Background and Objectives: Although a number of studies have empirically evaluated the use of extracts from several indigenous plants against insect pests, but little is known about the use of extracts from invasive alien plants to control these noxious species. This study investigated the repellent and insecticidal activities of the root extracts of two invasive alien plants, Chromolaena odorata and Mimosa diplotricha against Macrotermes species. Materials and Methods: Four concentrations of the aqueous root extracts of the two plants were tested on the worker caste of Macrotermes species using the filter paper impregnation technique after which percentage repellency was monitored for 30 min. In a similar experimental setup mortality was monitored for 12, 24 and 36 h. Results: The highest concentration [10% (w/v)] of the root extracts of C. odorata and M. diplotricha significantly repelled 98 and 100% of Macrotermes species, respectively, following a 30 min exposure period. Mortality of Macrotermes species caused by the root extracts of C. odorata and M. diplotricha was high and observed to be concentration and exposure time dependent. The highest concentration [10% (w/v)] of C. odorata and M. diplotricha root extracts accounted for 100% mortality against Macrotermes species after a 36 h exposure period. Following a 36 h exposure period, the median lethal concentrations (LC50) of C. odorata and M. diplotricha against the termites were 1.72 and 4.12% (w/v), respectively. Conclusion: This study elucidates the repellent and insecticidal activities of the root extracts of C. odorata and M. diplotricha for the first time and suggests that the root extracts of both plants can be used for the control of Macrotermes species.
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Despite the fact that termites play beneficial roles in ecosystem functioning, they are also very destructive and are a major threat to crops and household properties1,2. Crops such as yam, cassava, sugarcane, groundnuts, sorghum and maize are prone to infestation and damage by termites3. In addition, termites also attack stored grains such as maize and rice3. Termites are commonly responsible for the mortality of tree seedlings in forestry, plantations and also cause a considerable damage to buildings and other wooden structures such as fence poles and utility poles4.
Macrotermes species (Blattodea: Termitinae) are members of the fungus growing termites belonging to the subfamily Macrotermitinae. In Africa, Macrotermes is and remains a serious pest of some agricultural crops such as cassava, citrus plants, coconut, coffee, groundnut, sorghum, sugarcane, sweet potato and tree plantations3,5. They are thought to be responsible for the majority of crop damages and high levels of tree mortality in forestry4. Furthermore, damage to stored products by pests including termites provide entry for secondary infestation by pathogens especially Aspergillus spp., consequently causing indirect yield loss and contamination of products6.
The fascinating results produced by synthetic insecticides against pest such as termites, have been overshadowed by recent debates surrounding their hazards to human health and effects on non-target organisms amongst other issues7. Furthermore, their frequent usage sometimes result in the development of insecticide resistance in target species. The challenge of finding sustainable alternatives to these synthetic insecticides has led to the bio-prospecting of plants with repellent and toxicological properties6. Natural products of plant origin have been reported to be useful and desirable tools in pest management because they are effective8,9. Several experiments using plant extracts and powders in human and animal health protection, agriculture and in household pest management have been particularly promising. For example, the potential toxicological (pesticidal) activities of some plants such as Azadirachta indica (L.) (Meliaceae), Chrysanthemum indicum (L.) (Asteraceae), Cineraria folium (L.) (Asteraceae), Anchomanes diffuronis (Schott) (Araceae), Aframomum melegueta (K. Schum) (Zingiberaceae), Jatropha curcas (L.) (Euphoriaceae) and Annona muricata (L.) (Annonaceae) against pests of medical, agricultural, economic, storage and veterinary importance have been adequately documented10-13.
