Pesticidal plants are proven to be perfect alternative against insect pests. For many years, indigenous Africans have been using the available pesticides at their disposal from plants and other organisms for different purposes including insect pest control [22] - [24] . However, findings from the developing countries show that small scale farmers suffer from poor knowledge on the use of introduced synthetic pesticides [25] - [27] and this is associated with detrimental effects to them and the environment. Therefore, research on pesticidal plants is needed to strengthen local knowledge that farmers have been aware of since long time in history.

In Africa, many of the pesticidal plants are found without difficulty. For example, they grow in the wild or even at homesteads and in farm boundaries. In this case, there is almost no cost of growing the plants. Therefore, it is worth promoting their use because synthetics have negative impacts as they are costly to eco-health and the economy [28] - [30] . A better choice to help growth of African agriculture would be to start from the baseline knowledge on pesticidal plants of the Africans themselves. This can be achieved through the improvement of the available knowledge on the use of pesticidal plants that would even disappear if not used and disseminated widely.

Synthetic pesticides are known for their toxicity to the environment and to non-target organisms including wildlife, insects and human beings [31] [32] . Organochlorines, organophosphate and carbamates are the major groups of chemicals that are used as synthetic pesticides in developing countries despite the ban of others like DDT [33] . Their impacts threaten the food safety systems [34] , human health [35] [36] and the environment.

Synthetic pesticides have been reported to reduce population among birds [33] , and insects [37] [38] . The fact that synthetic pesticides are less selective treatments to insects in agro ecosystem practices gives a caution on the impacts that pesticides may have to non-target and beneficial insects [39] . Human beings are strongly affected during application and handling of the chemicals in the farms. Reports have revealed several types of cancer that results from improper use of synthetic pesticides such as Leukemia, Lung cancer, Pancreatic cancer, Colon and Rectal cancer, Lymphohematopoietic cancer, on-Hodgkin lymphoma, Bladder cancer, Breast cancer, multi Plemyeloma, Prostate cancer, Kidney cancer and Oral cavity cancer [40] . Pesticides are reported to enter into the cells and alter cell’s cycles and hence resulting into some cancer [32] . Pesticides are also known for the ability to disrupt endocrine systems in humans and wildlife [41] [42] . Apart from cancer; skin pill off, hardness in breathing, stomach ache and vomiting as well as farmers collapsing have also affected users [43] . These effects then result to high health costs [44] .

Generally, environmental organisms including plants, fish, birds, snakes and insects are affected massively. Farmers in Malaysia reported 80% reduction in number of fish in two cropping seasons during a study conducted to identify impacts of pesticides in paddy farming [42] . Studies also show biota uptake of pesticides [25] which also imply soils contamination. There are therefore more detrimental effects caused by using synthetic pesticides for various agricultural purposes.

Indigenous knowledge refers to the informal knowledge, skills and practices that are obtained not in schools, universities and research institutes but rather in local heritable ways normally in rural areas [45] . Developing countries are rich in such knowledge [46] . Indigenous knowledge on insect pest control is perceived as important because it was witnessed as useful in food security and survival of the users long before the invention of synthetic pesticides [45] . However, maintenance of the indigenous knowledge is found to be difficult because the majority of farmers have turned to modern agriculture that involves use of synthetic pesticides.

Pesticidal plants have been used for more than 150 years ago [47] . Majority of African small holder farmers have been using various botanical pesticides to control insect pests. For example, in the Victoria basin in Uganda, farmers have used Capsicum frutescens, Tagetes spp., Nicotiana tabacum, Cypressus spp., Tephrosia vogelii, Azadirachta indica, Musa spp., Moringa oleifera, Tithonia diversifolia, Lantana camara, Phytollacca dodecandra, Vernonia amygdalina, Aloe spp., Eucalyptus spp., [22] . In addition to the pesticidal plants, farmers in Tanzania have been using other products such as cow’s urine, cow dung, and ashes [23] . However, in these areas, there exists little information that this knowledge is used in an effective way in comparison to ancient practices. Research based efforts need to be undertaken to retrieve the knowledge.

During farming, indigenous knowledge in insect pest control involves direct spraying, intercropping pesticidal plants with the crops to be protected, and also using the botanicals based on the synthetic formulations [48] . Researchers have come up with some application techniques. This includes: the use of the freshly ground leaves, mixed and soaked overnight [19] [49] . Also, boiling plant parts and adding soap for extraction [14] has been practiced. [19] used fresh leaves pounded and mixed with water and 0.1% soap to make 3% w/v of the extract. [21] used another technique where sun dried plant materials were soaked in acetone and stirred for 30 minutes. Thereafter the mixture was left for 24 hours, filtered and stored under 4˚ temperature before use.

