Roman Pavela

Essential Oils as Ecofriendly Biopesticides? Challenges and Constraints

Recently, a growing number of plant essential oils (EOs) have been tested against a wide range of arthropod pests with promising results. EOs showed high effectiveness, multiple mechanisms of action, low toxicity on non-target vertebrates and potential for the use of byproducts as reducing and stabilizing agents for the synthesis of nanopesticides. However, the number of commercial biopesticides based on EOs remains low. We analyze the main strengths and weaknesses arising from the use of EO-based biopesticides. Key challenges for future research include: (i) development of efficient stabilization processes (e.g., microencapsulation); (ii) simplification of the complex and costly biopesticide authorization requirements; and (iii) optimization of plant growing conditions and extraction processes leading to EOs of homogeneous chemical composition.

Larvicidal effects of some Euro-Asiatic plants against Culex quinquefasciatus Say larvae (Diptera: Culicidae)

Methanol extracts of the aerial parts from 62 Euro-Asiatic plant species were tested for larvicidal activity against the mosquito Culex quinquefasciatus Say (Diptera: Culicidae) under laboratory conditions. The fourth larval instar was tested. The extracts of the plants Seseli pallasii and Schisandra chinencis displayed the highest larvicidal activities with LD50 6 and 15xa0ppm, respectively, followed by Arthemisia campestris, Verbena officinalis, and Imperatoria osthruthium with LD50 23, 38, and 49xa0ppm, respectively. The appraised value of LD50 for two species was between 51–100xa0ppm; eight species had lethal doses from 101 to 500xa0ppm, 13 species showed lethal doses from 501 to 1,000xa0ppm, and 34 species did not show lethal doses for low mortality (LD50u2009>u20091,000xa0ppm).

Acute toxicity and synergistic and antagonistic effects of the aromatic compounds of some essential oils against Culex quinquefasciatus Say larvae

The efficacy of 30 aromatic compounds and their mutual binary combinations was assessed for acute toxicity against the larvae Culex quinquefasciatus. Based on comparison of the lethal doses, thymol and p-cymene were selected as the most effective (LD50u2009=u200918 and 21xa0mgxa0L−1, respectively, and LD90u2009=u200925 and 30xa0mgxa0L−1, respectively). Although the LD50 for terpinolene and trans-anethole was also estimated at 21xa0mgxa0L−1, their LD90 was significantly higher compared to the substances above (245 and 34xa0mgxa0L−1, respectively). In total, 435 binary combinations were tested, of which 249 combinations showed a significant synergistic effect, while 74 combinations showed a significant antagonistic effect on mortality. Only nine substances were identified as being able to create a synergistic effect with more than 20 substances: limonene, trans-anethole, 4-allylanisole, carvacrol, isoeugenol, menthone, carvone, borneol, and camphor. The highest synergistic effect on larval mortality was achieved for the combinations: eugenol and isoeugenol, carvone and carvacrol, carvone and 4-allylanisole, carvone and α-terpineol, carvone and menthone, limonene and trans-anethole, limonene and menthone, α-pinene and menthone, β-citronellol and menthone, carvacrol and 4-allylanisole, carvacrol and terpineol, α-terpinene and trans-anethole, camphor and menthone, camphene and menthone, and 4-allylanisole and menthone. Significant differences between achieved mortality and the mutual mixing ratio were found for the five selected binary mixtures that had shown the most significant synergistic effect in the previous tests. The mixture of limonene and trans-anethole showed the highest mortality, with the mixing ratio 1:1; the mixture of eugenol and isoeugenol caused 90.2xa0% mortality, with the mixing ratio 1:3. One hundred percent mortality was achieved if carvacrol was contained in a mixture with carvone in a ratio >2. After a comparison of all our results, based on our experiments, we can choose two pairs that caused mortality higher than 90xa0% in concentrations lower than 20xa0mgxa0L−1: limonene and trans-anethole (with the mixing ratio 1:1), and carvone and carvacrol (with the mixing ratio 1:2–3). The information gained can thus be used in the development of new botanical insecticides based on essential oils (EOs) and particularly in the creation of formulations.

