Insect pest problems of sunflower with particular reference to the sunflower beetle, Zygogramma exclamationis (Fabricus), in Manitoba
Deedat, Yakub Daud
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Data for this study were collected during the summers of 1983 to 1985. Field and grower surveys were conducted to determine the status of the sunflower-insect pest complex as well as their management by growers. Sunflower beetle, sunflower midge, sunflower maggot, banded sunflower moth and sunflower moth were the most prevalent species and found in at least 10 of the 12 fields that were surveyed. However, pests of primary concern to growers were the sunflower beetle, the sunflower midge and cutworms. Cultural control did not play a major role in the management of these pests. More than 80% of the growers indicated that they use economic thresholds when available. However, data from the field survey on sunflower beetle showed that growers applied insecticides at well below the currently recommended threshold values. Insect densities were manipulated on individual plants in caged and open studies to determine the effect of defoliation of sunflower by sunflower beetle. Head diameter, yield and seed weight decreased, whereas oil content increased with increasing beetle densities. A curvilinear relationship between pest density and yield was obtained for adults and larvae. These quadratic polynomial regressions were used to estimate the economic thresholds which ranged from 2-3 adults and 5-10 larvae per plant. Sunflower plants were artificially defoliated at five growth stages using four levels of defoliation to simulate time of damage and time in insecticide application in relation to the sunflower beetle. Growth and yield components generally decreased with increasing levels of defolation. Plants in their early growth stages (V4-V8) were sensitive to defoliation. Plants showed compensation at low levels of defoliation. Depending on the duration of defoliation, a linear or a quadratic relationship between yield and defoliation was obtained. These relationships were used to calculate defoliation thresholds for individual growth stages. A sequential decision plan for the control of sunflower beetle larvae was developed from studies of larval dispersion in growers' fields. Iwao's patchiness regression technique was used as a basis for the sequential model since it provided a consistently good fit to the data. The plan should minimize sampling efforts and provide a reliable method of assessing whether control is required.