Pests Of Stored Products

Annually various diseases, pest, weed and other pest cause substantial losses in the yield and quality of crops in Bhutan. Besides the pre-harvest and harvest loss, a heavy loss is experienced during post-harvest stage. Most importantly in our country a considerable amount of grain is lost during storage, when the grain is being stored for off-season consumption or while the farmer awaits a selling opportunity or a rise in prices. Post harvest losses to the storage pest are gaining more recognization worldwide. Some insects cause serious grain damage by developing inside kernels and feeding, and some other insect species feed and hide inside cracked grains, making their detection very difficult, while some species are fed on damaged grains or fines. Many pest of stored products belong to the order Coleoptera and one of the most destructive secondary insect pests of durable stored products is the maize storage weevil, Sitophilus zeamais.

Hence, keeping in mind the future demand of maize,it is very essential to achieve sustainable production and to preserve the produce for increasing futureneeds. There are some major threats to the harvested produce from stored grain pests like Sitophilusoryzae Linnaeus, Sitophilus zeamais Motschulsky, Tribolium castaneum Herbst and Rhyzoperthadominica Fabricius etc. Among these pests, maize weevil, S. zeamais (Motschulsky) is one of the majorpests in field and storage. The maize weevil causes extensive damage to maize in storage, especially intropical and subtropical regions (Throne, 1994), causing serious losses to many poor farmers who storegrains on farm for use as food and seed without any chemical protectants. The huge post harvest lossesand quality deterioration caused by this pest is a major obstacle for achieving food security in developingcountries (Rouanet, 1992).

Initial infestations of maize grain occur in the field just before harvest andinsect pests are carried to the store where the population builds up rapidly (Adedire and Lajide, 2003).Maize (Zea mays) is one of an important cereal crop of the country which has been lost to the storage maize weevil, Sitophilus zeamais. Stored grain insect pests have been damaging our economy by infesting agricultural stored products and responsible for worldwide loss of 10 - 40% in the stored grains annually. Minimizing post-harvest losses is one of the crucial things a farmer needs to ensure to achieve optimum benefits in maize production. According to Wyss, Luka, Pfiffner, Schlatter, Uehlinger and Daniel in principle of organic farming use of biocontrol measures come in third phase following the cultural practices and management of habitat being first and second phase respectively (2005).

Sitophilus zeamais damage

Piper nigrumFor management of insect pests many plant products such as essential oils and solvent extracts have been screened for their repellent, antifeedant, toxic and growth inhibitory activities against stored grain pests (Maliq and Naqvi, 1984; Matthews, 1993).Materials and methodsSite/ study areaThe experiment will be done in the laboratory of College of Natural Resources, Lobesa Punakha. Plant material and extraction procedurePiper nigrum seeds will be obtained from broadleaved forest of Northern Gelephu (Dobhan), Bhutan at an elevation of 1350 masl. The matured seeds will be grinded to a fine powder in a electric grinder. Soak 500 gram of powder in 1 litre of petroleum ether solvent. Then put the solution 2 litre dark glass jar for 3 days with a continuous shaking. After 3 days filter it with guaze followed by filter paper till the clear solution is obtained. Then the obtained solution will be evaporated in the rotary evaporator at 50°C. The weight of the residue will be measured and keep it in the refrigerator at 4°C till use.

During an experiment, the extracted crude will be dissolved in the same solvent to get a 10% (w/w) stock solution. Then dilute it to obtain 1.25, 2.5, and 5% solution. Pest/ insect cultureAdult of Sitophilus zeamais, will be obtained from naturally infested maize seed from any farmer’s grain storage in Punakha or Wangduephodrang. Four hundred gram of maize grains will be placed in one-liter glass jars covered with perforated lids. About 200 adult insects will be separately introduced into the glass jars containing the maize grain at the room temperature of 30±3 and 70±5 relative humidity. Each glass jar where the adult insects oviposite will be allowed for progeny emergence. Progeny emergence will be monitored daily and those emerged on the same day will be transferred to fresh grain in glass jars with lids and kept at the experimental conditions until sufficient number of such insects were obtained.

Repellency Repellency experiments will be carried out in glass Petri dish. Different test solutions i.e., 1.25, 2.5, and 5% of Piper nigrum extract will be kept ready. Whatman no. 1 filter paper will be cut into two halves and each solution will be applied to one half of a filter paper as uniform as possible by using pipette. The other half of the filter paper will be treated with petroleum ether alone. Dry the treated filter paper to evaporate the solvent completely. The two halves will be attached with cellophane tape and placed in the glass Petri dish. Twenty adult flour beetles (4-6 days old) will be released at the centre of the filter paper disc and then sealed tightly. For each treatment/ treatment 4 replicates will be set. Number of insects present on two halves will be observed and counted and recorded as repelled and unrepelled after 4 hours of experiment setting.

Larval mortality

For evaluation of larvicidal activity newly molted S. zeamais will be exposed to various concentration of crude extract. Ten larvae taken from laboratory culture will be placed with 4 gram of maize flour in Petri dish. The filter paper strips (1 cm2) one in each Petri dish treated with different concentration of P. nigrum extract will be pasted on the inner surface of the cover and finally Petri dish will be made airtight. Each treatment will be replicated four times. After 24 h of fumigation in darkness, larval mortality was recorded.

Adult mortality

The toxic effect will be tested on well-fed adults. Ten adult insects will be taken from laboratory culture and placed with 4 g of wheat flour in Petri dish. The filter paper strips (1 cm2) one in each Petri dish treated with different concentration of P. nigrum will be pasted on the inner surface of the cover and finally Petri dish will be made airtight. Each treatment will be repeated 4 times. After 24 h to crude extract exposure in darkness, adult mortality will be recorded.

Developmental inhibition

Developmental inhibition potential of P. nigrum will be carried out against 4th instar larvae of maize weevil in the laboratory. For this ten larvae will be exposed to different concentrations of crude extract in glass Petri dish for 24 h. Treated/exposed larvae will be transferred to fresh Petri dish containing flour material. For each concentration four replicates will be set for the test and control to evaluate developmental inhibition. Number of larvae survived, pupae transformed from survived larvae and adult emerged from transformed pupae will be recorded. For developmental inhibition insects will be regularly observed for 40 days.Data analysis Chi-square test will be applied to establish the repellent activity of the oil tested (Sokal and Rohlf, 1973). Linear regression analysis will be performed to show all dose response relationship (Sokal and Rohlf, 1973). Analysis of variance (ANOVA) will be performed to test the equality of regression coefficients (Sokal and Rohlf, 1973).