Toxicity of new insecticides against pomegranate aphid, Aphis punicae

Transcription

1 International Research Journal of Applied and Basic Sciences 2013 Available online at ISSN X / Vol, 4 (3): Science Explorer Publications Toxicity of new insecticides against pomegranate aphid, Aphis punicae Mohammad Rouhani *, Mohammad Amin Samih, Hamzeh Izadi and Elham Mohammadi Department of Plant Protection, Vali-e-Asr University of Rafsanjan, Rafsanjan, Iran *Corresponding author Rouhani_valiasr@yahoo.com ABSTRACT: Aphis punicae P. (Hemiptera: Aphididae) is one of the most important pests in pomegranate orchards in Iran. Laboratory evaluation of imidacloprid, thiamethoxam, thiacloprid and flonicamid on mortality of A. punicae under controlled conditions was conducted. Different concentrations of each pesticide were prepared with distilled water. First instar nymphs were sprayed by Potter Spray Tower. The LC 50 value for imidacloprid, thiamethoxam, thiacloprid and flonicamid were calculated: 0.24 µl/ml, 0.31mg/ml, 0.48 µl/ml and 0.05 mg/ml, respectively. Probit analysis data revealed that the sensitivity of the insects to the pesticides was imidacloprid > thiacloprid > flonicamid > thiamethoxam. The results showed that imidacloprid and thiacloprid at 1 µl/ml, thiamethoxam at 0.35 mg/ml and flonicamid at 0.1 mg/ml had the highest mortality. Keywords: Aphis punicae, Flonicamid, Imidacloprid, Thiacloprid, Thiamethoxam. INTRODUCTION The pomegranate tree, Punica granatum is attacked by several insect species which decrease the quality and quantity of its product. Aphids are among the most serious and widespread pests in pomegranate orchards. The pomegranate aphid, Aphis punicae P. (Hemiptera: Aphididae) is one of the key pests in pomegranate orchards in Iran. This species is well known for its ability to reduce plant vigor, facilitate the growth of mould on leaves, and consequently reduce crop quality and yield. Both adults and nymphs feed on leaves, inflorescences and fruits (Moawad and Al-Barty, 2011). Insecticides still remain a very important component among the strategies for effective control of pomegranate aphid in Iran. One potential solution to this pest s problem may be the use of systemic insecticides, particularly if control from a single application can carry over multiple aphid generations and years. Systemic insecticides are absorbed by the tree s root system and circulated throughout the rest of the plant. Use of neonicotinoid insecticides soar above organophosphates, carbamates, and pyrethroids for pest control (Matsuda et al., 2001; Rogers et al., 2007). The neonicotinoids are a new insecticide class which includes the commercial products imidacloprid, thiacloprid and thiamethoxam. These insecticides are important to agriculture because of their activity against sucking insects (Iwasa et al., 2004; Anikwe et al., 2009). Imidacloprid, thiacloprid and thiamethoxam have a chloro-substituted heterocyclic group, either a chlorpyridinyl or chlorthiazolyl, joined to a second heterocyclic ring (Iwasa et al., 2004). Imidacloprid is a fast acting neonicotinoid insecticide in the chloronicotinyl nitroguanidine chemical family and acts on the insect nervous system by attaching to the acetylcholine binding sites called nicotinergic receptors on the receiving nerve cells. This mode of action prevents transmission of information at those binding sites, leading to a lasting impairment of the nervous system and eventually the death (Matsuda et al., 2001; Millar and Denholm, 2007; Sone et al., 2009). Thiamethoxam is a second-generation neonicotinoid compound with stomach and contact activity. It is classified as a non-repellent neonicotinoid and belongs to the thianicotinyl subclass. This insecticide has a similar mode of action of nicotine that interferes with the nicotinic acetylcholine receptors in the insect s nervous system (Maiensfisch et al., 2001). Thiacloprid is a neonicotinoid insecticide. It is closely related to imidacloprid andt works by disrupting the nervous system by acting as an inhibitor at nicotinic acetylcholine receptors (Osterauer and Kohler, 2008). It has