Although a number of studies have empirically evaluated the use of extracts or powders from several indigenous plant species against insect pests including termites14-16, but little is known about the use of extracts from invasive alien plants to control these noxious species9,16. Chromolaena odorata is an invasive weedy shrub native to the Americas17. Its natural habitat range stretches from southern Florida, USA to Northern Argentina including the Caribbean Islands. The weed is known to be invasive in some countries in South-East Asia, West Africa, East, Central and Southern Africa and parts of Australia17-19. Following the weed’s introduction into Nigeria in the late 1930s, it is now known to be widespread in the southern parts of Nigeria where it is recognized as an agricultural weed as well as a beneficial plant20. Mimosa diplotricha is a fast growing, annual (short-lived) or perennial shrubby leguminous vine native to the Americas21. Although, the nativity of the weed has been traced to Brazil, its natural habitat range in the Americas stretches from southern Mexico to northern Argentina including the Caribbean Islands21. Following the introduction of the weed into Nigeria in the late 1980s, it is now known to be present in most states in southern Nigeria where it threatens agriculture and biodiversity conservation and livelihoods21. The repellent and insecticidal activities of the root extracts of the invasive alien plants, C. odorata and Mimosa diplotricha C. Wright ex Sauvalle (Mimosaceae) against termites are unknown. Therefore, the objective of this study was to investigate the repellent and insecticidal activities of the aqueous root extracts of C. odorata and M. diplotricha against the worker caste of Macrotermes species.
MATERIALS AND METHODS
Collection and preparation of plant powders: Fresh roots of C. odorata and M. diplotricha plants were collected from an open farmland at Dentistry quarters, within the vicinity of University of Benin Teaching Hospital (UBTH), Benin city (6°39'N, 5°56'E), Nigeria. Following collection, the roots were chopped separately into pieces, washed with running water and shade-dried for about 7 days and thereafter oven-dried at 60°C for 72 h. The dried roots were blended into a fine powder using an electric blender (Braum Multiquick Immersion Hand Blender, B White Mixer MR 5550CA, Germany) and then preserved at room temperature in an air-tight and water-proof container for further use.
Extract preparation: Different concentrations of the root extracts of each plant were prepared based on weight per volume (w/v), as 2.5, 5.0, 7.5 and 10.0 g of the grinded root powder of each plant material was mixed with 100 mL of water in separate flasks to obtain a crude extracts of four different concentration levels of 2.5, 5.0, 7.5, 10.0% (w/v), respectively. Each of the flask containing different concentrations (2.5, 5.0, 7.5, 10.0% (w/v) of the root extracts was shaken thoroughly for 30 min to ensure quick and even distribution of the solutes (= root powders) in the solvent (= water) .
Insect collection: Macrotermes species worker caste were collected from termite mounds in an open farmland at the Faculty of Agriculture, University of Benin, Benin city, Nigeria. At the site of collection, termite mounds were dug up using a spade and mounds containing termites were put in a plastic box. The termites were dusted off the mound using camel brush and placed in a polythene plastic box measuring 22×17×7cm3. Plant materials were added to the plastic box as feeds for the termites. Then the top part of the box was covered with a Muslin cloth to allow adequate air circulation (in and out of the box) and also to prevent the escape of the termites. Moistened wad of cotton was placed into the plastic box to maintain the required moisture level (more than 60%) for termite survival. The box carrying the termites was then transported to the Laboratory of the Department of Animal and Environmental Biology, University of Benin, Benin city, Nigeria and placed in a cool dark area until needed for the experiment. Termites were used 24 h after collection. Both the repellency and mortality tests were conducted in the Laboratory of the Department of Animal and Environmental Biology, University of Benin, Benin city, Nigeria, between October 2016 and March 2017.
