The grain weevil, Sitophilus granarious (L.) is a major problem in storage
causing significant annual losses of wheat all over the world (Khanam et
al., 1990). Grain weevil can cause losses to wheat in strorage either directly
through consumption of the grain or indirectly producing hot spots
causing migration of moisture and thereby making grain more suitable for other
pests (Chowdhury and Pathak, 1990). The damaged wheat grain become unfit for
consumption and planting. The use of chemical pesticides to protect cereal crops
against the attack of grain weevil in storage may cause serious health hazards
(Bhaduri et al., 1989; Talukder and Howse,1994). In developing countries,
the indiscriminate, regular and routine use of synthetic pesticides for the
control of insect pest has led to a number of problems, such as insect pest
resistance, resurgence, environmental pollution, ecological imbalance and residues
in market produce. On the other hand, botanical pesticides are safe, less hazardous
and biodegradable (Talukder and Howse, 1993,1995). These facts have drawn researchers
interest in plant and plant products as source of pesticides and during the
last few years, a number of investigators isolated and identified chemical compounds
from leaves and seeds of many plant species as insect antifeedants. The potential
antifeedant activities of neem oil have been reported for several insect pests,
both in storage and field (Ramzan,1994; Seck, 1994; Xu-Hanhony and Zhao-Shanhuan,
1994). This study was undertaken to evaluate the efficacy of botanical products
as grain protectants on the feeding response and survival of Sitophilus granarious.
Materials and Methods
An experiment was conducted in the laboratory of the Department of Entomology, Bangladesh Agricultural University (BAU), Mymensingh. The grain weevil, Sitophilus granarious (L.) were collected from the LSD godown of Jamalpur and reared on a diet of wheat (var. Balaka) grain in jar .Wheat grains were collected from villages near Bangladesh Agricultural University, Mymensingh Campus. Grain weevils were reared in the laboratory at 27-300C temperature with 70-75% relative humidity. Fifty pairs of adult grain weevil (2-6 days old) were introduced in jar containing the rearing media of wheat grain. The jars were sealed and a maximum of 7 days were allowed for free mating and oviposition. Then the parent stocks were removed and the egg containing media were transferred to pre-conditional media of wheat grain in the breeding jar. The jars were covered with muslin cloth secured with rubber bands to prevent the contamination and escape of insects. Seeds of neem, safflower and sesame and leaf of nishinda, eucalyptus and bankalmi were collected from BAU campus for oil, powder and extract preparation. Babla wood was also collected for preparation of ash.
|| Surface protectant effect of different oils (v/w) on grain
weevil, Sitophilus granarius L.
|Values followed by different letters within a column are significantly
different at the 5% level of probability (DMRT)
||Residual toxicity of different leaf powders and wood ash on
grain weevil, Sitophilus granarius L.
|Values followed by different letters within a column are significantly
different at the 5% level of probability (Duncans multiple range test)
||Direct toxicity effect (by dipping method) of different leaf
extracts on grain weevil, Sitophilus granarius L.
|*Values followed by different letters within a column are
significantly different at the 5% level of probability (Duncans multiple
*HAT = Hour after treatment
Oil, Powder, Ash and Extract preparation: Thousand g seeds each of neem, sesame and safflower were ground separately in blender and soaked overnight in water. The soaked seeds were boiled for 2 h and cooled. Scums formed over the liquid was collected carefully keeping the residue below. The collected scum was boiled for extraction of oils.
Approximately 500 g of leaves of nishinda, eucalyptus and bankalmi were separately washed after their collection and then air-dried in the shade. Then they were ground in an electric grinding machine and passed through a 25-mesh sieve to obtain fine dust.
Babla wood was dried in the sun. Then the babla wood was burnt, ash was cleaned and collected from the oven. After cooling, the ash was put in the jar and covered it air-tight to prevent the absorption of air moisture. A 25 diameter sieve was used to obtain fine ash.
Ground leaves (10g) of nishinda, eucalyptus and bankalmi were separately mixed with 50 ml acetone and the mixture was then stirred for 30 min. in a magnetic stirrer and left to stand for next 24 h. The mixture was then filtered through a fine cloth. The filtered solution was then boiled for solvent evaporation at 80-850C in a water bath until the constant weight of extract was gained. Extracts were preserved in tightly corked bottles and stored in a refrigerator until their uses for insect bioassay.
Surface protectant effects: Oil of neem, sesame and safflower were mixed
separately with wheat grain @ 2.5, 5.0, 7.5 and 10.0 ml kg-1 seeds
. The oils were diluted with petroleum ether. Conical flasks containing 40 g
grains and oil mixture were shaken manually to ensure uniform application of
oils with the wheat grain. After proper shaking, the seeds were taken out and
air-dried for 1 h at room temperature. Treated wheat grains (10 g) were kept
in each plastic pot. There were 15 treatments, each being replicated thrice.
