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Abstract
The cotton leafworm, Spodoptera littoralis is one of the most destructive polyphagous Lepidopterous pests in Egypt, attacking several crops such as cotton, corn, peanuts, soybean and vegetables.
The intensive use of conventional insecticides for controlling this pest caused environmental problems including insect resistance, excessive persistence of residues and human health hazards.
So, there have been efforts during the past two decades to develop biorational insecticides affecting the target insect pests with minimum effect on natural enemies and the environment.
The present investigation was devoted to evaluate the toxicity of certain insecticides belonged to different groups which are differing in its modes of action against the cotton leafworm, Spodoptera littoralis larvae. Also the objective of this investigation extended to study the delayed effect of sublethal concentrations of the tested insecticides on certain biological aspects of S. littoralis. Also to study the effect of the sublethal concentrations of tested insecticides on some biochemical components of the 4th instar larvae such as total soluble protein, beside esterases, transaminases, phosohatases, chitinase and phenoloxidase enzyme activities. These attempts were elucidate to rationalize the using of insecticides via IPM program on cotton crop.
Ι. Toxicity Of Certain Insecticides On The 4th Instar Larvae Of Spodoptera littoralis Under Laboratory Conditions:
The toxic action as initial lethal effect of investigated compounds to 4th instar larvae of S. littoralis was evaluated under lab-conditions. The toxicants used were chlorpyrifos (organophosphorus insecticid), lufenuron (chitin synthesis inhibitor compound), spinosad (an aerobic fermentation product of the soil actinomycete bacterium Saccharopolyspora spinosa) and azadirachtin (the main powerful compound isolated from the neem tree, Azadirachta indica A. Juss) against the 4th instar larvae of S. littoralis.
Toxicity of tested insecticides were evaluated using leaf dipping technique using serial diluted concentrations of each of tested insecticide.
It was found that chlorpyrifos insecticide achieved acute toxicity at 24 hrs post treatment. Whereas the toxicity of non-traditional insecticides increased with time post treatment.
a. Median lethal concentration (LC50):
The LC50’s of chlorpyrifos was 30.85 ppm after 24 hrs of treatment, it was found that LC50s of lufenuron, spinosad and azadirachtin after 48 hrs of treatment were 21.75, 85.48 and 489.26 ppm, respectively.
The relative potency based on the least effective compound (azadirachtin), showed that lufenuron was the most toxic compound followed by chlorpyrifos and spinosad insecticide.
b. Slope:
It was found that the slopes of chlorpyrifos and spinosad insecticides were relatively close. This indicate some sort of parallelism in the conc.-response relationships of two compounds.
The highest slope was that of spinosad indicating a high conc.-response relationship. The lowest slope among the investigated toxicants was that of lufenuron insecticide.
2. Latent effect of certain insecticides on some biological aspects of Spodoptera littoralis:
This part of study aims to investigate the latent effect of LC25 of the certain insecticides on certain biological aspects; larval weight, larval duration, pupation, pupal weight, pupal period and emergence percentage of S. littoralis.
2.1. Latent effect of certain insecticides on larvae of the cotton leafworm, S. littoralis :
It was observed that, after treatment the 4th instar larvae of S. littoralis with LC25 concentration of chlorpyrifos, lufenuron, spinosad and azadirachtin treatments, the gained weights of surviving larvae were 0.28±0.42 gm, 0.24±0.07 gm, 0.19±0.06 gm and 0.26±0.06 gm which represent 15.15, 27.27, 42.42 and 21.21%, of control larvae (0.33±0.04 gm) respectively. The average of larval periods were 9.6±0.70, 8.89±0.07, 9.94±0.83 and 9.1±0.08 days for chlorpyrifos, lufenuron, spinosad and azadirachtin treatments in order. In other words LC25 concentration of the four mentioned toxicant caused larval period prolongation by 16.79, 8.15, 20.92 and 10.71% of control larvae (8.22±1.09 days), respectively.
2.2. Latent effect of certain insecticides on pupae of the cotton leafworm, S. littoralis:
Surviving larvae from the treatments resulted 86.67%, 51.3% 83.33% and 67.67% pupation for chlorpyrifos, lufenuron, spinosad and azadirachtin respectively compared with control resulted 100% pupation.
The mean gained weights of developed pupae were 0.27±0.04 gm, 0.26±0.04 gm, 0.23±0.05 gm and 0.22±0.05 gm which represent 3.57, 7.14, 17.86 and 21.43% of control pupae weight of 0.28±0.03 gm, for chlorpyrifos, lufenuron, spinosad and azadirachtin treatments respectively.
