Pests that Damage Stored Food | Zoology

Almost all stored food stuffs are liable to insect injury. It is usual to refer to almost all insects which infect products as “weevils” even though many not only are not weevils, but are not beetles at all. Storage pests are those associated with stored grains and milled cereals. There are several hundred insect species of storage pests but some 50 of them have been occasionally found to cause serious damage.

The four pests described below are considered to be the most serious under Indian conditions:

1. Rice Weevil (Sitophilus Oryzae):

This is the most destructive insect pest of grain in the world. It is called the rice weevil only because it was found to be infesting rice when it was described for the first time. Actually, it infests very large varieties of stored grain and is cosmopolitan in its distribution but it is generally much more injurious in warm humid countries. It is a reddish brown weevil about 3 mm in length and its head has a slender pointed forward projection with a pair of stout mandibular jaws at its extremity.

Unlike its near ally, the granary weevil (Sitophilusgranarium L.), it is a strong flier and it has been known to fly from the godown to the fields in the vicinity where it begins to infest the grain in the field. The life-span of the adult weevil is four to five months. The mother weevil makes a small cavity in the grain by means of its powerful jaws and then lays in is a small egg which is just plastic enough to fit in the cavity made by the mother who thereafter covers the egg with a gelatinous fluid.

The tiny fleshy white legless grub hatching out from the egg bores down into the grain wherefrom it emerges only on reaching the adult weevil stage. The grubs feed rather voraciously on the content of the grain but leave the shell intact. The full-grown larva makes a pupal cell therein. At the end of the pupal period which may last from a few days to a few weeks the adult bores its way out of the grain. Each weevil lays 300 to 400 eggs and the life cycle is normally completed within about four weeks.

Thus several generations are possible during a normal storage season and therefore the severity of infestation and damage goes on increasing with the passage of time. From the viewpoint of control, the weakest point in the life of this pest is that it is unable to breed in the grain moisture content is less than 9 per cent. Hence, if one can afford to dry the grain and keep it dry during storage, it is almost certain that the grain will not suffer any damage due to this pest.

It may, however, be pointed out in this connection that if the grain moisture is at the marginal level, say between 9.5 percent and 10 per cent in some pockets of the bulk, the weevil can raise both moisture and temperature by means of water and heat produced by itself and thus create a microclimate suitable for itself. Often serious cases of grain-heating in this way have been reported. If keeping the grain dry is not practicable, then fumigation is the only feasible proposition as soon as the infestation reaches an intolerable level.

2. Lesser Grain Borer (Rhizopertha Dominica):

The beetle pest (Rhizopertha Dominica) is also cosmopolitan in its distribution and very serious in respect of damage to stored grain. In some places it is considered to be next to Sitophilusbut in other places it is reported to be causing more damage than even Sitophilus. It belongs to a family of wood-borers and it is considered to have developed a liking for stored grain when it came in contact with grain as an insect infesting the walls of wooden grain containers.

In the adult stage, it is a small polished, dark-brown or black beetle about 3 mm in length and a slender cylindrical body with its head turned down under the thorax. Both the adult and the larval stages are equally injurious to the grain. This species was first described by Fabricius in 1972 from a shipment of seed and roots carried from India to South America and therefore its original home is said to be India where from it has spread practically to all parts of the world. The adult is a strong flier and can spread with great rapidity.

A few peculiar characteristics of this pest are as follow:

(a) At times this insect is found near the bottom of bins for which it seems to be well adapted,

(b) The adults are rather fragile and get damaged by mechanical turning of the grain stored in bulk,

(c) It can penetrate the sidewalls of wooden bins as it belongs to a family of wood-borers, and

(d) As a causative agent for heating the grain it is somewhat less effective than Sitophilus, the carbon dioxide production by adult Rhizopertha being roughly half of that produced by a Sitophilus adult. In fact, in a mixed infestation four Rhizopertha are considered to be equal to one Sitophilus.

A mother-beetle is capable of laying 300 to 500 eggs which are deposited either near the embryo end on the grain or simply dropped in between grains. The larvae hatching from these eggs are quite active and they soon enter the grain generally near the comparatively soft embryo end and pass the rest of their life therein.

Some, however, remain feeding on the powdered starchy material without entering the grain. The larva undergoes three or four moults and then pupates inside the grain itself. As a result of larvae feeding inside, the whole grains often get reduced to a sort of mere hollow broken shells.

The total life-cycle from egg-laying to adult emergence may be completed in about a month, although this period can be very much prolonged depending on the temperature. As regards control, the characteristics of this pest are more or less like those of the rice weevil, and therefore similar control measures can be adopted for this pest also.

