Ecological Engineering and Habitat Manipulation for Boosting Natural Enemies of Crop Pests

Ecological Engineering and Habitat Manipulation for Boosting Natural Enemies of Crop Pests
Pest, Disease and Weed Management

Soumik Dey Roy

Assistant Professor at Brainware University

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The Integrated Pest Management (IPM) concept as a philosophy arose in the 1960s, aiming to offer solutions to the problems of pesticide overuse and their effect on the environment. By providing an alternative to chemical, synthetic plant protective products, IPM relied on using several complementary tactics to protect the plants. 

IPM struggled to change farmers’ mindset, emphasizing tactics to suppress pests and reduce crop damage to practices of making the agroecosystem more pest-resilient. One way of further advancing the ecosystem management approach in IPM is through the understanding that crop health and sustainable yields depend on multiple factors like the soil health, the beneficial organisms (both on soil and the air), the availability and balance of nutrients and water, the quality of the propagation material and the agricultural practices applied. Robust, healthy plants can better withstand external threats and pressure (e.g., insects) and recover faster (Altieri & Rosset, 1996).

To achieve food security and environmental well-being in the long term, we need to understand these ecosystem services better and integrate their management into modern, productive, and environmentally friendly crop production systems. The expansion of monocultures has decreased the abundance and activity of natural enemies, reducing the option for natural biological control and making the agroecosystem increasingly vulnerable to pest invasion and outbreaks.

Monocultures generally do not constitute good environments for natural enemies (Andow, 1991). Such simple crop systems lack many of the resources, such as refuge sites, pollen, nectar, and alternative prey and hosts, that natural enemies need to feed, reproduce, and thrive. Normal cultural activities such as tillage, weeding, spraying, and harvesting can seriously affect farm insects. To the pests, the monoculture is a dense and pure concentration of its basic food resource – many insect herbivores boom in such fertilized, weeded, and watered fields. 

Many factors underlie the vulnerability of monocultures to pest invasions:

  • Decreased landscape diversity: There has been a consistent trend toward simplification that entails: 

(1) the enlargement of fields, 

(2) the aggregation of fields, 

(3) an increase in the density of crop plants, 

(4) an increase in the uniformity of crop population age structure and physical quality and 

(5) a decrease in inter- and intra-specific diversity within the planted field.

  • Decreased on-farm plant diversity: Many ecologists have conducted experiments testing the theory that decreased plant diversity in agroecosystems allows a greater chance for invasive species to colonize, leading to enhanced herbivorous insect abundance (Andow, 1983).
  • Pesticide-induced insect outbreaks: Many examples of insect pest outbreaks and/or resurgence following insecticide applications are reported in the literature (Pimentel & Perkins, 1980). Pesticides, especially when issued, can lead to even bigger problems (pests develop resistance to specific pesticide active compounds). More than cover500 species of arthropods have become resistant to a series of insecticides and acaricides (Van Driesche & Bellows, 1996).
  • Fertilizer-induced pest outbreaks: Luna (1988) suggested that crops’ physiological susceptibility to insects may be affected by the form of fertilizer used (organic vs. chemical-synthetic). Most studies evaluating aphid and mite response to nitrogen fertilization found a positive correlation between the increase in nitrogen rates and aphid and mite numbers. The example of the green peach aphid, Myzus persicae, is characteristic (van Emden., 1966), but there are more than 135 studies confirming this too (Scriber., 1984). 

Crop management practices should incorporate measures promoting landscape diversity to mitigate these negative impacts arising from landscape simplification. 

“Ecological Engineering” can be an effective tool for this purpose. 

Ecological Engineering and Habitat Manipulation for supporting crop pest control

Ecological engineering can be defined as the practice of joining the economy of society to the environment symbiotically by fitting technological design with ecological self-design. Ecological engineering is a conscious human activity and should not be confused with the more recently developed term “ecosystem engineering”. This refers to how other species shape habitats via their intrinsic biology rather than conscious design.

Habitat manipulation is an emerging strategy to enhance biological control in an agro-system by preserving or enhancing plant diversity or providing adequate refugia for pest’s natural enemies. It involves adjusting the cropping system to increase the numbers and actions of natural enemies of crop pests. Habitat manipulation is another form of conservation biological control. 

Conservation biocontrol aims to preserve the natural enemies available in the ecosystem to manage crop pests effectively. Contemporary habitat manipulation has its genesis in practices that have been used to promote generalist predators in agricultural systems for centuries. 

An example of an early habitat manipulation technique is the use of straw shelters to provide temporary spider refugia and overwintering sites during cyclic farming disturbances; used by Chinese farmers for over 2000 years and is still in use today also (Dong & Xu, 1984). Another example is connecting bamboo canes between citrus trees to enable predatory ants to move between the trees to control caterpillar pests (Van-Emden, 1989).