Repellency bioassay: The filter paper impregnation technique15 was used to perform the repellency bioassay. Whatman No. 1 filter paper cut into two equal parts (half part treated and the other half left untreated) were placed inside a Petri dish of 8.5 cm diameter. The distance between the two halves was 1 cm. The treated half had 1 mL of one of four treatments (2.5, 5, 7.5 and 10% (w/v)) of the botanical extracts of either plants while the untreated half was treated 1 mL of water. Both treatments were administered using a 2 mL syringe. Five replicates were used for all four concentrations. Ten worker termites were introduced into the center of the Petri dish, specifically at the point of demarcation between the treated and untreated filter papers and the Petri dish was placed in darkness to minimize the effect of light on the termites. The numbers of termites on both the treated and untreated filter papers were recorded from each Petri dish 30 min after treatment application. Repellency was determined based on the number of termites which stayed on the extract-treated filter paper. Percentage repellency was calculated using the Eq.:
|U||=||Number of termites on the untreated filter paper|
|T||=||Number of termites on the treated filter paper|
Mortality bioassay: To perform the mortality bioassay, the filter paper impregnation technique was employed15. Whatman No. 1 filter paper was placed inside a Petri dish of 8.5 cm diameter. The filter paper was treated with 2 mL of one of four concentrations [2.5, 5.0, 7.5 and 10.0% (w/v)] of the aqueous root extracts of C. odorata and M. diplotricha. Ten termites were placed in a Petri dish treated with one of the four treatments of the aqueous root extracts. Each treatment was replicated five times. The experiment was laid out using completely randomized design (CRD). A control treatment where tap water was used to treat the filter paper was also set-up for comparison. All experimental units were placed in darkness at a temperature of 26±3°C and relative humidity of 60-70% for 2 days. Termites’ mortality was recorded at 12 h interval until 100% mortality was recorded. Mortality (%) was calculated using the following Eq.:
Statistical analysis: Control treatments, where the termites were not exposed to the aqueous root extracts of C. odorata and M. diplotricha plants caused 0.0% repellency and mortality; hence the controls were not included in the statistical analyses. The percentage repellency of the different concentrations of C. odorata and M. diplotricha root extracts [2.5, 5.0, 7.5 and 10.0% (w/v)] was analyzed with a Generalized Linear Model (GLZ) assuming a normal distribution with an identity link function. When the overall results were significant in the GLZ analysis, the difference between the treatments means was compared using the Tukey’s Honest Significant Difference (HSD) test. The relationships between percentage mortalities of Macrotermes species and concentrations of M. diplotricha root extract across all three exposure times was analyzed with linear regression analyses, as was the relationships between mortalities (%) of Macrotermes species and concentrations of C. odorata root extract across all three exposure times. The relationships between mortalities (%) of termites and exposure durations for the two different plant species across the all treatment concentrations were analyzed using linear regression analyses. The effects of the root extracts of the two plants and the concentration levels the extracts on termite mortality was analyzed using General Linear Model Analysis of Variance (GLM ANOVA). Probit regression was used to calculate the concentration of the root extract estimated to cause 50% mortality (LC50), the temperature causing 50% of tested individuals to die in a given period. With the exception of the GLZ, GLM ANOVA and probit regression that were performed using SPSS statistical software, version 16.0 (SPSS, Chicago, IL, USA), all other analyses were performed using Genstat 12.0 (VSN International, Hemel Hempstead, UK) and Microsoft Excel.
Repellency test: Following a 30 min exposure period of Macrotermes species to different concentrations of the root extract of M. diplotricha, percentage repellency against the termites significantly differed (GLZ: Wald χ2 = 9.242, df = 3, p = 0.026) with the 2.5% w/v concentration exhibiting the least percentage repellent activity (78%) (Fig. 1a). There were no significant differences in percentage repellent activities between the 5.0, 7.5 and 10.0% (w/v) treatments, although the 10.0% (w/v) appears to have exhibited the highest percentage repellency (100%) (Fig. 1a). Following a 30 min exposure of Macrotermes species to different concentrations of the root extracts of C. odorata plants, percentage repellency (against the termites) did not differed (χ2 = 0.490, df = 3, p = 0.921) (Fig. 1b). Although percentage repellency did not differed between the treatments, the 10.0% (w/v) concentration exhibited the highest percentage repellency (98%), while the 2.5% w/v treatment exhibited the least repellency (86%) (Fig. 1b).
Mortality test: Termites’ mortalities were very high irrespective of exposure times and concentrations of M. diplotricha root extract with mortality ranging between 76 and 100% (Fig. 2a-c). Irrespective of exposure time (12, 24 or 36 h), linear regression analyses showed significant positive relationships between percentage mortalities of Macrotermes species and concentrations of M. diplotricha root extract as mortality increased with increased concentrations of the root extracts (Fig. 2a-c).
