ARTIGO

The use of Laurus nobilis and Mentha pulegium essential oils against Sitophilus zeamais (Coleoptera: Curculionidae) on stored maize

Utilização de óleos essenciais na proteção de milho armazenado contra Sitophilus zeamais (Coleoptera: Curculionidae)

Graça Barros^1,*, Ana Magro¹, Carlos Conceição², Olívia Matos³, António Barbosa⁴, Margarida M.S.M. Bastos⁵ and António Mexia⁶

 

¹ BIOTROP, Instituto de Investigação Científica Tropical, Tapada da Ajuda, Edificio Ferreira Lapa 3ºPiso, 1349-017 Lisboa, Portugal. E-mail: gracafragabarros@gmail.com. ^1,* Author for correspondencemico. E-mail: anamagro@gmail.com

² Universidade José Eduardo dos Santos, UJES, Huambo, Angola. Email: carlosconceicao_6@hotmail.com.

³ Instituto Nacional de Investigação Agrária e Veterinária, I.P., Quinta do Marquês, 2780-159 Oeiras, Portugal. E-mail: oliviamatos@sapo.pt

⁴ Rolão Gonçalves Limitada, 1950-196 Lisboa, Portugal. E-mail: affbarbosa@rolaogoncalves.com

⁵ LAPABE, Departamento de Engenharia Química, Faculdade de Engenharia da Universidade do Porto, Rua Dr. Roberto Frias 4200-465, s/n, Porto, Portugal. E-mail: mbastos@fe.up.pt

⁶ LEAF-Linking Landscape, Environment, Agriculture and Food, Instituto Superior de Agronomia, University of Lisbon, Tapada da Ajuda, 1349-017 Lisboa, Portugal. E-mail: amexia@isa.ulisboa.pt

 

ABSTRACT

Laboratory studies were carried out to evaluate the effects of some essential oils from Laurus nobilis and Mentha pulegium against Sitophilus zeamais on stored maize. The concentrated essential oils at different volumes of 0.5 µL, 1.5 µL, 2.5 µL, 5.0 µL and 10 µL, were poured on filter papers with 2 cm Ø each. For diluted oils, the fixed volume of 15 µL of different concentrations of 1:150v/v, 1:100 v/v, 1:75 v/v, 1:50 v/v and 1:10 v/v either in methanol or n-hexane were used to impregnate the filter papers. Treatments with the concentrated oils were more effective. All the concentrations used from M. pulegium provided 100% adult mortality and no progeny production were achieved. L. nobilis has revealed 100% adult mortality at 3.185 µL/cm². Regarding the treatments with diluted oils once again the oil from M. pulegium provided 100% adult mortality at concentrations of 1:50 v/v and 1:10 v/v. L. nobilis was not effective at any of the concentrations used. There were no significant differences between the solvents used.

Key words: botanical insecticides, essential oils, Laurus nobilis, Mentha pulegium Sitophilus zeamais.

 

RESUMO

Para avaliar o efeito dos óleos essenciais de Laurus nobilis e Mentha pulegium, no controlo de insetos adultos de Sitophilus zeamais, em milho armazenado, foram efetuados vários ensaios laboratoriais, utilizando-se óleos essenciais concentrados e diluídos, em metanol e n-hexano. Os tratamentos com os óleos essenciais concentrados apresentaram melhores resultados do que com os diluídos. Verificou-se 100% de mortalidade e ausência de descendência (F₁) em todas as concentrações testadas de óleo essencial de M. pulegium. No caso de L. nobilis só se obtiveram aqueles mesmos resultados com a concentração mais elevada. Em relação aos óleos essenciais diluídos, os valores de 100% de mortalidade e ausência de descendência (F₁) só foram atingidos para a concentração de 1:10 v/v, com M. pulegium e em ambos os solventes. O óleo de L. nobilis não foi eficaz em nenhuma das concentrações ensaiadas, não apresentando diferenças entre os solventes utilizados.

