Study On Some Factors Affecting Offspring Sex Ratio Of The Predacious Mite Amblyseius Pseudolongispinosus (acari: Phytoseiidae)

The predacious mite Amblyseius pseudolongispinosus is one of the most important biological agents in China. It has been applied successfully in controlling several spider mites, such as Tetranychus urticae and T. cinnabarinus. Since the alteration of predatory mite sex ratio would affect it's mating competition and reproductive strategy, the regulatory in population fluctuation during the mite production should be paid more attention to. It was reported that the progeny sex ratio of phytoseiid mite differentiated under various conditions. Thus, phytoseiid females maybe have the potency to control over the sex ratio of their offspring. Under the laboratory conditions, it was studied that the effects of prey density, predator density, predacious mite's age, proportion of males and females, residence time of males and temperature on the sex ratio of predatory mite Amblyseius pseudolongispinosus. The results are as follows.1. Egg density of T. cinnabarinus had a profound effect on progeny sex ratio of Amblyseius pseudolongispinosus. The sex ratio of the progeny tended to shift in favor of females as the levels of prey availability were increased. A highly significant difference was detected in the total number of eggs produced which differs according to the prey density. As the prey density decreased the total eggs produced and daily rate of female were decreased. The oviposition periods shortened. When the prey density was 5 eggs per 19.63 cm2, the female percent of progeny was 54.65%, while the prey density increase to 25, the percent of female offspring reached 68.98%.2. Under the abundant food conditions, the proportion of female offspring was decreased with the predatory mite density increasing. When 1, 2, 3 and 4 predatory mite(s) was/were given in experimental setup, the proportion of female offspring averaged 77.05%,73.99%,71.59% and 69.44%, respectively. On the other hand, as predatory density increased the total eggs laid by females increased, but have a lower per capita rate of oviposition. Most of eggs were laid daily when only one predatory mite in experimental setup (32.40), and the least was 4 predatory mite (24.87).3. The age of predatory mite also related to the progeny sex ratio. In our experiment females and males which are divided into four age groups (0-1d,5d,10d,20d). Then they mate with the newly hatched male or female respectively. Results indicated there was a shift in sex ratio towards an increase number of female offspring with neonate females mating with male in age increasing, that was, the female offspring were 69.60%, 71.13%, 71.52% and 73.64% when neonate female mated with males which were in age of 0-1d, 5d, 10d and 20d respectively. However, the female sex ratio of offspring decreased with the neonate male mating with females in age increasing. The reproduction of Amblyseius pseudolongispinosus can be started at any period throughout the whole life of the female. The highest number of eggs and the longest oviposition period were naturally recorded in young females or females in different ages mating with neonate males.4. The proportion of females in offspring was reduced when temperature was either too high or too low, especially under the highest temperatures. The relationship between the female sex ratio (y%) and temperature (x) could be described with parabola formula, that is, y=-0.13x2+0.40x+64.04. The highest female ratio was got at the temperature of 20℃. At 30℃the fecundity was the highest.5. Male and female proportion in a fixed density and residence time of males also had an impact on the progeny sex ratio. With the proportion of females increasing, the ratio of female in the offspring was reduced and total eggs produced were increased, but as the number of females increased gradually, eggs laid per female on average were decreased. Compared to in multiple mating, pre-oviposition shortened, and oviposition extended, both fecundity and female proportion increased in single mating.6. The female sex ratio in Amblyseius pseudolongispinosus is always over 60% under all conditions, as is determined by its biological characteristics. However, it can be more or less altered both with the environmental conditions and the predatory mite biological state itself. According to the results, one hypothesis is put forward to, that is, the population increase of A. pseudolongispinosus in ideal envirionmental condition come true by the increasing female sex ratio of its offspring.