Biological activity of essential oils from seven Azorean plants against Pseudaletia unipuncta (Lepidoptera: Noctuidae)
Abstract
Ten essential oils from seven Azorean plant species were evaluated for their insecticidal, ovicidal, feeding-deterrence and growth inhibition activities against Pseudaletia unipuncta. The oils of Laurus azorica (leaves), and Juniperus brevifolia (leaves) showed strong moderate insecticidal effect on fourth-instar larvae causing 93.3% and 46.7% mortality, respectively. Juniperus brevifolia (leaves), L. azorica (leaves), Persea indica (leaves), Hedychium gardnerianum (leaves) and Pittosporum undulatum (fruits and leaves) significantly affected the hatching of P. unipuncta eggs (<8% eclosions). Five oils showed significant feeding deterrent activities (L. azorica, 92.4%, J. brevifolia, 93.6%, P. undulatum leaves, 95.5% and fruits, 83.8% and H. gardnerianum, 88.2%). All of the essential oils tested, significantly inhibited the larval growth after 5 days of feeding on the treated diet. Essential oils from L. azorica and J. brevifolia were the most potent growth inhibitors among the oils tested, producing a decrease in the initial larva weight (−14.8 and −14.5 mg, respectively). Our results indicate that L. azorica (leaves), J. brevifolia (leaves), P. indica (leaves), H. gardnerianum (leaves), and P. undulatum (leaves and fruits) can be exploited for the development of bioactive compounds as a new source of agrochemicals. Further emphasis on isolation and identification of active constituents can be useful to develop new environment-friendly insect control agents.
Introduction
The armyworm, Pseudaletia unipuncta (Haworth) (Lepidoptera: Noctuidae), is a polyphagous insect and an important pest of graminaceous crops, including pasture, in Europe (Bues et al. 1986) and in North America (McNeil et al. 1996, 2000). It is the most important pest of grass pastures in the Azores Islands (Portugal) (Vieira et al. 2003). Infestations occur mainly in summer and early autumn, sometimes requiring the use of synthetic insecticides which has led to environmental problems (Tavares 1992; Vieira 1992; Vieira et al. 1994). To avoid these problems, it is necessary to search for alternative methods of pest control.
Prior to the discovery of synthetic pesticides, plant or plant-based products were the only pest managing agents available to farmers around the world (Kamaraj et al. 2008). The search for biologically active compounds from natural resources has always been of great interest to scientists looking for new sources and models for development of ecologically and environmentally friendly insect control agents (Isman 1999, 2000; Lahlou 2004; Isman and Akhtar 2007). Essential oils, found in a variety of plants, have been shown to contain biologically-active constituents, with antimicrobial (Bravo et al. 1997; Roussis et al. 1998; Chandramu et al. 2003), antifungal (Lalman 1980) and insecticidal (Lahlou 2004; Kamaraj et al. 2008) properties. Insecticidal activity of essential oils has been shown against a variety of insects especially noctuids including Heliothis armigera (González-Coloma et al. 1992), Spodoptera exigua (Fraga et al. 1997), S. frugiperda (Roel et al. 2000), S. litura (Shukla et al. 1997; Hummelbrunner and Isman 2001), S. littoralis (El-Aswad et al. 2003), and Trichoplusia ni (Jiang et al. 2009).
Regarding native plant communities, Laurisilva, a humid evergreen broadleaf laurel forest, was considered in the past to be the predominant vegetation form in the Azores (Borges et al. 2009). After human settlement, other types of vegetation cover have become progressively dominant. Presently, they include pastureland, production forest (mostly with Cryptomeria japonica), mixed woodland (dominated by non-indigenous taxa), field crops and orchards, vineyards, hedgerows, and gardens (Gaspar et al. 2009).
