Laboratory bioassays were conducted to investigate the bioactivity of powders, extracts, and essential oils from
Grain storage has often resulted in quantitative and qualitative losses due to physical, chemical, and most importantly biological factors such as pests which may be birds, rodents, fungi, or insects [
Once infestation is established pest insects cause gradual and progressive damage leading to losses in weight, nutritional, organoleptic, and aesthetic quality of stored grains. Osuji [
Several methods are used in controlling insects in stored grains, including physical (smoking, sun-drying, heating), cultural, biological (male insect sterilization, natural enemies, resistant grain varieties), and chemical (synthetic and natural products) methods. The most common and widely used is the chemical method involving mainly the use of synthetic insecticides.
Several workers have reported the successful wide scale use of synthetic organic insecticides, commencing with the organochlorines in the middle 1940s, followed by the later use of organophosphates, carbamates, pyrethroids, avermectins, and others. Insecticides most commonly used to protect stored grains from insect pests include aluminium phosphide, lindane, methyl bromide, ethylene dibromide, edifenphos, pirimiphos methyl, permethrin, malathion, sumithion, chlorpyrifos methyl, chlorpyrifos, propoxur, fenithrothion, dichlorvos, bromophos, fenvalerate, bioresmethrin, phenothrin, and deltamethrin [
The observed overreliance on insecticides was mainly, due to their initial quick action, ease of use and general efficiency in reducing pest populations and damage. However, there are limitations to their use mainly the deleterious side-effects to nontarget species including humans and the development of resistant strains of pests [
Due to the foregoing reasons, there has been a need to search for new insecticides with novel mechanism of action. In this regard, many scientists have reasoned that it is advantageous to investigate natural products as a source of degradable insecticides that may turn out to be safer to humans and the rest of the environment than the synthetics.
In the present study, garlic,
The cloves of garlic
Test plant materials were used against test insect species in four formulations, namely, powder, aqueous and ethanol extract of powders, and essential oils prepared as described below. To prepare the powder, plant parts were first dried slowly to constant weight in a wooden cabinet (
Aqueous and ethanol extract were each prepared from the powder. In each case, 500 g of plant powder was steeped in 1 L of water or ethanol that served as solvent, for 24 hrs. The mixture was then passed through Whatman No. 1 filter paper (15 cm diameter). The filtrate in each case was stored in a labelled Kilner jar while the residue was reextracted with water or ethanol, respectively, and all filtrates combined for each treatment. Each of the combined filtrates was then dried over a water bath at
Cowpea weevil,
Twenty active 0–3-day-old
Similar sets of experiments as described above were carried out, but this time grains were treated by dipping them for approximately 30 secs in different concentrations (0.5–16 g/L) of each plant extract.
Fumigation bioassays were carried out in 1 L airtight Kilner jars using the method of Don Pedro [
Fumigation of
Another similar experiment was set up with the arrangement described above using 6–8-day-old hatched eggs (i.e, 1-2-day-old larvae) since eggs hatch into larvae after 6 days of incubation. A batch of 20 cowpea seeds, each of which had one 6–8-day-old eggs were placed in fumigation chamber having 7 cm diameter filter paper impregnated with various concentrations of test oils. After 24 hours of fumigation, the cowpea seeds were transferred into ventilated plastic cups and left for 21 days. Each treatment and control was replicated four times. Mortality was assessed by dissecting each cowpea seeds to recover dead (or living) larvae.
Forty undamaged cowpea grains were treated by dipping for approximately 30 secs in predetermined concentrations (0.5 to 8.0 g/L) of aqueous extracts of either
Similar experiments were carried out using concentrations (0.8 mL/L to 12.80 mL/L) of essential oil of
In all bioassays insects were counted as dead when they failed to move any part of their body after prodding with fine brush bristle.
Quantal responses (mortality) of
The 48 hr
Acute (48 h) toxicity of test plant materials against
Formulation | Test plant species | Regression equation | DF | Slope ( | ||
95% Confidence Limits | 95% Confidence Limits | |||||
Powder | 9.661 (7.957–11.691) | 70.143 (50.983–96.317) | 4 | |||
(g/kg) | 26.293 (20.485–33.632) | 501.742 (293.804–854.42) | 4 | |||
Aqueous | 0.110 (0.087–0.137) | 1.30 (0.80–2.17) | 3 | |||
extracts (g/l) | 0.411 (0.314–0.510) | 4.017 (2.788–6.659) | 5 | |||
Ethanol | 0.219 (0.181–0.261) | 1.297 (0.959–1.803) | 3 | |||
extracts (g/l) | 0.863 (0.687–1.072) | 12.955 (7.624–28.913) | 3 |
DF: Degree of Freedom; SE: Standard Error.
The aqueous extracts were more toxic to
There was no significant difference in the toxicity of
Fumigant toxicity of test essential oils to
Test insect species | Test plant species | Regression equation | DF | Slope ( | ||
(95% Confidence limits) | (95% Confidence limits) | |||||
Adults | 15.46 (12.44–19.153) | 3 | ||||
23.144 (18.403–29.059) | 3 | |||||
Eggs | 14.536 (11.826–17.953) | 3 | ||||
20.844 (15.589–28.232) | 3 |
DF: Degree of Freedom; SE: Standard Error.
Fumigant toxicity of essential oil of Allium spp against
The persistence of the toxicity of aqueous extracts of both test plant species is shown in Figure
Persistence of test plant extracts against
The potency of the oils from the two test plant species remained only for 12 hrs, after which it was lost rapidly. The potency of
Persistence of test plant essential oils against
The results demonstrate that although
The extracts of
The solvent used in extracting plant materials for insecticidal potency is highly important as our present study shows. Ethanol extracts were less toxic than the aqueous extracts. This agrees with earlier reports that aqueous extracts of garlic
The ovicidal action of the essential oils from test plant species have been demonstrated in this study. This indicates that