The pest populations have a tendency to fluctuate as a result of their inherent characteristics as influenced by the environmental factors. The degree of influence of various environmental factors determine the magnitude of increase or decrease in numbers of a pest population.
The rate of change in a pest population is determined by the fecundity, speed of development and are not inimical to survival, promote increase, and those having a reverse influence cause a decline in numbers. The same factor may be favourable in case of one population but may become unfavourable for another. It is thus necessary to consider the influence of various factors with respect to particular pest population.
The environmental factors may be grouped into two main categories, i.e. abiotic and biotic. Among the abiotic factors, it is primarily the physical factors such as temperature, moisture and light that have a direct influence on the populations of insect pests.
These factors also influence the pest populations indirectly by modifying the biotic factors. The biotic factors include food and other populations, primarily the natural enemies of pest populations.
Abiotic Factors:
The climatic factors exercise a dominating influence on the development, longevity, reproduction and fecundity of insect pests. It is well known that densities of pest populations fluctuate with the prevailing weather conditions such as temperature, moisture, light and wind.
Extremes of temperature, humidity or rainfall cause mortality among the pest and its natural enemies. The chances of an insect population to survive and reproduce first increase and then decrease as the population is exposed to unfavourable low range through the optimum into the unfavourable high range.
1. Temperature:
The insects are all poikilothermic, i.e., they have no precise mechanism for regulating the temperature of their bodies. Their body temperature, therefore, follows more or less closely that of the surrounding medium. There is a fairly well defined favourable range of temperature for every insect species within which it is able to survive.
This temperature range is determined by the prevailing temperature at which the normal physiological activities of the insect take place. This narrow band of temperature has been called the preferred temperature or temperature preferendum. Exposure to temperatures beyond the favourable range, whether low or high, may retard growth and development of the insect or may even cause its death.
The upper lethal limits are usually between 40° and 50°C, but some insects, such as stored product and desert insects, can withstand temperatures in the neighbourhood of 60°C. The lower lethal limits vary widely and may lie below the freezing point of water. In the laboratory, some insects have been reported to tolerate temperatures as low as -70°C.
The departure from the optimal range on both sides is tolerated to some extent, depending upon the physiological adaptations of the conceived populations. The reaction to changed temperature depends upon the suddenness of the change. In case of a gradual change, the insects become conditioned or acclimatized.
The rate of acclimatization is dependent on the duration for which they are conditioned. The total heat required for the completion of physiological processes in the life- history of a species is considered as constant, a thermal constant. Within the favourable range, the thermal constant is not affected by the level of temperature.
Exposure to lethal low or high temperatures may result in instant killing and even the survivors may fail to grow and reproduce normally. The duration of exposure to such a condition is important and the harmful effects of exposure to sub-lethal temperatures may be manifested at some later critical stage, i.e., ecdysis and pupation.
Some of the insect species when exposed to extremes of temperatures beyond the favourable range may become dormant, i.e., undergo hibernation or aestivation, which are reversible processes as the individuals may resume activity on being exposed to favourable temperature.
The insect populations can be grouped into three categories according to their responses to low temperature:
i. Those which cannot survive for any considerable time if the temperature falls below the lower limit of the favourable range. They cannot become dormant and hence must either develop or die. Such species have originated from the tropical or subtropical climates, e.g., locust.
ii. Species which have a stage in the life-cycle adapted to survive exposure to low temperature and the other stages resemble those in the above category in lacking the capability to become dormant at low temperature. Such species have originated from the temperate climates, e.g., Helicoverpa sp. and Agrotis sp.
iii. This group has a diapause stage and they have also originated from the temperate climates, e.g., many lepidopteran borers.
The species adapted to live in temperate climates, hibernate in a particular developmental stage and frequently this very stage and none other is capable of undergoing diapause. Some other species depend for their survival during winter on the insulating protection of the hibernacula such as debris, soil, plant remnants, snow and ice, and also on their ability to withstand exposure to low temperature through under-cooling.