Presently, there are about 1000 species of vascular plants, of which no more than 300 are indigenous (Silva et al. 2005), including 72 endemic taxa. Juniperus brevifolia (Seub.) Antoine and Laurus azorica (Seub.) Franco (Lauraceae), Ilex perado Aiton ssp. azorica (Loes.) Tutin and Picconia azorica (Tutin) Knobl. (Oleaceae), species are considered important for the Macaronesian flora (including the Azores Islands), as are plants that only exist in these islands, and as such need to be preserved (Cardoso et al. 2008). Persea indica (L.) Sprengel is a Macaronesian species considered as an early introduction into the Azores that was valued for its timber in the past. On the other hand, Hedychium gardnerianum Ker-Gauler (Zingiberaceae) and Pittosporum undulatum Ventenat (Pittosporaceae) are among the worst invaders in the Azores archipelago (Silva et al. 2008).
The essential oils from J. brevifolia, P. undulatum and H. gardnerianum have previously been investigated (Silva et al. 2000; Medeiros et al. 2003). The objective of this research was to identify insecticidal activity of selected essentials oils from some endemic/introduced Azorean plants against P. unipuncta.
Materials and methods
Plant materials
Leaves of endemic species L. azorica, J. brevifolia, P. azorica, I. perado, and leaves, flowers and/or fruits of introduced species P. indica, P. undulatum and H. gardnerianum, were collected from different locations on S. Miguel Island (Portugal). Leaves, flowers and fruits were randomly picked from healthy plants, placed in plastic bags and immediately brought to the laboratory and stored at −20°C. Plant identification were achieved by Botanists from the Carlos Machado Museum Herbarium and from INOVA (Instituto de Inovação Tecnológica dos Açores), where voucher specimens are stored (table 1).
| Family | Scientific name | Common name | Plant Part | Voucher number |
|---|---|---|---|---|
| Aquifolioceae | Ilex perado subs. azorica (Loes.) Tutin | Azevinho | Leaves | INOVA-83 |
| Cupressaceae | Juniperus brevifolia (Seub.) Antoine | Cedro | Leaves | INOVA-38 |
| Lauraceae | Laurus azorica (Seub.) Franco | Louro | Leaves | INOVA-19 |
| Oleaceae | Picconia azorica (Tutin) Knobl. | Pau-Branco | Leaves | MCM21022 |
| Zingiberaceae | Hedychium gardnerianum Sheppard ex Ker-Gawler | Conteira | Leaves | INOVA-78 |
| Lauraceae | Persea indica (L.) Sprengel | Vinhático | Leaves Flowers | MCM21023 |
| Pittosporaceae | Pittosporum undulatum Vent. | Incenso | Leaves Fruits Flowers | INOVA-79 |
Analyses of the essential oils
Hydrodistillation was carried out in a modified Clevenger, for 4 h, apparatus with a water-cooled oil receiver. The collected oils were stored at −1°C until analysed. GC analyses of the oils were carried out using a VARIAN model 3400 equipped with a flame ionization detector (FID). Column temperature was programmed to change after 1 min, from 45°C to 130°C at a rate of 2°C/min and then to 220°C at a rate of 1°C/min. Injector and detector temperature was maintained at 250°C. GC-MS was performed on a Varian Chrompack Saturn model GC-MS 2000 according to conditions described previously (Medeiros et al. 2003). Compounds were identified by comparing their retention times to those of standard compounds, with peak enrichment by co-injection with standards whenever possible. Retention indices were calculated (Dool and Kratz 1963; Davies 1990) by using a mixture of C8–C30n-alkanes and compared with known retention indices described by others (Davies 1990; Adams 1995; Roussis et al. 1998; Choo et al. 1999; Couladis et al. 2000; Raina et al. 2002; Maia et al. 2005) to facilitate identification. Quantitative data was obtained by the peak normalization technique using integrated FID responses.
