After China and India, Iran is a third chief producer of potato (Solanum tuberosum) in Asia so that its cultivation in Iran is about 146,000 ha with an annually production of 4,300,000.1 The Colorado potato beetle, Leptinotarsa decemlineata Say (Coleoptera: Chrysomelidae), is a key pest of potato throughout the world. It has been considered as most destructive pest in Ardabil and Hamadan provinces at north-west of Iran.2 Its adults and larvae feed on the leaves and stems of potato plants. Further, the adults considered as a vector of plant diseases.3 Numerous chemical insecticides have been used to the management of L. decemlineata. Along with the negative side-effects of chemical pesticides such as environmental contamination and threat to human health, there are several studies indicating the resistance of L. decemlineata to the most classes of insecticides such as organochlorine, organophosphorus, carbamate, and pyrethroid ones.4567 So, there is an urgent need to introduce an eco-friendly and effective method to control this pest. Plants have undergone a co-evolution with their natural enemies including insects, in which plant secondary metabolites are developed against them. Essential oils as demotic secondary metabolites evinced a major role in the plant-enemies interactions. Plant essential oils have a complex mixture of aromatic and aliphatic components which affected by several endogenous and exogenous factors such as environmental conditions, plant genetic and extraction method.8 These materials have been characterized with low or non-toxicity to fish, birds and mammals. They are easily biodegrade in the environment because of their volatile quiddity. Further, pest resistance to the essential oils is lower than synthetic chemicals because of their multiple modes of action and several bio-effects.910 Consequently, essential oils considered as potential bioagents to the insect pest management.1112 Although lemongrass [Cymbopogon citratus (DC) Stapf (Poaceae)] is native of Southeast Asia, it is usually used by folk medicine in the numerous countries.13 Several biological effects of lemongrass such as anti-amoebic, antifungal and antidiarrheal activities have been documented.141516 Terpenic compounds such as citral and its isomers have high amount in the C. citratus essential oil and it was found that the biological effects of this oil have been related to these components.8917 The aim of the present study is to evaluate the toxicity of C. citratus essential oil against the damaging stages of L. decemlineata; larvae and adults. Findings may be useful to recognize a biorational and safe agent for management of L. decemlineata.

Results indicated that the essential oil of C. citratus showed significant toxicity against both larvae and adults of L. decemlineata after 24 and 48 h exposure times while there was not any mortality in the control groups. Comparison of mean mortality by Tukey's test revealed there are statistically significant differences between mortality of larvae and adults of insect against essential oil concentrations and exposure times (Fig. 1).

The results of Probit analysis are shown in Table 1; The LC₅₀ values with 95% confidence limits were 10.32 (9.17 – 11.72) and 7.76 (6.80 – 8.74) µl/ml for larvae after 24 and 48 h exposure time, respectively. These values were, respectively, calculated as 6.27 (4.82 – 8.15) and 4.35 (3.24 – 5.62) µl/ml for adults. The LC₅₀ values for both stages decreased with increasing of exposure time and there are statistically significant differences between these values after 24 and 48 h (Table 1).

Further, based on regression analysis and according to R² values, a positive correlation between logarithm of concentrations and mortality has been achieved for both larvae and adults (Fig. 2).

Chemical composition of C. citratus essential oil from different country was assessed in the recent studies (Table 2). According to the Table 2, different isomers of citral including as α-citral, β-citral, geranial and neral, geraniol, myrcene, neryl acetate, caryophyllene, caryophyllene oxide, geranyl acetate, verbenol, 1,8-cineole, and β-pinene were the main components, which all are among terpenic compounds. Toxicity of such terpenic compounds was approved against L. decemlineata and some of other insect pests.1719 Further, toxicity of some essential oils was evaluated against L. decemlineata in the recent years. For example, toxicity of Eugenia caryophyllus (Sprengel), Mentha spicata L., Myrtus communis L., Ocimum basilicum L., Satureja khuzistanica Jamzad and Thymus daenensis Celak essential oils were evaluated against the 4th instar larvae and adults of L. decemlineata. All essential oils were toxic against both stages in which the essential oil of S. khuzistanica was the most efficient.20 Toxicity of four Achillea species including A. biebersteinii Afan., A. coarctata Poir., A. wilhelmsii Koch and Achillea biserrata Bieb. was investigated against the adults of L. decemlineata by Cakır et al.21 L. decemlineata adults were affected by the concentrations and exposure times of all essential oils. They concluded that observed toxicity might be caused by the occurrence of main components or by synergistic effect of them with the other compounds. In the present study, the insecticidal activity of C. citratus essential oil was approved against the larvae and adults of L. decemlineata. This effects may be related to being some terpenic compounds specially citral, geranial, neral, geraniol, myrcene, and 1,8-cineole. Furthermore, the insecticidal effects of C. citratus essential oil were reported against some major insect pests such as Aedes aegypti L., Tribolium castaneum Herbst, Sitophilus oryzae L., Zabrotes subfasciatus (Boheman), and Musca domestica L., which support the results of present study for insecticidal potential of this oil.2223242526 One of the most attractive features of plant essential oils is that they are low-risk materials to mammals and other non-target organisms. Essential oils have short residual half-lives. So, their consumption can reduce risks to foraging pollinators such as honeybees. Furthermore, because of being several components with multiple modes of action against pests, pest resistance to these bio-agents will be low. Based on the previous findings C. citratus essential oil has a great potential for management of some insect pests and according to the our results it was a sound candidate as a new bio-pesticide for L. decemlineata management. However, additional studies should be made for enhancement of its toxicity against L. decemlineata and other economical insect pest, improvement its stability in the field application and reducing its cost problem.