SUBLETHAL EFFECTS OF THREE DIFFERENT INSECTICIDES ON Hadronotus pubescens (HYMENOPTERA: SCELIONIDAE), AN EGG PARATISITOID OF Riptortus pedestris (HEMIPTERA: ALYDIDAE)

Abstract

Riptortus pedestris (Hemiptera: Alydidae) is one of the most damaging insects of leguminous crops in Eastern Asia and has become a new pest in Bangladesh. Hadronotus pubescens (Hymenoptera: Scelionidae) is one of the active egg parasitoids of R. pedestris available in Bangladesh. Spinosad, pyriproxyfen and pymetrozine+ nitenpyram are commonly used to manage the stink bug populations. In this study, sublethal effects of spinosad, pyriproxyfen and pymetrozine+ nitenpyram were investigated on the parasitized host eggs by H. pubescens at different days after parasitization. At 1, 7 and 15 days after parasitization by H. pubescens, the parasitized eggs were diped in spinosad, pyriproxyfen and pymetrozine+ nitenpyram solution. All of the three insecticides reduced the number of emerged parasitoids, emergence rate, sex ratio and increased the mortality rate as well as development time of both male and female. The maximum number of emerged parasitoids was reduced by spinosad. On the other hand, it reduced the emergence rate by 41.72% and increased the mortality rate by 41.72% at 15 days after parasitization. Pyriproxyfen showed the lowest reduction in emergence rate and lowest increase in mortality rate at 1 day after parasitization. Spinosad reduced the sex ratio of H. pubescens by 0.09 compared to control at 1 day after parasitization. The least reduction was showed by pymetrozine+ nitenpyram in both 1 day and 15 days after parasitization. However, pymetrozine+ nitenpyram showed the maximum increase in development time of both male and female parasitoids at 1 day after parasitization, whereas the minimum increase was also caused by pymetrozine+ nitenpyram at 7 days after parasitization. Therefore, it can be said that, spinosad showed the strongest toxicity and most potential sublethal effect on the H. pubescens egg parasitoid. The sublethal effect of pyriproxyfen was the weakest of all insecticides applied. So, pyriproxyfen can be suggested to use in mung bean and soybean fields as it showed less toxicity and sublethal effect on the host eggs parasitized by H. pubescens.