The combination of these two protective mechanisms prevents mortality, but in the case of incomplete protection exposure to severe cold period may lead to eradication of the insect population. This may be more relevant in the case of introduced natural enemies.
2. Moisture:
A constant supply of moisture is essential for metabolic reactions as well as for the dissolution and transport of salts. The water content in insects varies from less than 50 per cent to more than 90 per cent of the total body weight. Variation occurs between different species and even between different stages in the life-cycle of the same species.
Soft bodied insects such as caterpillars tend to have comparatively large amount of water in their tissues, whereas many insects with hard bodies tend to have somewhat lesser amounts. Active stages commonly have higher water content than dormant stages.
The range of moisture required is not so broad as in the case of temperature. Most of the insect species are capable of maintaining their body water at fairly constant levels while living under varying conditions. For most of the species, food in the shape of plant or its products is the source of water, and they have the adaptation to cope with conditions of excessive moisture and shortage of water.
The other sources of gaining moisture are direct drinking of water or absorption through the integument. The loss of water from the body is prevented by insect cuticle having a waxy layer. A number of adaptations-morphological, biological and physiological-in nature help insect populations in overcoming unfavourable conditions of excessive moisture or acidity.
As in temperature, the phenomenon of humidity preferendum also operates in insects, and it helps insects to congregate in suitable places. The humidity preference is influenced by the prevailing temperature. Most adverse effects of moisture are due to its scarcity or absence.
Exceptionally dry air may prove lethal because some insect species may not survive the loss of even a small percentage of body water for a long time. Those in aestivation or diapause lose a large proportion of body water without adverse effects. One insect, the larva of an African chironomid midge, Polypedilum vanderplanki Hint, can tolerate dehydration and suspension of metabolism for several years.
The exposure to excessive moisture can prove harmful to insect populations in the following ways:
i. By adversely affecting the normal development and feeding activity of insects.
ii. By encouraging disease-causing microorganisms such as fungi, bacteria and mycoplasma, and thereby causing mortality among insects.
iii. Excessive moisture in insect body during winter reduces its capability to withstand exposure to low temperature and thus leads to an adverse effect on its cold hardiness.
Moisture also influences the speed of development and fecundity of most insect species. In some species, these activities are accelerated by excessive moisture, while in others they get retarded. Studies conducted in Punjab, India showed that sugarcane black bug Cavelerius excavatus (Distant), multiplied m ire rapidly at high humidity (90% R.H.), whereas relative humidity above 70 per cent was harmful for multiplication of cotton jassid, Amrasca biguttula biguttula (Ishida). For eggs and pupae of spotted bollworm, Earias insulana (Boisduval), both very high (around 100%) and low (<40%) relative humidities were not conducive for development of these stages.
3. Light:
Light is an essential ecological factor for many biological processes such as orientation or rhythmic behaviour of insects, bioluminescence, periodicities of occurrence and periods of inactivity. Light acts as a token stimulus by enabling insects to regulate and synchronize their life-cycles with the change in seasons.
Unlike temperature, light is a non-lethal factor and it has specific direction in its flow. A characteristic feature of light is its quantitative shift from a minimum to a maximum and vice-versa in a short time. The properties of light that influence insect life are intensity or illumination, quality or wavelength and duration of light hours or photoperiod.
The insects orientate to the source of light and thus reach the right place at the right time. Such phototactic behaviour of insects is altered and modified by a number of factors such as temperature, humidity or moisture and food. Several species of moths, leafhoppers and beetles are attracted to light at dusk or during the night, and this behaviour has been used extensively through light traps for observing brood-emergence or fluctuations in their populations. Direct sunshine may injure or kill an exposed insect largely because of heat and desiccation.
Photoperiodism influences the motor activity rhythms of insects such as locomotion, feeding, adult emergence, mating and oviposition, and also moulting and growth in some species. The reproductive cycle in most of the temperate-zone insects is so tuned that they reproduce only during favourable periods and the remaining period is passed in diapause state.
