Tuesday, April 16, 2019

Biology and ecology of some major insect pests of stored crops

In this unit you would be studying the biology and ecology of some insect pests of economically important stored crops with emphasis on crops grown in Nigeria.
At the end of the unit, you should be able to:
* mention some storage insect pests of some crops
* discuss the biology of these pests
* state the roles the pests play in the storage of the crops

3.1 Insects as storage pests
A lot of crop losses occur due to storage pests. They could account for up to 80% of damage to crops in storage
3.2 Introduction
Insects are a very serious problem in the storage of crops. Due to the low level of literacy by our local farmers there is the tendency for misuse of synthetic pesticides which could lead to a myriad of problems. The knowledge of the pest’s biology and ecology would assist in the prevention and reduction of storage pest problems.
3.3 Objectives
At the end of the unit, you should be able to:
* Mention some storage insect pests of some crops
* Discuss the biology of these pests
* State the roles the pests play in the storage of the crops
3.4 Stored produce of economic importance in the tropics
We would be looking at the life cycle of insect pests associated with some important storage crops namely: rice, (cereals) and groundnut (legumes).
3.5 RICE (Oryza glaberrima and Oryza sativa)
Oryza glaberrima is native to West Africa and is normally grown as a dry land crop with seasonal flooding. Oryza sativa is grown as either a dry land or irrigated crop. Rice is eaten by man, cattle and poultry and is also used in the manufacture of starch and beer. Rice has good storage qualities and can be moved from one place to another easily.  It is attacked by rice weevil and maize weevil in storage.
Rice Weevil (Sitophilus oryzae)
Maize weevil (Sitophilus zeamais)
(Coleoptera: Curculionidae)
They are some of the most destructive primary insect pests of stored grain. Both adults and larvae feed on the grain, which may often be damaged beyond use. They are most active under warm humid conditions.
Sitophilus zeamais is primarily a pest of maize but will attack rice, sorghum and other stored grains. Infestations can begin in ripening crops in the field and continue in the store. Sitophilus oryzae is primarily a pest of rice but will also attack maize, various cereals and their products, biscuit and pulses. This species is less likely than S. zeamais to infest ripening crops in the field. Both have been found in dried cassava.
DAMAGE: Adults and larvae feed on grains. Attack may start in the field and continue in the store. Larva tunnel and feed within the grain. After pupation adults cut 1.5mm diameter circular holes in the grain through which they emerge. Attack leaves the produce susceptible to moulds and caking. The grains are also tinted with insect excreta.
LIFE CYCLE: Females lay up to 150 eggs during their adult life. Oviposition occurs at temperatures between 15-35oC and at a grain moisture content of above 9.5%. The adult female bores a hole in the grain and deposits a single egg.. The hole is then sealed over with a gelatinous fluid. Each female lay 2-3 eggs per day during her life. Eggs hatch after about 8 days into legless, 4 mm long larvae. The larvae feed and develop over the next 6-8 weeks within the grain. They moult four times during this time. In small grains only one larva maybe present but larger grains such as maize can support 2-3 larvae in a single grain. Pupation takes place in the grain and can last 5-16 days. The adult emerges through a circular hole which it has cut out of the grain. The holes are characteristics of this pest. Adults may live for up to six months depending on conditions. There may be up to 7 generations in one year.
Non-Chemical Control: Sitophilus sp. cannot breed when grain moisture is 9% or less. Therefore grain stored in clean dry conditions is less likely to suffer attack. Infestation of stored maize can be reduced by storing it as unhusked cobs.
Chemical Control: Fumigation followed by storage in insect proof containers is also effective. Grains can be treated with pirimphos-methyl by spraying prior to storage to kill weevil infestations, or admixed with pirimphos-methyl dust.

