The damage through insect pests in crops has always been a difficulty in agriculture, causing humanity to search for various ways to control the population of insect pests. Biological control is one of the oldest methods that has been used to contain insect pests, with its first recorded usage in Ancient China, thousands of years ago. It relies on the natural enemies of the insect pests and is a method that has been included in the Integrated Pest Management. It has three main strategies, which all use predators or parasites as biological control agents. Importation, the most common of the strategies, is used to control invasive insect pests through imported natural enemies. The natural enemy that is imported has to have a range of qualities to ensure that the environment is not harmed by its introduction. Since the enemy is a neophyt as well, the establishment of it in its new environment can be a limiting factor to the successful control of the pest. Augmentation relies on enhancing native enemies by mass rearing and mass release. The third strategy, conservation, aims at regulation of the habitat to ensure a natural rise in the natural enemies’ populations.
Insect-borne damage in crops is diverse and enormous. Ranging from aesthetic damage from larvae in apples, transmitting plant-diseases such as aster yellows of carrots which is carried by certain locusts (Agrios. 2005) to destroying a significant amount of crop through consumption like the African Migratory Locust in 1931 (Bullen. 1966), insect pests can cause financial loss for producers and also famines.
The consequences of insect-pest damage has resulted in the fight to try, contain and control the pests has always been an important aspect in farming. Starting with natural remedies at the end of the 17th century such as nicotine extracts and evolving within the middle of the 20th century resulting in the production of synthetic insecticide (Tomizawa, Cashida. 2003). The control of insect pests and their direct and indirect damage seemed to be under control.
The prime example for chemical control during the 20th century is Dichlorodiphenyltrichloroethane. DDT is a broad range insecticide and works as a neurotoxin, causing death in larvae and adult insects by slowing down the closing of sodium channels in the nerve cells which leads to uncontrollable spasms (Vijverberg, van der Zalm.1982). It is cheap to produce, and it helped to control not only insect plant pests, but also to contain the diseases which were carried by insect vectors such as typhus and malaria (Dunlap. 1981).
DDT was banned by the end of the 20th century in all developed states of the western world. While it was deemed low in indirect toxicity for humans, the effects of the chemical on non-target insect species, partly important pollinators and partly on natural predators of insect pests such as the European spruce sawfly (Diprion hercyniae) (Carter, Kenney. 2018), had not been considered. As an organochloride, the chemical is lipophilic and easily bioaccumulated in the fat tissue of animals that consumed the insects that had died because of DDT (Connell, Lam et al. 1999).
The DDT scandal caused a deeper investigation in the environmental effects of synthetic produced chemical insecticides and while chemical insecticides were not abandoned due the low cost and broad efficiency spectrum, the control about how and when they should be used became a subject for discussion. This led to the establishment of the Integrated Pest Management, short IPM. It was incorporated in US law in 1972 (Acosta. 1995). The IPM consists of 6 basic components, which aim to add other common ways of insect control next to chemical control to the picture and shifted the focus on eradicating the pest to controlling (Sandler. 2010).
One of the components of IPM is biological control, a pest controlling method that had been around far longer than the chemical insecticides, more precisely since the establishment of agriculture in Ancient China (Shijang. 1983). While biological pest control was largely ignored during the arrival of the cheap chemical insecticides in the period of 1930 – 1960, the interest in it rose again after the DDT scandal and its aftermath. There are three basic strategies known in biological pest control: Importation, augmentation and conservation (Mahr, Ridgway.1993).
All three strategies concentrate on using predators and parasites as natural biological control agents to contain and control insect pest populations. When the predatory and parasitic insects is native to the environment and enhanced with additional rearing of offspring and mass release as suggested by Hartig in 1803 (van den Bosch, Messenger. 1982), the strategy is known as augmentation. If they are introduced as neophytes, the strategy is known as importation. If they are present and do not reach their full potential due to environmental restrictions, a reestablishment takes place which is known as conservation.
An early example of the usage of the import and establishment of non-native predatory insects is the Cottony Cushion Scale Project, which had the target to fight the invasive insect pest, Icerya purchasi, which had originated from New Zealand in 1868 and threatened the harvest of the Californian citrus industry for 20 years (Quezada, de Bach. 1973). The lack of natural predators and the not yet existent chemical insecticides exacerbated the fight against the pest. Only after their natural predators were imported from New Zealand, the predatory beetles Cryptochaetum iceryae and Rodolia cardinalis, the pest was under control (van den Bosch, Messenger.1982).
The imported biological agent must have certain qualities which enhance the chance of establishment in the new environment and to reduce the side-effects on biodiversity caused by the agent hunting non-target species. These qualities include:
- Narrow host range, enabling the agent to concentrate mostly on the targeted pest as food source.
- Climatic adaptability, since the establishment of the agent could fail if the agent is not able to survive the environmental conditions in its new habitat.
- Synchrony with host life cycle, to make sure that the agent is alive and active when the pest is. This, in combination with an increased reproductive potential is especially important if the agent is parasitic.
- Efficient search ability, to ensure that the agent can locate and attack the pest.
- Short handling time, since the increasing speed in consumption and reproduction, including a short juvenile phase in predators, makes the agent more effective.
- Survival at low host density, the efficiency of the agent should not eradicate the host, since the narrow host range would cause the agent population to collide if the host as its only food source is absent. (Latifian. 2016)
Up to this day, over 6000 importations of a variety of over 2000 agents have been implemented to contain insect pest and control the damage caused by the insect, both through consumption and the transmitting of diseases. Over half of them was focused on pests that target woody plants (Kenis, Hurley et al. 2017).
