An adult cottony cushion scale on a citrus leaf
Biological control of many insects is regarded as the result of a stable interaction between a host and one or more of its natural enemies as a natural enemy complex where the resultant pest damage is less damaging and more acceptable (Hassell & Comins, 1978; Huffaker & Messenger, 1964; Eilenberg et al., 2001). A natural enemy complex can be such effective biological control that George Compere stated that no insect is a pest of sufficient gravity in its native environ- ment to justify artificial control measures, as nature always provides a means of control through parasitic or predacious insects (Compere, 1961).
Biological control is constituted by the orchestration of either some, or a combination of classical (introduction), conservation and, or augmentative biological control (Heimpel & Mills, 2017; Van Lenteren, 2012a). Of these mentioned, classical biological control is, by a great margin, the most effective approach (Rosen & DeBach, 1979; Van Lenteren, 2012a).
In classical biological control, the progeny of the initially released agents affects the control of the subsequent generations of the target pest. The importation of such exotic arthro- pods is thus inoculated into an environment with the aim of establishing (Elzen & King, 1999; Eilenberg et al., 2001). A textbook example of classical biological control, the Vedalia beetle, Novius cardinalis Mulsant (Coleoptera: Coc- cinellidae), was introduced from Australia to California to control the cottony cushion scale,
Icerya purchasi Maskell (Homoptera: Margaro- didae), which was an introduced species that was devastating the citrus industry in Southern California (Doutt, 1958). This project was the first outstanding success in classical biological control and attracted much interest in this method of pest control (Kennett et al., 1999).
Also, the introduction of A. lingnanensis into California from China in 1947 against the California red scale has been the turning point against this serious pest as an effective biolo- gical control agent (Rosen & DeBach, 1979).
Although not many other introductions of natural enemies have been as successful as the case with N. cardinalis and A. lingnanensis, the number of successful schemes in citrus far surpasses that of any other crop (Kennett et al., 1999; Rosen & DeBach, 1979).
Adapted table from “Types of classical biological control programmes based on the history of the pest and natural enemy association and the geographical origins of the pest and natural enemy with some examples (Eilenberg et al., 2001; Barrows & Hooker, 1981; Doutt,1958; Compere, 1961; Van Lenteren & Tommasini, 2003)”.
With conservation biological control, naturally occurring beneficial arthropods, such as natural enemies or pathogens of pests, are conserved and often enhanced to play an improved role in attacking pests (Heimpel & Mills, 2017; Eilenberg et al., 2001).
A severe cottony cushion scale infestation.
However, although it is a prerequisite that the specific agricultural field scenario contains a healthy and diverse community of plants and arthropods for conservation biological control to contribute meaningful to pest management, it is not a guarantee of favourable outcomes (Jonsson et al., 2017; Tscharntke et al., 2016).
Agriculture is a drastic modification of a natural ecosystem towards a remarkably simple agro-ecosystem with a reduced biodiversity. This is achieved by means of mostly chemical and mechanical weed control leading to monoculture where, in most cases, only the cultivated crop plant species remains. Such a disrupted and eventually simplified ecosystem inflicts ecological stresses on beneficial arthropods in higher trophic levels on which conservation agriculture pivots (Michaud, 2018).
In practice, the use of selective rather than broad-spectrum pesticides, alternating strip treatments with approved insecticides, cultivation of cover crops, and the use of certain crop varieties would help to maintain conservation agriculture (Rill et al., 2008; Grout, 2000; Grout, 2012b; Bedford, 1968; Moore & Richards, 2002; Garcerá et al., 2013; Michaud, 2018; DeBach & Landi, 1961).
Augmentative biological control entails the deployment of artificially mass-reared beneficial arthropods in inundative or inoculative quantities and fashions to agro-ecosystems with the aim of controlling certain agricultural pests (Heimpel & Mills, 2017; Gontijo & Carvalho, 2020; Michaud, 2018; Elzen & King, 1999; Eilenberg et al., 2001).
With inundative augmentative biological control, success depends on the immediate agents inundatively released and not on their progeny to achieve control (Elzen & King, 1999; Van Lenteren, 2012a). The beneficial arthropod that is supplemented might already prevail in the relevant environment but lacks agent to multiply in the new environment and then its progeny is to suppress or control the target pest organism, but not to a permanent extent (Eilenberg et al., 2001).
When exotic beneficial arthropods are to be augmented, they should first be subjected to the relevant environmental risk assessment of the relevant authority (Gontijo & Carvalho, 2020; Van Lenteren, 2012b). The idea of augmentative biological control was first promoted by Speyer in 1926 after he realised that the parasitoid, Encarsia formosa Gahan (Hymenoptera: Aphe- linidae), could provide satisfactory control of whiteflies Trialeurodes vaparariorum Westwood (Hemiptera: Aleyrodidae) in greenhouses in Britain (Speyer, 1927).
The adult stage of the Vedalia beetle was introduced from Australia to California.
Many challenges still exist to improve the efficacy of augmentative biological control. The main challenges with augmentative biological control are insecticide selectivity and resistance, public policies and perceptions, and the quality and fitness of mass-reared insects (Van Lenteren, 2012b; Bellows & Morse, 1993; Vasquez & Morse, 2012).
South Africa is one of the few countries where the indigenous natural enemies of native scale insects were able to attack the introduced red scale of citrus, making it an ideal environment to pursue conservation biological control (Bedford & Grobler, 1981; Compere, 1961). Searle (1964) listed four parasitoid and 31 predator species, all of which are natural inhabitants of South Africa, as enemies of red scale.
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