Initiation of defence responses by plant extracts and their insecticidal role against the Russian wheat aphid in wheat
The Russian wheat aphid (Diuraphis noxia: Kurdjumov) is considered a harmful pest in South Africa and many other parts of world. The application of insecticides to host plants poses two unique problems as firstly it is too costly and the health risks associated with synthetic applications is usually met with consumer resistance. Plant species like Artemisia afra, Agave attenuata and Zanthoxlum capense are rich in bioactive compounds which are widely used in different industries. The bioactive compounds in the plant species and their possible role in providing protection against aphids served as the rationale for the basis of this study. In this study, the essential oil of Ar. afra and polar and non-polar extracts from the three plant species mentioned, were investigated for their repellent and insecticidal abilities towards the Russian wheat aphid (RWA) and their ability to induce a defence response in wheat. Water, methanol and dichloromethane (DCM) crude extracts were prepared from each plant using dried ground leaf material. The essential oil of Ar. afra was obtained through hydro-distillation from fresh leaf material. The identification of compounds present in the different plants through thin layer chromatography revealed the presence of terpenes and phenolics in Ar. afra extracts. In A. attenuata, steroids and saponins were highlighted as being present, while Z. capense extracts showed the presence of steroids, flavonoids and other phenolic compounds. Essential oil of Ar. afra and DCM extracts of the three plant species were analysed using gas chromatography coupled to mass spectrometry. In the essential oil, compounds found in high abundance were eucalyptol (12.87%), camphor (15%), α–thujone (29%), β-thujone (12%) and 1,8-cineole (7.42%). Alpha-thujone (29.47%) and β-thujone (37.36%) were identified as the major constituents in the DCM extract of Ar. afra. Compounds present in A. attenuata DCM extract revealed the presence of methyl jasmonate (6.4%) and 2,5-dimethyl-3,4-hexanediol (23%). The DCM extract of Z. capense consist of, among others, 2,4-dimethylhex-1-ene (25.98%) and E – (β)-farnesene (2.41%) . Laboratory bioassays were carried out to evaluate the insecticidal activity of the essential oil and other crude extracts against the RWA by spraying the insects directly with a 10 mg mL-1solution. Mortality was recorded at 1 h and 24 h after treatment. All the plant extracts and essential oil caused significant mortality of 50% or more after 1 h and at least 70% at the end of 24 hours. Ar. afra essential oil proved to be the most toxic with a LC50 – value of 0.396 mg mL-1. Initial screening for repellency of the essential oil and extracts was performed using a four arm olfactometer. Both polar and non-polar extracts repelled the RWA. However, the mean duration recorded in repeated 10 min cycles showed the The DCM extract and essential oil of Ar. afra, water extract of A. attenuata and DCM extract of Z. capense were highly repellent. The repellent properties and abilities of the essential oil and plant extracts to limit RWAs settling on susceptible and resistant wheat leaves, ensuing lowered disease symptoms after 7 days, were exploited in a leaf settling bioassay under glasshouse conditions. At 2 h.p.i, the essential oil and polar extracts of Ar. afra and DCM extract of A. attenuata reduced aphid settling significantly when compared to the control in the susceptible cultivar. The water extract treatment of Ar. afra, DCM extract treatment of A. attenuata maintained lower number of aphid settling on wheat leaves even after 72 h.p.i in the susceptible cultivar. In the resistant cultivar, no significant effects were observed, however the DCM extract treatment of A. attenuata still maintained to lower aphid settling after 72 h.p.i. The symptom analysis revealed that the extracts mentioned above also lowered the prevalence of disease in wheat after 7 days and the number of live aphids were less. The intiation of defence responses by the extracts of Ar. afra and A. attenuata was comparable to what was observed in the leaf settling bioassay. β-1,3-Glucanase and peroxidase activity increased with the application of the water and essential oil extracts of Ar. afra in both uninfested and infested plants during a 144 h period. Furthermore, an increase in peroxidase activity was observed when wheat was treated with the DCM extract of A. attenuata. The results obtained in this study, indicate that the plant extracts contain bioactive compounds that possess the ability to aid in the control of the RWA through various mechanisms and their integration in pest management strategies may prove to be beneficial.
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