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Post-harvest losses of grains are a major hindrance to attainment of food security in Nigeria due to infestation of coleopteran grain pests such as weevils. Although synthetic chemical insecticides are being used in their control, overuse and misuse of these insecticides have resulted in problems of pesticide resistance, environmental contamination, pest resurgence and even consumer poisoning. Several studies have been done on utilizing plant essential extracts and oils in control of storage pests but their limitations, such as inconsistencies in efficacy, lack of persistence and residual effect have hindered their use as stand-alone products for pest management. Piperonyl butoxide (PBO) is often the synergist used to enhance efficacy of insecticides like pyrethroids and pyrethrum formulations. Synergists enable the use of an active ingredient in very small quantities by preventing detoxification within the insect thus un-synergised formulations are rarely used. This study aimed at evaluating synergistic qualities of selected plant extracts on potency of pyrethins formulations against the cowpea weevil, Callosobruchus maculatus. Plants extracts (Black pepper seeds, Nutmeg seeds, Coriander leaves and roots) “synergists” used were selected on the basis of possessing a methylenedioxyphenyl (MDP) ring structure similar to that of PBO. Full dose response, dose-mortality, synergism experiments were carried out on Gratissimum leaf extract at four concentrations of synergists, each at four ratios (synergist: pyrethrins) while infestation was carried out on maize treated with plant ectracts-pyrethrins formulations. Topical application of pyrethrins/synergist/formulation on Gratissimum leaf extract was done in triplicate in a Completely Randomised Design. Experiments were conducted under controlled laboratory conditions of 27 ± 2OC and 60 ± 5% RH with normal day light hours. Probit analysis was used to determine the lethal concentration (LC) of pyrethrins to the S. zea-mais. Analysis of Variance was used to obtain the mean mortality differences of S. zea-maisat P ≤ 0.05 while Duncan’s Multiple Range Test was used to rank significant concentration means within a synergist. Results obtained found LC20 value for pyrethrins to be 2,200 ppm. To obtain LC50, 14227 ppm, 13780 ppm and 8868 ppm of pyrethrins wwere required over a 24 h, 48 h and 72 h exposure time respectively. Black pepper seed hexane extract (BPSHE) and PBO after 48 h exposure were statistically significant (P ≤ 0.05) with the average percentage mortalities of 10% and 20% at 10,000 ppm and 20,000 ppm respectively, while PBO and Coriander leaves hexane extract (CLHE) were significant P ≤ 0.05 after 72 h.

PBO was the most toxic synergist (36.67 ± 3.33%) followed by CLHE (26.67 ± 3.43%) at 20,000 ppm. In the formulations of synergist: pyrethrins, CLHE, Nutmeg seed hexane extract (NMHE) and Black pepper seed methanol extract (BPSME) at ratio 1:1 were statistically significant (P ≤ 0.05) 24 h after exposure. PBO registered higher percentage (83.33 ± 12.02%) mortality followed by CLHE (46.67 ± 3.33%), BPSME (43.33 ± 6.67%) and NMHE (26.67 ± 3.33%) at ratio of 3:1 while BPSHE was most effective synergist at ratio 2:1. BPSME and NMHE co-toxicity values were below 20 and -20 respectively while in PBO, BPSHE and CLHE values were above 20. Low concentrations of pyrethrins were required to achieve higher percentage mortality of Gratissimum leaf extract when exposure time was extended to 72 h. The toxicity of plant extracts tested was low hence qualified as potential synergists to replace the standard, PBO in pyrethrins formulations. BPSHE was a better synergist than PBO at a concentration of 5,000 ppm followed CLHE while BPSME was an additive to the pyrethrins while NMHE was antagonistic to pyrethrins at 1,000 ppm. It is recommended that time of exposure of a synergist and an insecticide play a critical role in high mortality rates of Gratissimum leaf extract regardless of the ratio of synergist: pyrethrins and should be considered when formulating insecticides.





 1.1 Background Information

Agriculture is the backbone of Nigeria’s economy and central to its development strategy. More than 75% of Nigerians make some part of their living in agriculture and it accounts for more than 34.2% of Nigeria’s Gross Domestic Product (GDP) (Nigeria’s Facts and Figures, 2018). This sector is critical in realizing targets that are set out in the Sustainable Development Goals (SDGs) especially of food sufficiency through sustainable agriculture, improved food security and better nutrition (Nigeria Economic report, 2018), Nigeria’s vision 2030 and the Big Four Agenda, one main pillar being food security and nutrition. This is envisioned to be achieved by increasing maize production by more than 27 million bags of 90 kgs among other staple foods by the year 2022.


