Insects have highly varied habits and habitats. Their size ranges from a millimetre to about 12 inches long and their food and feeding habits are equally variable. Insects are known to occur in subzero temperatures to the inhospitable hot springs. Owing to such varied preferences, insects’ rearing requires specific techniques for different types of insects. Therefore, it becomes important to be aware of the basic requirements of the insect species before it could be reared under the laboratory conditions. Generally the following 4 requirements should be taken care of in order to develop a successful rearing technique for a particular species.
1. FOOD: For phytophagous insects, the host should be found out and the plant material required by the insect should be placed in the cage. Food supply should be fresh and changed everyday to prevent decaying. Unused food material as well as the faecal matter should be removed everyday and the cage should be cleaned. Different species require different types of food material, which should be provided in sufficient quantity throughout the period of rearing.
For example, Papilio demoleus can be reared on citrus leaves, Erias vitella on lady fingers and Leucinodes by providing it with brinjal fruit. In some cases an artificial diet, if available, works very well if natural food is not available in certain season. Polyphagous species can generally be reared on cruciferous or leguminous vegetables as for example Heliothis armigera and Spodoptera litura can be reared on a variety of vegetables and pulses.
2. TEMPERATURE: Majority of insects breed at temperatures ranging from 25-35oC and undergo diapause at lower temperatures. However, there are species, such as Heliothis armigera that can be reared at room temperature even in winter months.
Successful rearing can be done during the natural breeding season of a particular species when conditions are optimal. But on small scale insects can be reared in rooms having temperature control devices like BODs. Special care has to be taken while rearing temperature-sensitive species. Stored grain pests can generally be reared in small spaces where temperature can easily be maintained. For some species lower temperatures and high humidity has to be maintained in summer.
3. HUMIDITY: A vast majority of insects prefer high humidity (80-90% RH) for their natural development. Humidity can be increased in cages by spreading water-soaked sponge or wet sand at the bottom of the cage, particularly in summer. A large number of insects are easier to rear in rainy season due to naturally available optimum temperature and humidity.
4. SPACE: Space requirements differ according to the species to be reared. For example, while butterflies and moths require plenty of space for nuptial flying, stored grain insects can breed in small chambers over even in glass jars. Cages are, therefore, designed keeping in mind the specific space requirements of the insects to be reared. Generally larger and flying insects require more space as compared to the smaller species. Some species are cannibalistic and must be reared in separate chambers.
An account of rearing methods of some insects of economic importance is given below.
Rearing field-collected larvae to adult stage
Field-collected larvae of plant feeding insects can be placed in a jar or wide-mouthed bottle along with the feeding material from which they were collected. The opening of the jar should be closed with a muslin cloth for sufficient aeration. Feeding material should be changed everyday and the container cleaned to avoid fungus growth and other infections.
The larvae will grow and enter pre-pupal stage, when they will stop accepting food and will restlessly look for suitable place for pupation. No feeding material should be provided after that and the container should be left clean. Sometimes bits of crumpled paper or corrugated cardboard placed inside the container will help the searching caterpillars to find suitable places for spinning cocoons.
Pupae should be kept at suitable temperature for proper development. Adults will emerge after completion of the pupal period. Adults may be fed on 30% honey and water solution if required to be kept alive for egg-laying. Many species can survive and mate in captivity and lay eggs on the host plants provided in the cages. Exopterygotes are easier to rear as their nymphs as well as adults require same food and habitat and need lesser care. Gregarious insects such as aphids, hoppers and bugs can easily be maintained in large numbers in cages.
Multipurpose Rearing Cage
Multipurpose rearing cage is designed to breed various kinds of insects in laboratory. It contains a cuboid wooden frame with a firm wooden bottom and a sliding glass door in front as shown in the diagram. Wire mesh is fixed on the back and the top sides, while on the two lateral sides, muslin cloth with a sleeve attached to it in the middle is attached, so that hands could be inserted through the cloth sleeves and insects could be easily handled inside the cage without danger of their escaping out.
