Effect of spinetoram seed treatments on SFB infestation and cabbage damage rates. The effects of spinetoram seed treatments were evaluated in both controlled and field environments. At controlled environment, the cabbage damage rates by SFB were found to be 5.08%, 0%, 20.50%, and 60.20%, respectively for group A (spinetoram applied and coated), group B (spinetoram applied, coated, and with net covering), group C (net covering with no seed treatments), and control (CK) on day 4. The damage rates were significantly higher in group C and CK as compared to group A or B (F=87.59, P< 0.001). The damage rates were found to be increasing with the increase in infestation time. On days 7 and 10 the damage rates were significantly higher in the control, group B, and group C compared with group A. On day 31, damage rates were significantly lower in group A as compares with other groups, which shared no significant difference among them (Figure 1).
On the other hand, the field environment studies revealed that on day 4, there was no significant difference found in-between group A and B, and group A and C among the field population of SFB. Conversely, a significant difference in the SFB numbers was found among group C, B, and CK (df=3 F=20.80 P༜0.001). On day 7, all groups were significantly different based on the infested SFB numbers on them. We have taken data from day 4 to day 31 on the filed evaluation. Interestingly, post-day 7 (from day 10 to day 31), there was no significant differences in the infested SFB numbers were observed among the groups A, C, and CK, whereas significant differences were found among the groups B and A, C, CK (Figure 2).
Degradation dynamics of the residual spinetoram in soil and different plant parts of flowering cabbage. The spinetoram soil residues were estimated by using the external standard method, where the standard curve equation was deduced to be Y=1501571X, with R2=0.9997, and detection limit 0.005ppm. On days 1, 4, 7, 10, 13, 19, 25, and 31, the spinetoram residue was detected to be 0.22, 0.37, 0.86, 0.88, 0.63, 0.23, 0.09, 0.04 mg/kg, respectively in the soil. These results showed that the spinetoram residues were increasing in soil for the initial days of treatment, that is, from 0.22 to 0.88 mg/kg on day 1 to day 10. Subsequently, the spinetoram residual amounts started to decline gradually up to day 19, and then the amount was declined sharply reaching 0.04 mg/kg on day 31 (Figure 3).
A similar method of the spinetoram residues estimation was employed to evaluate the insecticide residue in the different plant parts of flowering cabbage. The results revealed that spinetoram residue in the roots to be 9.13, 4.97, 0.95, 0.29, 0.09, 0.08 mg/kg on days 7, 10, 13, 19, 25, and 31, respectively. Likewise, in stem the amount of spinetoram residue was estimated to be 1.74, 0.20, 0.03, 0.0051, 0.0023, and 0.0018 mg/kg on days 7, 10, 13, 19, 25, and 31, respectively. In leaves, the spinetoram residual concentration was found to be 0.74, 0.45, 0.05, 0.0039, 0.0037, and 0.0014 mg/kg on days 7, 10, 13, 19, 25, and 31, respectively. From the results, it is clear that the highest residual spinetoram was recorded in the roots as compared to other plant parts. Furthermore, the residual amounts decrease in all parts of the flowering cabbage with the progress in the days-post treatment (Figure 4).
Estimation of LC 50 and half-life of spinetoram. Based on the different concentrations of spinetoram solutions (30 mg/L to 480 mg/L) and the respective SFB mortality rate the LC50 was estimated. The LC50 value was found to be 170.315 ppm (R2=0.951), while the toxic regression equation derived to be Y= -3.647+1.634X. On the other hand, the half-life of spinetoram was found to be different in soil and different cabbage parts (Table 1). The half-life of spinetoram post-pelletized seed sowings in soil was found to be 2.82 days, while in cabbage roots, stem, and leaves were 4.21, 5.77, and 3.57 days, respectively.
Table 1
The half-life of spinetoram in the soil and different parts of the flowering cabbage.
Source | First order kinetic equation | R2 | Rate constants | Half-life (in days) |
Soil | Ct=4.0485e−0.1490t | 0.998 | 0.149 | 4.65 |
Root | Ct=25.5993e−0.2088t | 0.914 | 0.2088 | 3.32 |
Stem | Ct=2.7959e−0.2717t | 0.844 | 0.2717 | 2.55 |
Leaf | Ct=3.0461e−0.2718t | 0.886 | 0.2718 | 2.55 |