The induction of diapause, a genetically determined state of suppressed development and the manifestation of which is induced by environmental factors, is influenced by photoperiod in most of the lepidopteran insects. Photoperiodic responses in insects also influence polymorphism, ecological adaptations and phonological synchronization with the sources of food.
4. Oxygen and Carbon Dioxide:
Insects can tolerate a wide range of oxygen and carbon dioxide. Some insects can survive several days in the absence of oxygen by reducing their metabolic rates and utilizing the oxygen in their tissues. Excessive carbon dioxide causes varied reactions in different insects.
Some insects can live in an atmosphere with high carbon dioxide for several days. However, an excess of this gas in the atmosphere causes growth retardation in many insects. If the environment is high in carbon dioxide, the spiracles of insects tend to remain open, which may lead to excessive water loss.
5. Air Currents:
Air currents are of great value to insect displacement and, therefore, affect population changes by influencing the numbers into or out of an area. Most insects will not undertake flight when the speed of wind exceeds the normal flight speed. However, insects are rarely blown about at random but have evolved such patterns of behaviour which enable them to exploit the wind to achieve their migratory needs.
Strong flying insects tend to fly with the winds during migrations and are displaced long distances as in case of spruce budworm moths. Many insects which are weak fliers also have specific behaviour patterns enhancing their opportunity to migrate to specific areas with the help of wind. The air currents carry aphids, leafhoppers and scale insects to far-off places and thus are instrumental in their dispersal.
Air currents may be directly responsible for the death of insects. Firstly, severe wind coupled with heavy rains may cause mortality. Secondly, movement of air above a surface where evaporation is occurring (e.g. insect cuticle) increases the gradient of water vapour concentration and hence tends to increase the rate of evaporation.
6. Water Currents:
Water currents often determine which species of insects would inhabit a particular area. The various genera of mayflies may be classified into still- and rapid-water forms. The legs and bodies of these insects are appropriately adapted. Black fly larvae fasten themselves to stones or other stationary material in the water. Caddisflies attach their cases to submerged objects. The mosquito larvae are unable to survive in moving water.
Another important feature of currents in the aquatic environment involves the circulation of dissolved gases, salts and nutrients. For example, caddisfly and mayfly larvae may be found under conditions of relatively high oxygen concentration, and midge and black fly larvae at somewhat lower concentration, while certain mosquito and other fly larvae at very low concentrations.
7. Edaphic Factors:
Edaphic factors include the structure, texture and composition of soil along with its physical and chemical characteristics. Each soil has a distinctive flora as well as fauna of fungi, bacteria, algae, protozoa, rotifers, nematodes, molluscs, arthropods, etc.
Some of these organisms help in maintenance of soil fertility through nitrogen fixation while others are responsible for return of the essential elements back to the soil by decomposition of dead organic matter. In humus formation, earthworms, millipedes, dipteran larvae, slugs and snails play important role in breaking up and division of litter.
Several properties of the soil like texture, moisture, drainage, chemical composition and physiography (topography) affect the distribution and abundance of insects. Soil texture varies from hard-packed clays to loose sands. Few insects dwell in hard- packed types, because they are unable to push or dig their way through them.
The loams allow digging and burrowing operation and are usually favourable in their characteristics like moisture, drainage and organic matter. The cutworm, Agrotis ipsilon (Hufnagel), larvae live in soil of fairly light texture in which they move around freely in response to daily or seasonal temperature and moisture changes.
Drainage and texture together exert considerable influence on the distribution of insects which pass part of their life in soil. The wireworms in wet arid land become important pests of potato, onion, lettuce and many other crops grown in irrigated fields.
Chemicals naturally present in the soil affect both the abundance and distribution of phytophagous insects. Deficiencies of mineral elements, resulting in similar plant deficiencies, inhibit the growth of some insects. Nitrogen deficiency lowers the productivity of some species of insects but results in outbreak of others.