Life cycle of rice weevil Adult rice weevil

Adults of both rice and maize weevil

GROUNDNUT (Arachis hypogaea)
Groundnut or peanuts as they are sometimes called are one of the most valuable legume crops of the tropical and sub-tropical countries. They are exported mainly for their oil. It is also used in the making of margarine, soaps, cooking and salad oils. It is attacked by a number of pests which include, Groundnut Bruchid, Khapra beetle, Rust Red flour beetle, Confused flour beetle and Tropical Warehouse moth.
GROUNDNUT BRUCHID (Caryedon serratus) (Coleoptera: Bruchidae)
This insect is a serious pest. It is regularly found in groundnut-growing areas where nuts are attacked as soon as they are harvested and left to dry. Groundnuts are especially susceptible when stored in the shells. The insects can also breed on tree legumes including tamarind and locust bean.
DAMAGE: Larvae bore through the groundnut shell and enter the seed where they feed and develop. Before pupation larvae cut an exit hole, 3 mm in diameter to escape from the seed. Damage caused by the insect increases with each generation and after 13 weeks in store weight loss of seeds can be up to 38%. The product becomes contaminated with insect excreta and is unfit for human consumption. Adult insects do not cause damage.
Non-Chemical Control: Drying seeds to a moisture level below 9% deters beetle attack.
Chemical Control: Pirimphos-methyl, give effective control. Stores should be inspected at regular intervals and any insects present controlled as soon as possible before the population increases and causes serious damage.

Adult Caryedon serratus
KHAPRA BEETLE (Trogoderma granarium Everts) (Coleoptera: Dermestidae)
It is a serious pest of oilseeds, previously damaged cereals and some pulses. In Nigeria, they are pests of groundnuts especially in the north. It is found mainly in large scale stores. It can survive at a moisture content of less than 2%. Once this pest becomes established in a store it is difficult to remove.
DAMAGE: The larvae bore into grains where they feed leaving the grains hollow. The insects also contaminate the products with their moults and frass. Oil extracted from infested groundnut is also contaminated.
LIFE CYCLE: Females lay about 40-70 eggs over a period of 3-12 days, depending on the temperature. Larvae take three weeks or more to develop and go through 4 or 5 moults. They live within the grain or hide in small crevices in the storage container. Two types of larvae are produced: those that are able to diapause and those that are not. In adverse conditions diapausing larvae leave their food material and seek out a sheltered position, such as crevice. While diapausing, the larvae are very resistant to effects of contact insecticides and fumigants. When diapause is complete the larvae will pupate. Pupation lasts for 3-5 days. Development from egg to adult usually takes about 4-6weeks, but under poor conditions can take as long as a year.
Non-Chemical Control: Storage areas should be free of crevices where the insects can shelter. The hair on the larvae allow it to become attached to sacks used for transporting the grain. Therefore sacks made from synthetic materials rather than rough fibres should be used for moving grain.
Chemical Control: Large scale stores can be treated by fumigation with methyl-bromide using appropriate precautions.

Adult Trogoderma granarium
3.6 Conclusion
Insects cause a lot of damage in storage which leads to losses in storage. To prevent these losses, the understanding of their life cycle will help in the prompt control of these insects.
4.0 Summary
It is expected that at this stage you would be able to mention some storage pests of some economic crops and their control. You should also be able to use the life cycle of these insects in their control.
5.0 Study Questions
1. Mention one crop of economic importance and list two insect pests associated with it.
2. Discuss the chemical control Caryedon serratus
3. Discuss the life cycle of Trogoderma granarium
4. Discuss the damage caused by Sitophilus oryzae

