Status of Resistance of Bemisia tabaci (Hemiptera: Aleyrodidae) to Neonicotinoids in Iran and Detoxification by Cytochrome P450-Dependent Monooxygenases.

Nine Bemisia tabaci (Gennadius) populations were collected from different regions of Iran. In all nine populations, only one biotype (B biotype) was detected. Susceptibilities of these populations to imidacloprid and acetamiprid were assayed. The lethal concentration 50 values (LC50) for different populations showed a significant discrepancy in the susceptibility of B. tabaci to imidacloprid (3.76 to 772.06 mg l-1) and acetamiprid (4.96 to 865 mg l-1). The resistance ratio of the populations ranged from 9.72 to 205.20 for imidacloprid and 6.38 to 174.57 for acetamiprid. The synergistic effects of piperonylbutoxide (PBO) and S,S,S-tributylphosphorotrithioate (DEF) were evaluated for the susceptible (RF) and resistant (JR) populations for the determination of the involvement of cytochrome P450-dependent monooxygenase and carboxylesterase, respectively, in their resistance mechanisms. The results showed that PBO overcame the resistance of the JR population to both imidacloprid and acetamiprid, with synergistic ratios of 72.7 and 106.9, respectively. Carboxylesterase, glutathione S-transferase and cytochrome P450-dependent monooxygenase were studied biochemically, for the purpose of measuring the activity of the metabolizing enzymes in order to determine which enzymes are directly involved in neonicotinoid resistance. There was an increase in the activity of cytochrome P450-dependent monooxygenase up to 17-fold in the resistant JR population (RR = 205.20). The most plausible activity of cytochrome P450-dependent monooxygenase correlated with the resistances of imidacloprid and acetamiprid, and this suggests that cytochrome P450-dependent monooxygenase is the only enzyme system responsible for neonicotinoid resistance in the nine populations of B. tabaci.

Oxydemeton-methyl resistance, mechanisms, and associated fitness cost in green peach aphids (Hemiptera: Aphididae).

Susceptibility to oxydemeton-methyl and imidacloprid, and the inhibitory effects of oxydemeton-methyl and some organophosphate compounds on acetylcholinesterase (AChE) and carboxylesterase activity were studied in two populations (Karaj and Rasht) of green peach aphids, Myzus persicae (Sulzer). Results show that the Karaj population was resistant to oxydemeton-methyl but susceptible to imidacloprid. The esterase activity of the resistant and susceptible populations suggests that one of the resistance mechanisms to oxydemeton-methyl was esterase-based. The inhibition assay shows that the AChE of the Karaj population is less sensitive to oxydemeton-methyl and paraoxon derivatives. Regarding the paraoxon derivatives, the smaller paraoxon side chain is more potent against the modified AChE than against the AChE from the susceptible strain. Fertility life table parameters of green peach aphid populations resistant and susceptible to oxydemeton-methyl also were studied under laboratory conditions. The standard errors of the population growth parameters were calculated using the Jackknife method. Results showed that susceptible strain exhibits a significantly higher r(m) than the resistant strain, probably because the resistant strain had a higher generation time than the susceptible strain. These results suggested that the resistant Karaj strain may be less fit than the susceptible strain.

Determination of pymetrozine residues in cucumber.

An experiment was carried out to investigate the residues of pymetrozin in cucumber under field conditions. Plots of cucumber (cv. Daminus) were sprayed with pymetrozine (Chess, WG50) at two different rates; 0.5 and 1.0 kg/ha. Samples were collected 2 hours and 1, 2, 3, and 4 days after spraying and pymetrozin residues were determined using high performance liquid chromatography with UV detector. Two hours after spraying, the residues was 0.326 mg/kg for 0.5 kg/ha treatment. The residues decreased to 0.013 mg/kg on the third day and no residue was detected four days after spraying. In 1.0 kg/ha treatment, the residue was 0.627 mg/kg, two hours after spraying, which decreased to 0.028 mg/kg, on the third day, but no residues was found in samples which collected four days after spraying. In farm plots which received 0.5 kg/ha pymetrozine, the residues in peels and peeled cucumber were 0.359 and 0.308 mg/kg respectively, two hours after spraying, but in plots which sprayed at 1.0 kg/ha, the residues were 0.648 and 0.609 mg/kg respectively. Furthermore the residues in peels showed a significant difference with those of peeled and unpeeled cucumber under field conditions.

Toxicity of fenitrothion, an organophosphorus pesticide, against summer population of sunn pest, Eurygaster integriceps Put. (Hemiptera: Scutelleridae).

The sunn pest are a group of insect representing several genera of the shield bug (Scutelleridae) and stink bug (Pentatomidae) families, with the species Eurygaster integriceps being the most economically important. The sunn pest is a major pest of wheat and barley in central and western Asia and it accounts for annual losses of 20-30% for barley and up to 100% for wheat. In the recent decades problems have intensified largely due to change in farming practices. Thus this pest threatens food security and reduces the stability of traditionally wheat-based agricultural systems. In order to control pest, the major organophosphorus insecticide used is fenitrothion. So, the aim of the current study was to determine toxicity of fenitrothion against summer population of adult sunn pest. Insecticide toxicity was measured using a topical bioassay. To do so, the toxin was dissolved in acetone and six concentrations of toxin including 25, 50, 100, 150, 175 and 200 ppm were prepared. Insects were treated on the pronotum with one micro litre aliquots of insecticide in acetone (for treatments) or acetone alone (for control). The dose-mortality relationship was assessed from above mentioned doses with 30 insects treated per dose. After treatment, insects were maintained at laboratory conditions and mortality was recorded after 48 hours. Results were corrected for control mortality with Abbott's formula and then analyzed and toxicity determined. The obtained results showed that LD10, LD50, and LD90 were 34.7, 90.43, and 235.5, respectively.