Insects
Pseudaletia unipuncta (eggs and fourth-instar larvae) were obtained from an established laboratory colony maintained for more than 10 generations. Larvae were reared on an artificial diet, (Poitout and Bues 1974), free of antimicrobial compounds, and adults were supplied with honey/water solution (10% honey). Insect colonies were maintained in laboratory at 23 ± 1°C and 70 ± 5% relative humidity under a L16 : D8 photoperiod. Fourth-instar larvae of P. unipuncta were used in this study, with an average weight of 40–50 mg. Eggs of P. unipuncta were collected on strips of vegetable parchment paper placed inside oviposition cages.
Contact toxicity
The filter paper impregnation method was used to examine contact toxicity. Bioassays were conducted in sterile 5 cm diameter Petri dishes containing one impregnated filter paper disc (Whatman no. 1) with 100 μl of an appropriate concentration (160 μg/cm2) of essential oil. The filter papers were air dried for 15 min. In the control group, filter papers were treated with ethanol only. Ten fourth-instar larvae of P. unipuncta were confined to each Petri dish, without food. About 24 h later, one piece of artificial diet was added to each Petri dish. For each treatment, four replicates of 10 larvae were used. The vials were kept in the incubator and mortality assessed during 72 h after treatment. Larvae were considered dead if they did not respond to prodding with a dissecting pin.
Ovicidal bioassay
About 0–24 h old eggs of P. unipuncta laid on strips of vegetable parchment paper from the laboratory colonies were disinfected in 5% formaldehyde solution for 20 min and rinsed with distilled water. These strips were then cut up into sections containing 40–50 eggs. Five replications were used for the ovicidal assay. The strips containing the eggs were dipped, with the help of tweezers, in a 100 mg/ml essential oils solution (placed in 1.5 ml eppendorfs) for 2 s and exposed to air for 15 min to completely evaporate the solvent. Two sets of controls were used: one set of control eggs was dipped in ethanol only, whilst the other was deposited over an untreated paper disk. After 12 days, the number of unhatched eggs was counted and ovicidal activity was calculated as percentage. This methodology was adapted from Prajapati et al. (2003).
Antifeedant bioassay
The antifeedant activity of the essential oils was tested against the fourth-instar larvae of P. unipuncta by diet-no-choice methods, adapted from El-Aswad (2003). The stock solutions were dissolved in ethanol at the concentrations of 300 mg essential oils per ml (w/v). Ten microlitres of the sample was added to a piece of artificial diet (average weight 150 mg), so that each gram of diet contained about 20 mg of essentials oils at the concentrations tested. Two sets of controls were utilized. One control group was treated with ethanol only, while the other was not treated at all. Bioassays were conducted in sterile 9 cm diameter Petri dish containing one moist filter paper disc (Whatman no. 1) to prevent desiccation, in which treated or untreated artificial diet was placed. Each treatment involved three replicates with 10 fourth-instar larvae per replicate. The larvae were pre-weighed and starved for 6 hr prior to the bioassays. Larvae were allowed to feed on treated or control diet continuously for 5 days. The difference in the weight of artificial diet before and at the end of the experiment was the amount of diet (mg) consumed by insects.
Growth-inhibition bioassay
A growth-inhibition bioassay was conducted against fourth-instar larvae of P. unipuncta on an artificial diet following the same procedure as used for the antifeedant bioassay. Freshly moulted and pre-weighed larvae were used on the treated and untreated diet and allowed to feed for 5 days. The weight of each larva was taken daily until the end of the experiment.
Statistical analyses
The antifeedant effects were calculated as feeding deterrence index, FDI (%) = [(C–T)/(C)] × 100, where C is the weight of diet consumed in control and T is the weight of diet consumed in the treatment (Huang et al. 1997). Growth inhibition was calculated from the equation: [(CL– TL)/(CL)] × 100, where CL is the larval weight gain in the control and TL is the larval weight gained in the treatment (El-Aswad et al. 2003).
The data obtained for insecticidal, ovicidal, antifeedant, and growth-inhibition of the essentials oils was subjected to analysis of variance (anova) procedure after arcsine transformation. Means with significant variance and F-statistic were separated by Tukey’s multiple range tests using spss statistic package, software version 15.0 for Windows (SPSS 2006).
