Sogatella furcifera (Horvath, 1899)

Liu, Lei-Lei, Dai, Ren-Huai, Yang, Hong & Jin, Dao-Chao, 2016, Sublethal effects of triazophos on the life table parameters of Sogatella furcifera (Hemiptera: Delphacidae), Florida Entomologist 99 (2), pp. 292-292 : 292-

RELATIVE TOXICITY OF TRIAZOPHOS TO S. FURCIFERA

The sensitivity of the 3rd instar nymphs and adults was determined for each rice stem impregnation method, and detailed results are shown in Table 1 . Based on this method and 5 replications, the LC50, LC25, and LC10 values of triazophos to 3rd instars 72 h afer treatment were 13.63, 5.86, and 2.74 mg [AI] per L, respectively (P <0.05) ( Table 1 ). The sublethal concentrations LC25 and LC10 were used as the reference in further experiments ( Table 1 ).

NYMPHAL DURATION OF S. FURCIFERA

The F1 progeny in populations derived from adults that had been treated as 3rd instar nymphs with sublethal concentrations of triazophos had longer development periods compared with the control population ( Table 2 ). The entire nymphal periods in F1 populations afer treatment of the P generation with LC10 and LC25 were delayed by 0.92 and 3.16 d, respectively, compared with the control population. Compared with the control, every nymphal instar at LC10 was prolonged, except for the 3rd instar. The development period of every instar at LC10 was shorter than of those at LC25. The nymphal durations at LC25 were significantly longer than at LC10 and the control (P <0.05). These results indicated that sublethal concentrations had a more negative effect on S. furcifera nymphal duration at LC25 than at LC10 when the parental generation was fed on rice seedlings treated with triazophos.

INFLUENCE OF TRIAZOPHOS ON THE LIFE TABLE PARAMETERS OF S. FURCIFERA

The results in Table 3 show that the S. furcifera populations from F1 to F2 increased 33.69 and 27.63 times when the parental genera tion had been treated with triazophos at LC10 and LC25, respectively, whereas the population increased 43.44 times in the control. The survival rates of neonates to 3rd instar and from 4th to 5th instar were sinificantly lower when the parental generation had been treated with triazophos compared with the control (P <0.05).

The emergence rate and copulation rate were both reduced afer the parental generation had been treated with sublethal concentrations of triazophos. The emergence rate of white-backed planthoppers whose parents had been treated with LC10 showed no significant variation compared with the control and LC25 treatments, whereas the control and LC25 treatments showed significant differences between them (P <0.05). The copulation rates of white-backed planthoppers whose parents had been treated with triazophos at LC10 and LC25 were 73.6 and 68.8%, respectively, and were significantly lower than in the control (P <0.05). The female ratio showed no obvious variation (P <0.05) and was approximately 50% in both treatments and the control ( Table 3 ). The percentage of hatching neonates in the F2 generation was 86.9, 79.6, and 77.2% for the control, LC10, and LC25, respectively. The variation in emergence rate between the control and both treatments was similar.

The fecundity (eggs per female) of white-backed planthoppers whose parents had been treated with distilled water, LC10, and LC25 was 154.1, 179.0, and 203.2, respectively. Sublethal concentrations of triazophos significantly increased the reproductive parameters, especially at the higher sublethal concentration (LC25) (P <0.05). These results show that sublethal concentrations of triazophos can stimulate the reproductive output of mated adults. However, the relative fitness of white-backed planthoppers whose parents had been treated with LC10 and LC25 decreased to 0.78 and 0.64, respectively. The relative fitness reached four-fifhs and three-fifhs of that of the control and suggested that treatment with triazophos could result in decreased fitness of S. furcifera relative to the control.

Discussion

Rice planthopper control programs rely primarily on the spraying of chemical insecticides. The effects of insecticides on insects are twofold: mortality resulting from the direct toxic effect of the compound; and the effect of sublethal doses of insecticides on insect life history parameters and behavior ( Kumar & Chapman 1984). Sublethal effects can be defined as physiological or behavioral in nature and act on individuals that survive exposure to a toxic compound at sublethal concentrations ( Desneux et al. 2007). Sublethal effects result when insects absorb insufficient amounts of the pesticide to cause death. Sublethal doses of insecticides have been shown to cause latent toxicity, enzyme induction, stimulatory and inhibitory effects on reproduction, and altered behavior and physiology ( Moriarty 1969).

Contrasting results to ours were found in tests of toxicity of insecticides to 3rd instar nymphs of N. lugens using the rice stem-dipping method and various concentrations of insecticide ( Samer et al. 2009). LC2.5 and LC10 concentrations of imidacloprid clearly stimulated the reproduction in N. lugens . However, fecundity at LC50 and LC90 was significantly less than in the control ( Samer et al. 2009). Stimulation of insect fecundity induced by sublethal concentrations of insecticides is a common phenomenon, particularly in homopteran insects and in mites ( Ge et al. 2010). The present study showed that sublethal concentrations of triazophos can increase fecundity in S. furcifera . The fecundity (eggs per female) of the planthoppers treated with LC25 was 131.9% that of the control, suggesting that sublethal concentrations of traziophos could significantly stimulate reproduction in S.furcifera . The mechanisms by which the fecundity of S. furcifera is increased by triazophos application should be considered if its use in controlling rice stem borers and paddy borers is to be continued. Qin et al. (2013) deduced that the relative fitness values of N. lugens were 0.55, 0.41, 0.21, and 0.09 following treatment with LC20, LC30, LC40, and LC50, respectively, of the insecticide paichongding (1-[(6-chloropyridin-3-yl)methyl]-7- methyl-8-nitro-5-propoxy-1,2,3,5,6,7-hexahydroimidazo[1,2-α] pyridine).

Pesticide-induced homeostatic modulation has been suggested as a term to broadly include both hormesis and stimulatory effects of pesticides on non-target pests. The specific role played by pesticide-induced homeostatic modulation in inducing pest outbreaks in agroecosystems is difficult to evaluate because other complex environmental factors are probably also involved ( Cohen 2006). Hormesis is a biphasic dose-response phenomenon characterized by low-dose stimulation and high-dose inhibition ( Calabrese 2008). It is a dose- and time-dependent phenomenon in which effects are displayed as a result of exposure to a certain sublethal dose range of an applied pesticide. Our study shows that treatment with various concentrations of insecticide has differing effects, including changes in nymphal duration and life table parameters. Although, on the whole, the population of the next generation of S. furcifera may decline, various sublethal concentrations of triazophos significantly stimulated reproduction. Resurgence in arthropod pest numbers following applications of chemical pesticides could be the result of disruptions of the biological equilibrium by elimination or drastic reduction of arthropod species that regulate the pest population or compete for food or space within a given ecological niche, and thus represent either direct or indirect effects on the pest population ( Hardin et al. 1995).

In conclusion, the organophosphorus insecticide triazophos could stimulate reproduction in S. furcifera as demonstrated by treatments with specific sublethal concentrations in the laboratory. This effect may be due to stimulatory effects on the physiological and biochemical mechanisms involved in reproduction and growth. Alternatively, insecticide applications to insect host plants are known to result in improvement of the plants’ nutritional quality, thus leading to better reproduction in the insects feeding on them ( Suri & Singh 2011). Field application of insecticides may result in sublethal levels at some time afer application. Understanding how individual insects and their populations respond to insecticides should help in the selection of appropriate and improved treatment plans. In particular, future research should focus on exploring the molecular mechanisms that stimulate reproduction in S. furcifera planthoppers when treated with sublethal concentrations of triazophos.