Most insects are poikilothermic, that is with body temperature more or less directly varying with environmental temperature, and thus heat drives 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 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. 6.13), and prediction of the larval feeding periods, generation length, and time of adult emergence under variable

Fig. 6.13 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 of10.5, -2.9, and 11.3°C. (After Samson & Blood 1979.)

Fig. 6.13 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 of10.5, -2.9, and 11.3°C. (After Samson & Blood 1979.)

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 (also degree-days) (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 night conditions or reduce or increase daytime heat. Thus, predictions of insect life-cycle events based on extrapolation from laboratory to field temperature records may be inaccurate. For these reasons, the laboratory estimates of physiological time should be corroborated by calculating the hour-degrees or day-degrees required for development under more natural

Box 6.2 Calculation of day-degrees

Day-degrees (or degree-days) can be estimated simply (after Daly et al. 1978) as exemplified by data on the relationship between temperature and development in the yellow-fever mosquito, Aedes aegypti (Diptera: Culicidae) (after Bar-Zeev 1958).

1 In the laboratory, establish the average time required for each stage to develop at different constant temperatures. The graph on the left shows the time (H), in hours, for newly hatched larvae of Ae. aegypti to reach successive stages of development when incubated at various temperatures.

2 Plot the reciprocal of development time (1/H), the development rate, against temperature to obtain a sigmoid curve with the middle part of the curve approximately linear. The graph on the right shows the linear part of this relationship for the total development of Ae. aegypti from the newly hatched larva to the adult stage. A straight line would not be obtained if extreme development temperatures (e.g. higher than 32°C or lower than 16°C) had been included.

3 Fit a linear regression line to the points and calculate the slope of this line. The slope represents the amount in hours by which development rates are increased for each 1 degree of increased temperature. Hence, the reciprocal of the slope gives the number of hour-degrees, above threshold, required to complete development.

4 To estimate the developmental threshold, the regression line is projected to the x axis (abscissa) to give the developmental zero, which in the case of Ae. aegypti is 13.3°C. This zero value may differ slightly from the actual developmental threshold determined experimentally, probably because at low (or high) temperatures the temperature-development relationship is rarely linear. For Ae. aegypti, the developmental threshold actually lies between 9 and 10°C.

5 The equation of the regression line is 1/H = k(T° - T'), where H = development period, T° = temperature, Tt = development threshold temperature, and k = slope of line.

Thus, the physiological time for development is H(T ° - Tt) = 1/k hour-degrees, or H(T ° - T ')/24 = 1/k = K day-degrees, with K = thermal constant, or K value.

By inserting the values of H, T°, and Tt for the data from Ae. aegypti in the equation given above, the value of K can be calculated for each of the experimental temperatures from 14 to 36°C:

Temperature (°C) 14 16 20 24 28 30 32 34 36 K 1008 2211 2834 2921 2866 2755 2861 3415 3882

Thus, the K value for Ae. aegypti is approximately independent of temperature, except at extremes (14 and 34-36°C), and averages about 2740 hour-degrees (or degree-hours) or 114 day-degrees (or degree-days) between 16 and 32°C.

conditions, but using the laboratory-estimated developmental threshold, as follows.

1 Place newly laid eggs or newly hatched larvae in their appropriate field habitat and record temperature each hour (or calculate a daily average, a less accurate method).

2 Estimate the time for completion of each instar by discarding all temperature readings below the developmental threshold of the instar and subtracting the developmental threshold from all other readings to determine the effective temperature for each hour (or simply subtract the development threshold temperature from the daily average temperature). Sum the degrees of effective temperature for each hour from the beginning to the end of the stadium. This procedure is called thermal summation.

3 Compare the field-estimated number of hour-degrees (or day-degrees) for each instar with that predicted from the laboratory data. If there are discrepancies, then microhabitat and/or fluctuating temperatures may be influencing insect development or the developmental zero read from the graph may be a poor estimate of the developmental threshold.

Another problem with laboratory estimation of physiological time is that insect populations maintained for lengthy periods under laboratory conditions frequently undergo acclimation to constant conditions or even genetic change in response to the altered environment or as a result of population reductions that produce genetic "bottlenecks". Therefore, insects maintained in rearing cages may exhibit different temperature-development relationships from individuals of the same species in wild populations.

For all of the above reasons any formula or model that purports to predict insect response to environmental conditions must be tested carefully for its fit with natural population responses.

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

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