Plant materials have demonstrated toxicity against a range of pests when applied to reduce losses and damage before and after harvest. Most targeted parts of the plant include leaves, roots, flower buds and stem bark (Isman 2020). The crude extracts, pure compounds and essential oils extracted from different plant parts largely constitute these plant-based chemicals, which exhibit different gradients of selective mortality and antifeedant effects on insects by disrupting their consciousness, larvicidal effects or inhibition by gustatory responses. A list of plants tested against a wide range of pests is presented in Table 1, whilst Table 2 shows selected plants tested for insecticidal activity against mealybug species.

Aromatic plants such as Mentha spp., Lavendula spp., Cedrus spp., Pinus spp., Eucalyptus spp., A. indica, and Citronella spp. and many more are depots of repellent and insecticidal chemicals. The repellency and insecticidal properties of extracts from these plants may be attributed to the characteristic permeation of insect exoskeleton, crevices and other materials that may act as shields to synthetic insecticides (Chandi & Kaur 2021). The insecticidal mechanisms of plant extracts, such as pyrethrum extracts, may include hyper-activating insect voltage-gated sodium channels, which could result in insect paralysis and death (Liu et al. 2021). In some cases, phytochemicals and essential oils from plants may act as chitin synthesis inhibitors by blocking the development of chitinous exoskeleton, making insects susceptible to early mortality because of indiscriminate interference with insect hormones during moulting (Chandi & Kaur 2021). The insect repellent activities of volatile compounds from plants have been demonstrated to involve receptor proteins at several binding sites that enable the transformation of chemical signals into electrical signals, which are then picked up by electroantennographic recordings during receptor-repellent interactions (Pulido et al. 2022). Pulido et al. (2022) further demonstrated in proteomic and electrophysiological experiments that repellent exposure disrupts ionic channel activity and modifies neuronal synapses and energy production processes in mosquitoes. Repellents may interfere with or mask the perception of host-attractant signals by exciting repulsion receptors (Liu et al. 2021).

Compared to studies on insect mortality, relatively few investigations have focused on the repellency of plant extracts (Peschiutta et al. 2018). The repellent activity of botanical extracts largely depends on the methods of extraction and type of solvent used (Jaleel et al. 2020). Several plant species, their extraction methods, and solvent types have been tested against various plant species (Table 3). In a study by Singh (2012), methanolic leaf extracts of A. indica, Eucalyptus globulus and Ocimum basilicum showed 97.0%, 93.0% and 88.0% repellency, respectively, against cotton mealybug (Phenacoccus solenopsis) after 24 h. The repellent effect was attributed to active compounds in these plants. For example, the bioactive alkaloid azadirachtin and other tetranortriterpenoids found in A. indica are responsible for its strong repellency (Guchhait et al. 2025). Similarly, Sombra et al. (2022) reported that compounds such as alkaloids, phenols and esters may act at multiple sites within insect physiology, exhibiting biocidal, repellent, antifeedant and developmental disruption activities.

Several plants, including Azadirachta indica, Eucalyptus globulus, Prunus persica and Polyalthia longifolia, have been reported to exhibit significant repellency against mealybugs (Peschiutta et al. 2018). In another study, Baliyarsingh, Mishra and Rath (2021) reported high repellency of leaf extracts of Andrographis paniculata against Tribolium castaneum (red flour beetle). In addition, Roonjho et al. (2013) tested the toxicity and repellency of different solvent extracts (petroleum ether, ethanol and acetone) of Prunus persica, Sonchus oleraceus, Silybum marianum, Polyathia longifolia and Eucalyptus globulus against cotton mealybug. Findings from the study indicated that the ethanol extract of P. persica exerted 72.5% repellency against the cotton pest, whilst other plant extracts exhibited lower repellency compared to P. persica.

The main categories of botanical products include pyrethrum, rotenone, neem and their essential oils (Sarwar 2023). Among these, pyrethrum and neem are derived from plants that are widely used to produce some of the most commercially available botanical pesticides (Table 4). For example, rotenone is a flavonoid compound found in the roots of several plant species and is known for its insecticidal properties (Shivkumara et al. 2019). Nicotine, ryania and sabadilla are other plant-based insecticides with documented insecticidal potency; however, their mechanisms of action are not well understood (Campos et al. 2019; Peschiutta et al. 2019).

