BIOLOGICAL PE|ST CONTROL PRECEPTS
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& the Average Abundance of Plants
Table 11.1(Examples showing host densities)
[Please refer also to Selected Reviews
Homopterous Insects as Biological Control Candidates
Scale insects, mealy bugs, whiteflies and aphids have been targets of numerous biological control projects. The first example of a scale insect being controlled biological was, of course, the cottony-cushion scale, Icerya purchasi Maskell, in California around 1880. Novius (Rodolia) cardinalis (Mulsant) gave complete control in 24 additional countries. Cryptochaetum iceryae (Williston) was also variously involved. Other examples include the citrophilus mealybug, Pseudococcus fragilis Brain, in California; the coconut scale, Aspidiotus destructor Signoret, in Fiji, Mauritius and Principe; Green's mealybug, Pseudococcus citricolus Green, in Israel; the red wax scale, Ceroplastes rubens Maskell, in Japan; the coffee mealybug, Planococcus kenyae (LePelley), in Kenya; and the citrus snow scale, Chienaspis citri (Comstock), in Florida and Peru, the woolly whitefly, Aleurothrixus floccosus (Maskell); the walnut aphid, Chromaphis juglandicola (Kaltenbach), and the blue aphid, Acrythosiphon kondoi Shinji, in California
Many additional homopterous insects were controlled either completely or substantially, and some partially. Such terms to describe control levels, although imperfect, still are used widely today. Scale insects alone account for nearly one-half of all projects against insects where some degree of success was evident. By 1973 about 67% of all complete successes, 31% of all substantial successes, and 43% of all partial successes involved scale insects. Homoptera as a whole contain about two-thirds of all successes.
The unusual biological control success rate with Homoptera may reflect a greater amount of effort; but it also indicates that these insects are more amenable to biological control in that about 78% of all attempts against them were successful. They are frequent invaders, and therefore deserve more attention than other insects.
Certain biological characteristics make Homoptera especially good candidates for biological control. Most are sedentary in habit and distributed in colonies. There is chronological continuity of all life stages in a population throughout the year in most species; and there is a certain degree of population stability conferred by perennial host plants upon which they occur. Parasitoids and predators are more likely to reach full effectiveness on this type of host population. It is thought that the 78% success figure could be much higher if efforts were continued on previous candidates, as was the case with the red scale work in California, which involved the work of two generations of scientists.
Examples That Demonstrate Precepts
Cottony-cushion scale--Icerya purchasi Maskell-- This species was accidentally introduced into California around 1868 and became extremely serious 19 years later (1887). Australia happened to be the native home as determined by the scarcity of this pest there. Introductions in 1888-89 were made of Novius cardinalis and Cryptochaetum iceryae.
Cryptochaetum was first regarded as the most promising species; however, Novius outperformed it in the commercial, drier areas of citrus. Complete control was achieved by the end of 1889.
More recent studies by Jose Quezada (1973) showed that both natural enemies are effective. Novius is dominant in desert areas and displaces Cryptochaetum in competition. Cryptochaetum is dominant on the coast and tends to displace Novius. The two species co-exist in the intermediate zones.
This example lends support to the precept of multiple introductions of natural enemies: as many potentially effective natural enemies of a pest as possible should be introduced. The most efficient in a given habitat will tend to displace the others and produce better overall control. Competition is not generally regarded as an incumbrance to the overall effectiveness of natural enemies in biological control
Florida red scale--Chrysomphalus ficus Ashmead.-- This species invaded Israel around 1910. There it was attacked by an indigenous ectoparasitoid, Aphytis chrysomphali (Mercet) whose effect was negligible. Pteroptrix (Casca) smithi (Compere) and Aphytis holoxanthus DeBach (--originally thought to be A. lingnanensis Compere) were introduced from Hong Kong in 1956-57. Complete control was achieved in 2-3 years with A. holoxanthus on the coast, and partial control in the hot Jordan Valley.
The importance of biosystematics to biological control is illustrated in the history of these introductions. The original Hong Kong material consisted of several parasitoid species, none of which were identified prior to establishment in the field. A mixture of parasitoids was later thought to have been in the initial imported material (even including some phytophagous thrips, because they had the appearance of Aphytis spp. to the investigators). More than one species of Aphytis entered this way undetected because of difficulties of separating the various species. It was thought that the California red scale parasitoid, Aphytis cohnei DeBach, and the purple scale, Lepidosaphes beckii (Newm.), parasitoid Aphytis lepidosaphes Compere entered Israel in this way.
Pteroptrix smithi had no apparent effect on initial biological control of Florida red scale in Israel. it dispersed through the range of its host, but there is no evidence that it detracted from the effectiveness of A. holoxanthus. Rather, it is regarded as a complement to overall biological control of the scale. Aphytis chrysomphali was entirely displaced by A. holoxanthus, and another parasitoid Habrolepis fanari DeLucci & Traboulsi, which entered the scene later.
