One of the biggest challenges that confront beans and grains farmers and merchants in the country is how to effectively control weevils and beetles in their stored products.
This has led to the indiscriminate use of excessive chemicals by many grains farmers and merchants to preserve these produce for future sale; as a result, most of the crops have high pesticide residues harmful to human health.
Two years ago, the European Union (EU) banned Nigerian beans because of the high level of dichlorvos pesticide residue at 0.3, which was beyond the 0.1 acceptable limit in the beans.
Other chemicals like Chlorpyrifos, Cyhalothrin, Dimathoate, Trichlorphon, Omethoate and so on were also discovered in high, unacceptable levels in dried beans for export.
These pesticides were either applied in the farm during cultivation or at storage to preserve the beans from weevils
PICS bags solve lots of problems
To tackle this problem, the Purdue Improved Crop Storage (PICS) system was developed by Larry L. Murdock, professor of Entomology at Purdue University, USA and scientists in Cameroon in a research funded by a USAID and the university’s International Programs in Agriculture.
The PICS consists of two high-density polyethylene plastic bags, and an outer sack of nylon that protects them. It can store grains safely for eight to 12 months without the produce being damaged by insects.
To make the technology available to farmers in Nigeria, a company based in Kano State has been given the license to produce the bags for farmers and merchants for grains and beans storage.
The system reduces the amount of damage the weevils inflict upon stored beans and other grains.
Murduck, on the Purdue University website, stated that the PICS bags are uninhabitable environments for beetles and weevils,” adding that “the keys to the success of PICS are that insect reproduction is essentially stopped quickly and most eventually die of dehydration.”
Dr Tahirou Abdoulaye, an Agricultural Economist with the International Institute for Tropical Agriculture (IITA), Ibadan stated that the “PICS bags simply eliminate the need to use any chemicals for storage.”
Extension expert offers advice on storage
The block extension supervisor for Tudun Iya Funtua Zonal Office, Katsina State Agricultural and Rural Development Authority (KTARDA), Malam Ahmad Idris, noted that they don’t use or recommend chemicals for maize storage, adding that the only thing a farmer needs to do is to make sure the maize is properly dried before storing it.
He explained that maize is properly dried with the onset of the harmattan season – that is from the beginning of November – and that the maize seeds are allowed to dry on their cobs until the cob turns brownish before storing.
For cowpea (beans), Malam Ahmad stated that because of insects and other pests, some chemicals may be used for storage. However he recommends that farmers use Purdue Improved Cowpea Storage (PICS) bags which suffocate all insect pests during storage as no air is allowed into the bag.
Chemical control
Dr Rabiu Adamu, an entomologist from the Crop Protection Department of ABU, Zaria, told Daily Trust that although there are many insecticides that can kill insect pests, only few can be used to control storage insect pests because of strict regulations on their use on foodstuffs.
Insecticides that need a long time to degrade and leave residues in the products are unsuitable for use on stored produce.
He described the major storage pests of maize as maize weevil Sitophilus zeamais and lesser grain borer Rhizopertha dominica, while for cowpea it is cowpea weevil, Callosobruchus maculates.
Dr Adamu offered the following tips on the application of chemicals in maize and cowpea storage:
Methods of control of maize and cowpea storage pests
There are two major ways to apply insecticides in order to control storage pests:
By mixing the maize grain with the grain, the pest is killed when it gets in contact with the insecticide protected grains. Recommended insecticides include Dust such as Actellic Dust. Emulsion concentrate can also be used such as Actellic EC. Wettable powders can also be applied such as Carbaryl to protect the grains.
Through fumigation, in which the insecticide in gaseous form penetrates the stored product, insects are killed when they inhale the poisonous gas. Example is the use of Aluminum or magnesium phosphide tablets.
Recommended rates
For Dust formulation, Pirimiphos Methyl (Actellic) can be used for shelled maize/cowpea grains, at a rate of 200-500g per 1000kg grains. Dust is a mixture of insecticide with inert powder. It is mixed with grains at the time of storage before the produce is bagged. The produce to be mix with the dust insecticide must be dry and must be evenly mixed.
Dust can also be used on floors around the bottom and top of the storage containers. Dust insecticide when properly used could provide protection of 6-9 months. After period of storage the produce must be thoroughly cleaned of the dust before it can use for consumption.
Actellic EC can be mixed with water to be sprayed on grains prior to storage. 0.5 – 1 litres in 5- 10 litres of water per 1000 kg of grains.
The grains must be thoroughly mixed with the spray liquid and must be properly dried before being bagged for storage.
Wettable powders such as Carbaryl (Sevin) can be applied at the rate of 10g per 1000kg of grains. However the powder is not popular with farmers because it is not highly effective against common storage pests of maize and cowpea.
The fumigant leaves no marks or discoloration on the stored produce. However it should be noted that fumigant kills only insects which are already in the grain.
Fumigants are gases and therefore self-dispersing and non-persistent is the most reliable chemical control of storage pests.
They have several advantages over contact poisons due to their ability to penetrate into mass of grains and not leaving residues.
For fumigants to be effective, they must be applied where there is air-tight storage condition in sacks, containers or buildings. For bagged produce, the rate is 1-2 tablets per 100 kg grain sack. The tablet is wrapped in small tissue paper and placed in the middle of the sack.
The fumigant can provide protection for up to six months provided the storage container is airtight.
For any recommended chemical to be effective against storage pests of maize and cowpea:
The grains must be harvested as soon they are matured, to avoid infestation in the field because most of the maize and cowpea storage pests are field to store pests.
Grains must be properly dried; below 12 percent moisture level before storage.
Repair all cracks in walls, floor and roofs to deny insect pests hiding places and dust the stores/ granaries with appropriate contact insecticides such as Permethrin or Pirimiphos Methyl before storing produce.
Clean granaries and stores before filling with new grains. A farmer can buy these chemicals from reputable agro chemicals dealers or retailers in towns and cities. They can also buy in large quantities from the manufacturers, distributors or their agents in the big towns