Data from insecticidal assay were used to calculate LT50 (the exposure time required to obtain 50% mortality of the insects) values.
Results
Analyses of the essential oils
Most of the oils are rich in terpenes, with monoterpenes present in higher percentage in almost all the oils, followed by sesquiterpenes and diterpenes (table 2). The compounds referred to as ‘others’ are mainly alkanes such as octacosane, non-acosane and triacontane. The compounds with the smaller percentage are oxygen-containing diterpenes.
| Chemical class | Endemic | Introduced | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Ilex perado (Leaves) | Juniperus brevifolia (Leaves)1 | Laurus azorica (Leaves)2 | Picconia azorica (Leaves) | Hedychium gardnerianum (Leaves)3 | Persea indica | Pittosporum undulatum 3 | ||||
| (Leaves) | (Flowers) | (Leaves) | (Flowers) | (Fruits) | ||||||
| Monoterpene hydrocarbons | 0.4 | 89.5 | 45.4 | – | 29.5 | 10.4 | 0.3 | 1.6 | 82.0 | 87.8 |
| Oxygen-containing monoterpenes | 0.5 | 0.4 | 28.6 | – | Trace | 5.0 | 0.6 | – | 12.0 | 5.6 |
| Sesquiterpenes hydrocarbons | 3.9 | 1.1 | 6.0 | 13.7 | 47.1 | 55.8 | 70.4 | 61.5 | 1.3 | 4.5 |
| Oxygen-containing sesquiterpenes | 11.0 | 0.6 | 2.5 | – | 17.0 | 14.7 | 11.2 | 16.5 | – | 1.1 |
| Diterpenes hydrocarbons | 17.4 | 5.1 | – | 82.0 | – | 3.6 | – | 10.7 | – | – |
| Oxygen-containing diterpenes | 0.4 | 0.6 | – | – | – | – | 0.1 | – | – | – |
| Others | 57.3 | 1.1 | 8.8 | – | – | – | 12.7 | 6.5 | 1.0 | – |
- 1 J. brevifolia: Silva et al. (2000).
- 2 L. azorica: Pedro et al. (2001).
- 3 H. gardnerianum and P. undulatum: Medeiros et al. (2003).
Contact toxicity
The most active essential oil in contact toxicity bioassays (72 h post-treatment) was L. azorica (leaves) causing 93.3% of larval mortality, followed by leaves of J. brevifolia (46.7% mortality) and fruits of P. undulatum (18.9% mortality) by impregnated method at a concentration of 160 μg essential oil/cm2 of filter paper (table 3). No insecticidal activity was observed with the essential oils from P. indica (flowers and leaves), P. azorica (leaves), H. gardnerianum (leaves), P. undulatum (leaves and flowers) and I. perado (leaves), in the impregnation method.
| Essential oils (parts used) | n 1 | Mortality at 72 HPT (%± SEM) (160 μg/cm2) | LT50 (95% CL)2 | LT90 (95% CL)2 | Slope (± SEM) | Intercept (± SEM) | H 3 |
|---|---|---|---|---|---|---|---|
| Juniperus brevifolia (leaves) | 40 | 46.7 ± 4.8 | 84.5 a (72.1–110.5) | 449.9 a (256.1–1554.2) | 1.8 ± 0.34 a | 1.6 ± 0.62 | 0.44 |
| Laurus azorica (leaves) | 40 | 93.3 ± 2.4 | 27.1 b (20.8–32.8) | 92.3 b (67.4–171.9) | 2.4 ± 0.36 a | 1.6 ± 0.56 | 2.56 |
| Pittosporum undulatum (fruits) | 40 | 18.9 ± 5.1 | – | – | – | – | – |
- 1Number of insects tested excluding controls.
- 2LT values and 95% confidence limits (CL) expressed in the number of hours required to kill insect larvae. LT values and slope within a column followed by the same letter are not significantly different based on non-overlapping 95% CL.
