The insecticidal and repellent activities of extracts of three Allium spp. (Amaryllidaceae) against grapevine mealybug (Planococcus ficus L.)

Mange, Tokozani

The insecticidal and repellent activities of extracts of three Allium spp. (Amaryllidaceae) against grapevine mealybug (Planococcus ficus L.)

Author(s)

Mange, Tokozani

Date Issued

2006

Type

master thesis

Publisher

Cape Peninsula University of Technology

Abstract

Grapevines are susceptible to a wide variety of pests, including the grapevine mealybug. It is one of the most economically important pests of wine grapes. It causes sooty moulds on grape clusters and transmits the grapevine leafroll-associated virus. One of the primary methods for controlling this pest in the vineyard is the application of synthetic insecticides. However, these synthetic insecticides harm natural enemies, the environment, and human health. In modern agriculture, Plants are increasingly being recognised as an important source of insecticides. Many plant-based insecticides are used as alternatives to reduce the reliance on chemical insecticides. Hence, researchers have been developing new and safer pest control methods. Plant-based insecticides have been traditionally used worldwide, particularly by subsistence/smallholders. Allium species are among the most extensively researched plants for their sulphur compounds and other bioactive compounds. The first part of this study, in Chapter One, begins with the background and rationale of the study, followed by a systematic literature review in Chapter Two, focusing on the use of plantbased products screened for pest repellence and insecticidal properties against the vine mealybug, which revealed that several plant species from different families have insecticidal properties against the vine mealybug pests. Chapter 3 is the experimental part of this study, consisting of two bioassays: toxicity and repellency, which evaluated the toxicity of Allium species (A. sativum, A. cepa, and A. porrum) against the vine mealybug. The three Allium spp. were store-bought, the garlic and onion were peeled, and the leek was rinsed and dried with a paper towel. The plants were crushed to a paste-like consistency and dissolved in various solvents (DCM, absolute ethanol, Acetone, and Distilled water) at room temperature (25 ℃) for 24 hours. The extracts were filtered and allowed to dry at room temperature (25 ℃). To perform the toxicity bioassay, the extracts were re-dissolved in extra-virgin olive oil to obtain a 25% w/v concentration, and this mixture was vortexed for 10 minutes. Fifty microlitres of each 25% w/v extract was applied to a 2x2 cm packet filter paper and allowed to dry for 5 minutes. A camel-hair brush was used to transfer the insect into the packet, and all sides were stapled. Five packets, each containing one insect, were placed in a petri dish and stored in a Perspex box containing saturated sodium solution in a 100 mL glass beaker, in a darkroom at room temperature and 60% RH. Data was collected after 10 minutes. The results showed a significant difference in insecticidal efficacy among the plant species (DF=2; P < 0.05). Garlic exhibited the highest potency against vine mealybug, particularly when extracted with DCM. The results showed that the DCM garlic extract was significantly more toxic (DF = 3; χ 2 = 23.09; P < 0.05), inducing 92% insect mortality at a concentration of 25% w/v. The DCM extract of garlic was further tested at different concentrations (25% w/v, 12.5% w/v, 6.25% w/v, 3.125% w/v) and at 0% (negative control) and compared to extra virgin olive oil. The highest corrected mortality rate was 92% of the vine mealybug, achieved at 25% w/v concentration. The mortality rate was concentration-dependent. The choice repellence bioassay evaluated the repellent activity of Allium spp. (garlic, onion, and leek) against the vine mealybug. The same plant preparation method was followed as for the toxicity experiment, except that the extracts were redissolved in solvents (DCM, Acetone, absolute ethanol, and Distilled water) to achieve a 25% w/v concentration and tested in a choice repellence bioassay. The results showed a significant difference in repellent efficacy. The acetone garlic extract was significantly more repellent than the other treatments (DF = 3; χ 2 = 19.95; P < 0.05). The distilled water extract of garlic and the organic soap exhibited 42% and 38% repellency, respectively, at 25% w/v. These findings demonstrated that garlic extracts have anti-grapevine mealybug activity and could be incorporated into integrated pest management.

Additional information

Thesis (MAgric)--Cape Peninsula University of Technology, 2026

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