P. S. Messenger

Use of Life Tables in a Bioclimatic Study of an Experimental Aphid-Braconid Wasp Host-Parasite System

to environmental extremes of drought and flash floods. 2. The natural life span for most Rhinichthys oscults in the Chiricahua Mountains is less than 3 years. None was found to reach 4 years. 3. High mortality rates during summer drought must be interpreted, in part, as an acceleration of death among older fish since most fish are expected to die before they are 3 years old. Among younger fish, drought directly elevates the mortality rate. 4. Mortalities during a drought are caused directly by the disappearance of water and indirectly by starvation of the fish which are crowded into reduced habitat with inadequate food. 5. Flash floods are an important cause of mortality among fish of the year if they occur while the fish are very small. The greatest potential loss of fish of the year occurs in late summer when the initial flash flood induces major reproduction and is then followed by another flash flood. 6. Flash floods are not a significant cause of mortality among older fish. 7. Temporary waters may persist in some sections through a period of wet years. The populations of fish inhabiting them are derived from upstream sections during a flash flood. All fish that were located in downstream temporary sections in the spring of 1960, were one-year-olds. 8. Temperatures do not rise to lethal levels in any of the flowing streams. In exposed shallow isolated pools, the temperatures may become lethal for oldcr fish, but not for fish of the year. 9. Predators play a minor role. Only one, the garter snake, Thamnophtis cyrtopsis, is common and apparently important. 10. Although fish populations are greatly reduced by a year of drought, there is no immediate threat of their local extinction.

Fecundity, Reproductive Rates, and Innate Capacity for Increase of Three Parasites of Therioaphis Maculata (Buckton)

Three species of hymenopterous parasites of the spotted alfalfa aphid were reared over a range of constant temperatures. At each thermal level, life—table data were obtained for each species. These data were used in computing certain statistics concerning reproduction and rates of potential population increase which are valuable in assaying the effectiveness of each species as an aphid parasite. The temperatures studied ranged from 10 to 35°C, and the relative humidity was held as constant as possible between 40 and 60%. Artificial illumination and photoperiods were identical in all studies. The studies showed that the braconid, Trioxys utilis Muesebeck, generally has the highest mean total fecundity of the three species, although the aphelinid, Aphelinus semiflavus Howard, produced more eggs in the temperature range of 18 to 22°C. The gross reproductive rate and net reproductive rate of T. utilis were also higher than the other parasites throughout most of the temperature range studied, although again A. semiflavus showed higher reproductive rates between 24 and 27°C. The innate capacity for increase, rm, of T. utilis was decidedly the highest of the three parasites at all temperatures. The braconid, Praon palitans Muesebeck, was inferior tot he other two parasites in nearly all phases of these life—table statistics. Its inclination to enter a facultative hibernal diapause at relatively mild temperatures, plus its intolerance of higher temperatures, which the other two species endured, limited its thermal range of effectiveness. Statistics such as total fecundity, gross reproduction, rate, net reproduction rate, and innate capacity for increase are discussed relative to their value in indicating the potential effectiveness parasites can be expected to possess in the field.

Laboratory Studies on Competition Among Three Parasites of the Spotted Alfalfa Aphid Therioaphis Maculata (Buckton)

Three species of solitary endophagous parasites of the spotted alfalfa aphid were tested under constant temperature conditions to obtain information on the reproductive behavior and competitive ability of each in relation to the others. The braconid, Trioxys utilis, was found at both 21 and 27°C to be a very effective competitor because of its ability to find hosts rapidly, to oviposit rapidly and frequently, because of the ability of its larvae to compete successfully with other larvae within the host, and because of its habit of parasitizing young aphids before they become attractive to other parasite species. However, it tends to be wasteful of eggs, in the laboratory at least, since under some situations it does not readily discriminate parasitized from unparasitized hosts. Corresponding with this propensity toward superparasitism is its habit of multiple parasitism. A second braconid, Praon palitans, is somewhat less efficient in its search ability, oviposition rate, and host discrimination than T. utilis. It tends to waste eggs through excessive amounts of superparasitism. It also multiple—parasitizes hosts freely. It parasitizes older host stages, which if already parasitized by other species places it at a disadvantage. However, the larvae of this species are excellent competitors within multiple—parasitized hosts. The third parasite, Aphelinus semiflavus, is inferior to the other species in its slowness to find and parasitize hosts. However, it discriminates to a high degree parasitized from unparatized hosts, and tends to avid wastage of eggs through superparasitism or multiple parasitism. Larvae of this species, when engaged in internal competition with other species through multiple parasitism by the latter, generally fail to survive.

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).