**3.4 Microwaves**

Microwaves are electromagnetic radiations with short-wavelength; which has an excellent microbial destruction potential when compared to other conventional chemical methods. Microwave technology is now a highly adopted process by most grain industries for disinfection [8, 13]. They provide protection on grains from insects [10], storage fungi, and field fungi [12]. However, treatment with the use of microwave can induce several adverse effects on seed germination and can affect grain quality. These adverse effects of microwaves are due to variations in heating caused by the difference in cold and hot spot temperature [9].

### **3.5 Fumigation**

Fumigation is a very active pest control technique. Phosphine gas for example is used to kill grain pests at every growth level of their life cycle; this is inclusive of pests with high resistance ability. Nonetheless, the phosphine gas application level needs to be up to 300 parts per million (ppm) and sustained at this level for a minimum of one week at 25°C or more. Alternatively, at a temperature of 25°C or less, a 200 ppm concentration of phosphine gas should be maintained for 10 days for effective and efficient destruction of pests that destroy legumes. Phosphine application exists in two forms; they include bag chains and tablets. There are also a number of ways with which each choice can be adopted effectively in a gas-proof secured silo. Bag chains are also considered a very safe system that assures one of not having any fumigant residue on the grain nor having the operator harmed in whatever way. The next form that phosphine exists in tablet form and is the most widely used and accepted. There exists a third approach in phosphine application which involves the use of a phosphine blanket and is mostly used for very large storages of above 600 tones. The application of phosphine and the concentrations to be used depend on the silo (which should be gas-tight and sealable) volume used for the fumigation. The phosphine concentration to be used is strictly determined based on the volume of the silo rather than the quantity of grain in the silo [13].

An airtight-covered silo especially one that passes the half-life pressure test must have to remain sealed through the entire fumigation period in other to attain a perfect fumigation result with the use of phosphine tablets and/or bag chains. In an airtight-sealed silo, fumigation is expected to last for 7 days with a temperature of above 25°C, and 10 days if the temperature falls between 15 and 25°C. Nonetheless, if the temperature in the silo is less than 15°C, pests particularly insects will be inactive and phosphine is not usually effective at such low temperatures. Based on the ineffectiveness of phosphine at temperatures lower than 15°C, phosphine application is not advisable at temperatures lower than 15°C. The silo must remain closed when fumigation is on and should only be accessed by personnel with suitable personal protective equipment (PPE) as it is dangerous for the operator. Constant opening of the silo is also detrimental to the effectiveness of the fumigation process considering the fact that the phosphine gas concentration and absorption rate would have been reduced below the lethal level recommended for pests' destruction. Recommendations for the phosphine label came to be as a result of detailed

testing by the industry, in other words, making use of phosphine as indicated on the label will ensure perfect results [13]. Phosphine is rated high as a very reliable fumigant for the control of pests in grain storage facilities and other production enterprises [13]. Nevertheless, there has been a continuous misuse of fumigants with a resultant effect of poor pest control and the development of resistance in certain species of pests. More so, just as the continuous use of herbicides that has the same principle of action advances weeds being resistant, continuous use of phosphine could lead to grain pest resistance. Nonetheless, in the case of herbicides, the development of resistance by pests can yearly be circumvented by alternating the chemicals used. The same cannot be said for stored grain fumigation as options are limited and where available, they are not cost-effective [13]. In other words, it is best to avoid the resistance of phosphine by using it as instructed.

Other fumigants and a controlled atmosphere may be used for stored grain pests but they are often high in price. However, to prevent resistance of stored grain pests, phosphine sealed in a silo that is impermeable to gas should be used.

#### **3.6 Controlled atmosphere**

In spite of the fact that phosphine is the common most used gas fumigant, there exist other gas fumigants for controlling pests in stored grain. These alternatives are however more expensive than phosphine and still require a gas-tight, sealable silo but they offer other options for resistant pest species. Nitrogen (N2) and carbon dioxide (CO2) have the advantage of being nonchemical control alternatives. Because nitrogen and CO2 methods of control change the balance of natural atmospheric gases to produce a toxic atmosphere, they are hence referred to as controlled atmosphere (CA) [13].