Tick repellents and acaricides of botanical origin: a green roadmap to control tick-borne diseases?

Arthropods are dangerous vectors of agents of deadly diseases, which may hit as epidemics or pandemics in the increasing world population of humans and animals. Among them, ticks transmit more pathogen species than any other group of blood-feeding arthropods worldwide. Thus, the effective and eco-friendly control of tick vectors in a constantly changing environment is a crucial challenge. A number of novel routes have been attempted to prevent and control tick-borne diseases, including the development of (i) vaccines against viruses vectored by ticks; (ii) pheromone-based control tools, with special reference to the “lure and kill” techniques; (iii) biological control programmes relying on ticks’ natural enemies and pathogens; and (iv) the integrated pest management practices aimed at reducing tick interactions with livestock. However, the extensive employment of acaricides and tick repellents still remains the two most effective and ready-to-use strategies. Unfortunately, the first one is limited by the rapid development of resistance in ticks, as well as by serious environmental concerns. On the other hand, the exploitation of plants as sources of effective tick repellents is often promising. Here, we reviewed current knowledge concerning the effectiveness of plant extracts as acaricides or repellents against tick vectors of public health importance, with special reference to Ixodes ricinus, Ixodes persulcatus, Amblyomma cajennense, Haemaphysalis bispinosa, Haemaphysalis longicornis, Hyalomma anatolicum, Hyalomma marginatum rufipes, Rhipicephalus appendiculatus, Rhipicephalus (Boophilus) microplus, Rhipicephalus pulchellus, Rhipicephalus sanguineus and Rhipicephalus turanicus. Eighty-three plant species from 35 botanical families were selected. The most frequent botanical families exploited as sources of acaricides and repellents against ticks were Asteraceae (15xa0% of the selected studies), Fabaceae (9xa0%), Lamiaceae (10xa0%), Meliaceae (5xa0%), Solanaceae (6xa0%) and Verbenaceae (5xa0%). Regression equation analyses showed that the literature grew by approximately 20xa0% per year (period: 2005–2015). Lastly, in the final section, insights for future research are discussed. We focused on some caveats for future data collection and analysis. Current critical points mainly deal with (a) not uniform methods used, which prevent proper comparison of the results; (b) inaccurate tested concentrations, frequently 100xa0% concentration corresponded to the gross extract, where the exact amounts of extracted substances are unknown; and (c) not homogeneous size of tested tick instars and species. Overall, the knowledge summarized in this review may be helpful for comparative screening among extensive numbers of plant-borne preparations, in order to develop newer and safer tick control tools.

Mosquitocidal activities of thyme oils (Thymus vulgaris L.) against Culex quinquefasciatus (Diptera: Culicidae)

Oil samples from four chemotypes of Thymus vulgaris were tested for mosquitocidal activity. Oil compounds were analyzed by gas chromatography-mass spectrometry (GC-MS). The different compositions of the oils varied in their biological activity as well. Larvicidal efficiency was determined for the most significant oil thymol type (T), namely the lowest doses LD50 32.9 and 14.2xa0mg/l for the third and fourth instars, respectively. The oils also showed very good efficiency with respect to mortality and to the percentage of adult emergence upon short-term exposure in water treated by lethal doses of individual oils. Although the larvae were left in treated water for only 5xa0h, mortality corresponding to LD50 was determined after 24xa0h, i.e., between 49% and 63%. Furthermore, mortality increased significantly in relation to time of exposure, and total mortality of the larvae at the end of their development was about 90%. Such significant mortality was also naturally reflected in the total emergence of adults. In the control sample, there was 77% adult emergence from the larvae, whereas in oils, there was only from 5.3% to 16% emergence. The greatest fumigate efficiency was found for T and linalool chemotypes, with an LC50 for 24xa0h of exposure being 1.1 and 1.8xa0mg/l, respectively. Essence T was also most efficient in the tarsal test, with LD50 of 44xa0μg/cm2 and LD90 of 63xa0μg/cm2. High antioviposition efficiency was found in all tested oils.

