Inhibition Of Fungicides On Yeast-like Symbiotes In The Brown Planthoppers, Nilaparvata Lugens(Sta) And Its Application

Yeast-like symbiotes(YLS) are endosymbionts that are closely related to the growth, development and reproduction of their host, the brown planthopper(BPH), Nilaparvata lugens(St?l)(Hemiptera: Delphacidae). In order to understand the relationship between the population of YLS in BPH and the survival rate of BPH, effects of seven different fungicides and two fungicide/insecticide mixtures on the number of YLS by haemocytometer and real-time quantification PCR and mortality of BPH were determined. Then the life table and Morris-Watt mathematical model were used as the methods of comprehensive evaluation to analysis the effect of screening fungicide/insecticide mixtures on population growth of BPH. The results are as follows:1. Seven kinds of fungicides were chosen to test their inhibiting effects on the total numbers of YLS in BPH. 5 fungicides : 70% fluopicolide & propamocarb hydrochloride SC, 50% iprodione SC, 40% pyrimethanil SC, 70% propamocarb hydrochloride AS and 70% propineb WP had no inhibition on YLS in BPH. Interestingly, 27% toyocamycin & tetramycin P & tetrin B & tetramycin A and 75% trifloxystrobin & tebuconazole WG, were found to significantly reduce the number of YLS in BPH, subsequently causing a high mortality of BPH. The results indicated that with the decrease of YLS number in BPH, the mortality of BPH was greatly enhanced.2. The two effective fungicides were each mixed with a commonly used insecticide-imidacloprid, and the fungicide/insecticide mixtures could cause a marked reduction in YLS number in BPH, resulting in a significantly higher mortality of BPH than did the imidacloprid alone. The results demonstrated a high dependence of the BPH survival rate on the number of YLS harbored in BPH fat-body cells. It implies that using specific fungicides as an additive to imidacloprid for controlling BPH could be a novel way to enhance the efficacy of insecticide, minimizing the use of imidacloprid in paddy fields.3. The impact of fungicide/insecticide mixtures on population growth of BPH was investigated by the life table and Morris-Watt mathematical model. The results showed that fungicide/insecticide mixtures had significantly inhibiting effects on the population growth of BPH. The trend index of BPH population growth in two treatments was obviously lower than that of positive control. The strongest inhibitory effect on BPH appeared in the egg hatch stage and the ovipositional stage of BPH adults.Our studies demonstrated the effectiveness of some fungicides on the abundance of YLS harbored in BPH and the mortality of BPH. This can provide a new way to suppress BPH, which is through inhibiting YLS by fungicides. So this approach will not only improve the inhibitory effect of imidacloprid on BPH, but also delay the development of BPH’s resistance to imidacloprid.