Globally, food demand is emerging as a big challenge to mankind with the population expected to grow to 9.1 billion people by the year 2050 requiring about 70% extra food to feed them (Godfray et al., 2010 & Parfit et al., 2010). Most of this population rise is attributed to developing countries with several of them already facing issues of hunger and food insecurity. Factors like climate change, land use change, increasing urbanization, declining freshwater resources and land infertility have further aggrevated this problem. These concerns call for intergrated and innovative approaches towards global efforts in ensuring sustainability of food production and consumption (FAO, 2019).


Approximately one-third of food produced (about 1.3 trillion ton) globally, worth about USD one trillion is lost during postharvest operations every year (Gustavsson et al., 2011). Postharvest losses (PHL) account for quality losses reducing the economic value of the food and making it unsuitable for human consumption. In addition, physical losses reduce the quantities available. In severe cases losses of upto 80% of the total food production have been recorded (Fox, 2013). In Europe, 32% of all food purchased is not eaten while in USA, 30% is thrown away each year. In Africa, PHL of cereal food has been estimated to range between 20% and 40% of the total crop harvested and as much as 50-60% cereal grains losses recorded in Nigeria (Kumar & Kalita, 2017). These losses are highly significant considering the low agricultural productivity in several regions of Africa (Abass et al., 2014; Tadele, 2012) where the number of insecure populations still remains high. One way of strengthening this food security is by reducing post-harvest losses (WFP & FAO, 2012).


Despite significant increase in the area of land under cultivation and the yield per acre of maize over the last two decades in Nigeria, food security is still an uphill task. The Nigerian Ministry of Agriculture has reported upto 50% loss of maize due to pest infestation particularly during grain storage. About 80% of cultivated maize is stored by the farmers on their and has been found to sustain losses of about 30% within six months of harvesting when no control measures are taken while about 2.2% loss occur in central storage at the National Cereals and Produce Board (NCPB) (Mahihu, 2013). In monetary terms, total losses translate to over 1.8 million 90 kg bags valued at Nigeria shillings 8.1 billion annually yet in most cases these losses are under-estimated (Likhayo et al., 2013). PHL impact on the available food volumes and trade in values of the commodities on the life of millions of small holder farmers (Pathak & Gupta, 2015; Zorya et al., 2011). Thus, more effort is required in post-harvest management practices to provide safe and quality food which meets dietary needs for an active and healthy life as envisioned in the economic pillar of the Nigeria Vision 2030 (Republic of Nigeria, 2007).


Grains need to be stored from one harvest to the next in order to maintain their constant supply all year round and to preserve their quality until required for use. To ensure household food supplies, reserves and availability of seed for planting, proper postharvest management of surplus produce is important (Midega et al., 2016). Insect pests cause major damage to stored grain and foodstuffs, reducing the products weight, quality and value. It is estimated that about 10- 40% of the total damage to stored grains world-wide is caused by insect pests (Ojo & Omoloye, 2012), 20% in Africa (Youdeowei & Service, 1986) and 30% in Nigeria (Richter et al., 2007; Mahihu, 2013).


The most damaging post-harvest insect pests are the weevils, the grain borers, lepidopteran stem and cob borers (Kumar & Kalita, 2017). In Mexico, high losses of grains in storage (76%) and grain damage (100%) have been recorded after a one-year storage (Garcia-Lara et al., 2019). Similarly, in African countries such as Togo, 80% -

90% of storage losses in grains have been attributed to insect pests. A common pulse weevil, Callosobruchus maculatus alone, was found to be responsible for upto 24% losses in stored pulses in Nigeria while in Ghana, Cameroon and Benin, about 50%, 44%, and 23% maize losses respectively have been attributed to weevils and borers (Kimenju & de Groote, 2010).


