of systemic insecticides, measured over the 14-day period. The efficacy of insecticides applied against B. fusca decreased from 98 % on day 2 to 30 % on day 14. The effect of Break-Thru S240 on the distance of movement of spray applications down into whorl leaves were determined by applying different dosages of the organo-silicone at a single water volume as well as applying the organo-silicone at a single dosage with different water volumes. This resulted in increased movement of 9 and 12 % compared to the control treatment respectively when Break-Thru S240 was applied at the registered dosages of 100 and 200 ml ha¯¹. The addition of Break-Thru S240 to water volumes of 100 and 200 1 ha¯¹ resulted in an increase in the distance of movement down whorl leaves of 12.5 % compared to 50 1 ha¯¹. This study provided data on the effect of Break-Thru S240 on spray applications that will be used for product registration purposes.