7.1. Toxicity of Essential Oil against C. maculatus
When toxic potencies of L. dentata EO against C. maculatus adults was studied through two inhalation and contact tests to evaluate their insecticidal activity against this chickpea pest, lavender EO exhibited a very significant insecticidal effects in both contact and inhalation tests (Figure 7 and Figure 8). In fact, mortality of adult C. maculatus increased with greater doses of EO and durations of exposure. The least concentration (1 μL/L) of EO of L. dentata caused lethality 23.33 ± 5.77% of adult C. maculatus, in both tests studied. Mortality increased considerably (p < 0.05) with the duration of exposure up to 96 h. At greater concentrations, total mortality (100%) was observed from the first 24 h in chickpea bruchid adults treated with 10 μL/L and 20 μL/L of the EO in contact and inhalation tests respectively. No mortality (0%) was observed in the control jar.
Figure 7. Mortality (means ± SD) of C. maculatus adults exposed to a contact toxicity test of different concentrations of L. dentata EO. Values with different letters are significantly different (p < 0.05).
Figure 8. Mortality (means ± SD) of C. maculatus adults exposed to an inhalation toxicity test of different concentrations of L. dentata EO. Values with different letters are significantly different (p < 0.05).
Concentrations of the lavender EO that cause 50% (LC50) and 95% (LC95) mortality in C. maculatus adults during 24 h were also calculated (Table 4). The biocide effect was more important in the case of the contact test, for which the LC50 (LC50 = 4.01 μL/L air) was less than during the inhalation test (LC50 = 5.90 μL/L air).
Table 4. LC50 and LC95 (μL/L air) responsible of mortality of C. maculatus adults in contact and inhalation toxicity tests after 24 h treatment with L. dentata EO.
| Bioassays |
LC50 (μL/L Air) |
LC95 (μL/L Air) |
Chi-Square (X2) |
| Inhalation test |
05.90 |
74.83 |
64.68 |
| Contact test |
04.01 |
16.48 |
62.80 |
7.2. Effects of Essential Oil on Fecundity and Emergence of C. maculatus
When effects of L. dentata EO on oviposition by females and emergence of new C. maculatus individuals were also studied, the number of eggs laid was inversely proportional to concentrations of EO (Figure 9 and Figure 10). The least concentration (1 μL/L), resulted in a decrease in oviposition to 22 ± 7 eggs/female, corresponding to 89.39% lesser fecundity, compared to the control. At the greatest concentration (20 μL/L), mean number of eggs laid per female was significantly less at 1.67 ± 1.15 eggs/female, which is equivalent to 99.2% less oviposition. Mean oviposition of unexposed, female C. maculatus was 207.33 ± 12.5 eggs/female.
Figure 9. Fecundity of females (mean values of eggs laid ± SD) and Emergence (means ± SD) of C. maculatus adults after a direct contact toxicity test with different concentrations of L. dentata EO.
Figure 10. Inhibition of fecundity and emergence (means ± SD) of C. maculatus adults after a direct contact toxicity test with different concentrations of L. dentata EO.
Alternatively, the number of emergences was significantly less in a dose-dependent manner with concentrations of EO to which they were exposed (Figure 9 and Figure 10). At the least concentration of 1 µL EO/L, the number of emergences of C. maculatus individuals after embryonic development in chickpea seeds was 3.33 ± 0.58 individuals compared to 163 ± 7.94 individuals in the control, which corresponds to 97.96% inhibition of emergence. Exposure to 5 µL EO/L resulted in 100% inhibition of emergence of new individuals.
7.3. Repellent Activity of Essential Oil against C. maculatus
When the repellent activity against
C. maculatus insects was also tested, the EO of lavender, based on the classification of McDonald (1970), exhibited only moderate repellent activity (
Table 5)
[29]. The rate of repulsion was dose-dependent with a maximum of 43.33 ± 5.77% observed after 120 min of exposure to the concentration of 0.315 µL/cm
2 of the EO and a mean repulsion of 34.44% for the same period.