2 activity against sucking insects such as aphids, whiteflies, some jassids and various species of beetles, weevils and leaf miners (Ciglar and Bari, 2002; Yun-long et al., 2006). The novel pesticide, flonicamid was developed in 2000 as a selective agent against aphids and other sapsucking insects. The mode of action has been identified as blocking the A type potassium channel with the biological effect of suppressing feeding and movement by aphids (Morita et al., 2007). Flonicamid is also nontoxic to beneficial arthropods, so the use of this pesticide is ideal for pest management programs (Hengel and Miller, 2007). This study was aimed at designing assay to assess the effectiveness of imidacloprid, thiamethoxam, thiacloprid and flonicamid in suppressing pomegranate aphid, A. punicae. MATERIALS AND METHODS Insects A colony of the test insect, A. punicae, was established in the insectary from field collected, disease-free aphids and reared under constant temperature of 26±2 o C, 65±5% RH with a photoperiod of 14 hr photophase and 10 hr scotophase. Aphids were reared on pomegranate sapling in glasshouse. Insecticide Commercial formulation of imidacloprid (Confidor, 0.25% SC, Bayer CropScience), thiamethoxam (Helix XTra WG 25%, Syngenta Crop Protection), thiacloprid (Alanto, SC 48% w/w, Bayer CropScience) and flonicamid (Teppeki %96 WG, ISK Biosciences, Belgium) were used. Based on preliminary tests, graded concentrations of each pesticides were prepared with distilled water (0.1, 0.17, 0.31, 0.56 and 1 µl/ml for imidacloprid and thiacloprid, 0.20, 0.23, 0.26, 0.30 and 0.35 mg/ml for thiamethoxam, and 0.02, 0.02, 0.05, 0.07 and 0.1 mg/ml for flonicamid). Bioassay For each test, three leaves of the same size were selected from disinfested pomegranate tree and washed in distilled water and putted in Petri-dishes (9 cm in diameter) lined by filter paper. Fifteen one-day old first instar nymphs were transferred into Petri dishes and sprayed with 2 ml of aqueous emulsions of different concentrations of each insecticide. The spray was applied at 10 mbar using Potter Precision Spray Tower (Burkard Manufacturing Co. Ltd., Rickmansworth Herts, UK). The experiments carried out with three replications while each of them consisted of ten nymphs. Distilled water was used as control. Treated nymphs were maintained in a climate chamber and reared on fresh leaves. Mortality counts were made after 24 hr. Statistical Analysis Data were analyzed with SPSS 16 software and Duncan's multiple range tests to compare effects among treatment. The results were expressed as means (±SE) of untransformed data and considered significantly different at P < Probit analysis was conducted to estimate LC 50 values with their fiducially limits by Probit Analysis-MSChart 2011 software. The LC 50 ratios with their lower and upper 95% confidence limits were used to determine significantly difference between susceptibility of pesticides. Their dose-response lines were compared to determine parallelism and equality between them. Furthermore g factor (95%), heterogeneity and t ratio were studied (Robertson et al. 2007). RESULTS The variance analysis showed a significant difference at 1% level between the effect of imidacloprid (F 5,12 = 51.9, P = 0.00), thiamethoxam (F 5,12 = 21.9, P = 0.00), thiacloprid (F 5,12 = 29.3, P = 0.00) and flonicamid (F 5,12 = 59.5, P = 0.00) on A. punicae. In the all tested pesticides, mortality of first nymphal instar increased with increases in the pesticide concentrations. In other words, mortality of A. punicae increased as a function of the pesticide concentrations (Tables 1, 2, 3 and 4).

