An Analysis of the Effects of Temperature upon the Growth and Reproduction of Dysdercus fasciatus Sign. (Hemiptera, Pyrrhocoridae). I.—The Intrinsic Rate of Increase

Abstract

In laboratory studies of Dysdercus fasciatus Sign., populations were exposed to constant temperatures of 15, 20, 25, 30, 35 and 40°C. and a relative humidity of 60 per cent., and to 28°C. and 70 per cent. R.H. After hatching, the insects were supplied with cotton seed as food and water to drink. Development of eggs was incomplete below 25°C., of nymphs below 20°C., and of both above 30°C. To allow comparison of growth and rate of reproduction at temperatures outside the embryonic range, this stage was ignored in calculating values of the intrinsic rate of increase, r, and the mortality rate and length of life were measured from the time of hatching, the eggs being incubated under optimum conditions (those at 28°C.) and the freshly hatched nymphs then moved to the appropriate temperature. The value of r was found to increase with increasing temperatures up to 30°C., and to do so more rapidly between 25 and 28°C. than between 20 and 25°C. The age-schedule of deaths for populations living at 28°C. showed a distribution of deaths that was almost ideal, in that their highest frequency did not occur until the reproductive period was advanced. The principal deviation from this ideal was due to the high mortality rate found in the first instar. At 20°C., the death rate approached constancy throughout the life-span of the population. The interval between generations decreased with increasing temperature, the rate of decrease being approximately 4·6 days per 1°C. rise in temperature.Further analysis of r was made by considering separately the growth and reproductive characteristics of the insect, and the effect of temperature upon them.The rate of post-embryonic growth, as measured by live weight, increased with temperature, within the range of 20–30°C. At 15°C., growth did not proceed beyond the third instar, or beyond the fifth instar at 35°C. The principal cause of the increase in growth rate was the decrease and eventual disappearance of the periods between initial ecdysis and start of growth, and between the end of growth and final ecdysis, the rate at which growth actually took place differing but slightly.The size of the insect just after the final ecdysis had an important effect on r because the number of eggs laid by small insects (mean weight 68·6 mg.) was only about half that of large ones (mean weight 100·2 mg.). Any decrease in size observed with increasing temperature was slight compared with that produced by restricting the insect's access to drinking water.Copulation and oviposition occurred in adults maintained at any of the temperatures except 15 and 40°C. The percentage of females that laid eggs differed according to the temperature, the highest recorded being 90 per cent, at 28°C. and the lowest 30 per cent, at 20°C. The total time spent in copulation also varied with temperature, being least (9 days) at 28°C. and most (26·3 days) at 20°C. The change of weight of the female after moulting largely represented growth of the ovaries; at 15°C., this is slow and not completed before the female dies; at 40°C. the ovaries do not mature; at other temperatures tested (20–35°C.) growth is fairly rapid, the ovary being full grown in about four days.The fertile female lays her eggs in batches, the maximum observed being six batches, at approximately three-day intervals. A few females survived the laying of the sixth batch of eggs by a period longer than their interoviposition period. At 28°C., the preoviposition period and the time spent in copulation were less, more egg-batches, and hence more eggs, were produced and a higher proportion of females mated, than at any other temperature. In the unmated female, the average life was twice that of the mated one, the preoviposition period was the same, the interoviposition period approximately three times as long, the egg-batches half as numerous, and the number of eggs per batch nearly the same. All fertile females were observed to lay eggs, but only 20 per cent, of the unmated ones did so. At 35°C., mated females produced one or two batches of eggs; unmated ones produced none, and although the ovary matured normally up to the fourth day, the yolk was subsequently reabsorbed, this process being complete by the 14th day.These studies have indicated that in D. fasciatus the decrease in the value of r with decreasing temperature is caused by a rise in pre-reproductive mortality and a fall in reproductive capacity, as well as by the increase of the generation length, which other workers have found to be of predominating importance in the case of other insects.