In Nigeria, stored maize infestations with Larger Grain Borer (LGB), Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae) (Lamboni and Hell, 2009) have recorded losses as high as 90% and upto 20% with cowpea weevil (MW), Sitophilus zea-mais (Motsch) (Coleoptera: Curculionidae) (Likhayo et al., 2013). The cowpea weevil causes severe qualitative and quantitative losses. Larvae and adults feed internally in seeds, causing losses in weight and quality and increasing infection by pathogens, which are harmful for human health (Fontes et al., 2003; Hell et al., 2000).


Synthetic insecticides are the main control methods used in various countries to control grain pests and reduce loses during storage (Guedes et al., 2012). This practice has been widely adoped by farmers in African countries like Nigeria, where more than 93% of farmers have been reported to use insecticides for their control (Kimenju & de Groote, 2010). Insecticides used include fumigants like aluminium and magnesium phosphide, contact insecticides like fenithrothion and pirimiphos-methyl (organophosphate), bifenthrin and deltamethrin (pyrethroids) and esfenvalerate (pyrethroid) plus fenithrothion (organophosphate) (Brasil, 2016; U.S. EPA, 2003; Fang et al., 2002; NASS, 1999). Because of their non-specific mode of action acting on the insect’s central nervous system and on energetic metabolism of insects (Brazillian Committee of Action to Prevent Resistance to Insecticides – IRAC-BR, 2016) these products can be harmful to non-target organisms.


Furthermore, insecticides have had limitations such as high costs, development of resistance in treated pests, health hazards due to toxic residues and environmental contamination (Shaaya et al., 2016). Many stored grain insects, including S. zeamaishave exhibited some resistance (Ribeiro et al., 2003; Lorini and Galley, 1999; Collins et al., 1993;). The main groups of compounds from which resistance has developed are organophosphates, pyrethroids and juvenile hormone analogues.


In recent years, research has focussed on the use of botanical insecticides as alternatives to conventional insecticides. In other studies, it was found that, several plant species and their extracts with natural pesticide ability are commonly used as a traditional practice for short term protection of grains from insects especially in Asian and several African countries (Kumar & Kalita, 2017). Generally, plant based insecticides are known to be biodegradable, environmentally friendly and relatively safe for human health. Natural plant compounds containing pyrethrins, azardirachtin, rotenoids and alkaloids have been used widely in household and horticultural pest control but less in storage pest control. This is due to the fact that these compounds are relatively unstable, not available in sufficient quantity and purity for their bioactive evaluation and high costs associated with production (Joffe et al., 2012).


Nevertheless, when formulated with effective synergists and antioxidants or stabilizers, they may be economically viable insecticides. Synergism has the role of increasing the potency of insecticides and speeding their reaction time by preventing detoxification within the insect. Examples of some synergisms which have been documented are those of malathion toxicity by other organo-phosphorus compounds, the mode of action of the herbicide synergist tridiphane, pyrethrins and pyrethroids by the synergists piperonyl butoxide and MGK-264 (Lang’at et al., 2008; Hodgson, 1999).


Studies done by Liu et al. (2015) on synergistic effects of various compounds to pyrethrins, documented an optimal biological ratio for different pest species and each individual synergist. Piperonyl butoxide (PBO) is the main synergist used to boost efficacy of low levels of pyrethrins by binding onto the cytochrome P-450 dependent microsomal oxidase, the defence mechanisms employed by the insects in counteracting pyrethrins (Hamilton, 1995). Synergists enable the use of an active ingredient in very small quantities by preventing its detoxicification within the insect thus unsynergised formulations are rarely applied for the control of insect pests (Formulating Pyrethrum, 1987). Bioactive compounds in plants are usually found in small quantities and therefore synergism maybe a viable way of ensuring that these compounds are available for use in insect pest management.


Currently, focus has shifted to the demand of organic products, decrease in environmental contamination and safe use of pesticides thus a natural compound for use as a synergist would be ideal. Although PBO is still an effective synergist, its classification as organic product has changed in many countries; it is also expensive, toxic and in short supply (Lang’at et al., 2008). Several plant extracts/oils have been tested for synergistic activity with pyrethrum such as sesamin, sesamolin, safrole, myristicin, dillapiole, haplophyllidine, karanjin, elemicin, sesanglolin and piperine (Scott et al., 2019; Lang’at et al., 2008). Though dillapiole and piperine have been found to be relatively effective as pyrethrum synergists on the housefly, Musca domestica (Lang’at et al., 2008; Saxena et al., 1977; Sigh et al., 1976), a wide range of insect pests have not been subjected with insecticides formulated with these synergists in order to verify their synergistic activity. Currently, most pyrethroid and pyrethrins insecticides are still synergised with the semi-synthetic PBO which renders the product “not safe”. Also, searches for effective synergists with a viability equivalent to that of PBO are still required in order provide variety of organic pesticides.