The use of biological agents that possess the qualities mentioned earlier, has several advantages compared to chemical and even non-chemical control methods mentioned in the IMP. Those advantages become more visible when the targeted pest is not a classical agricultural pest but a pest of certain trees growing in either a forest or artificially in urban territory. While the use of insecticides is banned in cities and forest worldwide due to the difficulty of maintaining the chemical balance of the environment and of ensuring the safety on non-target organisms, other management strategies like the manual control strategy also work poorly in wooden environment (Pschorn-Walcher. 1977). The biological agents are most often also insects or microbial agents such as Bacillus Thuringiensis and nematode agents, and are therefore small, mobile and effective. Their success-rate in general is influenced by environmental challenges and the reaction of the target against the agent, because the agent is a living being instead of an insecticide used in chemical control or a net used in mechanical control. The first step to success is the establishment of the species in its new habitat, which is managed by 33 % of introduced species. The second step, the targeting and containing of the pest has a success rate of around 10 % (Kenis, Hurley et al. 2017). It is notable that the success rate in woody areas is significantly higher as presented in Figure 1, standing by around 36 % for establishment and around 13 % for successful control of targeted pests. This could be caused by a broader variety of available niches.
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Figure 1: Percentages of (A) introductions of parasitoids and predators leading to establishment (B) introductions of parasitoids and predators leading to success (C) target species successfully controlled in classical biological control programs worldwide until 2010
Augmentation, the second main strategy of biological control, relies on population regulation by natural predators and parasites (Mahr, Ridgway.1993). Establishment in a new habitat is not an issue. It is based on the knowledge of the existence of natural predators and the acknowledgment that the numbers of the individuals of the predator species is not sufficient to keep the pest under control. This could be due to environmental challenges for the predator species, which keep them from generating enough surviving offspring (Nafiu, Dong, Cong. 2014). This strategy can be used against native and non-native pests, since native predators can adapt to the prey as a new food source. Since this process may take time, importation is often used as a first try to control the invasive pest, but the establishment of the new predator species might be unsuccessful as in the fight against the fall webworm (Hyphantria cunea) (Yang, Wang, Zhang. 2014).
The fall webworm is a moth, native to North America and spread to Europe and Asia during the 1950’s. It is a general feeder and can adapt easily to new habitats. While it does not harm fully grown trees, younger trees are vulnerable to defoliation, which can cause severe damage and lower the chance of survival. The importation of a primal parasite from North America to target the pest had been unsuccessful twice (Clausen. 1978, cited by Yang, Wang, Zhang. 2014). Therefore, the strategy shifted on augmentation and investigation showed that the fall webworm was targeted by two predators and 23 parasitic wasps. Through further analysing the different potential agents, for their ecology, reproduction, life circle and efficiency in targeting the pest, the researchers determined an endo-parasitic wasp as the most promising biological control agent (Yang. 2003). Due to lack of a low host range, the number of individuals needed to successfully control the pest was determined to be very high. The total number of wasps reared for mass release between the years of 1986 and 2012 in the areas with the highest pest density was 325.54 billion for an area of 235,000 ha. They were released on a yearly basis since 2000. In combination with the high density and the parasitic and predatory behaviour of the other natural enemies of the fall webworm, the success-rate rose to 96 % and since 2004 the pest has been under complete control (Yang, Wang, Zhang. 2014).
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Figure 2 Percent of defoliation caused by consumption through adult Japanese beetles
The mass rearing and seasonal release of insects and other biological control agents for immediate reduction of the pest population is known as inundative release and is one of two methods in augmentation (Mahr, Ridgway.1993).
The second method is known as inoculation and is a preventive method in which smaller amounts of most microbial or nematode agents are released to prevent the establishment of an economical damaging pest population (van den Bosch, Messenger. 1982). A popular example for inoculation is the release of a microbial agent against the Japanese beetle Popillia japonica. The Japanese beetle is an invasive pest in the east of USA and, as a general feeder of crop and ornamental plants, causes damage through consumption (compare Figure 2). The adult beetle targets over 300 wild and cultivated plant species and especially damage corn and soybean crops, while the larvae, living in the soil, only targets the roots of short grass species. The microbial agent chosen for the control of the pest is a natural soil bacterium known as Paenibacillus popilliae. Its exclusive host is the Japanese beetle in which is causes ‘Milky disease’ in the larvae. The disease kills the larvae and upon its death, it releases around 2 billion spores of the bacteria (Shanovich, Dean et al. 2019). This causes a high density of the bacteria and is a successful way to prevent the establishment of large beetle populations. Because the microbial agent only and solely targets the pest, the effects on the environment is minimal.
The third strategy is known as conservation. Conservation aims to maintain or enhance the efficacy and survival of native enemies of the pest. Instead of mass rearing or implementation of smaller numbers, the focus of this strategy is placed on preventing the loss or re-establish the habitat of the natural enemies, minimising disturbance caused by pesticides and other anthropological causes (Begg, Cook et al. 2016). Successful conservation interventions in the fight against pest include the enhanced efficiency of parasitoids of pollen beetles in oilseed rape (Thies and Tscharnthke. 1999, cited by Beeg, Cook et al. 2016). The process of conservation can be seen as an enhancement of biodiversity and the reduction of intense cropping, and, while its generally slower and less precise in its methods, can also lead to successful pest control.
All three biological control strategies aim to contain the damage caused by insect pests without having a negative long-term impact on the environment. While insecticides are still used more frequently in agriculture, the prospect of fighting the pests in a more natural manner is a method that can help re-establish the natural balance. Further research may enhance the success-rates and establish a broader interest of agricultural farms in focusing the insect control methods on biological sources instead of chemicals.