Antifungal efficacy of some natural phenolic compounds against significant pathogenic and toxinogenic filamentous fungi

In terms of food safety, species of the Fusarium, Aspergillus and Penicillium genera are considered the most significant because they produce the great majority of known mycotoxins. Developing resistance against commonly used fungicides have become a critical problem in area such as agriculture, the storage and production of food and even in human medicines. The need for research and development of new alternative antifungal treatment based on natural antifungal substances is obvious. Here, the antifungal efficacy of 21 phenolic components of essential oils and plant substances were tested against these filamentous fungi with respect to their different molecular structures. Minimum inhibitory concentration values MIC₅₀ and MIC₁₀₀ were successfully estimated for 15 substances by means of probit analysis. Thymol and carvacrol were evaluated as the most effective. The MIC₅₀ values for thymol ranged from 30 to 52 μg mL(-1). The MIC₁₀₀ values for thymol ranged from 76 to 255 μg mL(-1), respectively. For carvacrol, the MIC₅₀ values ranged from 37 to 76 μg mL(-1), and the MIC100 ranged from 131 to 262 μg mL(-1). The results also revealed differences in the efficacy of phenols depending on molecular structures and different inter-species sensitivity.

Mosquito control with green nanopesticides: towards the One Health approach? A review of non-target effects

The rapid spread of highly aggressive arboviruses, parasites, and bacteria along with the development of resistance in the pathogens and parasites, as well as in their arthropod vectors, represents a huge challenge in modern parasitology and tropical medicine. Eco-friendly vector control programs are crucial to fight, besides malaria, the spread of dengue, West Nile, chikungunya, and Zika virus, as well as other arboviruses such as St. Louis encephalitis and Japanese encephalitis. However, research efforts on the control of mosquito vectors are experiencing a serious lack of eco-friendly and highly effective pesticides, as well as the limited success of most biocontrol tools currently applied. Most importantly, a cooperative interface between the two disciplines is still lacking. To face this challenge, we have reviewed a wide number of promising results in the field of green-fabricated pesticides tested against mosquito vectors, outlining several examples of synergy with classic biological control tools. The non-target effects of green-fabricated nanopesticides, including acute toxicity, genotoxicity, and impact on behavioral traits of mosquito predators, have been critically discussed. In the final section, we have identified several key challenges at the interface between green nanotechnology and classic biological control, which deserve further research attention.

In vivo and in vitro effectiveness of Azadirachta indica-synthesized silver nanocrystals against Plasmodium berghei and Plasmodium falciparum, and their potential against malaria mosquitoes

Malaria transmission is a serious emergence in urban and semiurban areas worldwide, becoming a major international public health concern. Malaria is transmitted through the bites of Anopheles mosquitoes. The extensive employ of synthetic pesticides leads to negative effects on human health and the environment. Recently, plant-synthesized nanoparticles have been proposed as highly effective mosquitocides. In this research, we synthesized silver nanoparticles (AgNP) using the Azadirachta indica seed kernel extract as reducing and stabilizing agent. AgNP were characterized by UV-vis spectrophotometry, SEM, EDX, XRD and FTIR spectroscopy. The A. indica seed kernel extract was toxic against Anopheles stephensi larvae and pupae, LC50 were 232.8ppm (larva I), 260.6ppm (II), 290.3ppm (III), 323.4ppm (IV), and 348.4ppm (pupa). AgNP LC50 were 3.9ppm (I), 4.9ppm (II), 5.6ppm (III), 6.5ppm (IV), and 8.2ppm (pupa). The antiplasmodial activity of A. indica seed kernel extract and AgNP was evaluated against CQ-resistant (CQ-r) and CQ-sensitive (CQ-s) strains of Plasmodium falciparum. IC50 of A. indica seed kernel extract were 63.18μg/ml (CQ-s) and 69.24μg/ml (CQ-r). A. indica seed kernel-synthesized AgNP achieved IC50, of 82.41μg/ml (CQ-s) and 86.12μg/ml (CQ-r). However, in vivo anti-plasmodial experiments conducted on Plasmodium berghei infecting albino mice showed moderate activity of the A. indica extract and AgNP. Overall, this study showed that the A. indica-mediated fabrication of AgNP is of interest for a wide array of purposes, ranging from IPM of mosquito vectors to the development of novel and cheap antimalarial drugs.