3 Table 1. Mortality (mean±se) of Aphis punicae treated with imidacloprid Concentration (µl/ml) Mortality (%) mean ± SE ± 0.57 a ± 0.57 b ± 0.88 c ± 0.66 c ± 0.33 d ± 0.00 e significant difference at 5% Table 3. Mortality (mean±se) of Aphis punicae treated with thiacloprid Concentration (µl/ml) Mortality mean ± SE ± 0.57 a ± 0.66 ab ± 0.57 b ± 0.57 c ± 0.57 c ± 0.00 d significant difference at 5% level Table 2. Mortality (mean ± SE) of Aphis punicae treated with thiamethoxam Concentration (mg/ml) Mortality (%) mean ± SE ± 0.57 a ± 0.66 a ± 0.66 a ± 0.88 b ± 0.33 bc ± 0.00 c significant difference at 5% level Table 4. Mortality (mean ± SE) of Aphis punicae treated with flonicamid Concentration (mg/ml) Mortality mean ± SE ± 0.57 a ± 0.33 b ± 0.66 c ± 0.33 cd ± 0.57 d ± 0.00 e significant difference at 5% level The highest mortality of imidacloprid (73.33%), thiamethoxam (46.66%), thiacloprid (53.33%) and flonicamid (66.66%) was observed at the highest concentration of each pesticide i.e. 1 µl/ml, 0.35 mg/ml, 1 µl/ml and 0.1 mg/ml, respectively. Statistical analysis between maximum concentration of pesticides (F 4,10 = 70.12, P = 0.00) showed that all pesticides significantly decreased density of A. punicae compared with the control (Table 5). As shown in Table 5, the tested pesticides could be categorized in two groups i.e. imidacloprid and flonicamid in one group and thiamethoxam and thiacloprid in another group. If there was significant difference between the effects of these two groups against first instar nymphs of A. punicae but this difference was no significant between insecticides of each group. Table 5. Effect of the highest concentration of the test insecticides against first instar nymphs of Aphis punicae Pesticide Concentration Mortality mean ± SE Imidacloprid 1.00 µl/ml ± 0.57 a Thiamethoxam 0.35 mg/ml 7.00 ± 0.57 b Thiacloprid 1.00 µl/ml 8.00 ± 0.57 b Flonicamid 0.10 mg/ml ± 0.57 a Control ± 0.00 c significant difference at 5% level The LC 50 values for imidacloprid, thiamethoxam, thiacloprid and flonicamid after 24 h were estimated 0.24 µl/ml, 0.31 mg/ml, 0.48 µl/ml and 0.05 mg/ml, respectively (Table 6). The LC 50 value comparison using the LC 50 ratio (0.50) and their lower and upper 95% confidence limits ( ) showed that there was significant difference between LC 50 value for imidacloprid (0.24 µl/ml) and this value for thiacloprid (0.48 µl/ml). In compression between thiamethoxam and flonicamid, the LC 50 ratio (5.61) with their lower and upper 95% confidence limits ( ) showed that there was significant difference between the LC 50 value for thiamethoxam (0.31 mg/ml) and flonicamid (0.05 mg/ml) (Table 7). Table 6. Evaluation of imidacloprid, Thiamethoxam, Thiacloprid and flonicamid against first instar nymph of Aphis punicae Pesticide Slope (±SE) LC 50 Limits 95% Chi square (2) Imidacloprid 2.13 ± µl/ml Thiamethoxam 5.83 ± mg/ml Thiacloprid 1.40 ± µl/ml Flonicamid 3.14 ± mg/ml

4 Table 7. The LC 50 ratio and lower and upper 95% confidence limits between pesticides Pesticide LC 50 ratio Limits 95% Imidacloprid - Thiamethoxam Imidacloprid - Thiacloprid Imidacloprid - Flonicamid Thiamethoxam - Thiacloprid Thiamethoxam - Flonicamid Thiacloprid - Flonicamid It is also clear from Table 7 that there was significant difference between the LC 50 value for imidacloprid (0.24 µl/ml) and flonicamid (0.05 mg/ml), thiacloprid (0.48 µl/ml) and thiamethoxam (0.31 mg/ml), thiacloprid (0.48 µl/ml) and flonicamid (0.05 mg/ml), but there was no significant difference between the LC 50 value for imidacloprid (0.24 µl/ml) and thiamethoxam (0.31 mg/ml). For the all pesticides the statistic t ratio was > 1.96, the g factor was < 0.5 and the heterogeneity factor was < 1. DISCUSSION In the present study, bioassay was followed for assessing the insecticidal activities of four insecticides i.e. imidacloprid, thiamethoxam, thiacloprid and flonicamid on the pomegranate aphid, A. punicae. Usually larger doses of pesticides inflict the highest mortality on pests. In this research, insect mortality increased significantly with increase in pesticide concentrations. It is also well obvious from our results that imidacloprid and flonicamid have the excellent performance to control of A. punicae. Difference in toxicity of the tested pesticides observed in the present study might be due to the difference in their mode of action and formulation. Imidacloprid, thiamethoxam and thiacloprid belong to neonicotinoid insecticides that act on the central nervous system of insects by binding at a specific site, the postsynaptic nicotinic acetylcholine receptor. Their action causes excitation of the nerves and eventual paralysis which leads to death. As a group, they are effective against sucking insects. Imidacloprid has a wide range of target pests and sites. It is a systemic with long residual activity. Thiamethoxam's chemical structure is slightly different than the other neonicotinoid insecticides, making it the most water soluble of this family. Because of