AB 2111 - POST HARVEST BIOLOGY

Effect of Light on Growth and

Development of Stored Product

Insects

AG/12/FT/001 - W.H.G.S.Bhagya

AG/12/FT/007 - S.A.I.Jayasiri

AG/12/FT/009 - K.G.U.M.Kosgolla

AG/12/FT/012 – N.H.G.T.Madushani

AG/12/FT/014 – K.V.S.Manoj

AG/12/FT/017 – S.R.C.Perera

AG/12/FT/024 – W.A.G.E.Wijelath

Date: 12/02/2015

Practical No: 01

Title: Effect of light on growth and development of stored product insects.

Objectives:

Assessment of population growth of Rice weevil (Sitophilus oryzae) when the

populations grow under different coloured lights.

Introduction:

After Harvest, Agricultural grain commodities remain vulnerable to damage by insects and

the economic costs of this damage increase as raw grain is turned into value-added products

while moving through processing and distribution channels. Costs beyond direct damage

also can accrue when insects infest processed commodities, including loss of customer

good will, health hazards associated with allergens in food, sanitation and chemical

treatments, and the consequences of unsatisfactory food safety inspections. A stored food

product can become infested anywhere during the process from production until it arrives

to home. However, stored food is most likely to become infested in stores or in homes.

Most of the stored food insects also are pests of stored grain or other commodities and may

be relatively abundant outdoors. Food products that are left undisturbed on the shelves for

long periods are particularly susceptible to infestation. However, foods of any age can

become infested.

Some of most common stored product insects are Indian meal moths (Plodia

interpunctella), Meal moths (Pyralis farinalis), Drugstore beetles and cigarette

beetles (Lasioderma serricorne and Stegobium panicum), Flour beetles (Tribolium

confusum and T. castaneum), Warehouse and cabinet beetles (Trogoderma spp.), Granary,

rice, and maize weevils (Sitophilus spp.) and Bean weevils (Acanthoscelides obtectus). In

this practical we observed the effect of light on growth and development of Rice weevil

(Sitophilus oryzae).

Rice weevil, Sitophilus oryzae, (Coleoptera, Curculionidae) attacks rice, maize, and other

small grains. Adult lays eggs on the grain. Legless grub feeds inside the grain and pupates

within the grain. Adult is small, reddish brown weevil which has a prominent snout. It

completes the life cycle within 26-32 days depending on the temperature.

Growth of an insect population is affected by biotic and abiotic factors. Among the abiotic

factors temperature and humidity are the most important factors. Light: intensity and

colour, also effects on population growth. Colour of the light can be easily manipulated to

manage the population growth of stored product insects which has a practical applications

in proper packaging and proper storage.

Materials:

Rearing containers (glass), black colour papers, cellophane sheets (red and yellow), seeds

(rice), tally counter, dissecting microscope, electronic balance and adult Rice weevils

(Sitophilus oryzae) were used in this practical.

Method:

The containers were cleaned and dried. The cellophane (red and yellow) and black papers

were attached around the containers to prepare treatments as follows:

Treatment 1 (T1) – Control

Treatment 2 (T2) – Containers with black colour papers

Treatment 3 (T3) - Containers with red cellophane

Treatment 4 (T4) - Containers with yellow cellophane

Five containers were prepared for each treatment. The rice seeds were cleaned using a cloth

and 150 g of rice seeds were added per container. The adult Rice weevils were observed

under the stereo microscope and healthy, active and approximately similar body sized

individuals were selected from the given infected grain sample. Ten insects were

introduced for each container and the containers were closed with keeping cloth in place

using rubber bands. The containers were stored at the room temperature covering by a glass

structure. The set up was dismantled after 4,8,12 weeks and counted the number of adult

insects. The damages were observed visually. The dead Rice weevils were removed and

other insects were placed with same seeds in the same container and the experiment was re

settled.

Observations

Table 1: The variation of the population of rice weevil with time

Replicate No. 1st week 4th week 8th week 12th week

T1R1 10 10 10 31

T1R2 10 11 27 58

T1R3 10 15 16 38

T1R4 10 11 16 30

T1R5 10 12 12 41

T2R1 10 8 8 18

T2R2 10 10 15 47

T2R3 10 11 17 55

T2R4 10 12 18 35

T2R5 10 11 8 26

T3R1 10 10 22 55

T3R2 10 10 14 45

T3R3 10 10 9 9

T3R4 10 10 11 29

T3R5 10 10 11 17

T4R1 10 11 10 2

T4R2 10 12 12 43

T4R3 10 12 9 23

T4R4 10 11 17 39

T4R5 10 10 10 23

Results

When the readings obtained was analyzed using SAS analysis software, the following

results were obtained.

4th week

t Grouping Mean N TRT

A 11.8000 5 1

B A 11.2000 5 4

B A 10.4000 5 2

B 10.0000 5 3

8th week

t Grouping Mean N TRT

A 16.200 5 1

A 13.400 5 3

A 13.200 5 2

A 11.600 5 4

12th week

t Grouping Mean N TRT

A 39.600 5 1

A 36.200 5 2

A 31.000 5 3

A 30.400 5 4

Using the above results, a graph was drawn using Excel 2013 as follows.

Discussion

The name rice weevil suggests that this beetle was primarily associated with whole grain

rice early on. As the term weevil indicates, this insect belongs to the family Curculionidae,

known as snout beetles because of the narrowed, forward projecting head and tiny

mandibles. The rice weevil is considered by many to be the most important stored product

pest. It has worldwide distribution but primarily in warm climates. In the United States, it

is usually confined to stored grain north of North Carolina, but is widely distributed in field

and stored grain in the south.

