Introduction
One of the greatest global challenges of the twenty-first century is
meeting the increasing demands for human food production while
minimising adverse impacts on biodiversity and ecosystem health (Godfray
et al. 2010). This challenge is particularly critical in sub-Saharan
Africa (SSA) where the population is predicted to double over the coming
decades (Rosegrant et al. 2009) and food production is hampered both by
climate change impacts (Lobell et al. 2011) and significant yield losses
caused by crop pests (Lenné 2000; Oerke & Dehne 2004). For example, the
invasion of the fall armyworm (Spodoptera frugiperda ), which has
caused crop losses of about $3 billion a year in SSA, has become one of
the most important threats to maize production (Stokstad 2017). The fall
armyworm is also a cause of major damage to other crops including rice,
sorghum, millet, cabbage, and tomatoes (CABI, 2018), demonstrating the
vulnerability of smallholder farming to crop pests.
Conventional synthetic pesticides have severe limitations as a means of
pest control in SSA because they are economically inaccessible for a
large portion of smallholder farmers in the region (Ahissou et al.
2021). Pesticide residues also put human and livestock populations at
risk from contaminated food and forage (Nesser et al. 2016; Jepson et
al. 2020). Furthermore, synthetic pesticides may lead to resistance
within pest populations (Sawadogo et al. 2020), and have negative
impacts on non-target organisms, such as pollinators and natural
enemies, and the ecosystem services that biodiversity provides in the
production of food (Losey et al. 2006; Chaplin-Kramen et al. 2011;
Kennedy et al. 2013). If the reduction of natural enemy populations is
greater than that of the pest, this may lead to the resurgence of pests
following pesticide applications (Janssen and van Rijn 2021), which is a
widely reported problem associated with synthetic pesticides (Guedes et
al. 2016).
Biological control methods (hereafter biocontrol), which employ natural
enemies of crop pests, have been promoted globally as an alternative
approach to pest control, and are often used as part of an integrated
pest management strategy (Giles et al. 2017; Baker et al.2020). Extensive evidence is available on the responses of natural
enemies to the landscape composition surrounding crop fields (Karp et
al. 2018), which reveals that landscape effects are a key driver of pest
regulation by natural enemies. Recent syntheses show consistent positive
responses of natural enemies to landscape complexity (Chaplin-Kramen et
al. 2011), a reduction of natural pest control in simplified landscapes
(Rusch et al. 2016), and higher natural enemy populations in complex
versus simple landscapes (Bianchi et al. 2006).
In SSA, smallholder farmers have employed conservation biocontrol
methods and botanical pesticides for the control of crop pests
(Sporleder & Lacey 2013). Conservation biocontrol methods include
intercropping, push-pull and field margins (Table 1). Growing evidence
highlights the potential of biocontrol interventions to reduce pest
incidence and increase yield (Amoabeng et al. 2020; Farsia
Djidjonri et al. 2021). For example, push-pull technology has
been shown to be effective against a range of crop pests, particularly
maize stemborers (Midega et al. 2018), and plant-based botanical
pesticides can reduce pest incidence and enhance yield in vegetables
crops (Mpumi et al. 2020; Odewole et al. 2020).
Although biocontrol interventions may provide sustainable and accessible
alternatives to synthetic pesticides, their adoption by smallholder
farmers has not been widespread (Ratto et al . 2022). This may be
due to knowledge gaps relating to their effectiveness and the factors
that lead to their success or failure, particularly in comparison to
synthetic pesticides. Biocontrol techniques have been applied to
numerous crops and targeted a wide variety of pests in the region, yet
there is a lack of understanding of how the effectiveness of biocontrol
varies across different crop types and pest taxa (Ratto et al.2022). Recent research in Tanzania found greater natural enemy diversity
in fields surrounded by intercropped fields, suggesting spatial flow of
potential biocontrol services across landscapes (Tripathi et al. 2022),
but the established relationship between landscape composition, natural
enemies and pest regulation is almost entirely based on studies carried
out in the global north, and very seldom in sub-Saharan regions where
farmers are most exposed to food insecurity caused by crop pests
(Steward et al. 2014). This represents a key knowledge gap; more
clarity is needed about the environmental factors affecting biocontrol
performance in sub-Saharan Africa to better assist in smallholder farmer
decision making, and to determine the broader indirect impact of pest
management options on biodiversity compared to synthetic pesticides,
both on a farm and at a landscape scale.
Quantitative analyses have been conducted on the performance of
biocontrol agents (Stiling & Cornelissen 2005), on the impact of
landscape context on augmentative biocontrol (Perez-alvarez et
al. 2019) and pest and natural enemy responses (Chaplin-Kramen et
al. 2011). However, none of these approaches have focussed specifically
on the sub-Saharan region, nor have they evaluated the efficacy of
different biocontrol interventions on crop pest populations and their
damage to crops.
Here, we aim to better understand the key factors driving the success or
failure of biocontrol interventions using quantitative meta-analysis.
Specifically, we posed the following questions:
(1) What are the effects of biocontrol interventions on the management
of insect crop pests in sub-Saharan Africa? (2) Are these effects
consistent across biocontrol techniques, crop types, target pests and
farming systems? (3) How does the effectiveness and impact of biocontrol
interventions on crop pests and non-target insects compare to synthetic
pesticides? (4) Does the surrounding landscape composition affect the
efficacy of biocontrol interventions?
We hypothesised that pest abundance and crop damage would decrease, and
yield would increase in crops subject to biocontrol interventions, that
the impact on natural enemy abundance would be less than that of
synthetic pesticides, and that these effects would be enhanced in fields
surrounded by greater landscape complexity.