Biological control – how nature is our friend and our foe

By Simran Patel

Pests threaten lives and livelihoods when they damage crops¹. While it’s easy for people to directly blame pests, outbreaks are the result of poor ecological decisions by humans. We grow monocultures in which every crop is susceptible to the same pest, and pests can hitchhike ships and planes to invade places they wouldn’t be able to reach otherwise². Farmers try and fight these enemies using broad-spectrum pesticides, which kill off-target insect species and infiltrate our food³. Plus, repeated pesticide usage over the last few decades has provided a selective pressure, so pests are now resistant to our defences². Fortunately, nature has an effective defence that has evolved over millions of years – natural predators and parasites. Biological control is when farmers introduce natural enemies to their land to drive down pest numbers. Although it has only recently gained traction as a pesticide alternative, ants were used as pest control in China before the Common Era⁴. Alas, humans have treated nature as our friend for way longer than as our enemy. 

There are 3 types of biological control. Classical control brings the natural enemy of an invasive species from their native habitat to the farm, and letting a self-sustaining population grow⁴. Since the population of the natural enemy or control agent grows on its own, classical control is low-maintenance¹. Augmentative biological control is higher maintenance, involving repeated introductions or one massive introduction to overwhelm the pest population⁴. Since the control agent for classical and augmentative control is from a different habitat, scientists need to ensure it won’t become invasive like the pest. There is a chance that the natural enemy will prey on animals other than the pest, called off-target effects. Off-target effects are more devastating in places with high endemism levels, such as island nations². To prevent this, the control agent needs to be tested on phylogenetically and ecologically similar organisms to the pest⁵. Research also needs to be done on whether the control agent is adapted to the foreign conditions, otherwise its population may die out⁶. Fortunately, nature can be our friend regarding off-target effects too: other organisms in the agroecosystem and abiotic factors buffer changes in pest, control agent and off-target population sizes². 

Besides, the third type of biological control removes the invasion worry altogether. Conservation control involves making the habitat around a farm optimal for any native enemies of the pest. This fosters a large population of the native enemy, from which they can disperse to the rest of the farm⁷. Conservation control involves growing plants in which the control agents overwinter, or nectar-producing plants for them to feed on⁴. It has the additional benefit of diversifying the farmland plant community, reversing the biodiversity losses from converting the land to monoculture⁸. While the ideal classical or augmentative control agent is specific to the pest and entirely dependent on it for survival, conservation control agents are generalists. This means the control agent won’t run out of food when the pest is exterminated, so its population will persist to prevent pest resurgence⁸. With all these benefits, Gontijo suggests conservation control principles should be applied to classical control agents, i.e. making patches of agroecosystem resemble their native habitat⁷. Thus, classical, augmentative and conservation control approaches can work in tandem to fight our pest foes without damaging our health or the environment.

Regardless of the control type, the biology of the pest and the chosen natural enemy are crucial success factors. Most crop pests are insects, from defoliators to frugivores. Although insectivorous birds are an option to control insect pests, predatory and parasitoid insects are most commonly used⁸. When an insect feeds on a crop, either it releases compounds as it feeds³ or the plant releases compounds signalling that it’s damaged⁴. These chemicals attract our friends the parasitoid wasps, which inject their ovipositors into the pests so eggs are laid inside³. For example, the wasp Doryctobracon longicaudata parasitises fruit fly larvae. Its ovipositor has to penetrate fruit and then find the larvae inside, which it does by sensing vibrations as the larvae feed on fruit³. The parasitoid eggs hatch inside the pest, and the parasitoid larvae eat the pests from the inside out. Some wasps parasitise pest eggs, such as Doryctobracon aerolatus with the aforementioned fruit fly. Others parasitise different life stages of the same pest, such as Cephalonomia stephanoderis eating eggs and adults of the coffee berry borer beetle⁸. By targeting different stages of the life cycle, fly adult development is reduced further³ and resurgence is prevented. 

Despite preventing pesticide resistance and pest resurgence, insect control agents are controversial. If insecticide was used on a farm before switching to biological control, residual insecticide could kill the control agent too – an off-target effect⁷. If the pest has a faster growth rate than the parasitoid, which is the case for the fruit fly³, the time spent in the preferred growth stage for parasitism may be too short and no flies will be parasitised. Although host-specific parasitoids reduce off-target effects, the specific conditions they need to be reared in may be expensive to maintain⁴. Finally, since insects can fly across national borders, continent-wide efforts may be required to control them. For example, the cassava mealybug Phenacoccus manihoti was combatted across Africa and Southeast Asia by introducing the host-specific parasitioid Anagyrus lopezi¹. Importing the parasitoid to each infested country probably required extensive and expensive paperwork², which wouldn’t have existed if humans didn’t view nature as the enemy. 

Thus, biological control lies at the intersection of plant science, entomology, ecology, and politics. Ensuring the sustainability of our food system will require more research in these fields, especially if we are to avoid introductions become invasive. But wasps alone won’t solve the systemic issues that made pest outbreaks so common in the first place – we need to acknowledge that nature is our friend and humans are the real foe.

References

1. Wyckhuys KAG, Wongtiem P, Rauf A, et al. Continental-scale suppression of an invasive pest by a host-specific parasitoid underlines both environmental and economic benefits of arthropod biological control. PeerJ 2018; 6: e5796.
2. Messing R, Brodeur J. Current challenges to the implementation of classical biological control. BioControl2018; 63: 1–9.
3. Paranhos BJ, Nava DE, Malavasi A. Biological control of fruit flies in Brazil. Pesquisa Agropecuária Brasileira; 54. Epub ahead of print 11 April 2019. DOI: 10.1590/S1678-3921.pab2019.v54.26037.
4. Wang Z, Liu Y, Shi M, et al. Parasitoid wasps as effective biological control agents. Journal of Integrative Agriculture 2019; 18: 705–715.
5. Lefoe G, Hauser CE, Steel J, et al. Systematic cultivar selection for weed biological control risk assessment. Biological Control 2022; 165: 104816.
6. Canavan K, Canavan S, Harms NE, et al. The potential for biological control on cryptic plant invasions. Biological Control 2020; 144: 104243.
7. Gontijo LM. Matching landscapes and managing habitats to enhance classical biological control of arthropods. Biological Control 2022; 169: 104896.
8. Escobar-Ramirez S, Grass I, Armbrecht I, et al. Biological control of the coffee berry borer: Main natural enemies, control success, and landscape influence. Biological Control 2019; 136: 103992.

Article written in June, 2022