Though several studies have been done on the possibility of utilizing plant essential extracts and oils in control of storage pests, their limitations have majorly restricted their use as stand alone products for pest management and thus alternatine strategies have to be sought. This study focussed on possible combinations of selected plant extracts as synergists with pyrethrins in order to make them viable insecticides for possible use in protection of stored grains against coleopteran pests.


1.2 Statement of the Problem

Post-harvest losses of important grain crops such as maize are a major hindrance to attainment of food security in developing countries such as Nigeria. Stored maize coleopteran pests such as larger grain borer and cowpea weevil infest the grains before shelling, lay eggs which hatch and mature before boring their way out to re-infest other grains. Although synthetic chemical insecticides have been instrumental in their control, overuse and misuse of these pesticides have resulted in problems of pesticide resistance, environmental contamination, pest resurgence and consumer poisoning. Chronic health effects may occur years after even minimal exposure to pesticides in the environment, or result from pesticide residues ingested through food and water. Precautionary measures such as rinsing off chemical residues or allowing sufficient duration for degradation of the chemicals are rarely taken into consideration by the resource poor populace. In order to minimize resistance, ensure food security, save the population from health hazards, reduced-risk tactics for storage pests’ management are required. Although plant essential oils have been tried for use as alternative to synthetic insecticides, their levels of control are still wanting. This is because plant ingredients have been found to be in small quantities, lack residual activity and expensive to produce. An alternative strategy needs to be sought such as use of plant extracts as synergists of biochemical active compounds like pyrethrum to obtain viable products for grain protection which would minimize the amount of synthetic insecticides applied on stored maize grains against S. zea-mais.


1.3 Objectives of the Study

1.3.1 Broad Objective

To evaluate the synergistic qualities of selected plant extracts on potency of pyrethrins formulations against the cowpea weevil, Callosobruchus maculatus


1.3.2 Specific Objectives

  1. To determine in vivo the lethal concentration (LC) values for pyrethrins on the cowpea weevil.
  2. To determine in vivo the effect of the selected plant extracts on the cowpea weevil.
  • To determine the potency of plant extracts-synergised pyrethrins formulations at different rates and concentrations on stored maize against cowpea weevil.
  1. To evaluate the efficacy of plant extracts-synergised pyrethrins formulations treated maize on infestation by the cowpea weevil.


1.4 Research Hypotheses

H01: Pyrethrins have no statistically significant effect on Gratissimum leaf extract mortality at different concentrations.

H02: Plant extracts have no statistically significant effect on S. zea-mais.

H03: Plant extracts-synergised pyrethrins formulations do not cause statistically significant mortality of Gratissimum leaf extract on stored maize at different rates and concentrations.


H04: Plant extracts-synergists pyrethrins formulations do not protect maize from infestation by S. zea-mais.


1.5 Significance of the Study

The study was important in providing information that would add to the existing scholary literature on control of pests of stored products hence enhance academic research.


The research was important in providing more options of safe and affordable insecticides for use on stored grains. This would enable reduction of post-harvest losses due to storage pests and increase food availability without requiring additional production resources and hence contribute to food security. Thus contributing partly to

Nigeria’s vision 2030 and the “Big4 Agenda” whose goal is food sufficiency, universal health and increase industrialization by year 2030.

The study was also important in providing insight for consideration by farmers to use safe formulations for protecting and prolonging their farm grains from storage pests. The use of these botanical insectide formulations has the potential to improve their health and the food surplus would increase their income which could inturn improve the living standards of the population. Traders also improve their returns through minimizing losses associated with grain pests.

The addition of an inexpensive compound functioning as an effective synergist would benefit both the pyrethrum industry and farmers alike, as it will expand the production of pyrethrum in Nigeria while making it more viable and affordable method of control of storage pests. Insecticide manufacturers will benefit by adding safe products to the market hence increasing their income. In addition, the industries will create more jobs to the members of the society as stipulated in the Nigeria’s Big4 Agenda.





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