|Fig. 1(a-b):|| |
Percentage (mean±SE) repellency of different treatments of (a) Mimosa diplotricha and (b) C. odorata root extracts against Macrotermes species following a 30 min exposure period. Means capped (following GLZ) with different letters are significantly different [after Tukey’s Honest Significant Difference (HSD) test: p<0.05]. Sample sizes are given in parenthesis
The root extract of C. odorata plants caused considerable mortalities in Macrotermes species irrespective of exposure times and concentrations with mortality ranging between 10 and 94% (Fig. 3a-c). There were significant positive correlations between percentage mortality levels of Macrotermes species and concentrations of the root extract of C. odorata plants across all three exposure trials (Fig. 3a-c). When termites were exposed to the root extracts of C. odorata and M. diplotricha for 36 h, the LC50 values were 1.72 and 4.12% (w/v), respectively. There were linear relationships between percentage mortalities of termites and exposure durations of the insects at the different concentrations of M. diplotricha root extract (2.5% w/v: R2 = 0.989; 5.0% w/v: R2 = 0.964, 7.5% w/v: R2 = 0.987, 10.0% w/v: R2 = 0.899) (Fig. 4a). Similarly, there were linear relationships between percentage mortalities of termites and exposure durations of the insects at the different concentrations of C. odorata root extract (2.5% w/v: R2 = 0.946, 5.0% w/v: R2 = 0.907, 7.5% w/v: R2 = 0.995, 10.0% w/v: R2 = 0.987) (Fig. 4b).
|Fig. 2(a-c):|| |
Relationships between mortality (%) of Macrotermes species and concentrations of Mimosa diplotricha root extract following (a) 12 h, (b) 24 h and (c) 36 h exposure period
Overall, termites mortality differed according to plant species (M. diplotricha versus C. odorata, GLM ANOVA: F1,39 = 32.18; p<0.001) and concentrations (F3,39 = 6.14, p = 0.002) (Fig. 5). With the exception of the equal mortality levels between M. diplotricha and C. odorata at the 10.0% (w/v) concentration, M. diplotricha exhibited significantly higher mortalities across concentrations compared to C. odorata (Fig. 5).
This study investigated the repellent and insecticidal activities of M. diplotricha and C. odorata aqueous root extracts on the worker caste of Macrotermes species. The study was undertaken as part of the global initiative to introduce and practice more sustainable environmentally friendly methods of controlling termites, a serious economic pest of agricultural crops, household furniture, forest trees, plantations and building structures. The results from this study revealed that the root extracts of M. diplotricha and C. odorata displayed some repellent and insecticidal activities against Macrotermes species.
|Fig. 3(a-c):|| |
Relationships between mortality (%) of Macrotermes species and concentrations of Chromolaena odorata root extract following a (a) 12 h, (b) 24 h and (c) 36 h exposure period
This study demonstrated that the aqueous root extracts of C. odorata significantly repelled the worker caste of Macrotermes species, although repellency was a function of concentration of the root extract used. Studies focusing on the repellent activities of the leaf powders or extracts of C. odorata against insect pests are not uncommon16,22. For example, cowpea grains treated with the leaf powder of C. odorata exhibited significant repellent activity against the adults of Callosobruchus maculatus (F.) (Coleoptera: Chrysomelidae)9. The high repellent activities (98%) demonstrated by the highest concentration (10% w/v) of the root extract of C. odorata is consistent with the findings of other authors15,16, who reported high repellency with increasing concentrations of the plant powders or extracts against insect pests including termites. The lack of a significant difference in termite repellency between all four concentrations of C. odorata root extract (2.5. 5.0, 7.5 and 10.0% w/v) not only suggests that low concentrations of the C. odorata can significantly repel Macrotermes species, but also demonstrates the excellent repellent properties of C. odorata root extract.
|Fig. 4(a-b):|| |
Relationships between percentage mortalities of Macrotermes species and exposure durations of termites to different concentrations of (a) Mimosa diplotricha and (b) Chromolaena odorata root extracts
|Fig 5:|| |
Percentage mortality of Macrotermes species treated with different concentrations of the root extracts of Chromolaena odorata and Mimosa diplotricha over a 36 h period. Means capped with different letters are significantly different (after Tukey’s HSD test: p<0.05)
The results from this study revealed that the root extract of M. diplotricha significantly repelled Macrotermes species and as with C. odorata root extract, the repellent activities were dependent on the concentrations of root extract used. Following a 30 min exposure period, the highest concentration of M. diplotricha root extract (10% w/v) significantly repelled 100% of Macrotermes species. The lack of a significant difference in repellency between the 5.0, 7.5 and 10.0% (w/v) treatments suggested that low concentrations of the M. diplotricha can also cause significant mortality against the investigated termite species. To date no studies exist on the repellent activities of M. diplotricha root extract against insect pests, consequently, this study is the first to document the repellent activities of M. diplotricha root extract.
