Robert van den Bosch

The defense reactions of three species of Hypera (Coleoptera, curculionidae) to an ichneumon wasp

Abstract The principal result of the investigation is to show that the parasite Bathyplectes curculionis exists in two strains, one in northern California parasitizing Hypera postica, and one in southern California developing on Hypera brunneipennis. About 94% of the eggs of the northern strain evoked a defense reaction in H. brunneipennis, 91% a reaction so strong that the parasite could not develop; about 50% of the eggs of the southern strain met a defense reaction in H. brunneipennis, but it was usually weak, and fewer than 15% of the parasite eggs were prevented from developing. Of the larvae of H. brunneipennis parasitized by the northern strain, not more than 10% would have been destroyed; of those parasitized by the southern strain, 100%. In Hypera postica less than 1% of the eggs of the northern strain, evoked a hemocytic reaction, and it was weak; but 15% of the eggs of the southern strain evoked a reaction and in some cases it was effective. Observations of secondary importance show that eggs of Bathyplectes that escaped encapsulation in one larva of H. brunneipennis were usually encapsulated when transferred to a second larva of the same species. Families of H. brunneipennis reared from isolated female weevils did not differ significantly in their reaction to northern parasites. A few instances were observed of larvae of Bathyplectes being encapsulated in H. brunneipennis. Eggs and larvae of Bathyplectes implanted in larvae of Hypera punctata were thickly encapsulated and melanized.

Biological control of insects by predators and parasites.

Abstract Serious problems are affecting the prevailing insect control strategy. The dilemma has been magnified by a neglect of biological control and disruption of natural enemy activity by the insecticides. A developing integrated control strategy promises a more effective role for biological control in the future.

The Ecological Basis for Biological Control

Biological control is a natural phenomenon that, when applied successfully to a pest problem, can provide a relatively permanent, harmonious, and economical solution. But because biological control is a manifestation of the natural association of different kinds of living organisms, i.e., parasites and pathogens with their hosts and predators with their prey, the phenomenon is a dynamic one, subject to disturbances by other factors, to changes in the environment, and to the adaptations, properties, and limitations of the organisms involved in each case (Huffaker and Messenger, 1964).

The History and Development of Biological Control

The purposeful control of insect and weed pests by biotic agents is a comparatively modern development, having become an effective technique in pest control only since about 1890. However, there are antecedent historical events that trace the evolution of some of the fundamental concepts in the development of biological control, and several of these events show the remarkable and perceptive insight of man into the workings of nature. Without these pre-nineteenth-century discoveries and conceptualizations, modern environmental science, to which biological control has made substantial contributions, would very likely have been much delayed. These discoveries and concepts include, among others, those of the balance of nature; population growth and limitation; natural control of numbers; the symbioses among different species, particularly those of plants, animals, and their natural enemies; and the roles such natural enemies play in the determination of abundance.

Life Table Analysis in Population Ecology

Given equivalent immigration and emigration, the rates of population births and deaths determine whether populations grow or decline. Immigration and emigration rates are, however, unlikely to be equal, and thus often influence the pattern of population growth. These notions are captured in Figure 7.1. Movements into and out of populations are difficult to assess in nature and are not dealt with directly, but the former would be analogous to births and the latter to deaths in this context.

Naturally Occurring Biological Control and Integrated Control

Faunistic surveys of agricultural, sylvan, or natural, undisturbed environments will disclose large numbers of herbivorous insect species that are of insignificant abundance, causing little or no harm to the plants growing in such habitats. Many of these insects are kept in check by native natural enemies. We describe this situation as naturally occurring biological control.

Economics of Biological Controls

Assessing the economic benefits and costs of imported biological control is difficult. Costs of research, quarantine, implementation, and overall organization are easy to measure, but many of the important benefits to agriculture and society are more difficult to quantify. The two obvious benefits of successful biological control projects may be seen in lower pest control costs to growers and increases in production. Even some partially successful projects (California red scale) requiring an occasional rerelease of natural enemies and the supplemental assistance of chemical and cultural controls may substantially reduce the total costs of control by 75% or more (DeBach, 1974). The total benefits to the ecosystem and the general public from lowered pesticide use are difficult to assess in monetary terms. What value can we place on a human life, a poisoned lake, contaminated groundwater, or future pesticide-induced cancers, mutations, or teratogenic effects? Taking a more practical view, biological control when successfully applied avoids many pesticide-induced problems such as pest resurgence, secondary pest outbreaks, phytotoxicity, pollinator mortality, pesticide resistance, and the above mentioned health problems. Clearly there is no easy way to estimate these benefits, but we all recognize that they are important. When an importation project is completely successful, the benefit accrues over future time, given continued production of one or more of the host crops.

Factors Limiting Success of Introduced Natural Enemies

Since the initial success against the cottony-cushion scale in California, by 1976 approximately 128 species of pest insects and weeds in many parts of the world have been completely or substantially controlled by imported natural enemies (Laing and Hamai, 1976). Despite this gratifying record, most attempts in classical biological control either have met with total failure, or have been only partially successful (Turnbull and Chant, 1961; Turnbull, 1967; Hall and Ehler, 1979). But this is not reason for despair, as the few limited successes have been of immense value, saving countless millions of dollars for growers and consumers, and have helped reduce pesticide use in agriculture. This record of course needs improvement, and only careful agroecosystem analysis of the factors limiting natural-enemy effectiveness will help show us the way.

Other Kinds of Pests and Other Biological Methods of Pest Control

On occasion, vertebrate species become pests when they are accidentally or purposely introduced from one area of the world to another. If the new habitat provides no natural population control, the population of the introduced vertebrate expands dramatically and an artificial means of population control must be implemented, either through harvesting or biological control. Examples of such introductions are: the marine toad, Bufo marinus L., in Australia, the African clawed frog, Xenopus laevus in Florida, the Norway rat, Rattus sp., throughout the world, and the European rabbit, Oryctolagus cuniculus L., in various regions of the world (Figure 11.1).

Microbial Control of Insects, Weeds, and Plant Pathogens

Microbial control is here defined as “The utilization of pathogens for the management of pest populations.” Nematodes, although not microorganisms, are considered to be “microbial control” agents because of the techniques involved in their utilization. Various types of microorganisms have been used in biological control, among them bacteria, viruses, fungi, and protozoa. The most important groups of microbial agents are discussed in this chapter. In the broadest consideration, pathogens are important naturally occurring biological control agents. In nature, microbial pathogens frequently cause epiphytotics and epizootics, which help maintain the balance of plant and animal populations. The role of microbes in the dynamics of organisms was observed early on by biologists, who eventually initiated studies to attempt to manipulate pathogens for pest control. This was the genesis of the microbial control tactic that in recent years has become a more significant weapon in the pest control arsenal. Microbial control has predominantly involved the artificial manipulation of pathogens, especially viruses, bacteria, fungi, protozoans, and nematodes formulated as sprays or dusts to suppress outbreaks or threatened outbreaks of pest arthropods (or weeds). Environmental manipulation (e.g., irrigation practice) is also utilized to enhance activity of certain pathogens and thus effect microbial control, although this is as yet a relatively poorly developed aspect of the tactic.