The control of the citrophilus mealybug, Pseudococcus gahani , by Australian parasites

Abstract

The citrophilus mealybug, Pseudococcus gahani[Pseudococcus calceolariae], Green, has been reduced from the status of a serious pest in California to one of almost negligible numbers by the introduction of the internal parasites Coccophagus gurneyi, Comp., and Tetracnemus pretiosus, Timb., from Australia [R.A.E., A, xx, 22, etc.]. The annual saving in Orange County alone, where more than 40, 000 acres of Citrus were infested, is estimated at anything from £100, 000 to £200, 000 (at par). Descriptions of these parasites and an account of their propagation, transport and biology are given, as well as brief notes on the less important enemies found in Australia, viz., Anusoidea comperei, Timb., a parasite that was not introduced into California; Midas pygmaeus, Black., which was introduced and is still maintained in some of the local insectaries and of which colonisations are occasionally made; Diplosis sp., which was introduced in 1928 but of which the influence is negligible, and Chrysopa ramburi, Cam., the establishment of which in California is doubtful. T. pretiosus deposits the majority of its eggs in a few days after emergence, whereas C. gurneyi oviposits over a period of about three weeks. After the first week, therefore, some of the ovipositing females of the latter attack mealybugs already inhabited by larvae of T. pretiosus or other parasites, or even of its own species. Almost invariably the egg is then placed upon or within the primary occupant. When developing on other parasites, the larvae of C. gurneyi grow much faster than when developing on the mealybugs themselves, probably as a result of using food that has already been elaborated by a prior inhabitant of the host. It has frequently been suggested that it is a mistake to introduce for biological control two or more insects attacking the same stage of the host, as the resulting competition is said to reduce the total effect below that produced by the more prolific species alone. It is not always found, however, that the parasite destroying the greater percentage of a host in its native home is the more valuable one for introduction; T. pretiosus, for example, proved the more effective of the two parasites in Australia, while C. gurneyi seems to be the more effective in California. The introduction of these two species also provided some interesting data on the practical aspects of the question of multiple introductions. When both are introduced simultaneously into a cage heavily stocked with mealybugs, C. gurneyi rapidly becomes dominant and completely eliminates T. pretiosus. The percentage of parasitism is then very high, with much overlapping. In the field it is found that if parasites are of any great value they will maintain the mealybug population at a low density, and as the density of the host becomes reduced the percentage of parasitism must be reduced also. The overlapping of the two species is naturally diminished in consequence, so that when the density of the host is low there is a minimum of overlapping and therefore slight effect of one parasite on the other. It seems reasonable to conclude, therefore, that these two parasites are more effective than either alone would have been. Under such conditions, each species destroys host individuals that would have escaped destruction by the other provided that the parasites have slightly different habits and habitats. Moreover, T. pretiosus is very scarce during the winter months, whereas C. gurneyi is active throughout the cold weather and develops two generations to one of its host, thus materially reducing the numbers of overwintering mealybugs that produce the most injurious spring generation. In places where the mealybugs are protected by ants, T. pretiosus, which oviposits very quickly, is less interfered with than the slower C. gurneyi. The percentage of parasitism in thousands of mealybugs collected from various parts of California during the past three years, ranged from 20 to 60 and occasionally reached 100. The actual number of mealybugs destroyed would, however, be considerably higher for many reasons, one of which is the abnormal migration induced by parasitism, which results in many parasitized mealybugs leaving the trees. As, moreover, there are approximately two generations of parasites to one of mealybugs, if 50 per cent. of a given generation of the latter are destroyed by one generation of parasites and 50 per cent. of the survivors by the next, there is a total destruction of 75 per cent. although only 50 per cent. would contain parasites at any one dissection. It is an important consideration that a high percentage of destruction of an insect by one factor, such as parasites, can be important or unimportant according to whether it replaces or does not replace some other cause of mortality. The percentage of parasitism means little, from the standpoint of control, unless correlated with host population density, yet estimates of parasitism are almost invariably given in percentage of hosts attacked, with no reference to host population density. It is a mistake to assume, because a parasite is destroying only a small percentage of its host when the host is scarce, that the parasite has no important effect on the maintenance of a low host population density; it may be the critical factor. There is no method known for measuring accurately the quantitive effect of separate environmental factors on the population density of a phytophagous insect. Conclusions must still be based on general field observation, and the contention that the disappearance of injurious infestations of P. gahani is due to the work of C. gurneyi and T. preitosus is based on the observation that without exception the absence or occurrence of serious infestations of the pest has been positively correlated with the presence or absence of the parasites.