Palavras-chave: inseticidas naturais, Laurus nobilis, Mentha pulegium, óleos essenciais, Sitophilus zeamais.

 

Introduction

Maize is one of the most important cereal in the world because is a staple food for millions of people especially in Africa and Central America. According to the World Bank (2011) in Kenya and Tanzania maize contributes over one third of people's daily caloric intake.

Traditional storage practices in developing countries cannot guarantee protection against major storage pests of food crops like maize, leading to 20-30% grain losses, particularly due to post-harvest insect pests and grain pathogens (Tefera et al., 2011).

Insect damage to stored grains has been of great concern to man throughout the ages. Storage pests, such as the maize weevil (Sitophilus zeamais), the rice weevil (S. oryzae), the red flour beetle (Tribolium castaneum), the bruchid beetle (Callosobruchus maculatus) and others cause quantitative and qualitative damage to grains (Padin et al., 2002). One of the most economically destructive maize storage insect in Sub-Saharan Africa is S. zeamais.

The control strategies against S. zeamais consist in the use of the traditional chemical control but the development of insect resistance to insecticides and consumer concern over the use of these products in food has resulted in the search for alternative methods of insect control. Consumers today expect a food product that is pesticide free or with low residue levels. The management of stored product pests, using substances of natural origin, is nowadays the subject of much research (e.g. Papachristos and Stamopoulos, 2002).

In this study two essential oils, of Laurus nobilis and Mentha pulegium, were tested to answer the questions about the fumigant toxicity of these essential oils, adult mortality, life cycle and the effects on the emergence of the F1 of S. zeamais, in order to establish new control practices with lower mammalian toxicity and low persistence in the environment.

 

Material and Methods

Maize and insects

The yellow maize grain was acquired from local commerce with an average moisture content of 14 ± 0.5%. Unsexed adult insects aged from 1 to 7 days were used. The stock cultures of insects and the biological tests were carried out in a single incubator at 27 ºC and 75± 5% r.h.

Plant material and essential oils

In this study the essential oil of L. nobilis used was obtained by hydro-distillation of air-dried leaves in a modified Clevenger-type apparatus for 3-h. The leaves were collected in the Agronomy College Campus. The extracted oil was dried over anhydrous sodium sulphate and stored in a sterilized amber bottle at 4 ºC until used. The essential oil of M. pulegium was supplied by the company “Segredo da Planta”.

Laboratory procedures

For the bioassays, plastic jars of 25 mL capacity containing 10 g of maize and 10 adult insects aged from 1 to 7 days old were used. The concentrated essential oils at different volumes of 0.5 µL, 1.5 µL, 2.5 µL, 5.0 µL and 10 µL, were applied on filter papers with 2 cm Ø each corresponding respectively to 0.159 µL/cm², 0.478 µL/cm², 0.796 µL/cm², 1.592 µL/cm² and 3.185 µL/cm². The treated filter papers were attached to the under surface of the plastic jars screw caps. The caps were screwed tightly and hermetically sealed with parafilm. Five replicates per treatment and control were set up. All the replicates were kept in an incubator at 27 ºC and 75% ± 5% r.h..

After 24-h of exposure the adult insects were examined for mortality and thereafter were re-examined after 48-h and 168-h, on maize grain. After the last observation at 168-h the adult insects were removed and presumably only the young eggs or the early stage of larvae remained in the maize kernels which were kept at 27 ºC and 75% ± 5% r.h. during three weeks for the F₁ progeny evaluation. The development index and the life cycle were also evaluated.

Similar procedures were also adopted to evaluate the effects of the diluted essential oils. The fixed volume of 15 µL of different concentrations of 1 :150 v/v, 1 :100 v/v, 1 :75 v/v, 1 :50 v/v and 1 :10 v/v of diluted oils either in methanol or n-hexane were used to impregnate the filter papers of 2 cm Ø each. After the evaporation of the solvents during 2 – 3 minutes the filter papers were transferred into the plastic jars which were hermetically sealed. Five replicates per treatment and each control (methanol, n-hexane and non-treated ambient) were set up.

