Phytochemical Analysis
The phytochemical screening of H.indicum leaf extracts revealed tannins, saponins, alkaloids, cardiac glycosides, terpenoids, flavonoids, and phenols. Saponins, flavonoids, and phenols were present in trace amounts in the hexane extract Table 1). Alkaloids, saponins, flavonoids, and steroids were found in both ethyl acetate and methanol leaf extracts. The insecticidal potential of plants is attributed to these phytochemicals, which cause significant insect mortality and are also responsible for the therapeutic benefits of medicinal plants. This aligns with the report of Shoge et al.[11], who noted that the wound-healing activity of H. indicum L., was due to tannins.
Insect Mortality
The H.indicum L extracts showed a definite level of toxicity against Sitophilus species. Tables 2–4 showed that the hexane leaf extract exerted potency on Sitophilus oryzae at concentrations of 0.01 g/mL to 0.08 g/mL from 24–96 h. However, the highest mortality of 36.67% at concentration of 0.08 g/mL 96 h postexposure was observed, while the lowest mortality exerted on Sitophilus oryzae was 1.7%. However, the standard chemical insecticide (cypermethrin 2.5 E.C at 0.5 ml) was observed to have caused mortality of up to 100% on S. oryzae at 96 h post exposure while the negative control caused 1.7% mortality 72 h postexposure which may likely be due to natural death. Similarly, the hexane extract caused the least mortality of 1.7% on Sitophilus granarius and the greatest mortality of 30% 96 h post exposure at 0.08 g/ml. The same trend was observed for maize weevil. Hexane extract caused the lowest mortality of 1.7%, while the greatest mortality of 71.7% was observed on Sitophilus zeamais. On the other hand, ethyl acetate extract caused the greatest and lowest mortality of 50% and 3.3% on rice weevil, but 43.3% and 1.7% on sorghum weevils, then 90% and 3.3% on maize weevils. However, similar trend was observed on the potency of methanolic extracts on the insect pests of these stored grains. As can be observed from the Tables 2–4, the lowest and highest mortality of 63.3% and 5% was observed for rice weevils, 90% and 3.3% for sorghum weevil, and then 96.7% and 3.3% for maize weevils. A slight statistical significant difference was observed in the degree of effectiveness between cypermethrin and the extracts at (P ≤ 0.05) significance level.
This was similar to the report of Okwute [1] in the evaluation of pesticidal activity of extracts from Piper guineense, Piper longum, and Piper retrofractu where he found out that the plants were active against Callosobruchus maculatus, Zonocerus variegatus L, and the mosquito larvae at 96–100% mortality rate in 48 h, and that the activity was due to the presence of guineense, whereas, piperine gave a synergistic effect. Cypermethrin has been very effective in controlling adult S. zeamais which agrees with the findings of Asawalam et al., [10] who reported 100% mortality to Sitophilus zeamais when treated with cypermethrin-stored maize. It can be suggested that the potency of the leaf extracts of H.indicum is due to the presence of phytoconstituents which may have disrupted the enzymatic and osmotic balance of the insects thereby leading to death or can also be attributed to interference in the oxidative phosphorylation of ADP to ATP thereby inhibiting normal respiratory activities of the weevils; resulting in suffocation and subsequently, death [12]. This study revealed the active potentials of this plant product as plant-derived insecticides against stored grain weevils, and provides a scientific rationale for the use of this botanical as alternative to synthetic insecticides in post harvest protection of stored grains. The treatment with methanolic leaf extract of H.indicum had the highest percentage mortality as well as reduced number of adult emergence and the least percentage weight loss. This was supported by the report of Adedire and Lajide [13] on Piper guineense belonging to the family piperacea and stated that the plant possesses some forms of insecticidal properties against the eggs of stored cowpea grains, (bruchid) which are capable of suppressing various developmental instars of Callosobruchus maculatus, while Fasakin and Aberejo [14] have also reported that pulverized plant material from P.guineense inhibited egg hatchability and adult emergence of Dernlestes maculatus during storage. Similar effects of plant materials as insect protectants have been observed in the treatment of cowpea and maize weevils [15] [12]. This study also agrees with the report of Okwute [1] who investigated three Sri Lankan plants' extracts: Plearostylia opposita (Wall); Alston (Celastraceae), Aegle marmelos Correa (Rutaceae), and Excoecaria agallocha (Euphorbiaceae), and found out that the plants are insecticidal, and for the first time three compounds were found to possess the daphnane orthoester skeleton which may be the constituent of ethyl acetate extract of E. agallocha and have been found to be insecticidal.