All mentioned application techniques demonstrated positive results in controlling certain insect pests. These efforts have been done in few parts of developing countries despite presence of pesticidal plants in diverse areas. These calls for diverse research of plants with pesticidal properties coupled with indigenous knowledge from different domains and develop tangible solutions on the use of pesticidal plants to control insect pests.

Lantana camara is reported to have toxicity effects to animals and is also a noxious plant species that has been cited as invasive in need monitoring [51] . Literatures report that Lantana causes less mobility, dehydration and constipation, congested heart and lung, nephrosis, general reproductive performance and teratology to mice [52] . Lantana is also reported to have a fumigant effects [53] [54] and has been revealed for its water purification potential. Lantana camara contains a compound known as ursolic acid stearoylglucoside (UASG) depicted in Figure 1, that is associated with toxicity and which includes triterpenoids in the apolar phase [54] .

In agriculture, Lantana camara has been used for the control of insect pests in stored grains [56] [57] . Lantana has also been tested for its repellent, antifeedant and toxicity against termites [58] . All these experiments are laboratory based. Hence there is a need for more practical research on the field to test for the effectiveness of L. camara against field insect pests. Furthermore, use, use, use of Lantana camara for beneficial effects will help reduce its invasive property to farm lands.

Tephrosia vogelii is widely used for control of pests and as a source of nutrients to the soil [59] . Leaf extract of T.vogelii is reported to exhibit toxicity against Tilapia nilotica [60] . This is practiced in remote areas of Africa, commonly regarded as illegal fishing [61] . It is reported that leaves of T. vogelii contain high amounts of rotenone and deguelin [62] responsible for the toxicity to fish. Tephrosia vogelii has however a great potential in agriculture in the control of insect pests and in soil enrichment through nitrogen fixation [59] .

Diverse compounds exist in the plant and according to [63] this plant consist of chemotype 1 (C1) and chemotype 2 (C2) (Figure 2) of which C1 is found to be active against insect pests. In additional to insect pest

control, mulches of the plant have increased maize biomass while decreasing the weed biomass [63] . Therefore, this makes T. vogelii to have herbicidal, pesticidal effects and again as a fertilizer [64] . More studies support the activity of T. vogelii in insects. [65] has confirmed its insecticidal, antifeedant and repellent effects against golden flea beetle that represent a group of insects.

Tephrosia vogelii is a legume, that has higher proliferation rates and this is potential to small scale farmers that has less cost to grow and process for insect pests control in agriculture.

Vernonia amygdalina is reported to contain useful compounds such as vernolide and vernodalol, epivernodalol [66] ; Figure 3), kolaviron [67] as useful phytochemicals.

Vernonia amygdalina is reported to be effective against bacteria, fungi and virus species [66] , as an anticancer, anti-malaria and as anti-diabetic antioxidant agent and also used as vegetable [68] - [70] .

Vernonia amygdalina has been reported as a pesticidal plant, in the treatment of been weevils where ethanoic extracts showed activity [71] . Several other compounds such as saponins and alkaloids, terpenes, steroids, coumarins, flavonoids, phenolic acids, lignans, xanthones, anthraquinones, edotides and sesquiterpenes have been identified from this plant [67] [72] . More studies on identification of useful compounds from V. amygdalina and testing their efficacy against field and storage crops is of paramount importance.

Tithonia diversifolia is a well known traditional plant in developing countries. It is renowned to have agricultural benefits such as higher phosphorous contents in the above ground biomass [73] , insecticidal effects [74] , anti-malarial, and anti-inflammation. It is also used as ruminant fodder [75] [76] . T. diversifolia contain many compounds. [77] found about 16 compounds while [78] review about 150 compounds of Tithonia and 150 compounds of Tithonia diversifolia. Some of them are shown in Figure 4.

T. diversifolia has been tested for its biochemical and toxicological effects. In China, T. diversifolia has been reported to treat, diabetes, hepatitis, and hepatocarcinoma although the mechanism involved is not yet understood [79] . In agriculture, chopped pieces of stem and leaves of T. diversifolia have showed a significant increase in soil P, Ca, CEC, K and soil organic matter content [73] .

Another study by [80] showed that Tithonia diversifolia has phytoremediation potential with a capacity to accumulate Pb and Zn from the soil to the shoots. This ability gives room for study on the possible capacity of the plant in the remediation of degraded soils from synthetic pesticides contamination.