This example illustrates the beneficial aspects of multiple introduction. Competition did not obviously deter from success in biological control. Aphytis holoxanthus evoked successful control in surrounding Arab countries and in other countries where it was subsequently introduced (e.g., Florida, Mexico, South Africa, Brazil and Peru).
In Israel, the California red scale has in recent years become a more serious pest. Previously, the Florida red scale was an effective competitor with California red scale. However, the suppression of the competitor by A. holoxanthus has allowed the California scale to increase. This illustrates the need for a multiple project approach in biological control.
California red scale--Aonidiella aurantii (Maskell)-- The red scale invaded California around 1868-75, and attempts to introduce natural enemies were begun in 1889. However, the most important natural enemies were introduced over 50 years later in 1948-47! Biological control of California red scale was considered a complete failure for those 50 years. During this "black out" period, Aphytis lingnanensis was not introduced from China because taxonomists thought it already occurred in California. When it was finally introduced in 1948, it was very effective in control and far superior to Aphytis chrysomphali with which it had been confused.
A long series of failures to establish imported natural enemies resulted because of inadequate taxonomic knowledge of the host scales. Several parasitoids were repeatedly obtained from the Orient from scales that were misidentified as the California red scale. Some failures were also the result of cryptic effects of host plant on certain endoparasitoids. The host plant imparted an intrinsic immunity to the scale. All these and more errors led to the conclusion that no effective parasitoids were present in the Orient.
After clearing the confusion, two endoparasitoids were introduced from China and established in California. These were Comperiella bifasciata Howard (1941), and Prospaltella perniciosi Tower (1947). Pteroptrix (Casca) chinensis (Howard) was not successfully established due to insufficient knowledge of its biology. This species might still be the final best bet.
Climate-related restrictions on Aphytis lingnanensis resulted in the importation of a better adapted species from India and Pakistan in 1956-57. Aphytis melinus DeBach, Aphytis fisheri DeBAch (a sibling species of A. melinus) was also introduced from Burma, but competition with the other Aphytis is thought to have precluded its establishment.
The percent parasitization in areas where the red scale is now held at low population densities by these parasitoids is only 15-20% on a year-round average. This gives an example of the uselessness of a percent parasitization figure, especially when it is known that the parasitoids kill a lot of the scales by probing and host-feeding actions. When a particular parasitoid population begins its activity on red scale in a citrus grove, parasitization is low and the proportion of living scales is high. As the percent parasitization approaches the "equilibrium" average of 15-20%, the proportion of live scales becomes low. Therefore, relatively small increases in parasitization are reflected by relatively great increases in red scale mortality.
Olive scale--Parlatoria oleae (Colvee).-- The olive scale became established near Fresno, California in 1934, where it was a major pest of many deciduous fruit crops and ornamental trees and shrubs. It spread over the entire Central Valley and down into portions of southern California. There are two generations per year, one each in the spring and autumn. On olive the autumn generation scales are direct pests of the fruit.
Aphytis maculicornis (Masi) was introduced from Egypt in 1949, followed by continued searching for natural enemies in Europe and Asia. Among the various parasitoids introduced there were some distinct strains of A. maculicornis. The Persian strain alone was effective, and it was colonized by the millions. The percent parasitization averaged about 90% at low scale densities (also about 90% of the original population density). However, this drastic reduction was not sufficient because even one scale per fruit was an economic loss. Aphytis maculicornis could not perform better because it was unable to tolerate the heat of summer, and winter was equally severe on its survival.
In 1957 two more parasitoids were introduced from Pakistan, namely Coccophagoides utilis Doutt and Anthemus inconspicuus Doutt. Coccophagoides was artificially spread by causing outbreaks of the host scales in orchards with DDT, in order to temporarily reduce the effects of A. maculicornis. Coccophagoides is endoparasitic with primary and hyperparasitic habits, where the males are produced hyperparasitically on females of the same species. It averages 40% parasitization and occupies the niche left open by A. maculicornis during summer, thereby contributing additional mortality to the autumn generation. Coccophagoides complements A. maculicornis, the latter being the superior parasitoid when weather conditions are right. This example illustrates another score for multiple introductions.
Rhodesgrass scale--Antonina graminis (Maskell).-- A biological control project was begun in 1962 in portions of the southeastern United States to control Rhodesgrass scale. Five species of parasitoids were introduced as follows: Anagyrus antoninae Timberlake from Hawaii; Xanthoencyrtus phragmitis Ferr. from France; Boucekiella antoninae (Ferr.) from France; Timberlakia europaea (Mercet) from France and Anagrus diversicornis Mercet from France. None of these species are known to have become established. A final introduction of Neodusmetia sangwani (Rao) from India did become established and finally controlled the scale (Schuster et al. 1971).
The females of N. sangwani cannot fly; therefore, the parasitoids were spread by airplane over the scale-infested terrain. Rhodesgrass yield comparisons between treatments was the most reliable measure of effectiveness, because percent parasitization by the parasitoid was not often dramatic.