The potency of plant-derived insecticides is determined by different mechanisms which involve targeting biological systems, such as endocrine, nervous and respiratory systems, as well as homeostasis, including water regulation. In the nervous system, botanical insecticides target acetylcholinesterase, tyramine and octopamine receptors, sodium and g-aminobutyric acid-gated chloride channels (Regnault-Roger, Vincent & Arnason 2012). Unlike conventional insecticides, which typically rely on a single active ingredient, botanical insecticides leverage the synergistic effects of heterogenous chemicals in plants, which influence both behavioural and physiological processes, providing a multifaceted mode of action. Their ability to interfere with insect physiology makes them a promising alternative that aligns with IPM (La Pergola et al. 2017).

Pyrethrum, derived from the Dalmatian pyrethrum plant (Tanacetum cinerariifolium L.) contains six active ingredients known as pyrethrins, which are insecticidal esters. These are categorised into two groups, namely Pyrethrins I and Pyrethrins II. They are fast-acting and cause immediate ‘knockdown’ paralysis in insects (Sarwar 2023). Their neurotoxic mechanism of action involves blocking voltage-gated sodium channels in the insect nervous system, ultimately leading to paralysis and death upon contact, whether applied as a spray or powder (Chen et al. 2018; Sarwar 2023). The insecticidal activity of the pyrethrins results from the synergistic action of the six ingredients. Pyrethrins are effective against a wide range of insect pests from various taxonomical orders. However, their moderate toxicity to mammals and non-target insect species limits their widespread application (Jeran et al. 2021).

Neem is the common name for Azadirachta indica Juss (Meliaceae), a tree native to the Indo-Pak subcontinent (Badshah et al. 2015). The tree contains several potent bioactive compounds, including azadirachtin, nimbin, meliantriol, desacetylnimbin, nimbidin, salannin and desacetylsalannin. Over 100 chemical compounds have been identified from neem, with azadirachtin being the most effective among them. According to Jababu, Kopta and Pokluda (2016); Sarwar (2023), Azadirachtin has two significant effects on insects. At the physiological level, it blocks the prothoracic gland from synthesising and discharging moulting hormones (ecdysteroids), which results in incomplete ecdysis in immature insects. In adult female insects, a similar mechanism results in sterility.

Neem exhibits multiple insecticidal properties, acting as an insect growth regulator, antifeedant and sterilant. It also affects insect vigour, longevity and fecundity. Neem has demonstrated effectiveness in controlling a wide range of agricultural insect pests. For example, a study reported by Majeed et al. (2018) found that the neem extract was the most toxic against adult citrus mealybugs, whereas Ali et al. (2017) tested several plant extracts against sucking insect pests and found neem seed extract to be more effective compared to extracts from other plants.

Essential oils are volatile hydrophobic compounds extracted from plants. These oils are highly concentrated heterogenous mixtures of 20–60 compounds, though some contain over 300 different constituents (De Sousa et al. 2023). Typically, two or three compounds are generally available in significant proportions (20% – 70%) and these are primarily responsible for the biological activity (De Sousa et al. 2023). High variability and diverse complexity of aromatic mixtures of essential oils complicate the understanding of their mechanisms of action. This shifts attention to the study of individual compounds (Verdeguer, Sánchez-Moreiras & Araniti 2020). The toxicity and ability of the lipophilic essential oil to penetrate the waxy cuticle of mealybugs and disrupt their physiological activities and morphological features make them more suitable than synthetic insecticides, which often encounter barriers when in contact with waxy cuticle (Avila et al. 2023; Karamaouna et al. 2013).

Over the years, chemical insecticides have been used by farmers in controlling pest infestations to minimise pre- and post-harvest losses. These chemicals control pests but leave behind adverse effects on humans like food poisoning, environmental pollution, ozone layer depletion, pest resistance, mutations and extended toxicity on non-target organisms. In most cases, these chemicals cause severe injuries to critical human organs such as the liver, heart, kidneys, lungs and blood capillaries (Murugesan et al. 2021). For instance, pyrethroids, a prominent group of synthetic insecticides, have been classified as endocrine disruptors because of their antiprogestagenic properties, oestrogenic activity and neurodevelopmental toxicity (Elser, Hing & Stevens 2022).