Landscape Engineering A Subset of Habitat Manipulation

Landscape Engineering: A Subset of Habitat Manipulation

A diversified agricultural landscape mosaic may sustain a broad diversity of natural enemies. Non-crop habitats are often favorable for natural enemies and act as source habitats from which the less favorable agricultural fields are invaded. Only when natural enemies have a year-round preference for non-crop habitats may they act as sinks relative to crop habitats. The extent to which a habitat functions as a source or sink depends on its quality and size relative to the surrounding habitats (Dunning et al., 1992). The benefit to the farmer of a diversified landscape in this context is increased when:

  1. the population of natural enemies is bigger and more diverse, 
  2. natural enemies substantially colonize arable fields, 
  3. they significantly reduce pest densities, thereby reducing damage levels and 
  4. increasing yield or quality and 
  5. benefits are more than the costs

Loss of biodiversity is a common problem-result of modern agricultural systems dominated by few types of annual crops (Stoate et al., 2001; Benton et al., 2003). Many organisms depend, at some point in their life cycle, on resources that annual crops cannot offer, and such species may be rare or absent in crop-dominated landscapes if their dispersal power is smaller than the distance to these other resources (Tscharntke and Brandl, 2004). Essential resources for biological control agents that are usually not available in annually plowed fields are pollen and nectar sources for adults of parasitoid Hymenoptera and Diptera, or perennial plant cover and an undisturbed soil surface for overwintering and larval development of a wide array of insects and spiders (Landis et al., 2000; Tscharntke 2000; Gurr et al. 2003).

“Landscape engineering” is the interdisciplinary application of engineering and other applied sciences to the design and creation of anthropogenic landscapes. The technique of modifying the crop landscape through various means is referred to as Landscape Engineering. It involves exploring the factors driving arthropod dynamics at the landscape scale.

The characteristics of the landscape significantly influence the prevalence of insect pests and the efficacy of natural enemy-based control measures. Essentially, landscape composition directly impacts pest abundance by influencing their distribution and reproduction and indirectly by disrupting their natural enemies. Additionally, the diversity and abundance of the natural enemy community are influenced by the composition of the landscape. A varied landscape fosters a greater diversity and abundance of natural enemies, resulting in more effective control of insect pests. Furthermore, diversified landscapes reduce pest populations, minimize damage, and enhance crop yield and quality. Diversified landscapes have the potential to affect both insect pest control and biodiversity conservation positively.

The underlying science behind landscape engineering follows the basic concepts of the “Push-Pull Strategy.”

The push-pull method operates by utilizing deterrent chemical cues to repel or deter pest insects (push) away from the main crop while concurrently employing highly attractive stimuli to draw the pests (pull) towards alternative areas of food such as trap crops (Khan et al., 1997). Although each component (the push and the pull) may not independently reduce pest populations below economically viable levels, their combination enhances the strategy’s effectiveness. Moreover, since the push and pull elements typically involve non-toxic substances, it is a practice that can be applied in biological control and organic agriculture (Khan & Pickett, 2004). While push-pull strategies are not novel, one of the most successful instances was recently developed in Africa for managing stem borers affecting cereal crops (Khan et al., 2001). Read more about that in the relevant article: The Push-Pull Strategy: Controlling Stemborers and Striga to Increase Corn Yields

Different Techniques of Landscape Modification

The various mechanisms of landscape modification are discussed below:

1. Alternative Hosts and Prey

Non-crop habitats maintain populations of alternative hosts and prey for the parasitoids and predators of crop pests (Denys & Tscharntke, 2002; Kozar et al., 1994; Pickett et al., 2000; Sotherton, 1984; Wyss, 1996). This enhances natural pest control by providing the natural enemies of pests with alternative hosts and prey during periods in which host and prey density is low in fields or by increasing the fitness of natural enemies. Many parasitoids and other natural enemies consume honeydew (Wäckers et al., 2005). The presence of sap-feeding alternative prey in non-crop habitats may, therefore, enhance the control of crop pests. However, habitats providing alternative hosts or prey may also accommodate pest species, thereby increasing pest populations. The different methods of providing alternative hosts are as follows:

a) Use of cover cropsCover cropping or living mulch is an important agro-ecological pest management practice in which annual or perennial herbaceous plants are planted before or after the main crop and used to cover the soil for a single season or the whole year (Reeves, 2017; Altieri, 2018; Kahl et al., 2018). 

b) Intercropping: Intercropping consists of multiple cropping systems in which two or more crops are grown at the same time in the growing season. It was reported that intercropping reduced pests and increased pests by 53% and 18% in experiments, respectively, in comparison to pure cropping. Intercropping acts on herbivores by partitioning their population between the crop and the intercrop, reducing pest pressure in the main crop (Root 1973). It also helps to deter or repel pests because non-crop visual or chemical cues change insect behavior, potentially reducing pest damage (González-Chang et al. 2017a). This form of habitat management also creates a physical barrier, restricting inter-plant pest movement or providing floral resources for the pests’ natural enemies (Smith & McSorley, 2000).