Sand or sponge could be spread on the floor and moistened periodically to provide optimum humidity to insects. A clay pot with host plant can be kept inside the cage by opening the front sliding door. Even twigs dipped in a water container can be used as food for larvae. Feeding material should be changed everyday and faecal material and dried plant material should be removed from the cage. Many types of insects, particularly the phytophagous species can be bred in this type of cage for several generations and a culture can easily be maintained for experimental purposes.
Rearing insects on potted plants
Some insects do not require much caring such as aphids and hoppers, and can be easily maintained for several generations by holding them on potted plants. Three or four sticks can be inserted in the soil of the clay pot vertically and a muslin cloth should be wrapped around these sticks, so that the insects are confined to the plant and are not allowed to escape.
Also a cloth barrier will protect them from predators. Only small insects can be reared by this method, such as aphids, leafhoppers, scale insects, mealy bugs and small beetles, whose space requirement is minimal. Sometimes Mylar plastic can be used in place of muslin cloth but there should be provision for aeration. This method is also commonly used to set up field experiments under natural conditions.
Rearing stored grain insects
Insects infesting stored grains, e.g. Trogoderma, Ephestia, Corcyra, Sitophilus etc. are comparatively easy to breed in captivity, as their requirements for food, space and humidity are not as rigid. Almost all of them can be easily reared through successive generations in wide glass jars in which food material has been filled to about half and the opening covered with muslin cloth.
Almost no further caring is required till the food supply is exhausted. However, low temperature induces diapause in stored grain insects and therefore suitable temperature should be maintained in winter months. The culture jars should be placed inside the water-filled trays to keep ants away from contaminating the cultures. Stored grain insects can be maintained in large numbers with minimum of efforts and are ideal experimental animals.
Rearing houseflies
In nature houseflies breed in cow dung or other decaying matter but in laboratory they can be reared on a paste made of milk powder or agar powder with a little yeast added to it. The mixture should be allowed to ferment for at least 24 hours. A tray containing this type of breeding medium can be placed in any type of cage in which adult flies can be confined for egg laying. Adult flies can be collected from the field with a sweep net and released inside the cage.
They will readily lay eggs in the rearing medium, after which they can be released. Whitish maggots will hatch from the eggs within 24 hours and will move about actively in the medium to feed and grow for about a week and then look for dry places for pupation. They will normally come to the surface and pupate in dry corners. Pupae are dark brown in colour and barrel-shaped. Adults will emerge from these pupae after 3-4 days. All stages of flies can be collected from such a cage and can be used in experiments.
Rearing mosquitoes
Mosquito larvae called wrigglers live in water where they feed on micro-organisms. Although they are aquatic and bear tracheal gills, they must surface frequently for breathing air. Adult females after emergence require blood-feed for the growth and maturation of ovaries. In laboratory, mosquitoes are reared in a large cage, preferably made of muslin cloth supported by wire skeleton.
A tray containing water is placed in the cage and bits of bread crump or wheat bran is added to water so that fermentation of this material will produce microorganisms in water, which will serve as food for the mosquito larvae. Pupae called tumblers also swim about actively in water but do not feed. The tray containing pupae should remain inside the cage for adult emergence. Adults emerge from the pupae and females must be fed on blood before they can lay eggs.
Males can be fed on 30% honey and water solution so that they can survive and mate with the females. In order to provide blood meals to the female adults, either rats or pigeons are commonly used which should be shaven or defeathered on some body parts for easy feeding by mosquitoes. A separate cage can be used to provide blood-feed or if the cage is sufficiently large, the animal can be placed within the same cage for about an hour. Care should be taken to tie the legs and wings of the animal before keeping it in the cage so that it is not able to run around in the cage and disturb or destroy things or cut holes in the cage.
Field-collected adults can be used to start the culture. But if pure cultures of a particular species are required, then the field-collected adults should be identified and sorted to species level before introducing them into separate cages. In winter higher temperature has to be maintained for the success of the culture.
The above rearing techniques cover majority of insects of common interest but specially designed cages are used for rearing particular insects for specific needs or for experimental purposes according to specific requirements.