Investigation on the optimal term of one-fold insecticide application for decreasing onion thrips ( Thrips tabaci Lindeman, Thysanoptera, Thripidae) damage on early white cabbage

An impact of the term of one-fold insecticide application to reduce the damage of the onion thrips in early white cabbage was established. During the growing season plants were treated with insecticide (abamectin) in three different terms (treatment 1: 9 June, treatment 2: 16 June, treatment 3: 23 June, and treatment 4: untreated). On the exterior leaves of cabbage heads, treated with the insecticide, statistically significantly lower mean index of damage was determined as compared to the untreated plants. No significant differences were found between three different terms of application. The majority of the economically important damages in all of the treatments was found between the 3rd and 6th exterior leaf in the head. The highest mean weight of heads (1517.3 g) and mean net weight of heads I = mean weight of heads - weight of damaged and removed leavesl (1166.3 g) were established in plants which were treated the last. These parameters were the lowest in untreated plants (1083.3 g / 805.6 g). The yield loss due to damaged leaves removal amounted from 22.9% (treatment 2) to 25.6% (treatment 4). Based on the results obtained in this research we concluded that yield loss due to onion thrips attack in plants with one-fold insecticide application is not substantially lower as in untreated plants (though statistically significant differences between them were established), because insecticide cannot reach the interior leaves in the head. Substantial differences in the total and net weight of heads between the treatments and especially between the treated and untreated plants are explained by the fact that feeding of numerous thrips populations in the heads and on the exterior cabbage leaves negatively affects plant physiology and yield. The highest average yield in plants which were treated the last indicates a possibility that insecticide also inhibits plant growth and development to a certain extent.