It was observed that for chlorpyrifos, spinosad and azadirachtin treatments, the average pupal duration were 14.67±2.60, 14.63±1.36 and 13.89±1.30 days, respectively. That means that, pupal periods were prolongated by 24.53, 24.19 and 17.91%, over control respectively. Treated larvae with lufenuron resulted pupae duration of 9.78±1.30 days representing 16.98% shortage compared with control pupae 11.78±1.30 days.
2.3. Latent effect of certain insecticides on adults of the cotton leafworm, S. littoralis:
The treated pupae resulted 87, 82, 75 and 83.33% adult emergence, for treatments of chlorpyrifos, lufenuron, spinosad and azadirachtin, respectively while it was 100% for the control.
The average adult longevities were 7.33±0.58, 8.28±1.03 and 8.22±0.83 days for chlorpyrifos, lufenuron and azadirachtin treatments respectively compared with control adult longevity of 7±0.87 days. Chlorpyrifos, lufenuron and azadirachtin toxicants prolongated the adult longevities by 4.71, 18.28 and 17.43%, of control in order. The adult longevity for treatment of spinosad was 5.88±1.31 days representing 16% reduction of control.
The hatchability percentages of egg masses deposited by adult developed from treatments were 48.3, 75.66, 63.33 and 58.25%, for chlorpyrifos, lufenuron, spinosad and azadirachtin treatments, respectively compared with control which was 95.34% hatchability.
3. Effect of LC25 of certain insecticides on some biochemical components of 4th instar larvae of Spodoptera littoralis:
This part of study deal with the effect of sublethal concentrations of the tested compounds on certain biochemical homogenate constituents of the 4th instar larvae of S. littoralis under laboratory conditions.
3.1. Effect of non-traditional insecticides on total soluble protein:
It was found that the treatment of the 4th instar larvae of S. littoralis with LC25 level of the tested non-traditional insecticides caused increasing of total soluble protein higher than the protein level of untreated larvae.
Also it was observed that the increasing of such biochemical constituents was time dependent in both lufenuron and azadirachtin treatments. The maximum increasing of the soluble protein was highly pronounced with spinosad at 24 hrs post treatment.
3.2. Effect of non-traditional insecticides on some enzymatic activities:
3.2.1. Phenoloxidase activity:
The results showed that treatment of 4th instar larvae of S. littoralis with LC25 of tested non-traditional insecticides reduced the activity of phenoloxidase in all treated larvae at 72 hrs post treatment. The maximum inhibition power was observed for spinosad treatment followed by lufenuron.
It could be concluded that the main effect of the above tested compounds are through disturbance of sclerotization process during molting of the larval stage of S. littoralis.
3.2.2. Chitinase activity:
The results indicated that spinosad showed the most inhibitory power on chitinase at 24 hrs post treatment than for azadirachtin treatment. While at 72 hrs post treatment the inhibition potency inverted for azadirachtin, it had the highest inhibitory power than spinosad.
3.2.3. Esterases activity:
It was found that spinosad having the highly inhibition power on AchE of treated larvae. On the contrary lufenuron and azadirachtin treatments induced the AchE activity at 24 hrs post treatment. A very less inhibitory power was observed for both compounds at 72 hrs post treatment.
Also it was observed that spinosad treatment having the highly inhibition power on non-specific esterases either at 24 hrs or 72 hrs post treatment followed by azadirachtin treatment, while lufenuron treatment showed a very lower power of inhibition on both kind of non-specific esterases.
3.2.4. Transaminases activity (GOT) and (GPT):
From the obtained results it could be seem that lufenuron had the most inhibitory power at 72 hrs post treatment followed by azadirachtin and spinosad for (GOT) activity. While for (GPT) lufenuron having the highly inhibition power followed by spinosad at 72 hrs post treatment.
3.2.5. Acid and alkaline phosphatases activity:
The results indicated that azadirachtin had the most inhibitory power at 24 hrs and 72 hrs post treatment for both enzymes for alkaline (Alk-P) and acid phosphatases (Ac-P).
3.3. Effect of chlorpyrifos insecticide on the total soluble protein and some enzymatic activities:
It was observed that the chlorpyrifos have a high potent inhibitory power either on AchE or on (α- and β-esterases). Also the inhibition effect was extended on (Alk-P) and on a very low inhibition power of GPT) While chlorpyrifos induced the activities of Ac-P and GOT with higher induction percentages.
It could be concluded that the mechanism of action of chlorpyrifos insecticide on S. littoralis larvae is mainly through the inhibition of (AchE) as well as (α- and β-esterases). While the effect on the other tested enzymes through inhibition of Alk-P and GPT or induction of Ac-P and GOT is come a secondary mechanism due to the chronic effect of its sublethal concentration.