Contact toxicity of insecticides against rice weevil, Sitophilus oryzae L. and its effect on progeny production

Abstract

Post-harvest losses caused by insect pests, particularly the rice weevil, Sitophilus oryzae, pose a significant challenge in food storage facilities worldwide. To combat this pest, synthetic insecticides and fumigants are widely used. However, effective contact insecticides are scarce. Hence, the present study explored the alternatives by evaluating the contact toxicity of various insecticides against S. oryzae using glass, jute, and floor tiles as surface substrates and further examining the effect on progeny production of promising candidate insecticides. Among the insecticides tested, malathion exhibited the highest toxicity on glass and jute surfaces regardless of the exposure period. On the other hand, spinetoram proved to be the most effective on tile surface with an 8 h exposure period. Among the alternate insecticides (spinosad, spinetoram, chlorfenapyr and lambda-cyhalothrin), spinetoram was most effective with LC₅₀ values of 155.8 and 116.9 mg/m² for 4 h and 8 h exposure, respectively, on tile surface; 204.6 and 129.0 mg/m² for 4 h and 8 h exposure, respectively, on glass surface; and 271.5 and 199.5 mg/m² for 4 h and 8 h exposure, respectively, on jute surface. Relative toxicity assessments revealed spinetoram to be 2.11 and 2.51 times more effective than deltamethrin on tile surface for 4 and 8 h of exposure, respectively whereas it was 1.14 times more effective than malathion on tile surface at 8 h exposure. Principal component analysis indicated a higher demand for insecticide doses closely associated with the structural properties of surfaces, particularly evident with jute surfaces. Furthermore, the effect on adult mortality and progeny production by malathion, spinetoram, and lambda-cyhalothrin revealed malathion as the most effective insecticide followed by spinetoram. Carboxylesterase, acetylcholinesterase, and Glutathione S-transferase (GST) activities were notably higher in deltamethrin-treated insects compared to other insecticides. The studies concluded that spinetoram can be considered an alternative to conventional insecticides for the management of S. oryzae under different storage conditions.

Keywords: Sitophilus oryzae; Contact insecticides; Exposure surface; Exposure time; Lethal concentration; Progeny production; Relative toxicity.

Fig. 1

Relative toxicity of different insecticides against Rice weevil Sitophilus oryzae on different surfaces with reference to deltamethrin/malathion. Relative toxicity values calculated as the ratio of LC₅₀ of deltamethrin / LC₅₀ of each selected insecticide is denoted by blue bars; Relative toxicity calculated as the ratio of LC₅₀ of malathion / LC₅₀ of each selected insecticide is denoted by green bars. The horizontal red line at a value of one (or 1) serves as a reference, with values above indicating greater effectiveness than deltamethrin (blue) and malathion (green). The illustration was developed using SPSS version 21.0. (SPSS Inc. Chicago, Illinois, USA). (Yellow colour filled triangle) shows highly effective insecticide at 4 h exposure period on that particular surface among selected insecticide in relative to deltamethrin, and (Pink colour filled star) shows highly effective insecticide at 8 h exposure period on that particular surface among selected insecticide in relative to deltamethrin; (Red colour filled circle) shows highly effective insecticide at 8 h exposure period on that particular surface among selected insecticide in relative to malathion.

Fig. 2

Biplot and contribution of insecticides to dimensions on different surfaces. (a) PCA biplot, Presentation of 12 variables viz., Malathion 4 h, Malathion 8 h, Deltamethrin 4 h, Deltamethrin 8 h, Spinosad 4 h, Spinosad 8 h, Spinetoram 4 h, Spinetoram 8 h, Chlorfenapyr 4 h, Chlorfenapyr 8 h, Lambda-cyhalothrin 4 h and Lambda-cyhalothrin 8 h; (b,c) contribution of 12 variables to dimensions one and two, respectively; PC1 and PC2 are represented on the horizontal and vertical axes, with a cumulative variance of 87.6% and 12.4%, respectively.

Fig. 3

PCA analysis considering insecticides and surfaces (Glass, jute, and tile).Visualization of correlation showing the influence of surfaces on the toxicity of insecticides used in this study. The distance of the variables from the center indicates the magnitude of influence; PC1 and PC2 are represented on the horizontal and vertical axes, with a cumulative variance of 95% and 4.1%, respectively.

Fig. 4

Cluster analysis shows the grouping of studied variables. Cluster analysis divided the whole group of variables into distinctly separated two major clades based on the similarity matrix: (Blue line) One was a cluster of jute surfaces, attributing variables encompassing six insecticides at two exposure periods and another (yellow line) was a cluster of tile surfaces and (black line) was a cluster of glass surfaces.

Fig. 5

Pearson correlation coefficients for the median lethal concentrations (LC₅₀) of various insecticides are presented in the upper half of the figure. These values indicate the strength and direction of the linear relationships between pairs of insecticides. A value close to 1 signifies a strong positive correlation, while a value near − 1 indicates a strong negative correlation; values around 0 suggest no significant correlation.

Fig. 6

Effect of insecticides and food source on progeny production of Rice weevil, Sitophilus oryzae; Insecticides (malathion, spinetoram, and lambda-cyhalothrin) applied to jute bags containing three commodities (maize, wheat, and rice). Adult weevils were continuously exposed for 21 days, followed by the removal of live and dead parental adults. Progeny reduction was assessed at 60 days post-exposure. The same colour bars sharing a common letter indicate non-significant difference in progeny reduction over control (Tukey’s HSD test).

Fig. 7

Detoxification enzyme activity expressed as micromole/min/mg of protein for Carboxyl esterase (A), Cytochrome P450 monooxygenase (B), Glutathione -S- Transferase (C), and as nmol/min/mg of protein for Acetylcholine esterase (D). Enzyme activity was recorded at 12, 24, and 48-h intervals. Statistically significant differences were determined by ANOVA and Tukey’s HSD test (p < 0.05), with shared uppercase letters indicating no significant differences within each insecticide across time intervals, and shared lowercase letters indicating no significant differences within each time interval across different insecticides. The absence of letters signifies no significant differences.