ANALYSIS OF SUCCESSES IN BIOLOGICAL PEST CONTROL
I. A thorough resume of biological control efforts and successes may be found in Clausen (1978). Another publication released in
1999 by the Academic Press, San Diego, CA that discusses in great detail some of the outstanding contributions to pest control
employing the biological control method.
II. The so-called Island Theory seems to be borne out in thee results, because a substantial portion of the more striking successes
in biological control have occurred on such islands as Hawaii, Fiji and Mauritius, and ecological islands such as portions of
A. One reason is that biological control work began early in such places, and a disproportionate amount of research and
importation was undertaken there in comparison to continents (excepting California).
B. The present record shows that about 60% of all the complete successes have occurred on continents; thus, the island
theory is no longer fully acceptable.
III. Parasitoids have been argued to be better than predators as biological control agents.
A. Because a predaceous larva consumes many host individuals during its lifetime and a parasitoid but one host, it might
appear that a predator is inherently more destructive and thus makes a better biological control agent.
B. However, analysis of the 139 species of entomophagous insects imported and established in the United States as of 1967
showed that 113 were parasitoids and 26 predators. This ratio has remained similar into the 1990's. Roughly twice as many
successes in biological control have resulted from parasitoid introduction in the United States. However, about four times
as many on the world scene.
C. The apparent superiority of parasitoids is the subject of contemporary debate and research. This may only reflect the fact
that parasitoids have received the greatest amount of attention in terms of the number of species introduced and the
number subjected to field analyses.
IV. Multiple versus "The Best" Species.
A. The question has arisen whether multiple importation of different natural enemy species attacking a given host and the
resulting interspecific competition among them produces a greater or lesser total host mortality than would be the
importation of the so-called "best" species allowed to act alone.
B. Analysis of past successes suggests that multiple species importation, whether made simultaneously or sequentially, have
nearly always resulted in enhanced biological control.
1. multiple introductions provide a series of natural enemies that can attack a sequence of host stages in any one habitat.
Here environmental changes may adversely affect one natural enemy yetfavor another, so that the latter natural enemy
may tend to compensate for the reduced efficiency of the former. Howard and Fiske made these points the basis of their
so-called sequence theory of multiple importations.
2. when several natural enemy species are established on a common host, they are more likely to parasitize that host over a
greater geographic range than a single species of natural enemy.
3. multiple introductions increase the chances of obtaining a species of natural enemy that can use alternate hosts to
overcome difficulties associated with seasonal fluctuation in pest abundance.
4. multiple importations favor the chance of establishing a truly superior species of natural enemy.
V. Clausen's 3-Host Generation / 3-year Rule.
A good exception is provided by the mymarid egg parasitoid, Patasson nitens imported from Australia into South Africa in 1926. Complete biological control of the eucalyptus weevil was achieved within the required three years in southern and southeastern parts of the country. However, in the northeastern highlands where conditions were less favorable to both host and parasitoid, several additional years were required for the parasitoid to bring about substantial control of the eucalyptus weevil. This example also nullifies the generalization that egg parasitoids alone would not prove capable of biological control.
VI. Single Larval Parasitoid Importations.
A good example of a single larval parasitoid working successful biological control is the tachinid, Ptychomyia remota, introduced into Fiji from Malaya in 1925, which resulted in the complete control of the coconut moth. This also illustrates a case where an area other than the native home of a pest produced a useful biological control agent, since Ptychomyia's natural host in Malaya was a related, but innocuous species of native moth.
VII. Single Pupal Parasitoid.
The imported cabbage worm controlled in New Zealand by Pteromalus puparum introduced from North America in 1933.
VIII. Other Generalizations.
Such generalizations as biological control being more likely to succeed against pests of perennial rather than short-lived annuals, against sessile or nonmotile pests, or against alien rather than native pests, must also be qualified. As with any generalization, there are exceptions to the rule. Analyses of the results of past efforts can provide useful guidelines.
IX. It will probably continue to hold that the number of successes attained in biological control in any one country is directly
proportional to the amount of research and importation work carried out there
Hawaii, California, the rest of the United States, New Zealand and Australia, as well as the former Commonwealth Institute of Biological Control, currently lead in the number of cases of successful biological control of insect pests and weeds brought about by imported natural enemies. This reflects the proportionately greater amount of biological control programs instituted by each of those countries where early impetus was provided by the proportionately greater losses that those countries have suffered from introduced pests.
X. There are of course many other countries reporting successful cases of biological control. Many of these are represented by only one or two successes that resulted largely from trans-shipments of biological control agents of proven value following their initial successful employment in other countries. Four insect pests that have been controlled in this manner in various countries are:
A. Cottony-cushion scale controlled by the Rodolia (Vedalia) beetle in 55 countries following its initial success in California.
B. Woolly apple aphid controlled by Aphelinus mali in 42 of 51 countries into which it was introduced following its initial
success in New Zealand.
C. White peach scale controlled by Prospaltella berlesei in 5 countries following its initial success in Italy.
D. Citrus blackfly controlled by Eretomocerus serius in 9 countries following its initial success in Cuba.
XI. Pest Groups.
A. Further analysis reveals that 55% of the 107 pest species brought under some measure of biological control through 1960
belong to the Homoptera, nearly 40% of which are scale insects.
B. 20% of the pests are Lepidoptera.
C. 17% are Coleoptera.
D. 8% belong to other taxa.
XII. Natural Enemy Groups.
A. Since a majority of successes have involved coccids, it follows that a large proportion of the natural enemies involved in
biological control success have been natural enemies of scale insects: Hymenoptera-- Encyrtidae & Aphelinidae
B. This grouping will probably change as more emphasis is given to nonhomopterous pests.
C. For weed control, Homoptera-Hemiptera, Thysanoptera, Coleoptera, Lepidoptera, Diptera and Hymenoptera.
XIII. It is suggested that biological weed control has registered a proportionately greater measure of success than biological
control of insect pests.
A. Only during the last few years has the method been used against weeds other than those infesting relatively stable,
B. Weeds engage in intense competition for space, water and nutrients with other plants, and the competitive advantage of
these other plants may be strongly favored by further additional insect injury to the weeds.
C. Plant injury by weed-feeding insects may be attended and intensified by the action of plant pathogens.
D. The work has been necessarily restricted to promising prospective biological control agents.
E. Unlike insect hosts, plants do not always die from the attack of a single insect. The greater numbers of natural enemies
that are thus generated at low host densities makes for a greater searching effectiveness on the part of biological weed
Bellows, T. & T. W. Fisher (eds.). 1999. Principles and Application of Biological Control. Academic Press, San Diego CA. 1046 p.
Clausen, C. P. (ed.). 1978. Introduced Parasites and Predators of Arthropod Pests and Weeds: A World Review. U. S. Dept. Agric., Agric. Handbk. No. 480., Washington, D. C. 545 p.