2. Alternative Sources of Pollen and Nectar

Adequate flower abundance and suitable vegetation structure are necessary to sustain a variety of insect populations (Zurbrügg & Frank, 2006). Consequently, altering structurally deficient habitats by introducing flowering plants and grasses can boost beneficial insect numbers in agricultural settings (Long et al., 1998; Kells et al., 2001; Rebek et al., 2005).

Seminatural habitats also act as sources of pollen and nectar, which are essential for many species (Pickett & Bugg, 1998; Wäckers et al., 2005). Several studies have shown that more diverse vegetation, including flowering weeds, for example, results in greater availability of pollen and nectar, leading to higher densities of carabid beetles (Lys et al., 1994), syrphid flies (Hausammann, 1996; Sutherland et al., 2001), and parasitoids (Patt et al., 1997; Powell, 1986). Many species take floral nectar, which can result in increased rates of parasitism (Powell, 1986). Extra-floral nectar is produced by various plants such as fava bean (Vicia faba) and cotton (Gossypium hirsutum) and is an important food source for adult parasitoids (Bugg, 1989).

To have the best result, it is essential to know the physiology and preferences of the natural enemies of interest so you can choose the most appropriate plant mix. 

3. Shelter and Overwintering Areas

Natural enemies of insect pests rely on shelter to survive environmental hazards like heat, cold, rain, or pesticide exposure. Adequate habitats support their foraging, resting, overwintering, or nesting activities. These habitats, found in various locations, including crop fields, can range from herbaceous and woody plants to human-made structures (Beane & Bugg, 1998). In some cases, specific shelters like lacewing houses have been constructed and tested to augment biological control efforts by increasing lacewing populations (McEwen & Sengonca, 2001). Perennial crop systems offer more stability for conservation biological control compared to annual systems, as resident natural enemy populations can persist across years. However, practices like harvesting entire fields of crops such as alfalfa disrupt this stability, prompting efforts to provide refuge for displaced natural enemies. Examples include augmenting leaf debris in orchards, wrapping tree bases with vegetable debris, or providing on-tree refugia in peach trees (Tamaki et al., 1968).

4. Agroforestry

Agroforestry integrates trees and/or shrubs with crops and livestock to optimize biological interactions on a landscape scale, aiming to balance ecosystem needs while meeting agricultural demands sustainably (Nair, 1993). Although research on pest interactions within agroforestry is limited, it is generally believed to mitigate pest outbreaks typical in monocultures. Various agroforestry designs can influence pest populations through factors like microclimate, nutrition, and natural enemies, with landscape complexity potentially enhancing predator and parasitoid populations by providing shelter and alternative food sources (Landis et al.). Studies indicate that landscape complexity and tree presence in agricultural systems can positively impact the biological control of specific pests (Perfecto et al., 2004; Bianchi et al., 2008; Tscharntke et al., 2011; Karp et al., 2013). Additionally, different tree species offer shelter for avian predators, contributing to effective pest management.

5. Beetle Bank

Providing suitable overwintering habitat within fields by creating a raised earth bed, termed beetle banks, sown with perennial grasses (Wratten, 1992). 

6. Growing of Repellent Plants

Grown either as a border crop or a main crop, these repel the pests away from the crop mainly due to the release of volatile repellent plant chemicals. Basil repels flies, mosquitoes, and tomato borer. Garlic repels beetles, aphids, weevils, spider mites, carrot fly. Radish deters cucumber beetle. Mint repel cabbage moth. Marigold repels beetles, cucumber beetles, and nematodes.


Ecological studies provided a strong theoretical knowledge base concerning how species are likely to respond to landscape context and the establishment of population dynamics at the landscape scale. However, such studies have generally not considered the diversity of cropping areas and their relative managements, assuming arable land to be homogenous. The science of habitat management is still in its infancy. Publications on the topic date from the first half of the century, but nearly 80% of the literature reviewed herein was published after 1990. While this is in part attributable to our intention to focus on recent literature, within the current decade, a marked trend toward increasing activity is evident. The international community of scientists engaged in this field appears well poised to meet the challenge of making agricultural pest management more effective and production systems more sustainable, as well as being increasingly compatible with nature conservation. Landscape engineering can and should be combined with other methods for better, long-lasting results. This strategy has significant potential for boosting the variability and population of natural enemies of crop pests. In the near future, these formerly separate branches of biological control will be merged to a synergistic effect in “integrated biological control’’.


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