6.0 Further Reading

University of Sydney.
Unit 1. Methods of controlling insect pests in the field and store
Types of insect pests control can be categorised into
1. Cultural control
This implies the purposeful manipulation of the environment to make it less favourable for insect reproduction and feeding. Cultural methods are based on modifications in time and manner of performing necessary operations in the production of crops etc. it is one of the cheapest methods of control available
This includes the use of various farm practices which directly or indirectly reduce pest populations. e.g.
a) Simple sanitation:-burning of refuse thereby destroying places in which the over wintering organisms may live. The proper care of garbage disposal e.g. when manure is cleaned up, housefly growth is retarded. Good drainage will help to control mosquitoes.
b) Soil tillage:-this will expose the ground insects to risks which lead to their destruction e.g. mole crickets.
c) Crop rotation:-the rotation of crops at the proper time will help to control pests of that crop. Timing of planting and harvest will often mean the between an average or a good yield.
d) Use of resistant varieties/planting date:-we can use varieties of crops that are resistant to pests so as to minimize damage. There are times the resistant varieties are combined with planting date to achieve efficient results e.g. Mayetiola destructor called the Hessian fly or barley midge almost wiped out wheat in USA but it was successfully controlled by using a resistant variety of wheat along with late planting.
2. Mechanical/Physical Control
This includes the simple fly swat, fly screen and mosquito nets, light traps ('zappers'), exclusion methods such as packaging and sealing (e.g. against storage pests), sifting and separation in flour mills and the use of temperature, humidity and gas regimes against storage and museum pests. It also includes drainage against mosquitoes and removal of bushes against Tsetse flies
3. Biological Control
Biological control implies the use of natural enemies to control insects. This includes the application of available predators, parasites or diseases, either natural, introduced or commercially available. e.g. Introduction of Australian ladybirds to USA citrus, spray applications of bacterial diseases against caterpillars.
They have been used with a high success rate e.g. the scale insect is a pest of cotton and it also feeds on citrus. It almost wiped out citrus in the USA but it was successfully controlled by using the ladybird beetle insect.
1. each natural enemy is specific for what it is to control
2. it is non-hazardous i.e. it doesn’t constitute any danger to the environment
3. it is economical when it is established
4. it is a more permanent kind of situation
1. it has limited usefulness i.e. limited to those organisms that have this natural predators
2. it has limited success because the natural enemies have to be identified and it has to be within ones reach.
3. in most cases it is very slow in action although it gives a more permanent result.
4. the cost is very high

4. Genetic Control
This includes male sterilisation techniques, selective breeding and genetic modification.
Male sterilisation techniques involve the mass rearing of a pest, laboratory sterilisation of males and their release into the wild with the purpose of swamping the wild male population leading to infertile egg laying. For this strategy to work, the females need to mate only once and not to be able to select against the sterile males.
This method worked extremely well with screw worm flies (the larvae of this insect is a very serious pest of cattle), leading to its complete eradication in USA. However, the same method did not work on sheep blow fly in Australia as the females selected against the sterilised males. The use of sterilant have certain specific advantages over the use of chemicals.
Advantages of sterilant over chemicals (insecticides)
Let us assume that a chemical is applied and it kills 90% of the male and female population, leaving 10 males and 10 females to mate and continue the next generation. Suppose on the other hand a sterilant with the same effect is applied, this means we would have 10 fertile females subject to the mating competition from 10 fertile males and 90 sterile males. Only one fertile female is left to produce the next generation.
There are certain criteria necessary for its success namely-
1) we are assuming the female will mate only once in a life time which is a right assumption
2) we are assuming the sterile males will enter into competition with the fertile males
3) we must have as much as possible, a geographically isolated population.

5. Chemical Control
Chemical control includes both behaviour modifiers and insecticides.
Behaviour modifiers
Attractants - cause insects to move towards their source. Examples include
Pheromones - secreted by insect, species specific - may be used to lay trails, or for aggregation, swarming, alarm or sexual attraction.
Food and oviposition attractants. E.g. Old fruit fly is attracted to ammonia (NH3) , flavouring essences and protein sources
Antifeedants – these prevent an insect from feeding and it subsequently starves to death
Antioviposition chemicals – they prevent female insects from laying eggs.
Repellents – these are used to ward off insects from where it is applied e.g. personal fly repellents
6. Attractants
Monitoring of pest situation. Traps give an early warning system in quarantine situations and are used in crops to monitor pest intensity to indicate when to apply insecticides e.g. coddling moth control.
Direct control
When baits are combined with a lethal trap or insecticide e.g. dark pots against the common fruit fly Drosophila melanogaster in orchards.
Mating disruption occurs when the environment is saturated with mating pheromone so that the male is confused and cannot find a mate. (used in control of oriental fruit moth or peach moth Grapholita molesta).

No comments:

Post a Comment