Esterase activity in summer population of sunn pest, Eurygaster integriceps Put. (Hemiptera: Scutelleridae).

A key constraint on increasing wheat production in Iran and some neighbouring countries is Sunn pest which cause severe damage to vegetative growth stage of plant in the early season. It also feeds on wheat grain in late growth stage of plants thus damaged wheat grains loose their bakery properties. Because of injecting protease enzymes into the grain during feeding, enzymes degrade gluten proteins and cause rapid relaxation of dough which results in the production of bread with poor volume and texture. Organophosphorus insecticides are the main pesticides used to control the insect pest. However, suitable reduction in pest population has not been achieved partly due to resistance to pesticides. Esterase plays crucial roles in insect physiology and detoxifies a broad range of xenobiotics including insecticides. Enhanced esterase activity is a major mechanism if insecticide resistance and has been detected in a number of insects. To evaluate esterase activity adult bugs were collected from wheat field in Karaj area of Iran and transferred to the laboratory. For biochemical assay, two adult bugs (either males or females) were homogenized in 500 microl Na-phosphate buffer pH 7.2. The homogenates were centrifuged at 14000 g for 10 minutes at 4 degrees C. The supernatants as the enzyme source were pooled and stored at -20 degrees C for later use. For enzyme assay, 300 microl of supernatant was mixed with equal volume of substrates (30 mM alpha-naphthyl acetate or 30 mM beta-naphthyl acetate) and incubated at 30 degrees C for 30 minutes. Then, 50 microl of fast blue solution (150 mg fast blue B in 15 ml distilled water plus 35 ml 5% SDS) was added and esterase activity was determined in a spectrophotometer at 595 nm. Data showed that there are no differences in esterase activity between male and female. However, There was significant differences between hydrolysis of substrates, alpha-naphthyl acetate and beta-naphthyl acetate. Insect esterase hydrolyzes alpha-naphthyl acetate much more than beta-naphthyl acetate.

Residues of profenofos in spring onion.

Profenofos is one of the commonly used insecticides in the control of Trips tabaci on spring onion in Iran. Residues of profenofos in spring onion were determined in two different fields under the same conditions. In the first field, onion plants were sprayed with profenofos (40EC) at the rate of 1000 g/ha. Spraying was repeated 2 weeks later. In the second field one spraying was preformed at the same rate. Spring onionwere sampled at different time intervals and analyzed for profenofos residues using a GC equipped with NPD detector. In the first field's samples, the residues were 0.097 and 0.025 mg/kg at 2 and 6 days after spraying, respectively. The residues declined to 0.002 mg/kg on the day twelve. Two days after the second spraying the residues was 0.27 mg/kg, which reduced to 0.032 on the day sixth. However the residues were not detectable 32 days after the second spraying. In the second field, residue levels were 0.193 and 0.043 mg/kg at 2 and 6 days after spraying. Residues, which found after 32 days, were less than 0.001 mg/kg. The rate of residue decay in the first field was higher than the second field.

Effects of different factors on the residues of heptenophos in cucumber.

The present study was conducted to determine the effects of various factors on residues of heptenophos in cucumber under field conditions. Heptenophos (EC50) was sprayed at recommended close (1/1000) and double dose (2/1000). The effects of peeling and repeated application on residues level were also examined. Samples were collected at different time intervals and residues of heptenophos were determined using GC. Six hours after application 9.96 and 2.5 mg/kg heptenophos were recovered on the cucumber from the above treatments respectively. The residues of heptenophos in cucumber decreased rapidly in both treatments and reached a low level after five days (0.2 and 0.1 mg/kg respectively). Two times application of heptenophos caused no insecticide accumulation in cucumber. This investigation revealed that peeling the fruits did not reduce total residues in cucumber.

Side-effects of three pesticides on the predatory mite, Phytoseiulus persimilis (Acari: Phytoseiidae).

Side-effects of three commonly used pesticides in Iran were evaluated on an introduced strain of the predatory mite. Phytoseiulus persimilis Athias-Henriot, reared for about 10 years without exposure to any pesticides. Application of pesticides was carried out either to detached bean leaves using a Potter tower at 1 mg wet deposit per cm2 or by a hand sprayer on bean plants until run off. According to an EPPO decision making scheme, pirimiphos-methyl was found to be harmful (E=90.8%) and heptenophos harmless (E=-3.7%) to the predatory mite in the residual initial toxicity tests. For determination of the hazard class of malathion a field test was found to be necessary (E=59.8%). Categories of 1, 2, 3 were determined for heptenophos, malathion and primiphos-methyl, respectively, using IOBC classification. Despite being harmful, it is possible to use pirimiphos-methyl 10 days before release of P. persimilis. Investigation of the contribution of both lethal and sub-lethal effects to total impact indicated the dominance of lethality in the case of pirimiphos-methyl, while malathion acted by both mechanisms. Heptenophos did not have negative effects on fecundity of P. persimilis but rather caused a higher rate of fecundity in comparison with the control. The mortality found in the heptenophos test was not significantly different from the control.