Environmental Effects On Development

The rate or manner of insect development or growth may depend upon a number of factors. These include the type and amount of food, the amount of moisture (for terrestrial species) and heat (measured as temperature), or the presence of environmental signals (e.g. photoperiod), mutagens and toxins, or other organisms, either predators or competitors. Two or more of these factors may interact to complicate interpretation of growth characteristics and patterns.

6.10.1 Temperature

Most insects are poikilothermic, that is with body temperature more or less directly varying with environmental temperature, thus heat is the force driving the rate of growth and development when food is unlimited. A rise in temperature, within a favorable range, will speed up the metabolism of an insect and consequently increase its rate of development. Each species and each stage in the life history may develop at its own rate in relation to temperature. Thus, physiological time, a measure of the amount of heat required

Fig. 6.12 Age-specific oviposition rates of three predators of cotton pests, Chrysopa sp. (Neuroptera: Chrysopidae), Micromus tasmaniae (Neuroptera: Hemerobiidae), and Nabis kinbergii (Hemiptera: Nabidae), based on physiological time above respective development thresholds of 10.5°C, -2.9°C, and 11.3°C. (After Samson & Blood 1979.)

Fig. 6.12 Age-specific oviposition rates of three predators of cotton pests, Chrysopa sp. (Neuroptera: Chrysopidae), Micromus tasmaniae (Neuroptera: Hemerobiidae), and Nabis kinbergii (Hemiptera: Nabidae), based on physiological time above respective development thresholds of 10.5°C, -2.9°C, and 11.3°C. (After Samson & Blood 1979.)

over time for an insect to complete development or a stage of development, is more meaningful as a measure of development time than age in calendar time. Knowledge of temperature-development relationships and the use of physiological time allow comparison of the life cycles and/or fecundity of pest species in the same system (Fig. 6.12), and prediction of the larval feeding periods, generation length, and time of adult emergence under variable temperature conditions that exist in the field. Such predictions are especially important for pest insects, as control measures must be timed carefully to be effective.

Physiological time is the cumulative product of total development time (in hours or days) multiplied by the temperature (in degrees) above the developmental (or growth) threshold, or the temperature below which no development occurs. Thus, physiological time is commonly expressed as day-degrees (D°) or hour-degrees (h°). Normally, physiological time is estimated for a species by rearing a number of individuals of the life-history stage(s) of interest under different constant temperatures in several identical growth cabinets. The developmental threshold is estimated by the linear regression x-axis method, as outlined in Box 6.2, although more accurate threshold estimates can be obtained by more time-consuming methods.

In practice, the application of laboratory-estimated physiological time to natural populations may be complicated by several factors. Under fluctuating temperatures, especially if the insects experience extremes, growth may be retarded or accelerated compared with the same number of day-degrees under constant temperatures. Furthermore, the temperatures actually experienced by the insects, in their often sheltered microhabitats on plants or in soil or litter, may be several degrees different from the temperatures recorded at a meteorological station even just a few meters away. Insects may select microhabitats that ameliorate cold

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