Table 5. Results of the repellent activity of L. dentata EO against C. maculatus depending on the treatment time.
| |
Repellent Activity at Different Doses of Essential Oil |
PR Average (%) |
Class * |
| 0.079 (µL/cm2) |
0.157 (µL/cm2) |
0.315 (µL/cm2) |
| 30 min |
13.33 ± 5.77 |
20.00 ± 10.00 |
26.67 ± 5.77 |
20.00 |
Moderately repellent (II) |
| 60 min |
20.00 ± 10.00 |
33.33 ± 5.77 |
40.00 ± 0.00 |
31.11 |
Moderately repellent (II) |
| 120 min |
26.67 ± 5.77 |
33.33 ± 5.77 |
43.33 ± 5.77 |
34.44 |
Moderately repellent (II) |
L. dentata EO exhibited significant insecticidal potency against
C. maculatus at several levels. Mortality, oviposition and rate of emergence were all affected. These results are consistent with those of Wagner et al. (2021)
[7], who observed a strong toxic potency of
L. dentata EO against
T. castaneum and
S. zeamais during 6 h exposures. With the same approach,
L. angustifolia EO exhibited significant toxic potency against two types of aphid pests of the chili and bean
M. persicae and
A. pisum, with a mortality of 100% after exposure to 2 µL/L in air
[30].
Insecticidal effects of
L. dentata EO against the pests studied can be attributed to its main constituents, camphor, eucalyptol and fenchone, all of which exhibited strong potency against insects of stored cereal and legume seeds, including
S. zeamais and
T. castaneum [31][32]. These results are consistent with those of other studies which have found that EOs, rich in camphor and eucalyptol exhibit significant toxic potency against phytophagous
[33][34]. Although the mechanism of action of lavender EO on insects has was not investigated directly in this paper, studies have reported the effect of terpenoids such as eucalyptol, and camphor, which affect the nervous system, by blocking the action of the acetylcholinesterase enzyme (AChE) of insects
[30][35][36][37]. In addition, a study conducted on the action of
M. arvensis EO tested by contact against adults
S. granarius, reported rapid paralysis and altered walking
[31][38]. During the same study, essences of
M. arvensis also induced spectacular physiological changes in treated insects, marked by up-regulation of the majority of differentially expressed proteins (DEP), which are involved in development and function of the nervous and muscular systems, protein synthesis, cellular respiration, and detoxification
[39]. These findings suggest that EOs can affect a wide range of biological processes, and highlight the repair mechanisms used by surviving insects to restore the harm caused.
A significant reduction in fecundity and emergence was also observed in the insect
C. maculatus, which was indicative of the strong ovicidal and larvicidal activity of
L. dentata EO. Indeed, the ovicidal effect of EO tested was probably caused by blockage of embryogenesis after penetration of volatile oils into eggs through the respiratory tract of
C. maculatus [40][41]. This is due to the direct toxicity of these compounds, which inhibit metabolic activity of eggs. This was the case for piperitone isolated from the EO of
C. schoenanthus tested on
C. maculatus eggs
[42] and β-asarone identified in the oil of
A. calamus tested on
C. chinensis,
S. oryzae and
S. granarius eggs
[43]. Further work on another bruchid species has demonstrated that the degree of sensitivity and vulnerability of eggs to the vapours of three EOs including
L. hybrida varied according to ages of eggs and stages of embryonic development
[44]. Alternatively, some reports have indicated that EOs have a sterilizing effect on eggs
[43].
Results of this study demonstrated total elimination of emergence when eggs were exposed to 5 μL EO/L, which could be explained by larvicidal effects of lavender EO and their major constituent linalool. This conclusion was consistent with previous results where young larvae (L1) of the cereal seed pest
T. confusum were most sensitive to toxic effects of
L. spica EO and linalool, with LC
50 = 19.535 μL/L air and LC
50 = 14.198 μL/L air, respectively during 24 h of exposure
[45]. In the same study, linalool caused greater mortality of eggs than did
L. spica oil at equal concentrations and reduced emergence of surviving adults, larvae and pupae
[45].
Repellency of
L. dentata EO was moderate at all concentrations tested. Efficacy of EO-based repellents is usually short-lived and related to their volatility. Furthermore, synthetic repellents tend to be more effective and/or persist longer than natural repellents
[46]. The degree of recursiveness of EOs is mainly due to their composition. Monoterpenes such as camphor, α-pinene, thymol and cineole are frequent components of a variety of EOs mentioned in the literature as repelling mosquitoes
[47][48].