POPULATION BALANCE IN THE HOST-PARASITE INTERACTING SYSTEM DEMONSTRATED BY PARTIAL REMOVAL OF POPULATION

Abstract

Using the azuki bean weevil, Callosobruchus chinensis, and its larval parasite, Neocatolaccus mamezophagus, it was examined whether the balance of interacting system of the host and its parasite exists when the density independent mortality such as artificial removal of constant percentage of the population has influence upon the host-parasite interacting system. Under the condition controlled at 30℃ and 75 per cent R.H., various percentages of both host and parasite populations (exp. HP), or host population (exp. H), or parasite population (exp. P) were removed from the original population of the host-parasite system in every generation respectively, and how the steady state is altered by these treatments was examined. The methods of removal are as follows : In the experiment HP, a fixed percentage of beans which contained such percentage of host and parasite as 25,50,90 of the level of the steady density were heated just before the host emergence and removed. In the experiment H, during about one week, the host was removed every day, every two days, every three days from the time of adult emergence. In the experiment P, when the host is a full-grown larva and the parasite can easily lay its eggs in the host from outside the bean, the parasite was removed every day, every two days, every three days the from the 14th day after host oviposition. In experiment H and P, percentages of such removing as mentioned above were calculated as 70,80,90 per cent of the control population. In any case of the removing, the host-parasite system maintains a steady level of population density (Figs. 1,2,3). But the steady state of each removed population changes its level from that of the unremoved population as shown in Table 1. When the host was removed, the parasite density did not so conspicuously fluctuate as the host density (Fig. 2), and when the parasite was removed the trend of fluctuation seemed to be the reverse (Fig. 3). Unless the host density attains a saturation level, the steady density of it depends upon only the intensity of competition in the parasite population. After the host population reached such a state, the steady state of the host becomes higher in a single population of host than in the host-parasite system (Fig. 6). This difference in the steady density is due to the action of the parasite, which is highly influenced by the host density or parasite density even under a constant physical condition. In the interacting system, though the parasite efficiency of host finding is dependent on the density of the host, the probability that the deposited egg of the parasite developes into an adult parasite in the next generation is dependent on whether it is able to get away with hyper-parasitism. The probability that the egg of the weevil develops into an adult weevil which will be the individual of final density of the host population in the next generation depends on whether host can survive from attack of the parasite. Of course, the probability that the egg of the weevil is deposited by the individual of final density of the host developes into the individual of initial density of the host in the next generation is only dependent upon the density effect of the host population. This is because the host population is subjected to attack by the parasite for only a fraction of its developmental period just before host emergence. In any case of the removing, the host-parasite system maintains a steady level of population density. But the steady state of each removed population changes its level from that of the unremoved population as shown in Table 1.