Economic analysis of habitat manipulation in Brassica pest management: Wild plant species suppress cabbage webworm
Introduction
Agricultural intensification has led to enhanced crop productivity per unit area (Bommarco et al., 2013). This has, however, been achieved with less attention to sustaining the natural processes that support agroecosystems (Woodcock et al., 2016). Removal of natural habitats and over dependence on agrochemicals have resulted in declines in beneficial insects and an increase in pest impact (Gurr et al., 2017; Potts et al., 2016). Adverse effects of synthetic insecticides on ecosystem have necessitated the search for alternative approaches to manage pests in agriculture (Bommarco et al., 2013). Minimizing the use of agrochemicals and integrating ecologically prudent practices may reduce their negative impact on ecosystem services (Bommarco et al., 2013). Further, in many less developed regions, insecticide use is not a viable option because of constraints imposed by access and cost (Tefera et al., 2016). So, here, non-chemical pest management strategies are of particular value if they are effective and affordable (Gurr et al., 2016).
Conservation biological control aims to realize the potential of endemic and naturalized predators and parasitoids by modifying the agroecosystem to remove constraints on their survival and activity (Griffiths et al., 2008). Different groups of arthropod predators, parasitoids and entomopathogenic organisms present in the agroecosystem can provide pest suppression (Gurr et al., 2017). The availability of pollen and nectar resources, as well as shelter and alternative prey, sustains and enhances the survival and performance of natural enemies that are often scarce in simplified agroecosystems (Isaacs et al., 2009). Often, natural vegetation in the agricultural landscapes does not provide sufficient floral resources at the right time and place, hence the need for local manipulation such as flower strips (Karp et al., 2018; Gurr et al., 2017). Optimal survival of predators and parasitoids depends on the availability of pollen and nectar from flowers (Gurr et al. 2017, 2018). Accordingly, there is need to manipulate the habitat to provide natural enemies with these key resources. Many natural enemies are omnivores (Kean et al., 2003) requiring both prey and plant resources to function effectively. It is important that the cropping environment is positively influenced to suit natural enemies in delivering pest suppression. Availability of shelter habitats among crops enhances the heterogeneity at the farm level and decreases the possibility of extinction of rare but potentially beneficial natural enemy species (Jonsson et al., 2015). Shelter at the farm scale also provides donor habitat for beneficial organisms during agronomic practices such as tillage, pesticide application and harvesting of crops.
Several forms of habitat manipulation for conservation biological control have been undertaken across the developed world including Australia, New Zealand, Western Europe and the US with some reported successes (Gurr et al., 2017). In the developing world including sub-Saharan Africa, whilst many smallholder farmers do not have the financial capacity to purchase chemical insecticides and insects continue to cause crop losses and imperil food security (Amoabeng et al., 2017), farmers fail to capitalize on the low-cost pest management option by promoting endemic natural enemies to effect pest control (Wyckhuys et al., 2013). Ecological information about importance of natural enemies and ways of exploring their potential in pest management is often non-existent, especially in developing countries including Africa (Wyckhuys et al., 2013).
A review a decade ago showed that plants from 35 plant families have been used in most of the habitat manipulation studies; with only four families, Apiaceae, Asteraceae, Fabaceae and Lamiaceae, having at least 10 species tested (Fiedler et al., 2008). Most of the studies have utilized one or more of just four plant species, Phacelia tanacetifolia Benth. (Boraginaceae), Fagopyrum esculentum Moench (Caryophyllales: Polygonaceae), Lobularia maritima L. (Desv.) (Brassicales: Brassicaceae) and Coriandrum sativum L. (Apiales: Apiaceae) in regions outside their places of origin. Selection criteria for plant species have focused on the effectiveness shown in earlier habitat management studies (Fiedler et al., 2008). In recent years, however, interest in native plants has increased. Pandey et al. (2018) showed the longevity of the parasitoids, Diaeretiella rapae (McIntosh), Cotesia glomerata (L.) (Hymenoptera: Braconidae), and Diadegma semiclausum (Hellen) (Hymenoptera: Ichneumonidae) exposed to flowers of Australian native plants was comparable with the longevity when exposed to the commonly used F. esculentum. Native species can outperform or provide similar resources as non-natives and have several advantages such as local adaptation, habitat perpetuity, and enhanced native biodiversity value (Fiedler et al., 2008).
Regardless of the intercrop plant species used for habitat manipulation, the intervention must ultimately result in pest suppression with an increase in yield and quality of crops to stimulate global patronage as a reliable pest management tactic for use in situations where insecticides are unavailable, unaffordable or undesired. Even though several studies have reported on the benefits of habitat manipulation to suppress pests, there are few that give experimental evidence to support the claim that habitat manipulation enhances yield and quality of crops and is a cost-effective pest management option (Amoabeng et al., 2020; Cullen et al., 2008).
While not a major pest in temperate regions due to its reduced activity in temperatures below 20 °C (Sivapragasam and Chua 1997), the cabbage webworm is an important cabbage pest in tropical and subtropical regions. A single H. undalis larva can cause the death of a whole plant or result in the plant forming multiple non-marketable heads (Mewis et al., 2002).
This study aimed to provide experimental evidence on the effect of habitat manipulation on abundance of H. undalis and on yield and quality of cabbage (Brassica oleracea var. capitata) as well as calculating the cost-benefit of this conservation biological control intervention for cabbage pests.
Section snippets
Experimental location and design
Field experiments were conducted at the Crops Research Institute (CRI), Kwadaso, Kumasi, Ghana (6°43′N, 1°36′W; 287m elevation) between January 2017 and March 2018. Cabbage seasons were June–August 2017 (major rainy season - season one), September–November 2017 (minor rainy season - season two) and December 2017–March 2018 (dry season – season three). Six wild plant species; Ageratum conyzoides, Tridax procumbens (Asteraceae), Crotalaria juncea (Fabaceae), Cymbopogon citratus (Poaceae), Lantana
Effect of habitat manipulation on abundance of H. undalis
All habitat manipulation treatments (intercrop) had lower numbers of H. undalis compared to the control. The larva of H. undalis (Fig. 2) typically attacks the plant from the growing point and bores through the stem.
In week one (and throughout) of the first season, the control treatment had significantly (F = 21.88, df 6, 20, P = 0.001) higher numbers of H. undalis per plant than all other treatments. Among the six treatments, Talinum and Lanta had significantly lower numbers of H. undalis in
Conclusions
The high cost: benefit ratios obtained in this study show the potential of habitat manipulation using these non-crop plant species for sustainable pest management in cabbage. More generally, this study provides motivation for studies in a wider range of geographical locations and crop systems to assess the utility of habitat manipulation for conservation biological control of insect pests. Whilst many such studies have reported benefits to natural enemy density and pest incidence, there is a
CRediT authorship contribution statement
Blankson W. Amoabeng: Conceptualization, Investigation, Formal analysis, Writing – original draft, Writing – review & editing. Philip C. Stevenson: Supervision, Writing – review & editing. Moses B. Mochiah: Supervision. Kwesi P. Asare: Formal analysis, Supervision. Geoff M. Gurr: Supervision, Conceptualization, Investigation, Writing – original draft, Writing – review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This manuscript emerged from the doctoral fellowship awarded to ABW by Charles Sturt University, Australia, through the International Postgraduate Research Scholarship (IPRS) and International Tuition Payment (ITP) programs. Thanks to Mrs AC Johnson for reviewing and editing the manuscript.
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