The rice weevil is usually confined to stored grain north of North Carolina. In the south,

adults fly from stored grain to infest the new grain crop in the field and the infestation

continues through storage. Rice weevils occasionally are delivered to pet shops and super

markets in birdseed containing infested sunflower seeds. Therefore, rice weevils are

1 4 8 12

T1 10.00 11.80 16.20 39.60

T2 10.00 10.40 13.20 36.20

T3 10.00 10.00 13.40 31.00

T4 10.00 11.20 11.60 30.40

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

40.00

45.00

Tota

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Week no.

Variation in Total Population with Time

T1 T2 T3 T4

sometime brought into the home in birdseed. It has been recorded attacking corn, wheat,

rice, beans, nuts, cereals, rye, buckwheat, stored cotton, wheat products of all kinds and

grapes. In addition, it will feed on apples and pears. Optimum conditions for rice weevil

activity are 80 to 86°F, 75 to 90% relative humidity and grain of 13.5 to 17.6% moisture

content. When disturbed, it will draw its legs up to its body and play possum. Adults can

fly and are attracted to lights.

Figure 1: The lifecycle of rice weevil

The rice weevil female chews a hole into a kernel of grain and lays an egg, sealing the

opening with a gelatinous material. Females can lay 300 to 400 eggs in their lifetime but

egg-laying is sporadic during the wintertime, with less activity at cooler temperatures.

There are 3 to 4 instars (growth stages), which require an average of 18 days for

development. The pupa stage requires an average of 6 days (range 3 to 9) and upon

transformation, the adult insect will remain within the kernel for 3 to 4 days until it tans

(hardens) and matures. The life cycle (egg to egg) may be as short as 32 days in the summer.

The adult may live for 3 to 6 months.

The Rice Weevil is a small dark brown weevil with a long snout, and small round dimples

on the thorax and ridges along the wing covers. There are two paler reddish brown patches

at each end of the wing covers (four spots altogether). The head and thorax is nearly as

long as the wing covers. The larvae are white or creamy white grubs with a small light

brown head. Also the larval stage is legless and humpbacked. Pupa is white in color with

free appendages and its segments are movable.

According to the results obtained from the above experiment, initially there was a

significant difference in the population of rice weevils grown under normal light conditions

and under red light. But with time there was no significant difference in the population of

rice weevils grown under dark, red, yellow and normal light conditions.

Hence it can be observed that there is no impact of light on the growth and development of

rice weevils with time.

The abundance and distribution of organisms in an environment are affected by abiotic and

biotic factors. All living organisms have certain habitats zones that they are restricted by

abiotic factors, outside of this zone they cannot live for long time. Within this zone,

populations of organisms show typical density fluctuations within limits. Although the

habitat is suitable for many organisms at the same time some species could be more

abundant in an area than other species. This could be associated with how well these species

adapt to their environment. A high reproductive potential, tolerance to climate and ability

to compete with others are among the factors contributing to adaptive superiority.

Competition can take place for different limited resources such as food, mating partner or

territories in nature. Generally stored product insects have a short life cycle. They are most

destructive pests of stored products. Nine hundred million ton of cereal grain are stored

around the world at once time and that annual losses because of insects and rodents reach

about 30% in Asian countries.

It was reported that during the development period of weevils, the larvae consume about

50% of the total weight of grain. Germinating capacity of the seed is also lowered in

severely infested grains and grains infested by weevils become more susceptible to

infestation by other associated pests like grain like grain beetles and mites. This huge

amount of loss could be associated with their biology because their all four larval stages

and pupal stage occur inside the grain. Therefore their control and elimination in storage is

very difficult.

The damage of rice stored in the warehouses is the end result of the operation of several

factors which include the biotic and abiotic factors. One of the most important biotic factor

is the insect pest of stored products while temperature, relative humidity, light and aeration

have been identified as the abiotic factors that influence the quality of stored product. These

abiotic factors influence both the physical and chemically damages of rice itself and the

growth and development of the insect pests. Rice weevil is one of the common stored

product pest that is majorly attacked to rice. The influence of light factor on the growth and

development of rice weevil was studied in this practical.

It has been found by researchers that the insect’s body temperature is affected by the

surrounding condition. This phenomenon is known as poikilotherms. The increase of

temperature in the container to the near of the optimum temperature would affect the rise

of the development and population growth of rice weevil. Furthermore it has been found

that the relative humidity was another important factor that, either directly or indirectly,

affected the development of rice weevil. However it has been widely known that relative

humidity in the storage is positively correlated with the moisture content of the materials

stored. Therefore increasing of the relative humidity will increase of the water content of

the stored materials. As the most stored insect pests prefer high moisture content of grain

stored, the damage of the store product will be more extensive.

Rice weevil would have a short developmental time and high population growth if there

are suitable temperature and relative humidity conditions. Under these conditions this

insect would be a potential and important pest of grain storage products. The change of

climate conditions, therefore, would lead to a change the temperature and might affect the

growth and development of stored product insect apart from light. This may interfere on

the result which was obtained by analyzing the population while providing different colour

lights.

Conclusion

Initially the growth and development of rice weevils is retarded by red light. But at the

latter stages of the life cycle, there is no impact of light on the growth and development of

rice weevil.

References

Anonymous, Rice Weevil (Sitophilus oryzae) [online], Available

at:http://www.ozanimals.com/Insect/Rice-Weevil/Sitophilus/oryzae.html [Accessed:

01/06/2015]

Anonymous, Rice weevil Sitophilus oryzae (L.) [online] , Available at:

https ://www. Grainscanada.gc.ca / storage - entrepose/pip-irp/rw-cr-eng.htm

[Accessed:02/06/2015]