- 3 H, heterogeneity factor (ϰ2/d.f.).
Correlation between larval mortality caused by the oil with time of exposure was significant for L. azorica (r = 0.988; F-test = 254.584; d.f. = 7; P = 0.000), and J. brevifolia (r = 0.957; F-test = 43.023; d.f. = 5; P = 0.003) leaves. Laurus azorica is three times more toxic than J. brevifolia. The estimated LT50 for L. azorica and J. brevifolia were 27.1 and 84.5 h, respectively (table 3).
Ovicidal bioassay
The egg hatch rate ranged from 1.1% to 98.7%, for J. brevifolia (leaves) and ethanol or untreated treatment, respectively, with significant differences among treatments (F-test = 70.271; d.f. = 11, 35; P = 0.000). Based on the ovicidal activity of essential oils at 100 mg/ml, the essential oils were grouped into two, those exhibiting high activity without significant differences between them (P > 0.05, Tukey’s multiple range tests), including leaves of J. brevifolia (1.1% eclosion), leaves of L. azorica (1.5% eclosion), leaves of P. indica (4.6% eclosion), leaves of H. gardnerianum (6.7% eclosion), leaves and fruits of P. undulatum (6.8% and 7.8% eclosions, respectively). The second group exhibiting very reduced activity including leaves of I. perado (96.0% eclosion), flowers of P. undulatum (97.0% eclosion), P. azorica (98.9% eclosion), and P. indica (100% eclosion) (fig. 1).
Effects of essential oils from endemic and introduced Azorean plants on the eclosion of Pseudaletia unipuncta eggs in the ovicidal bioassay. Means (± SEM) followed by the same letter are not significantly different (Tukey’s multiple range test; P < 0.05). 1 – Control, 2 – Ethanol, 3 –Persea indica (flowers), 4 –Picconia azorica (leaves), 5 –Pittosporum undulatum (flowers), 6 –Ilex perado (leaves), 7 –Pittosporum undulatum (fruits), 8-Pittosporum undulatum (leaves), 9 –Hedychium gardnerianum (leaves), 10 –Persea indica (leaves), 11 –Laurus azorica (leaves), 12 –Juniperus brevifolia (leaves).
Antifeedant activity
The antifeedant activity of the Azorean plants oils were tested against fourth-instar larvae of P. unipuncta. The larvae fed on the artificial diet treated with essential oils at a concentration of 20 mg/g diet, consumed less material after 5 days, than the larvae fed on the untreated diet (table 4). The FDI percentages ranged between 31.5% and 95.5% and showed significant differences among treatments (F-test = 36.467; d.f. = 9, 29; P = 0.000).
| Essential oils (parts used) | Diet consumed (mg/larva) | Feeding deterrence index (%± SEM) | ||
|---|---|---|---|---|
| Untreated | Treated | |||
| Endemic | ||||
| Ilex perado (leaves) | 412.7 ± 0.9 | 282.5 ± 7.7 | 31.5 ± 4.7 | c |
| Juniperus brevifolia (leaves) | 548.7 ± 13.9 | 32.9 ± 4.6 | 93.6 ± 2.2 | a |
| Laurus azorica (leaves) | 568.6 ± 13.2 | 40.9 ± 4.4 | 92.4 ± 2.1 | a |
| Picconia azorica (leaves) | 412.7 ± 0.9 | 181.6 ± 7.2 | 56.1 ± 3.5 | bc |
| Introduced | ||||
| Hedychium gardnerianum (leaves) | 547.6 ± 9.1 | 52.2 ± 4.7 | 88.2 ± 3.0 | a |
| Persea indica (leaves) | 386.2 ± 17.3 | 123.1 ± 10.3 | 67.0 ± 0.7 | b |
| Persea indica (flowers) | 420.7 ± 1.9 | 204.0 ± 6.8 | 50.5 ± 3.4 | bc |
| Pittosporum undulatum (leaves) | 545.3 ± 14.1 | 26.5 ± 4.5 | 95.5 ± 1.3 | a |
| Pittosporum undulatum (fruits) | 412.7 ± 0.9 | 65.9 ± 4.1 | 83.8 ± 2.1 | ab |
| Pittosporum undulatum (flowers) | 545.3 ± 14.1 | 331.0 ± 10.4 | 38.7 ± 1.8 | c |
- Each data point represents the mean of three replicates (10 insects per replicate). Ten fourth-instar larvae of P. unipuncta used at concentration of the 20 mg of essentials oils per gram of artificial diet. Means (± SEM) followed by the same letter in the column are not significantly different (Tukey’s pairwise comparison test; P < 0.05).