Moreover, it can be observed from Table 5 that all the treated seeds investigated showed percentage germinability of up to 80%. For instance, paddy rice treated with hexane leaf extract showed 81.67% germinability, while sorghum and maize given the same treatment showed germinability of 85%, and 86.67%, respectively. The negative control (untreated grains) showed 85% germinability level while all seeds treated in cypermethrin showed 75% germinability. In the same vein, Rice seed treated with ethyl acetate leaf extracts of H.indicum showed 85% germinability level. Sorghum, and maize, showed 88.33%, and 83.33% germinability levels, respectively. The same trend was observed in methanol extracts. Rice treated with methanol leaf extract of H.indicum showed 88.33% germinability level. The same was observed in sorghum and maize with percentage germinability of 85%, and 83.33%, while the negative and positive controls showed percentage germinability of 85%, and 75%, respectively. This shows that all the seeds were viable after treatments with various leaf extracts of H.indicm because they showed germination rate above 75% in wet planting condition. This agrees with the report of Okwute [1] who reported that cowpea seeds exposed to crude Dalbergia saxatalis showed viability with a germination rate of over 70% after five days of exposure to planting (wet) conditions. There was no statistical difference observed between all the treatments and controls at (P ≤ 0.05) significance level. The result so far indicated that all the seeds were viable after exposure to leaf extracts of H.indicum.
Furthermore, the effect of the plant extracts on adult emergence is shown in Table 6. The various plant extracts used in the study significantly suppressed the emergence of the weevils of stored grains when compared with the control. Adult emergence of rice weevil was least among the seeds treated with extracts of H.indicum, while the control significantly had the highest adult emergence of 35%. The highest adult emergence of 2% was observed in S.granarius, and S.zeamais exposed to methanol leaf extracts of H.indicum at 0.01 g/mL. There was no adult emergence observed in the treatments with positive control (cypermethrin). Insecticidal property of any plant extracts would depend on the active constituents of the plant material. The reduction in adult emergence and seed weight loss upon treatment with methanol leaf extract of H.indicum suggested that S.zeamais is affected on exposure to methanol leaf extract of H.indicum when
The percentage weight loss was also calculated as one of the parameters required to ensure the effectiveness of the plant extracts on the weevils. Weight loss was observed across all the treatments. It can be observed from Table 7 that the percentage weight loss in rice grains treated with hexane leaf extract was 0.63%, while that of sorghum, and maize were 1.56%, and 1%. The weight loss in rice grains treated with ethyl acetate leaf extract was 1.25%, while that of sorghum, and maize were 3.31%, and 6.50%. Similarly, the percentage weight loss in rice grain treated with methanol leaf extract of H.indicum was 1%, whereas, sorghum, and maize grains exposed to methanol leaf extracts of H.indicum had percentage weight losses of 2%, and 1.75%. Meanwhile, the control (untreated grains) also recorded a weight loss of 31.5%, but the percentage weight loss in grains treated with Cypermethrin was 0.25%. The noticeable increase in percentage weight loss in the control may be due to feeding effect of the weevils on the substrates due to the weevils population as compared to the grains treated with cypermethrin which recorded the lowest percentage weight loss due to high mortality effect exerted on the weevils’ population. It can be observed that there was no statistical significant difference between the percentage weight losses between all the grains treated with the extracts compared with the standard insecticide at (P ≤ 0.05) significant level, but a noticeable high statistical significant difference was observed between the treatments and control at (P ≤ 0.05) level of significance. According to Mills [19], the presence of insects in stored grains causes an additional rise in temperature because the insects' feeding behavior creates "hot spots" in the grains which allow a moisture build up inside the stored grains, leading to seed degradation and consequent weight losses. This was similar to the report of Santos et al. [17] who found that in Brazil, the presence of Sitophilus zeamais and Sitotroga cerealella in maize grains caused a decline in germination as the insects' developmental levels increased from 13% on the egg stage for Sitophilus zeamais and 10.9% for Sitotroga cerealella to 93% and 85%, respectively, at the adult stage. The phytochemicals present in the extracts are known to be important sources of toxicants against major insect pests.. This is in agreement with Hassanali et al.[18] who reported that the active constituent eugenol, extracted from Ocimum suave is an effective repellent against S.zeamais. The active constituent in these plant material appears to be responsible for their insecticidal properties against the stored grain weevils, just as the presence of active constituents in Nicotiana tabacum(nicotine) is attributed to contact, stomach and respiratory poisoning properties [19], especially in Zonocerous variaegatus. Since H.indicum is not a food plant and can be used cheaply, the weed plant used in this project was appropriate. It is inexpensively accessible as undesirable plant that farmers frequently have to pay to remove. Some plant compounds may have a pesticidal effect by changing the osmotic and enzymatic activity required for organisms to survive, ultimately leading to their death. To prevent biodeterioration, seeds must be stored correctly in cool, dry environments. Chemical treatment was more effective in controlling pests than plant treatments; however H.indicum leaf extract showed better results after 96 h compared to 72 h. But there is a lot of criticism against the usage of chemicals because of the detrimental effects they exerted on both the environment and humans [20]. Potential use of H. indicum against Sitophilus oryzae, Sitophilus granarius, and Sitophilus zeamais, has been suggested by the biopesticidal efficacy of the plant treatments on weevils of stored grains. This result aligns with the recommendation of Ogunnupebi et al.,[21]) on the use of potential natural products for crop protection and food preservation.