Factors Influencing Insect Abundance PDF

2. FACTORS FOR INSECT’S ABUNDANCE Dr. R. B. VADHER Associate Professor College of Agriculture Junagadh Agricultural University Khapat- Porbandar Factors influencing population growth a) Biotic factors or density dependent factors or organic factors b) Abi...

2. FACTORS FOR INSECT’S ABUNDANCE Dr. R. B. VADHER Associate Professor College of Agriculture Junagadh Agricultural University Khapat- Porbandar Factors influencing population growth a) Biotic factors or density dependent factors or organic factors b) Abiotic factors or density independent factors or physical factors  BIOTIC FACTORS The biotic environment includes individuals of same species or of the other species. The vital process such as growth, nutrition and reproduction depend upon the interaction between the individuals of the same species (intraspecific) or between those of different species (interspecific). The interaction or relationship between species may be beneficial to both, harmful to both or beneficial or harmful to one and neutral to other.  INTERACTION BETWEEN ORGANISMS 1) COMPETITION: The members of an insect species are likely to be competing with one another or with members of another species for limited resources like food, mates and suitable site for oviposition or pupation. Such competition operates whenever the population is increasing and the resources are limited. A. Intraspecific competition: Population of the same species competes for a resource is called intraspecific competition. It consist of aggregation of individuals of same species, concurrence, cannibalism, association of sexes, parental care and social life.  1. Aggregation Tendency of some insects to concentrate in numbers larger than found in normal distribution. They congregate under various circumstances mostly by their own movements. With the result they modify the micro climate as well as bring about changes in morphology, physiology and ecology of aggregating members and acting as very effective biotic factor.  2. Concurrence The majority of insects are solitary and live for themselves while others assembled together in mass during a part of whole of their life for hibernating, feeding or mating. The living in such groups is probably for protection which may neither be harmful nor beneficial to the species but under extreme conditions of overcrowding it may lead to struggle for the same requirement. Therefore, concurrence is harmful to the species as it results in reduction of food, space and contamination of food occurs through excretory products of crowded individuals.  3. Cannibalism Overcrowding between members of same species and exhaustion of food results in cannibalism. Cannibalism, also known as intraspecific predation, is the process of killing and consuming an individual of the same species. e. g. Helicoverpa armigera (Bigger larva feeds on smaller one) 4. Association of the sexes Sex ratio of insect plays a vital role in the buildup of their population. In case of housefly if the sex ratio is 1:1 at ordinary temperature then 7 there will be 2 x 60 flies at the end of breeding season. While at sex 14 ratio 3:1 (female: male) 3 x 30 flies will be produced.  5. Parental care It is well known that all insects take care that the eggs are laid in situations where conditions are favorable for their hatching as well as presence of sufficient food for newly hatched young ones. There are some insects which lay their eggs in protected places or cover with foreign materials or prepare brood nests to protect them from natural enemies. The cricket, Gryllotalpa and earwigs establish their brood nests in underground chambers in which mother lives with the eggs. Time to time, she picks up the eggs, licks them, clean the fungal spores and fights in defense of them.  6. Social life The honeybees, termites, ants and wasps have developed social life which is good for the colony members. This relationship is not for only living together in large number but is also involves the division of labor among the members for the welfare of the community as a whole. For example, in termites, the queen lays only eggs in her whole life, the workers attends all sorts of duties, including care of the eggs, young ones and of the royal pair (King and queen), building and cleaning the termitorium, growing of fungus gardens and procuring food. The soldiers protect the colony.  Interspecific competition The competition occurs between members of two or more species. Two or more competing species with identical requirements cannot coexist in a same place for a long time. e.g. when flour beetles Tribolium castaneum and Tribolium confusum were grown in the same jar of flour, one species eliminates the other. Under high temperature and RH conditions T. castaneum eliminates T. confusum and vice versa under low temperature and RH conditions. Odum (1971) described all types of relationships between organisms in to two groups viz., “positive” and “negative” interactions.  A. Positive interaction Symbiont: Both insects and its host live in close union without harmful effects (i.e. symbiosis) each member being known as symbiont. Commensal: One insect is benefited by living on or inside another insect without injuring the other and is known as commensal and it lives on the surplus food or the waste food of its host. The staphylinid beetles inhabit nests of Hymenoptera and utilize their stored food. When the commensal uses its host as a means of transport the phenomenon is termed as phoresy. e. g. Telonomus beneficiens parasitoid attaches themselves to the anal tufts of female moths of rice stem borer Scirpohaga incertulas (Walker) for their transport. The parasitoid parasitizes freshly laid eggs.  Mutualist: When both the symbionts are benefited by the association it is known mutualism e.g. Insect-insect mutualism: Ants and aphids, coccids. The ants provides shelter to these insects and they in turn supply honey dew to the ants which is relished by them as food. Similarly, scarabid beetles are met within the nests of termites. Insect-animal mutualism: termites and flagellate protozoans. Termite feeds on wood which being practically pure cellulose cannot be digested without the aid of flagellate protozoans present in their gut. The waste products after the cellulose is digested serve as food for the termites. Thus one cannot survive without other. Insect-plant mutualism: Honeybees visit flowers for obtaining nectar and pollen for their own use and doing so accidently carry pollen from one flower to another and bring about cross pollination.  II. Negative interaction (PREDATORS AND PARASITES): Predators: Predators are free living organisms that feed on other animals, their prey, devouring them completely and rapidly. If predator feeds on only one species of prey it is called obligate predator. If it feeds on more than one species then it is called facultative predator. More than one individual of prey required for predator to reach maturity e.g. lady beetle, Chrysoperla, syrphid fly, birds, fish, amphibians, reptiles, mammals and arthropods  Parasites An organism that is dependent for some essential metabolic factor on another throughout its all life stages. A parasite weakens or kills the host while feeding. Many parasites on a single host require only one part of one host to reach maturity. e.g. Mosquitoes, Trichogramma spp. When one individual host is attacked by two or more individual of same species of primary parasite the phenomenon is called super parasitism. When one individual host is attacked by two or more species of primary parasite the phenomenon is called multiple parasitism. Sometime a primary parasite is itself attacked by another parasite which is called hyper parasite and phenomenon is termed as hyper-parasitism.  Parasitoid An insect parasite of an arthropod that is parasitic in its immature stage killing the host in the process of development and adults are free living. e.g. Trichogramma spp., members of Tachinidae family (Diptera)  FOOD Insect depends directly or indirectly on plants for food. The quantity and quality of food play an important role in insects’ survival, longevity, fecundity, distribution, reproduction, speed of development etc. The availability of food to a particular population is influenced by number of individuals of the same species per unit quantity of food, individuals of other species and environmental factors. Quantity of Food Short supply of food causes intraspecific and interspecific competition Also affects parasitoids and predators of insects hosts whose food is of short supply Quality of Food This depends on nutritional availability of plants Crop varieties/species differ in nutritional status which affects insects Insects are heterotrophic; they cannot synthesize their own food but depend on plants/animals for food. Thank you