This successful biological control effort illustrates the importance of being persistent on ones efforts to secure additional parasitic species. It also shows how technology may hasten the control process, in this case spreading parasitoids by airplane. Finally, it is important to judge the success of a project not by the degree of parasitism but rather by the amount of control actually achieved.
Walnut aphid--Chromaphis juglandicola (Kaltenbach).-- The aphid was controlled in southern California with a strain of Trioxys pallidus (Haliday) introduced from France in 1959; and one decade later in northern California with a T. pallidus strain from Iran. The second introduction is thought to have been a sibling species as some reproductive isolation from the first species was detected. Complete control was achieved, as previously discussed. This is another demonstration of the importance of multiple introductions of different apparent strains of natural enemies from different climatic areas.
Orb-weaving spiders.--Interspecific competition between two orb-weaving spiders, Metepeira grinnelli (Coolidge) and Cyclosa turbinata (Walckenaer), was investigated by Spiller (1986), who selectively removed the predators. The estimated predation rate of small prey was higher when Cyclosa was alone than when both species were present, because when Metepeira was removed the density of Cyclosa became higher than the combined density of both spiders. This was because the consumption rate of small prey by Metepeira was very low compared with that of Cyclosa. The study suggested that a subset of predator species might be more effective in reducing prey populations than the entire natural guild (Spiller 1984a,b, 1986). The example argues against multiple introductions.
The Importance of Single Species in Determining the
Average Density of Plants and Animals
The presence of one or two species in the ecosystem is known to influence drastically the population density of plants and animals (Legner 1987). The realization of this is probably of one of the most difficult concepts to grasp for modern ecologists who through their broad experiences in measuring density dependent and density independent forces in nature appreciate the complexities of the ecosystem. It seems inconceivable that in the midst of all the interacting abiotic and biotic factors, only one or two organisms could ever be responsible for the average abundance of another organism.
Nevertheless, proof for this simplistic assumption is available from many sources. Breaking down the world's biota into terrestrial plants, aquatic plants, vertebrates, phytophagous insects and insects of medical and veterinary importance, Table 1 gives selected examples to demonstrate the importance of one or two species in accounting for tolerably low densities of other organisms. Many of the causative agents act as density dependent regulative forces which bear a reciprocal density relationship to their hosts, or as limiting forces which set an upper limit to the density that an organism can attain but which do not bear the close relationship of reciprocity.
If there are any doubts of the basic assumption that the presence of one or two organisms account for the observed low population densities of the various species listed in Table 1, the question may be asked, "What would happen to the average density of the controlled organism if the causative agent were removed?" Invariably, the answer would be simply that a rise in density would follow the removal.
It is apparent that the greatest number of examples are found among phytophagous insects, which is a reflection of the greater biological control effort against this group. Insects of medical and veterinary importance are just becoming favored targets for biological control as the desire to reduce pesticide use against them increases. Thus, we undoubtedly will see more successful cases in years to come.
Table 11.1 gives examples of the abundance of plants and animals dependent on the presence of one or a few species of organisms in the ecosystem.
The importation of new natural enemies from abroad is the single best approach to biological control. This approach needs much more emphasis in current biological control attempts. The search for natural enemies should extend throughout the entire range of distribution of the pest.
Accurate biosystematics are necessary as well as basic ecological studies of the pest-natural enemy complex at home and abroad. However, neither should retard the simultaneous importation of new natural enemies.
The most successful natural enemies have shown high host or prey specificity. They are often multivoltine with respect to their prey, and well adapted to the physical conditions of the pest habitat. They are also good searchers. However, there is no single best natural enemy for a given pest.
In most cases of complete biological control success, only one or two natural enemy species are involved. Different species or strains of natural enemies are usually required when the pest is to be controlled in a wide area with different climates. The interspecific competition between natural enemies in the areas of overlap has not been shown to be detrimental to regulation at a satisfactory control level, although theoretically there is a risk (Force 1974; Ehler 1979, 1982, 1985; Turnbull & Chant 1961).
Multiple importations of competing parasitoids and predators are a practical way to practice biological control, which has not been shown to be detrimental to overall host reduction. The so-called direct pests (e.g., olive scale) are suitable subjects for biological control although the probability of success with direct pests may be lower than with indirect pests.
Exercise 11.1--Would you defend the multiple species introduction approach for biological control? If so how? If not, why?
Exercise 11.2--How many biological control precepts can be identified?
Exercise 11.3--How is biosystematics necessary in biological control work? Give examples.
Exercise 11.4--Discuss in detail the hosts, natural enemies, and population dynamics associated with the biological control of the following: cottony-cushion scale, Florida red scale, California red scale, walnut aphid, olive scale, Rhodesgrass scale, navel orangeworm.
Exercise 11.5--Can you suggest a practical alternative to the designations "complete", "substantial" and "partial" success for biological control?
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