3. Khapra Beetle (Trogoderma Granarium):

This pest is cosmopolital in its distribution and assumes very serious proportions mostly in warm dry regions. Thus, in India, its damage is far less along the humid coastal regions than in the interior of the country. Another peculiarity of this beetle pest is that unlike other beetles the adult in this case in harmless and the grain is damaged only by the larval stage. Further, the first attack is generally at the embryo point of the grain but later when infestation becomes heavy, other parts or the grain are also badly damaged.

In fact, the grain damaged by Khapra and Rhizopertha loods more or less alike. The adult female stage is a small, rather oval, pale red-brown or black beetle about 2.5 mm in length and with indistinct markings on the wing-covers; the male is almost half the size of the female. One female lays about 125 eggs, which hatch into very prominent hairy larvae. The hairs are reddish brown, movable and they are grouped into a number of bundles along the body and a sort of tail at the hind end.

The larvae are extremely resistant creatures, resistant to insecticides, resistant to adverse temperatures and humidity and resistant to starvation for months and year. Besides the inbuilt protection in the form of hair and a hard waxy covering on the body surface, the larvae are in the habit of seeking refuge from an adverse environment in various cracks and crevices, concealed places behind broken plasters, in seams and meshes of jute bags, etc., and they can remain alive in such niches for several years without food.

In this stage, this insect is probably one of the few most resistant insects. Fortunately for us, however, this extreme resistance gets lost rather suddenly the moment the larva changes even to the prepupal stage. The pupal stage generally lasts for only one to three weeks and the adults are ready to start the next generation very soon after emergence. The whole life-cycle can be completed under favourable conditions in so short a time as four weeks but this period can get prolonged to as much as four year under unfavourable conditions.

This pest is most resistant to low humidity and low-moisture content of grain and unlike the rice weevil and Rhizopertha, this can breed in grain with so low a moisture-content that the drying of the grain as recommended for the other two pests is of no avail for checking Khapra infestation.

On the other hand, the weakest spot in the life economy of the Khapra beetle is that unlike the other two pests, it is highly susceptible, particularly in its egg, pupal and adult stages, to reduction in oxygen tension. Due to this peculiarity, the Khapra beetle is generally a surface-feeder and it is not able to breed beyond a particular depth below the grain surface. This characteristic can be best exploited for its control because an airtight container is quite a dependable insurance against damage by this pest.

As soon as the oxygen in an airtight container is even partially consumed due to respiration by grain or by insects, if the infestation in there, and the oxygen percentage in the air is reduced from about 21 to about 16 the environment inside the airtight container becomes unfit of Khapra breeding. Of course, if the container is opened frequently and fresh air is allowed to get in, then Khapra-breeding will not cease.

4. Sitotroga Cerealella (Olivier):

This grain and flour moth was reported for the first time in the Angoumois province of France hence known as Angoumois moth also. It is a noted pest of unmilled stored grains like wheat, maize, sorghum, barley, rye, Jwar and oat. It has a world-wide distribution and prefers mountainous areas where the climate is mild. It is reported from the foot hills of Himalaya in U.P. mostly on wheat store houses.

The larvae are the main destructive stages which cause damage to the kernels of the grain. The damage is maximum in the rainy season when there is dark damp and cloudy weather. The larvae enter the grain and grow inside the grain. The tunnel in the grain is filled with the excreta as well as secondary infections also attack in the case of severe infestation. Larvae close their entrance by silky web and a cluster of grain is formed. They generally infest during the milky stage of the seed and during the harvest period. In a bag generally outer or peripheral amount is damaged much.

The moth is a buff coloured, yellowish brown or dirty brown in colour. The adults are winged which is narrow and fringed. It is 1.5 cm. across the wing. The females have a longer life span than male and mate within 24 hours after their emergence.

The female lays eggs singly, in two or three in the cracks and crevices, holes of the grain or in the store houses where suitable place is seen-About 150-200 eggs are laid during the life. The eggs are small white or reddish in colour depending upon the time of egg laying and age of the eggs. The egg surface is ornamented with rounded tips and oval shape. They have incubation period of 4-8 days depending upon the temperature and humidity.

The newly hatched larvae migrate and bore into the seed. The larval period lasts for 2-3 weeks. A full grown larva is .5 cm. long cream coloured or dirty white with a shining brown black head.

The pupae are formed in the silken puparium. The pupa is bright red brown in colour. Pupal period lasts for 6-10 days.

The best control for this pest is completely dehydrated and dried grains should be stored. The store houses should be clean and without infestation.