The persistence of chemical insecticides in the environment is of great concern to environmental biologists as pest management becomes a costly enterprise for organic farmers. The choice of plant-based biopesticides as alternatives to synthetic insecticides created a compelling need to screen the insecticidal properties of several plant materials (Seiber et al. 2014). Thus, it became necessary to invent efficient means of minimising the contamination of synthetic insecticides through microbial degradation, chemical oxidation and photooxidation (Gajendiran & Abraham 2018). These approaches, coupled with the need to ensure human safety and safeguard the environment from chemical hazards, promoted the development and propagation of plant-based, eco-friendly chemicals that could be used as insecticides based on the pest management properties of novel compounds in the plants (Purkait et al. 2019).

The use of plant-based chemicals in pest management is supported by prominent epidemiological reports, including their biodegradability, reduced risk of toxicity to humans and non-target organisms, and low resistance development by pests (Adetuyi et al. 2024; Cantrell, Dayan & Duke 2012). High diffusibility and cuticular penetration of phytochemical insecticides are enhanced by the polarity of solvents used for extraction and various antifeedant mechanisms, such as distortion of typical neurological function capable of perceiving chemical phagostimulants, interference of ribonucleic acid synthesis pathways or by stimulating specialised receptors for feeding inhibition (Jimoh et al. 2023; Seiber et al. 2014).

The exposure time and concentration of plant-based insecticides play a significant role in determining their efficacy. According to Peschiutta et al. (2018), exposure time plays a critical role in determining the effectiveness of botanical compounds. However, findings by Rajagopal et al. (2022) suggest that whilst the concentration of extracts significantly contributes to adult insect mortality, treatment duration may not always have a notable effect (Rajagopal et al. 2022).

In contrast, several studies have reported that the efficacy of plant extracts generally increases with both higher concentrations and longer exposure periods (Badshah et al. 2015; Ramzi et al. 2022; Umair Sardar et al. 2018). For instance, Badshah et al. (2015) found that after a 1-week exposure at 3% concentration, mortality rates were recorded at 100%, 97%, 88% and 67% for neem seed n-hexane extract, neem seed acetone extract, neem oil and neem seed water extract, respectively. At a lower concentration of 1%, the respective mortality rates decreased to 68%, 62%, 56% and 29%. Similarly, Umair Sardar et al. (2018) observed that a 1% neem water extract resulted in a 21.66% mortality rate after 24 h, which increased to 33.33% after 48 h (Umair Sardar et al. 2018). These findings illustrate that both time and concentration can synergistically influence insect mortality. However, it is not always the case that an increase in the concentration of plant extracts increases the mortality rate.

The repellency and mortality rate increased proportionately with the concentration and time (Miah et al. 2018). According to Singh, (2012), after 24 h of mealybug release, the methanolic extracts (A. indica leaf, E. globules leaf and O. basilicum leaf extract) obtained the highest repellency of 97.0%, 93.0% and 88.0%, respectively. However, Erdemir and Erler (2017) reported that all the essential oils tested had a repellent activity in varying degrees, though the repellent activity was concentration- and time-dependent.

Despite a significant increase in academic publications recommending botanical insecticides as environment-friendly alternatives to synthetic pesticides, only a limited number of commercial products based on plant-derived pesticides are available. The preparation of these botanical formulations at the domestic level often requires basic technical skills, which may not be widely accessible (Ngegba et al. 2022; Tripathi 2021). One major limitation to the commercialisation of botanical pesticides is the availability of plant materials in large and sustainable quantities. The cultivation of source plants is constrained by land-use competition with food crops, especially in regions experiencing increasing food production demands (Seiber et al. 2014).

The study is only focusing on a single insect species (VMB). Hence, future research could explore the long-term effects of these plant-based chemicals with extended exposure time on other insect species especially close-relatives of the VMB. Additionally, it is imperative to investigate the mechanism of action of the insecticidal properties of plant extracts to provide valuable insights for their application in IPM strategies. Finally, it is crucial for researchers to continue to innovate with the view of promoting the formulation of eco-friendly anti-insect chemicals.