HISTORICAL PERSPECTIVE

- term biological control has been used in a variety of ways.
- many people have tried to expand the definition of the term possibly in an attempt to add legitimacy (environmental sensitivity) to their own particular research endeavours.
- some people include in biological control:

1). the development and selection of plants which are more resistant to insects e.g. secondary plant substances - plant resistance.

2). cultural techniques - crop rotation or habitat modification.

3). genetic techniques - sterile male/insect release.

4). "third and fourth generation pesticides" analogues of insect hormones, or compounds such as pheromones.

5). microbials which are applied in a manner so that the mode of action is different than that of a natural system, e.g. Bt the bacteria is applied like a chemical; the dose/formulation used eliminates any need for the pathogen to replicate in the host or become established in the environment.

6). most products of biotechnology are sometimes classified as biological control - new term(s) has been coined for these products e.g. biorational pesticides.

- may all be useful control techniques, alternatives to the use of chemical insecticides and all that implies while demonstrating a greater awareness of the biology of the system, but they don't fit the original definition of biological control.

- exclude from biological control, except for comparative purposes.

- described as part of biological insect pest suppression programs.

- include some as part of natural control.

- coined by Harry S. Smith in 1919 to apply to:

the control or regulation of pest populations by natural enemies, i.e predators, parasites and pathogens.

Handout 1 (upper) = components of natural control of population numbers.

see HANDOUT

density-independent factor: any factor causes mortality independent of the population density. e. g. heavy frost killing plants.

density-dependent factor: a population-regulating factor that changes in intensity with changes in population density. e. g. Intraspecific competition - how does this operate?

nonreciprocal: resource doesn't change but interactions of population using the resource change as their populations fluctuate.

e. g. space - aphids sensing crowding may produce migratory forms before aphid populations impact the plant - "controls" the numbers of users without changing the space available.

reciprocal: vary in numbers (magnitude) in response to changes in host numbers.

e. g. as the host population increases, predators, parasitoids and pathogens may increase and decrease as host poulation decreases.

- original definition restricts the term biological control to predators, parasites and pathogens, among all natural control factors as they relate to the control of insect pests.

- some argue that biological control must also be simply the reestablishment of interactions that once existed and once reestablished humans need to do very little.

- define more on a case-by-case basis.

- the earliest use of one insect species to control another insect species in agroecosystems probably predates the written record.

- as one might expect, the earliest records come from the Chinese.

e.g. book published in China around 900 A.D. describes the use of predatory ants to control caterpillars and large boring beetles by citrus growers.

ants, Oecophylla smaragdina F weaver ant

- build large paper nests in trees.
- contain large numbers of individuals.
- can be transported from wild to domestic trees.
- can be transported from tree to tree.
- alternatively poles set up between trees to facilitate movement among trees.
- become a source of revenue-farmers bought and sold colonies.
- a current practice as an alternative to chemical control (1987).

- around 1775, Yemenese transported predatory ants from the mountains to oases to feed on insects feeding on date palms.
- the major pest of the date palms were also ants.
- indicated their ability to differentiate between two similar species at least on the basis of their food sources.
- considered to be the first documented case of movement over a substantial distance of natural enemies for the purposes of biological control.
- two early examples of biological control demonstrate the use of one species of predatory insect to control another insect species, however the definition does not exclude other types of predators.

e. g. ,the red locust was a serious pest of sugarcane in Mauritius in the 18th century.

- mynah bird was imported from India in 1762 and by 1770; it was credited with the successful control of this locust.
- first documented case of international movement of a biological control agent.

- in Europe the early observers of natural history began to generate the information which later would be formalized as " biological control".

- readily visible for the reasonably careful observer of animal behaviour.

- another student of natural history, De Geer (deGeer) in the 1760's is reported to have said, "we shall never be able to guard ourselves against insects but by means of other insects".

PARASITISM

- 1602, first person to describe insect parasitism was Ulysses Aldrovandi published a description of the larvae of the braconid, Apanteles glomeratus, emerging from the cabbage butterfly, Pieris rapae (L.) where they spin their external cocoons. Unfortunately he misinterpreted these structures as the butterfly eggs.

- 1668, Francesco Redi described aphids parasitized by an ichneumonid, again not understanding the actual process however.

- 1701-1710 several people, including van Leeuwenhoek (microscopy), described the parasitic nature of the interactions between insect species including Aphidius sp. emerging from aphid.

- once nature of the parasite-host relationship was understood, descriptions of the biology of numerous parasites were published -1750's.

- One of the first to comment on the use of a parasitic insect to control a pest (1800) was Dr Erasmus Darwin.

- noted the destruction of cabbage butterfly larvae infestations after the deposition of eggs by an ichneumonid on the backs of caterpillars.

- also recommended the use of coccinellid beetles to control aphids in greenhouses.

Diseases in insects have been noted for a long time, certainly much before the nature of infectious disease was understood.

- early descriptions largely focused on insects that were of economic importance; e.g. diseases of honeybees and other insects were described by:

- Aristotle in his Historia animalium written some 2300 years ago.

- the Roman writer, Virgil, also commented on honeybee diseases some 300 years later.

- descriptions of diseases occurring in the silkworm were published in Japan around 1000 years ago, while the silkworm and related species were reared in China for at least 3000 years--possibly literature exists from these earlier periods.

- silkworms themselves can be considered to play a central role on insect pathology as a discipline, and one may even consider them as central to the development of the whole concept of infectious disease. With time the rearing of silkworms for the production of silk spread from Asia to Europe and North America. At various times devastating outbreaks of diseases destroyed large numbers of insects and uninfected stocks from one part of the world were sent to another part for considerable profit. In addition these diseases created a demand for causes and cures. One of the first individuals to take up the challenge was a man named Agostino Maria Bassi who examined silkworms with a disease known as calcino (in Italy) or muscardine (in France). He was able to demonstrate that the disease was caused by a "vegetable parasite" or fungus; and that the growth of the fungus leads to death of the silkworm and the production of an infectious agent which can be transmitted by inoculation, contact or contaminated food. He was even able to demonstrate that the infectious agent could be destroyed by certain chemicals.

- the date of these discoveries was 1833, although he didn't present his findings until 1834.

Louis Pasteur also was recruited to study the diseases of the silkworm, although not until around 1865. Pasteur was able to isolate several different micro-organisms from silkworms each of which caused diseases. These included bacteria and protozoa and he also described a disease which we now know to be caused by a small RNA virus, which of course he could not identify. Certainly one would suspect that the research Pasteur conducted on insect diseases aided in his efforts to understand vertebrate diseases.

The first scientific field test of microbial control was conducted by a Russian, Krassilstschik, who used the fungus against the sugar beet curculio- 50-80% mortality in test plots.