Major topographic factors which affect biogeography of insects include height of mountain chains, steepness of the slope, directions of mountains and valleys, and exposure of the slope. These features also affect the climate of an area, thus influencing the distribution of certain insects. The mountain range and large bodies of water such as seas act as physical barriers to the spread of insects.
Biotic Factors:
Under natural conditions, organisms live together influencing each other’s life directly. Such vital processes 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 interdependencies between insects themselves as well as between insects, other animals and plants exist throughout the whole life, or are casual and temporary.
However, interdependency may exist between species which are taxonomically widely different such as between insects and bacteria, screw-worms and cattle, etc. The relationship between species may be beneficial to both, harmful to both or beneficial or harmful to one and neutral for the other.
The most important biotic components of the insect life-system are natural enemies and food:
1. Natural Enemies:
The natural enemies of insect pest populations include predators, parasites/parasitoids and disease causing microorganisms such as fungi, bacteria, viruses and rickettsiae. The natural enemies are also influenced by various environmental factors such as weather and hyper-parasitism. The degree of influence of various natural enemies on the pest population would thus vary.
The abundance of predators influences the abundance of their prey in field conditions. Predators respond to an increase in prey population density through numerical response (increase in the density of predators in a given area) and functional response (increase in consumption by individual predators).
Weather is known to determine the effectiveness of a natural enemy. For example, in the control of woolly apple aphid, with Aphelinus mali (Haldeman), the prevalent temperature in spring would determine the effectiveness of the parasitoid. Both the host and the parasitoid remain dormant during winter but the activity of host is resumed at a slightly lower temperature, and thus it is able to breed in the absence of the parasitoid in the early spring.
In such a situation, the parasitoid may fail to press heavily on the host population as compared with the one when relatively higher temperature prevailed during early spring. Similarly, the larval parasitoid, Apanteles flavipes (Cameron), of maize borer, after successful overwintering in the host larvae, resumes activity in the spring earlier than its host but is unable to parasitize this particular host during dry and hot summer. This is because of lethal effects of high temperature (above 35°C) and low humidity (below 60% R.H.) on the parasitoid.
2. Food:
Insects being heterotrophic depend directly or indirectly on plants for food. The quantity and quality of food play an important role in insects’ survival, longevity, distribution, reproduction, speed of development, etc.
The quantitative aspects of food may be expressed as absolute food shortage (when food is completely destroyed within a localized region) and effective food shortage (when food shortages may occur in patches throughout the distribution of a given insect population).
The causes of absolute and effective food shortages are numerous, some of them are:
i. There may be large number of the same individuals per unit quantity of food (intraspecific competition).
ii. There may be more than one species consuming the same food materials (interspecific competition).
iii. There might be other species that influence the food of a particular species without actually consuming it. For example, an epizootic of the fungus, Entomophthora, so reduced the number of the caterpillars of Plutella xylostella (Tinnaeus), that the predator, Angitis sp. suffered severely from a shortage of food when it had been abundant before the outbreak of the disease.
iv. Any environmental factor causing reduction in populations of particular plants or insects will probably cause reduction of not only the insect populations that utilize these plants or animals for food but also of parasitoids or predators of these animals. Monophagous insects are more likely to be affected by such reductions in food supply than polyphagous species.
The quality of available food greatly influences egg production, larval development, longevity and size of insects. Many insect species store sufficient nutrients during the larval stage to accomplish adult activities. The adult longevity of these species is of comparatively short duration and they commonly do not feed at all. Mayflies which live long enough to copulate and egg laying, depend entirely on these reserves. In other species, larvae may store nutrients sufficient for egg production, but the adults must ingest water and carbohydrates to survive.
There are differences with regard to suitability of a host at the species or even at the varietal level within the same crop. The differences in varieties/species as regards their suitability for a particular insect population are governed by their acceptability as host, nutritional adequacy and absence of metabolic inhibitors or agents toxic to pest species.