Data processing and analysis

Haryadi and Rahayu (2002) criteria was adopted to calculate the developmental index, using the following formula: DI = (ln F₁/ADD) x100, where DI = developmental index of S. zeamais; F₁ = number of adult insects emerged from F₁ and ADD = average duration of development.

For the analysis of the results it was used ANOVA statistical analysis and Tukey test for the means comparison.

 

Results and Discussion

The M. pulegium concentrated oil caused 100% mortality of S. zeamais after 24-h of exposure and complete inhibition of F₁ progeny at the lowest dosage of 0.159 μL/cm². The L. nobilis concentrated oil revealed 100% mortality and complete inhibition of F₁ progeny at the highest dosage of 3.185 μL/cm² after 168 h (Table 1).

The effects of L. nobilis in the duration of the development differed statistically between the dosage of 0.159 μL/cm² and the dosages of 0.478 μL/cm² and 0.796 μL/cm². This difference may be caused by suppression of feeding (Table 2). The results obtained in this study are according to Haryadi and Rahayu (2002). For the 1.592 μL/cm² and 3.185 μL/cm² dosages there was no progeny.

The M. pulegium diluted essential oil at 1:10 v/v and 1:50 v/v with n-hexane caused 100% mortality after 168-h and a complete inhibition F₁ progeny. For the dilution with methanol 100% of mortality after 168-h and a complete inhibition F₁ progeny was obtained with a dosage 1:10 v/v. The methanol and n-hexane diluted oil of L. nobilis presented a weak effect in the mortality and in the F₁ progeny (Table 3).

The reduction and the absence of F₁ progeny may be attributed to the repellent effects of the substances in the essential oils and extracts, which disturb the oviposition behaviour of the parent insects. Another possibility is that there are substances in the essential oils and extracts that are toxic to S. zeamais larvae during their early development.

The results obtained are according to similar experiments performed by other authors. The essential oil of L. nobilis at the concentration of 0.1-0.74 mg/kg caused decrease on the adult longevity and fecundity of S. granarius (Boeke et al., 2001). Rozman et al. (2007) have demonstrated the fumigant activity of some major constituents present in the essential oils of Lavandula angustifolia, Rosmarinus officinalis, Thymus vulgaris and L. nobilis against adult insects of S. oryzae, Rhyzoperta dominica and T. castaneum.

Lee et al. (2001) reported substantial efficacy of M. arvensis oil as well as its constituents, menthone, linalool and a-pinene against S. oryzae. Similarly, Varma and Dubey (2001) reported complete inhibition of S. oryzae and T. castaneum, through the treatment of wheat samples with M. arvensis essential oil.

Lee et al. (2003) proved that plant monoterpene pulegone in preliminary fumigant analyses successfully controlled insect pests, and recommended this substance as suitable fumigant because of their high volatility, fumigant efficacy and safety.

In conclusion, the effects of the essential oils from M. pulegium have revealed differences in the development and in the F1 progeny of S. zeamais, in comparison with L. nobilis oils. Mentha species has all the potential to restrict synthetic insecticides as well as to be used in IPM (Integrated Pest Management) for control of various insects pests.

More studies are needed to further define the repellent effect, antifeedant effect and toxicity to storage insect pests of the active principles present in those essential oils.

These compounds may be suitable as fumigants because of their high volatility, effectiveness, and their safety

 

Acknowledgements

This work was funded by FEDER through the Operational Competitiveness Programme –COMPETE-and by national funds through the Foundation for Science and Technology - FCT - in the framework of the project PTDC/AGR-ALI/119270/2010.

 

References

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Received/Recebido: 2015.03.06

Accepted/Aceite: 2015.05.14