CHAPTER 2 LITERATURE REVIEW
2.6.3 Insecticidal activity
2.6.3.1 Stored-grain pests (Sitophilus zeamaisand Sitophilus oryzae L.)
Sitophilus zeamaisand Sitophilus oryzae L. have been reported as some of the severe pests of stored grains and their products163-165. Sitophilus zeamais, although associated primarily with maize is capable of developing in all cereal grains and their products164. Sitophilus oryzae can also develop in different types of grains but mainly prefers soft varieties of wheat grains165. Sitophilus zeamais (Figure 2.16a) and Sitophilus oryzae,(Figure 2.16b),are similar in appearance. Their colours vary from dull red-brown to nearly black and are usually marked on the back with four light reddish or yellowish spots. They have fully developed wings beneath the wing covers; the maize weevil has more distinct coloured spots on the forewings, it is slightly larger in size and is a stronger flier. Their habits and life cycle are similar with a minimum life cycle of 28 days. The egg, larva and pupa stages occur in the grain kernels. The adults live an average of 4 to 5 months, and each female lays 300 to 400 eggs during this period.
(a) (b)
Figure 2.16.Sitophilus zeamaisand Sitophilus oryzae.
The weevils can survive in extreme cold and hot temperatures hence they are found all over the world. The maize and rice weevil causes similar damage to the grains they infest. The larvae feed within the kernel and consumes the endosperm. The adults leave a large, ragged exit hole in the kernel and feed on damaged kernels. The adult weevil gathers and reproduces in stored grains. This produces heat and moisture which can lead to mould development and invasion by other insect species. The grain can also be tainted with white, dusty excreta which contaminate the product as well as render it unpalatable. Infestations can result in reduction of weight and quality of grain as a result of the larvae and adult weevils feeding on the endosperm. Heating of grain also occurs which accelerates the development of the insects making the commodity liable to caking, moulding and even germination. Stored grain pests can be controlled physical treatments which include:
mechanical impact, physical removal and physical barriers to prevent the entrance of insects, abrasive and inert dusts, ionizing radiation, light and sound166. Stored grain pests
stored-product insects is between 25-33oC167. Fluidized beds, spouted beds, pneumatic conveyors, a counter-flow heat exchanger, high frequency waves, microwaves, infra waves and solar radiation have been used to satisfactorily disinfect grains using high temperatures but high costs are involved167. Stored grain pests can be controlled biologically by the use of living organisms, parasitoids and predators168,169. Laboratory tests have demonstrated that
and the rice weevil in wheat170. Although parasitoids and predators are potential stored grain pest control agents and even some of them have been sold commercially, their use may be limited by many factors171. Such factors include increased costs which make the method expensive; releasing enough parasitoids in large amounts of the grain may be difficult and hence the efficiency of the parasitoids in such situations may be poor and the presence of the parasitoids or predators in the end products will not be accepted in retail trade. Stored grain pests are mainly controlled by fumigation, a method where gaseous pesticides completely fill an area to suffocate or poison the pests within172. Phosphine and methyl bromide are the most common fumigants used for stored product protection173. However, insect resistance to phosphine and hence control failures have been reported in some countries174
phased out completely by most countries in the early 2000s175. In view of the problems with the current fumigants, there is a global interest in alternative strategies including the development of chemical substitutes, exploitation of controlled atmospheres and integration of physical methods176. Active research is being undertaken to exploit ozone as a potential quarantine treatment for controlling stored-product pests177. Ozone offers several safety advantages over conventional post-harvest pesticides. It can be easily generated at the treatment site using only electricity and air and hence there are no stores of toxic Anisopteromalus calandrae, a wasp, has the potential to control the maize weevil in corn
. In addition, methyl bromide was declared to be ozone-depleting and was
chemicals, no chemical mixing hazards and no disposal of left over insecticides or containers178. In addition, ozone has a short half-life, 20-50 minutes, it reverts back to oxygen leaving no residue on the product and if necessary ozone can be neutralized by thermal activated charcoal178. The major disadvantage of ozone is that it is corrosive towards most metals179. Plants may provide potential alternatives to currently used insect- control agents because they constitute a rich source of bioactive chemicals. The use of plants, plant material or crude plant extracts (botanical insecticides) for the protection of crops and stored products from insect pests is probably as old as crop protection itself180. Before the development of synthetic insecticides, botanical insecticides were major weapons in the farmer’s arsenal against crop pests and are still used to date in some societies especially in Africa181. For example, in Uganda and Kenya, farmers use the whole plant of Tagetes minutato protect maize stores 182while in Malawi, crushed tobacco leaves are mixed with grains during storage183. Natural compounds from plants have also been used as insecticides and as templates for commercial pesticides. Various monoterpenoids from plant essential oils have attracted attention in recent years as potential pest control agents due to their insecticidal, repellent and antifeedant properties184-187. Pyrethrins I and II,
that attack the nervous system of all insects188. When present in amounts not fatal to insects, they have an insect repellent effect189. They are the compounds upon which pyrethroid insecticides like permethrin 2.17b were designed and their actions are similar, (Figure 2.17).
2.17a, related esters from Chrysanthemum cinerariaefolium(pyrethrum), are neurotoxins
O
O Cl
Cl O
O O
O R
2.17a. 2.17b
Pyrethrin I, R = CH3
Pyrethrin II, R = CO2CH3
Figure 2.17.Pyrethrins andpermethrin