Undoubtedly, the insecticidal activity of C. odorata root extract against Macrotermes species reported in this study has not been previously documented, but studies reporting the insecticidal activities of the leaf or root powders and leaf extracts of C. odorata against insect pests are not uncommon9,16,23,24. For example, Uyi and Igbinoba16 reported that the root powder of C. odorata caused significant mortality (100%) against C. maculatus following a 72 h exposure period. In this study, the highest concentration (10% w/v) of the root extract of C. odorata caused 100% mortality against Macrotermes species, following a 36 h exposure period. The high mortality demonstrated by the highest concentration of C. odorata root extract is in agreement with the findings of other authors15,24,25. who reported high mortalities with increasing concentrations of either plant extracts, oils or powders against insect pests including pest of agricultural and economic importance.
Admittedly, mortality of Macrotermes species caused by the root extract of M. diplotricha was high and observed to be both concentration and exposure time dependent. Following a 36 h exposure period, the root extracts of M. diplotricha at the highest concentration (10% w/v) accounted for 100% mortality against Macrotermes species. The results from this study is consistent with the findings of other authors15,24,25, who reported high mortalities with increasing concentrations of plant extracts, oils or powders against insect pests. Although many studies often report that insect (including termites) mortality always increase with an increase in exposure period to plant extracts or powders15,16, but only a few performed regression analysis to empirically demonstrate significant correlations in insect mortality between concentrations of plant extracts (or powders) and exposure time. This study used linear regression analyses to show significant positive relationships between percentage mortalities of Macrotermes species and concentrations of C. odorata and M. diplotricha root extracts and empirically demonstrated that mortality increased with increased concentrations of the root extracts.
The high repellent and mortality exhibited by the root extract of C. odorata against Macrotermes species could be attributed to the presence of one or more bio-active compounds present in the roots of the plant. Studies on the phytochemical composition of the roots of C. odorata revealed that the roots of the plant contain secondary metabolites (= phytochemicals) such as alkaloids, phenols, flavonoids, saponins, cardenolides, anthraquinones and tannins26,27 and this might plausibly explain the reason for the high repellency and mortality of C. odorata root extract against Macrotermes species reported in this study. Although, the root extract of M. diplotricha demonstrated significant repellent and insecticidal activities against Macrotermes species, little or nothing is known about its phytochemistry. The high repellent and insecticidal activities of M. diplotricha root extracts might be attributed to the presence of phytochemicals as has been documented for other invasive alien plants27,28. Studies focusing on the insecticidal activities of the leaf, stem, flower and root extracts of a congener of M. diplotricha, Mimosa pudica (L.) (Mimosaceae) revealed that the various plant parts demonstrated significant insecticidal activity against the larvae of Aedes aegyptii (F.) (Diptera; Culicidae)29. The reason behind the insecticidal activity of Mimosa species could be attributed to the presence of alkaloids, flavonoids, tannins and sterols, terpenoids. Nevertheless, further studies on the phytochemical composition of the roots of M. diplotricha are necessary to either validate or rebut this conjecture.
The excellent repellent and insecticidal activities of C. odorata and M. diplotricha in this study suggests that the root extracts of both plants can be used as sustainable alternatives to conventional insecticides and extracts of indigenous plants, in the control of Macrotermes species in Nigeria and other countries where the pest is a menace. Furthermore, both C. odorata and M. diplotricha are agricultural nuisance and their use as bio-pesticides could help curb the invasiveness of these alien plants, thus making it a win-win situation for both agriculturists and farmers.
This study discovers the repellent and insecticidal activities of the root extracts of two invasive alien plants, C. odorata and M. diplotricha that can be beneficial for the control and management of Macrotermes species. This study uncovers the positive attributes of C. odorata and M. diplotricha that many researchers were unable to explore. Thus a new environment friendly insecticide can developed from the findings of this study.
We want to thank Ifeanyi G. Amolo, Edwin Uyamasi, Ogheneganre Akpodiogaga and Juliet Samugana for assistance during fieldwork. We also thank one anonymous reviewer for comments on the previous version of this manuscript.
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