Acute larvicidal toxicity of five essential oils (Pinus nigra, Hyssopus officinalis, Satureja montana, Aloysia citrodora and Pelargonium graveolens) against the filariasis vector Culex quinquefasciatus: Synergistic and antagonistic effects.

Mosquito vector control is facing a number of important and timely challenges, mainly due to the rapid development of pesticide resistance and environmental concerns. In this scenario, screening of botanical resources for their mosquitocidal activity may offer effective and eco-friendly tools against Culicidae vectors. Culex quinquefasciatus Say (Diptera: Culicidae) is a vector of lymphatic filariasis and of dangerous arboviral diseases, such as West Nile and St. Louis encephalitis. In this study, the chemical composition of five essential oils obtained from different plants, namely Pinus nigra J.F. Arnold var. italica (Pinaceae), Hyssopus officinalis L. subsp. aristatus (Lamiaceae), Satureja montana L. subsp. montana (Lamiaceae), Aloysia citriodora Palau (Verbenaceae) and Pelargonium graveolens LHér (Geraniaceae), was investigated by GC-MS analysis. Furthermore, it was evaluated their acute toxicity on larvae of C. quinquefasciatus. Then, the most effective oils were selected, in order to focus on the potential synergistic and antagonistic effects, testing them in binary mixtures on C. quinquefasciatus larvae. Results showed that the higher effectiveness was obtained by S. montana subsp. montana essential oil (LC50=25.6μL·L-1), followed by P. nigra var. italica (LC50=49.8μL·L-1) and A. citriodora (LC50=65.6μL·L-1), while the other essential oils showed LC50 values higher than 90μL·L-1. The larvicidal effectiveness can be enhanced by preparing simple binary mixtures of essential oils, such as S. montana+A. citriodora (ratio 1:1), which showed higher larvicidal toxicity (LC50=18.3μL·L-1). On the other hand, testing S. montana+P. nigra (1:1) an antagonistic effect was detected, leading to a LC50 (72.5μL·L-1) higher than the LC50 values calculated for the two oils tested separately. Overall, our results add useful knowledge to allow the employ of synergistic essential oil blends as effective, cheap and eco-friendly mosquito larvicides.

Insecticidal properties of phenols on Culex quinquefasciatus Say and Musca domestica L

Thirteen simple phenols and 8 phenolic acids were tested for toxicity to Culex quinquefasciatus larvae and Musca domestica adults. It was found that while the phenolic acids (except salicylic acid) showed little or no effect on acute toxicity, all the tested simple phenols caused mortality within 24xa0h after application. Lethal doses for acute toxicity of C. quinquefasciatus were successfully estimated for 12 substances using probit analysis. Thymol, carvacrol, 2-ethylphenol, and salicylaldehyde showed significantly the highest efficiency, for which the lethal doses LD50 were estimated as 30, 36, 38, and 43xa0μg/ml, respectively. Lethal doses for acute toxicity of M. domestica adults were successfully estimated for ten substances. Thymol, carvacrol, and 2,6-dimethoxyphenol showed significantly the highest efficiency, for which the lethal doses LD50 were estimated 53, 69, and 87xa0μg/fly, respectively.