Screening of high effect and low toxicity insecticides for controlling Thrips hawaiiensis Morgan

Abstract: 【Objective】Thrips hawaiiensis Morgan(Thysanoptera: Thripidae) is one of the most common flower-dwelling thrips, and considered as one of dominant insect pests of many fruits and vegetables in tropical and subtropical area. At present, this species damages many important plants worldwide, especially causes large loss on banana. Because of its undeniable invasive capacity, high fecundity and hidden habits, more attention is being paid to its introduction and spread around the world. Chemical control has been the primary strategy for this pest, especially in flower season in plantation, where a great number of specific treatments against T. hawaiiensi are applied. Nevertheless, in any given situation and crop, the range of compounds and formulations which are effective for the control of the thrips is quite limited. Recently, some growers reported control failures against the thrips, most possibly because insecticide resistance has developed. However, there is a fundamental lack of knowledge with regard to control efficiencies of novel insecticides against such thrips, it is critical for pest management in case of its population outbreaks. The current study aims at screening novel and effective insecticides for controlling T. hawaiiensis in banana plantation. The current study can help understand integrated pest management(IPM) of T.hawaiiensis by scientific and reasonable insecticide use.【Methods】Laboratory toxicity bioassays and field control tests were carried out for determining insecticide efficiency against T. hawaiiensis. Abamectin and acetamiprid were served as control, because the two insecticides were commonly used in the plantations.We also selected eight novel insecticides, including cyantraniliprole, chlorantraniliprole, sulfoxaflor,spinetoram, dinotefuran, metaflumizone, thiacloprid, spirotetramat. The above insecticides were purchased from market. Laboratory tests were conducted for determining the toxicities of insecticides by using leaf-tube residual bioassay. Both 2nd-instar larvae and adults of T. hawaiiensis from standard laboratory strain were conducted for determining LC50 through toxicity equation by DPS software. Field experiments were carried out in banana plantation in Chengmai, Hainan province, and both bud-spraying method with a sprayer and bud- injection method with a syringe were applied. The completely randomized block design was involved in the study. The experiments were started from the emergence of flower bud,and ended at the open of the 8th bud. During the experiment period, four times of insecticide application were carried out in May in 2015 with 4 days interval. We evaluated the control efficiencies and fruit damage rates depending on the number of black spots caused by T. hawaiiensis oviposition.【Results】Laboratory tests demonstrated that the toxicity( LC50) order on 2nd-instar larvae was spinetoram(0.19 mg·L^-1)>cyantraniliprole(1.76 mg·L^-1)>spirotetramat(2.09 mg·L^-1)>metaflumizone(4.66 mg·L^-1)>dinotefuran(5.34 mg·L^-1)>abamectin(7.35 mg·L^-1)>sulfoxaflor(8.57 mg·L^-1)>acetamiprid(10.42 mg·L^-1)>thiacloprid(14.95 mg·L^-1)>chlorantraniliprole(48.34 mg·L^-1). While this was spinetoram(0.68 mg·L^-1)>cyantraniliprole(3.94 mg·L^-1)>spirotetramat(7.59 mg·L^-1)>dinotefuran(10.25 mg·L^-1)>abamectin(11.42 mg·L^-1)>metaflumizone(11.97 mg·L^-1)>acetamiprid(13.99 mg·L^-1)>sulfoxaflor(14.95 mg·L^-1)>thiacloprid(26.66 mg·L^-1)>chlorantraniliprole(62.76 mg·L^-1)when the insecticides were used against adult thrips. The above results indicated that insecticides of spinetoram, cyantraniliprole, dinotefuran and spirotetramat showed higher toxicity to T. hawaiiensis when compared to acetamiprid and abamectin(control). It also revealed that larvae were more sensitive to insecticides than the adult. When bud-spraying method was used in the field experiments, only spinetoram and spirotetramat showed over 80% of control efficiency on T. hawaiiensis, followed by cyantraniliprole with 76.94%. The above three insecticides performed significantly better than abamectin and acetamiprid(control), while chlorantraniliprole was the lowest, with 37.78%. The control efficiencies of sulfoxaflor, dinotefuran and metaflumizone were almost similar to abamectin and acetamiprid, with 70.63%, 72.09% and 67.23% respectively, which indicated that those insecticides would also be used in the banana plantation to control T. hawaiiensis, except that thiacloprid and chlorantraniliprole appeared to be ineffective to control thrips when using bud-spraying method. With regard to bud- injection method, spirotetramat showed the highest control efficiency with83.14%, following by spinetoram and cyantraniliprole with 75.73% and 73.38% respectively. For abamectin and acetamiprid, the control efficiencies were 66.91% and 49.75%, while those declined to 37.86%,43.28% and 44.98% for dinotefuran, thiacloprid and metaflumizone. Furthermore, chlorantraniliprole and sulfoxaflor had similar effect on thrips when compared to conventional insecticides, abamectin and acetamiprid. The above results indicated that novel insecticides of spirotetramat, spinetoram, cyantraniliprole,chlorantraniliprole and sulfoxaflor could be recommended for controlling T. hawaiiensis on banana by using bud-injection method.【Conclusion】Laboratory tests indicated that toxicities of novel insecticides in-cluding spinetoram, cyantraniliprole, spirotetramat, metaflumizone and dinotefuran were higher than that of conventional insecticides, abamectin and acetamiprid. Field experiments demonstrated that spinetoram,spirotetramat and cyantraniliprole were more efficient than other insecticides when using bud-spraying and(or) bud-injection method. The novel insecticides spinetoram, spirotetramat and cyantraniliprole were considered as optimal pesticides for controlling T. hawaiiensis. Furthermore, to restrain the cardinal population of T. hawaiiensis, insecticides with periodic application should be conducted at the time of the emergence of flower bud. These findings suggest that rotations and mixtures of above insecticides may be an effective management strategy.