Five essentials oils showed strong antifeedant effects. Leaves of endemic plants J. brevifolia and L. azorica demonstrated 93.6% and 92.4% antifeedant effects. The introduced plants P. undulatum leaves and fruits showed 95.5% and 83.8% antifeedant effects and leaves of H. gardnerianum showed 88.2% deterrence. Ilex perado (leaves), P. undulatum (flowers), P. indica (flowers) and P. azorica (leaves) exhibited low antifeedant effects (31.5–38.7%) without significant differences among the essential oils tested.
Growth-inhibition bioassay
Growth inhibition activities of the endemic and introduced Azorean plants essential oils on P. unipuncta are shown in table 5. All of the essential oils tested inhibited the larval growth after 5 days of feeding on the treated diet with significant differences amongst treatments (F-test = 27.047; df = 9, 29; P = 0.000). The leaves of endemic plants L. azorica and J. brevifolia were the most potent growth inhibitors among the essential oils tested, producing a decrease in the initial larva weight (−14.8 and −14.5 mg, respectively), while the flowers of P. undulatum and leaves of P. indica showed the weakest growth-inhibition activity.
| Essential oils (parts used) | Mean weight gain (mg/larva ± SEM) | Growth inhibition (% ± SEM) | ||
|---|---|---|---|---|
| Untreated | Treated | |||
| Endemic | ||||
| Ilex perado (leaves) | 120.6 ± 6.3 | 57.3 ± 4.2 | 52.5 ± 3.8 | cd |
| Juniperus brevifolia (leaves) | 182.0 ± 7.7 | −14.5 ± 1.2 | 108.7 ± 2.2 | a |
| Laurus azorica (leaves) | 189.5 ± 8.0 | −14.8 ± 1.0 | 108.5 ± 1.8 | a |
| Picconia azorica (leaves) | 120.6 ± 6.3 | 25.0 ± 2.3 | 79.2 ± 2.1 | bc |
| Introduced | ||||
| Hedychium gardnerianum (leaves) | 159.1 ± 8.3 | −6.1 ± 3.6 | 105.2 ± 2.7 | ab |
| Persea indica (leaves) | 207.5 ± 4.9 | 29.4 ± 5.2 | 85.6 ± 1.7 | ab |
| Persea indica (flowers) | 162.6 ± 6.5 | 94.9 ± 6.8 | 42.7 ± 4.3 | d |
| Pittosporum undulatum (leaves) | 201.6 ± 6.6 | −10.7 ± 0.8 | 106.3 ± 1.7 | a |
| Pittosporum undulatum (fruits) | 120.6 ± 6.3 | −4.9 ± 3.2 | 102.5 ± 2.3 | ab |
| Pittosporum undulatum (flowers) | 201.6 ± 6.6 | 117.6 ± 6.7 | 42.5 ± 3.6 | d |
- Each data point represents the mean of three replicates (10 insects per replicate). Ten fourth-instar larvae of P. unipuncta used at concentration of the 20 mg of essentials oils per gram of artificial diet. Means (± SEM) followed by the same letter in the column are not significantly different (Tukey’s pairwise comparison test; P < 0.05).
