16.6 Biological control of arthropod pests

We have just described a range of arthropod diseases that can govern the development of arthropod populations in nature. They kill specific arthropods and in so doing produce an increased supply of infective agents that can kill more of the same arthropod. Put in those terms, and against the background of our need to produce crops that are subject to attack by numerous arthropod pests, and suffer diseases that are transmitted by other such pests, it is not at all surprising that great research effort is devoted to attempting to harness these diseases into biocontrol agents that can be used to attack those arthropods that we view as pests. Biological control has been defined as the practice by which the undesirable effects of a pest organism are reduced through the agency of another organism that is not the host, pest or pathogen, or man.

Synthetic chemical insecticides and acaricides have been the most important element of arthropod control programmes since the discovery of the insecticidal properties of DDT in the late 1930s. They remain the most important element today because they each:

  • have a broad spectrum of activity that makes them effective against several different types of arthropod;
  • are highly efficient;
  • are cheap and easy to produce;
  • are persistent, and continue to kill pests throughout the season or the field life of the crop.

Chemical pesticides have served us well in the latter half of the 20th century by reducing the economic damage caused by arthropod pests and by reducing the incidence of many of the diseases of plants and animals that are transmitted by arthropods because of this combination of properties.

But these same properties have also created the problems widely associated with use of chemical pesticides. Because they are highly efficient and cheap and easy to produce they were rapidly brought into heavy and widespread use. Their broad spectrum of activity lead to their use throughout the world and in all circumstances in which control of arthropod pests and/or vectors of human and animal diseases was an issue, even if only minor. Their persistence combined with this extensive use resulted in many of these pesticides becoming unnatural components of the natural environment, contaminating land and food and water supplies. The varied results included:

  • development of resistance in many pest populations;
  • emergence of new pests as old pests are removed;
  • elimination of natural enemies of pests;
  • disruption of natural ecosystems as non-pest organisms were also killed;
  • accumulation of pesticides through food chains to adversely affect bird and mammalian predators.

All of these negatives directed attention towards increased emphasis on alternative control agents and strategies, which include:

  • biological control (biocontrol) agents;
  • environmental management;
  • use of pheromones to control pest mating;
  • genetic modification to produce arthropod-resistant plants;
  • combination of these tactics into integrated pest management (IPM) programmes.

Naturally occurring pathogens of arthropods are good candidates as biological control agents and many species are already employed, at least on a small scale, to control arthropod pests in glasshouse crops, orchards, ornamentals, turf and lawn grasses, stored products, and forestry, and for moderation of vectors of animal and human diseases (see review by Lacey et al., 2001). There are several theoretical advantages of using microbial biocontrol agents although attempts to harness their potential so far have had comparatively minor commercial success. Among the undoubted advantages of biocontrol are:

  • efficiency and potentially high specificity;
  • natural
  • safety for humans and other non-target organisms;
  • reduced chemical pesticide use and consequent reduction of residues in food and the environment;
  • preservation of natural enemies of the pest;
  • maintenance of biodiversity in managed ecosystems.

But there are some negative aspects of biocontrol:

  • biocontrol agents can be costly and difficult to produce in quantity;
  • they can have short shelf life;
  • the pest must be present before the pathogen can be usefully applied (so prophylactic, or preventative, treatment is difficult).

The use of entomogenous fungi for biological control has been disadvantaged by the need for high humidity (80% and above) during the prolonged period required for fungal spores to germinate and then penetrate the surface of insects because, unlike bacteria, these fungi attack insects through their cuticle, not their digestive tracts. To try to overcome this problem researchers have developed oil-based and other formulations of fungal spores for use in biological control. Verticillium lecanii (Lecanicillium muscarium) is an effective biocontrol of Myzus persicae and other aphids on chrysanthemums because the crop is grown in a glasshouse in which humidity can be controlled. Furthermore, the crop is ‘blacked out’ with polythene sheeting from mid-afternoon until morning during the summer to control the initiation of flowering (it’s a ‘short-day’ flowering cycle) and this helps to create a high humidity around the plant in which the fungal spores can germinate and infect the aphids. A single spray of spores given just before ‘black-out’ gives satisfactory control of the aphid within 2-3 weeks. Spores of Verticillium lecanii can be used to control whitefly as well as aphids.

It is much more difficult to use entomogenous fungi for biological control in the field, although there is promise for biocontrol of pests that have an aquatic stage. Among 85 genera of entomopathogenic fungi only six species are commercially available for field application: Aeschersorzia aleyrodis, Beauveria bassiana, B. brongniarti, Metarhizium anisopliae, Paecilomyces fumosoroseus, and Verticillium lecanii (Hussain et al., 2012). Of importance is the fact that recent research has improved the prospect of using insect fungal pathogens for control of diseases such as malaria that depend on insect vectors (Blanford et al., 2005; Scholte et al., 2005; Thomas & Read, 2007). Recently, a specific fungal pathogen of anopheline mosquitoes, called Metarhizium pingshaense, has been genetically engineered to transmit transgenic insect-selective toxins derived from a spider (Lovett et al., 2019). Tests in near-to-field conditions (using a 600-m2 structure, called the MosquitoSphere, built like a greenhouse but with mosquito netting instead of glass) the fungus eliminated 99% of mosquitoes released into the MosquitoSphere within a month. This is amazingly effective; but genetically modified organisms can face steep regulatory obstacles, so the fungus is a long way from real-world use. Nevertheless, this sort of approach is bound to contribute to the new technologies that will manage insect disease vectors in the future.  

So, although several fungi are in use or have promise, the most widely used microbial control agent of arthropod pests now is the bacterium Bacillus thuringiensis (Lacey et al., 2015). This is used as the model for biocontrol mechanisms and advances in understanding its molecular biology, mode of action, and resistance management contribute to development of fungal control agents (mycoinsecticides), especially insights into their pathogenic process, enzymes involved with the penetration of the host cuticle and the role of insecticidal fungal toxins (Charnley, 2003). Transgenic plants expressing endotoxin genes from Bacillus thuringiensis have been generated to protect crops (tobacco, cotton and maize) against attack from pests. It is possible that the same technology could be applied to toxins produced by entomogenous fungi.

Increased use of microbial control agents as alternatives to broad-spectrum chemical pesticides depends on:

  • increased pathogen virulence and speed of kill;
  • improved pathogen performance under challenging environmental conditions (cool weather, dry conditions, etc.);
  • greater efficiency in their production;
  • improvements in formulation that enable ease of application, increased environmental persistence, and longer shelf life;
  • better understanding of how they will fit into integrated systems and their interaction with the environment and other Integrated Pest Management (IPM) programme components;
  • greater appreciation of their environmental advantages;
  • wwider acceptance by growers and the general public.

Future emphasis is likely to be on IPM programmes, especially those featuring mixed infections because interaction between pathogens can improve virulence and reproduction of the biocontrol species and improve host-pathogen dynamics (Hussain et al., 2012; Lacey et al., 2015; Ortiz-Urquiza et al., 2015).

Updated July, 2019