its greater water solubility, it moves readily in plant tissue. Thiacloprid acts as an agonist of the nicotinic acetylcholine receptor in the central nervous system, thus disturbing synaptic signal transmission. Thiacloprid is an acute contact and stomach poison, with systemic properties. Flonicamid is a novel insecticide of pyridincarboxamids that rapidly inhibits the feeding behavior of sucking insects. This insecticide has systemic and tranlaminar activity and its mode of action is different from that of neonicotinoids (Matsuda et al., 2001; Tomizawa and Casida, 2003; Saour, 2005; Millar and Denholm, 2007). From the tested pesticides, imidacloprid and thiacloprid were used in liquid formulation where as thiamethoxam and flonicamid in solid formulation. Imidacloprid with the LC 50 value of 0.24 µl/ml found more effective against A. punicae than thiacloprid with LC 50 value of 0.48 µl/ml. In other words, sensitivity of A. punicae to imidacloprid is two times more than thiacloprid. Result of our study also revealed that flonicamid with LC 50 value of 0.05 mg/ml is 6.2 times more toxic to A. punicae than thiamethoxam with LC 50 value of 0.31 mg/ml. In comparison between liquid and solid formulations, sensitivity of A. punicae to imidacloprid is times more than flonicamid. Thiacloprid is also times more effective against A. punicae than thiamethoxam. So, it could be concluded from these findings that sensitivity of A. punicae to liquid formulation of pesticides (even in the same group) is much more than solid formulation. In the last decade, many researchers have studied toxicity of pesticides on sap-sucking insects. Susceptibility to imidacloprid has been studied in different arthropods sucking species including mites (Bullock and Pelosi, 1993), aphids (Conway, 2003; Herbert et al., 2008) and whiteflies (Kumar et al., 2001; Ateyyat et al., 2009). The data obtained in this project with imidacloprid confirm a strong activity of this neonicotinoid against sucking pest insects with an LC 50 value of 0.24 l/ml against A. punicae. The high activity of neonicotinoids is also confirmed in other aphid species. Sadeghi et al. (2009) reported the LC 50 values of 100 g/ml after 24 h and of 0.03 g/ml after 72 h fort imidacloprid against A. pisum. Nauen and Elbert (1997) reported an LC 50 of 0.07 g/ml in artificial diet against a susceptible population of Myzus persicae and M. nicotianae, whereas for resistant aphids the LC 50 was 14 mg/l. Lowery and Smirle (2003) determined an LC 50 value of mg/l for imidacloprid when first instars of A. pomi were challenged for three days on treated apple leaf disks. In another leaf-dipping bioassay, the LC 50 values ranged between mg/l for imidacloprid against different clones of M. persicae and M. nicotianae that were collected from different locations around the world (Devine et al., 1996). Herbert et al. (2008) studied the effect of imidacloprid on phylloxera and reported that imidacloprid improved vine vigor with decreasing effect on density of phylloxera. Yu et al. (2010) showed that imidacloprid decreased fecundity of M. persicae.

5 The strong insecticide activity obtained with flonicamid concurs with the other reports in aphids (Morita et al., 2007; Sadeghi et al., 2009). Morita et al. (2007) reported that the LC 50 values ranged between 0.64 and 2.01 mg/l when different plants (Japanese radish, eggplant, wheat, Chinese cabbage seedlings) were sprayed with flonicamid against different aphid species, M. persicae, Aphis gossypii, Rhopalosiphum erysimi and Schizaphis graminum. Sadeghi et al. (2009) reported that flonicamid showed high toxicity against first-instar A. pisum nymphs with an LC 50 of 20.4 g/ml after 24 h, and of 0.24 g/ml after 72 h. Flonicamid rapidly inhibits the feeding behavior of aphids, i.e. within hours of treatment, without noticeable poisoning symptoms such as convulsion, and the aphids did not recover before dying. This rapid activity is promising as it can contribute in controlling virus transmission. Torres et al. (2003) reported that application of the thiamethoxam on cotton significantly reduced the mean number of immature whiteflies per sample in all treatments. Schroeder and Dumbleton (2006) reported that six weeks after sowing of rape seed, only thiamethoxam at 8 g ai/kg and imidacloprid at 14.4 g ai/kg had significantly lower adult aphid survival than untreated seed. These two treatments had significantly fewer aphid nymphs than untreated seed at the same sampling time. McCornack and Ragsdale (2006) investigated the efficacy of thiamethoxam to suppress soybean aphid populations in Minnesota soybean and demonstrated that in an excised-leaf bioassay, aphid mortality persisted 23 to 35 days after planting. In an in-field bioassay intact plants showed longer persistence of thiamethoxam and aphid mortality persisted 49 days after planting and mortality was significantly higher in the older leaves than in newly-expanded leaves. 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