Discussion
Secondary metabolites including various alcohols, terpenes and aromatic compounds (Phillips and Croteau 1999; Pichersky and Gershenzon 2002) contribute to the defence system of plants. They can discourage insects or other herbivores from feeding, can have direct toxic effects, or may be involved in recruiting predators and parasitoids in response to feeding damage (Baldwin et al. 2001; Kessler and Baldwin 2001). Antifeedants, compounds that deter feeding on plants by insect pests, may be used as important components in the integrated pest management strategies for agricultural pest control.
Our results indicate that the essential oils from leaves of J. brevifolia, P. indica, L. azorica and P. undulatum (leaves and fruits) were the most biologically active. According to Isman (2000) some essential oils have a broad spectrum of biological activities, and the action of these caused, possibly, the compound(s) that occurs in greater quantity in the extract or/and synergy between compounds. The amount and the diversity of the molecules active in Azorean oil plants are not the same for all the oil tested. P. undulatum (flowers and fruits), J. brevifolia (leaves), L. azorica are rich in monoterpenes (94%, 93.4%, 89.9% and 74%, respectively), while the other essential oils have low values.
The strong biological activities in the oils exhibited in our study can be attributed to the presence of monoterpenes and diterpenes, as shown by González-Coloma et al. (1999) and Lago et al. (2006) for P. indica and P. undulatum (leaves), respectively. However, there was a lack of correlation in the amount of monoterpenes and biological activity in the oil of P. undulatum (flowers) and it might be attributed to the antagonistic effects of the different compounds present in this essential oil.
Toxicity of essential oil of J. brevifolia (leaves) and P. undulatum (leaves) can be attributed to the presence of limonene (41–79% and 69%, respectively) and to α-pinene (6–40% and 2.8%, respectively) in these oils (Silva et al. 2000; Lago et al. 2006). The main components of the leaf oils of L. azorica and H. gardnerianum are α-pinene (15–37% and 17.5%, respectively), 1,8-cineole (12–31%) and caryophyllene (8.89%), respectively (Pedro et al. 2001; Medeiros et al. 2003). Toxicity of these compounds is demonstrated on Coleopteran (Bekele and Hassanali 2001; Ngamo et al. 2007) and Lepidopteran pests (Miyazawa et al. 1998; Hummelbrunner and Isman 2001).
Although the quantity of terpenes was lower in the essential oil from leaves of L. azorica, high insecticidal and ovicidal activities might have resulted from synergistic interactions between different components in the oil as evidenced by work done by Rodilla et al. (2008) and Jiang et al. (2009). Traboulsi et al. (2005) showed strong repellent and toxic effects for the essential oils of Laurus nobilis against the mosquito Culex pipiens molestus.
Some of these plant essential oils have been studied for the evaluation of insecticidal activity against other insects. González-Coloma et al. (1992) have demonstrated that extracts of the terminal branches of P. indica had strong insecticidal activity on two Lepidopteran species Macaronesia fortunata (Lymantriidae) and Heliothis armigera (Noctuidae). Jadhav et al. (2007) reported in vitro insecticidal activity against human head lice by the essential oil of Hedychium spicatum, which is closely related to H. gardnerianum, on a level higher than commercialized compounds used to fight this problem.
Our results indicate that toxicity, ovidicidal, growth inhibition and antifeedant activity of the essential oils from Azorean endemic plants (L. azorica, J. brevifolia and P. indica) and introduced plants (H. gardnerianum and P. undulatum) have potential for development as new effective insect control agents against P. unipuncta, one of the most important pest in Azores islands.
Since even closely related species can differ markedly in susceptibility to the same plant extract or pure allelochemical (Isman 1993; Akhtar and Isman 2004a,b), future studies should be aimed to determine the effect of these oils against other insect pests in the family Noctuidae, and also their effects on natural enemies.
Acknowledgements
We would like to thank M. Fernando Almeida, from the University of Azores for the insect production. We are also grateful to Nuno Rainha and two anonymous reviewers for their helpful comments and suggestions that both contributed for review and finalization of this manuscript.