Five pairs of grain weevil (5 female and male) were then released in each pot
and were closed with lid for next 7 days.
Residual toxicity test: Leaf powders of nishinda, eucalyptus, bankalmi and babla wood ash were mixed with wheat grain (rearing media) @ 2 and 3% by weight and kept overnight. Treated wheat grain (10 g) was put into separate plastic pot (3.5x4 cm). Control pots contained normal food only. Ten adult grain weevils were introduced at the centre of plastic pot containing food and closed with lid for next 7 days to allow them for oviposition.
Direct toxicity by dipping method: Leaf extracts of nishinda, eucalyptus and bankalmi were diluted with acetone to make 2, 4 and 6% solution . Five pairs of adult grain weevil (3 5 days old ) were taken at the centre of the filter paper and they were dipped for 3-5 S in diluted extract solution. Then the insects were returned to the petri dishes containing insect food ( 5 g wheat grain). Mortality were observed at 24, 48 and 72 h after treatment (HAT).
Weight loss and inhibition rate (%) were calculated using the following formulae :
|| Weight of undamaged seed
||Weight of damaged seed
|| Number of damaged seed
||Number of undamaged seed
||Number of insects in control pot and
||Number of insects in treated pot
Results and Discussion
Surface protectant effects: Surface protectant effect against grain
weevil has been evaluated by comparing the number of F1 adult emerged,
seed damage percentage, weight loss percentage and inhibition rate in the food
treated with 0.25, 0.50, 0.75 and 1.00% plant oils (Table 1).
The least number of F1 adults were emerged from the food were 34.50,
29.00, 7.50 and 3.25 respectively. Seed damage was also lower and significantly
differed for the food treated with 1.00% neem oil than other in all concentration.
The weight loss(%) was not significantly different at various concentration
of oil. But the lowest was 0.64 in safflower oil at 1% concentration identical
to neem (0.76) and sesame oil (1.11).The inhibition rate was significantly differed
at 0.25 and 0.75% concentration.
From this study it was observed that in all the parameters, the higher concentration of oil showed the lower infestation i.e., least number of F1 adult emergence for which the seed damage and weight loss and inhibition rate were less and higher, respectively. This finding was in strong agreement of Jood et al. (1993) who observed that the oil of neem acted as surface protectant of wheat grain against the larvae of Trogoderma granarium by reducing hatching (%) of grain.
Residual toxicity: Leaf powders and wood ash acted as seed protectant
of wheat grain were evaluated by comparing the number of F1 adult
emerged, seed damage rate, weight loss (%) and inhibition rates (Table
2). Residual toxicity showed that the lowest number of F1 adult
emergence (63.00), seed damage rate (25.75%) followed by nishinda and eucalyptus,
weight loss (7.14%) identical to nishinda and the maximum inhibition rate (45.32%)
were found from wheat grain treated with 2% wood ash. In case of 3% mixture
similar observations in F1 adult emergence, seed damage rate, percentage
weight loss and inhibition rate were observed. The lowest number of F1
adult emergence (56.50), least seed damage rate (18.75%) followed by nishinda
and eucalyptus, weight loss percentage (5.02%) identical to nishinda, eucalyptus
and bankalmi and the maximum inhibition rate (51.05%) identical to nishinda
were observed from the wheat treated with 3% wood ash. Between the two treatment
rates, emphasis was given on lower rate (2%) because it might be economically
profitable. The present findings agrees with the report of Gonzalez and Lagunes
(1986) who stated that leaf powder and wood ash were effective for reducing
seed damage rate.
Direct toxicity effect: Direct toxicity effects of three different plant leaf extract (2, 4 and 6%) were evaluated through mortality data at 24, 48 and 72 h after treatment (Table 3). It was found that the nishinda extract revealed the highest toxic effect and bankalmi extract showed the lowest toxic effect on grain weevils at 2, 4 and 6% rate of leaf extract. The order of toxicity of three extracts on grain weevils were as such nishinda> eucalyptus> bankalmi. Mortality(%) showed their proportional relation to the level of concentration with time. De-pedro and De-pedro (1994) observed that crude plant extracts were toxic to corn weevil and their findings are in accordance of the present results.
The botanical products are not only of low cost, but also have less hazardous effect. It is easy to process by farmers at village level using very simple methods. Therefore, the application of plant products will be ideally suited by farmers and consumers in developing countries like Bangladesh in storage pest management systems without using costly methods or expertise.