Production techniques

The occurrence of some insect species known as pests can cause losses in the field and in storage, as well as increases in the use of pesticides, as this is currently the main method of insect control in agriculture.

This way, the use of essential oils for the control of pests in agriculture has been discussed in numerous studies (Table 2). However, little is known about the action mechanisms and the active ingredient responsible for the effects they cause in insects. This fact is due to the wide range of compounds present in the essential oils, as their composition varies with plant species, and with some factors related to cultivation, soil and climate (Knaak and Fiuza, 2010).

Differences in composition and insecticidal activity of essential oils from plant species were found in the control of Tribolium castaneum using the essential oils from Cymbopogon citratus and Eucalyptus citriodora. The results indicated differences in composition and insecticidal activity of essential oils, demonstrating that the essential oil from Cymbopogon citratus caused higher mortality of insects (Olivero-Verbel et al., 2010). In the control of Ephestia kuehniella, Plodia interpunctella and Acanthoscelides obtectus with the use of essential oils from Origanum onites, Satureja thymbra and Myrtus communis was verified that the different compositions of the essential oils of the studied plant species caused distinct effects on each insect, and Acanthoscelides obtectus showed the lowest toxicity to the essential oils (Ayvaz et al., 2010).

Composition and insecticidal effect of essential oils can also vary between plants of the same species depending on the cultivation system. The essential oil from Salvia aucheri spp.

blancoana extract from wild and cultivated plants showed differences in its composition, especially of the major compounds camphor, camphene and α-pinene. Moreover, the essential oil extracted from wild plants was more effective in the control of Tribolium castaneum insects (Khiyari et al., 2014).

Table 2. Examples of studies on essential oils effects in insects

Essential oil Insect Action Author

Artemisia herba-alba

Sitophilus oryzae Insecticide Kim et al., (2013)

Ocimum basilicum Citrus

Tribolium castaneum Insecticide Nenaah (2014)

Thymus vulgaris

Artemisia vestita Sitophilus zeamais Insecticide Chu et al., (2010)

Francisco Wilson Reichert Jr., Maurício Albertoni Scariot, Jéssica Mulinari et al.

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The essential oils normally cause neurotoxic effects, influencing on insects nutrition, reproduction and behaviour, resulting in death or repellency. Such effects can be related to different action mechanisms, for example by means of GABA, synapse octopamine and the inhibition of acetylcholinesterase (Regnault-Roger et al., 2012). However, the effects caused by essential oils depend on several factors, such as the way in which they come in contact with the insect, which can be by inhalation, ingestion and contact, as well as the insect species (Regnault-Roger, 1997).

The main application methods of essential oils for the insects control found on literature are fumigation, ingestion and contact. Fumigation is a method that uses the volatilization of compounds present in the essential oil, so that they come in contact with the insects by inhalation or absorption through the pores. The ingestion and contact method is usually applied as food substrate, which is treated with the essential oil and made available to the insects. Thereby, the insects do the digestion of the essential oil in addition to the topical effect, so that there is absorption by pores and by inhalation (Hashemi and Safavi, 2012;

Yazdgerdian et al., 2015).

Franz et al., (2011) verified that essential oils of Cymbopogon citratus, Zingiber officinale and Mentha sp. applied by fumigation and contact resulted in mortality rates around 85% and greater than 90%, respectively, in the larger doses tested on Sitophilus oryzae.

Likewise, Chu et al., (2010) found insecticidal effects of Artemisia vestita essential oil on Sitophilus zeamais with contact and fumigation applications. In another study, Saroukolai et al., (2010) found that Thymus persicus essential oil applied by fumigation showed insecticidal activity against Sitophilus oryzae and Tribolium castaneum.

Liu et al., (2010) found insecticide potential of essential oils of Artemisia capillaris and Artemisia mongolia on Sitophilus zeamais insects when applied by fumigation and contact.

Also, Kiran and Prakash (2015) found that the essential oil from Rosmarinus officinalis applied by fumigation on Sitophilus oryzae and Oryzaephilus surinamensis insects caused toxicity and antifeedant action, and reduced by around 40% the acetylcholinesterase enzyme activity. Likewise, Yang et al., (2012) found that the essential oil from Allium sativum, as well as two of its major components, diallyl disulfide and the diallyl trisulfide, caused toxicity, inhibition of oviposition and behavioral change in Sitotroga cerealella insects.

In another study, Emamjomeh et al., (2014) found that the essential oil from Zataria multiflora has insecticide potential on Ephestia kuehniella when applied by fumigation. And the studies of Aref et al., (2015) demonstrated that the essential oil from Eucalyptus dundasii provided, besides toxicity by fumigation, effect on feeding and behavior of Rhyzopertha dominica and Oryzaephilus surinamensis.

The repellent action of essential oils on insects is not yet well understood. However it is known that it results from the individual and combined action of some compounds, such as α-pinene, limonene, citronellol, citronellal, camphor and thymol (Nerio et al., 2010). Caballero-Gallardo et al., (2011) found repellent action of essential oils from Tagetes lucida, Lepechinia betonicifolia, Lippia alba, Cananga odorata and Rosmarinus officinalis, as well as their major compounds applied alone, including limonene and α-pinene on Tribolium castaneum.

Licciardello et al., (2013) found repellent action of essential oils from Cymbopogon nardus, Origanum vulgare and Rosmarinus officinalis on Tribolium castaneum when applied to repellent films used as food packing. And Bougherra et al., (2015) found repellent action of Pistacia lentiscus essential oil on Rhyzopertha dominica, Sitophilus zeamais and Tribolium confusum. In other studies, Mishra et al., (2012) found repellent action of essential oils from

Eucalyptus globulus and Ocimum basilicum on Tribolium castaneum and Sitophilus oryzae.

However, repellent action of some compounds may not be effective for long because they are extremely volatile. The studies of Zhang et al., (2011) showed that limonene and citronellal compounds isolated from the essential oil from Cymbopogon distans reduced their repellent ability 4 hours after the application to Tribolium castaneum insect. The action provided by essential oils on insects such as mortality and repellency, can also be variable in relation to applied dose and exposure time of insects to the essential oil. The studies of Abdel-Sattar et al., (2010) demonstrated that the essential oil from Schinus molle caused mortality and repellency effects on Tribolium castaneum and Trogoderma granarium in accordance with the increase of the applied dose and exposure time. Similarly, Zapata and Smagghe (2010) found that increasing the dose and time of exposure of essential oils from Laurelia sempervirens and Drimys winteri on Tribolium castaneum, higher rates of mortality and repellency were achieved.

The repellent, insecticide and behavioral action exerted by the compounds present in the essential oils extracted from plants, makes these up as a technique for insect control in agriculture. Moreover, the huge variety of species with insecticide potential as well as the enormous range of compounds, provides a wide range of options for replacement of pesticides in agriculture. However, more studies are needed in order to better elucidate the mechanisms of action and the best methods of application of essential oils for insect control.