Discussion
In this study we identified the overall effectiveness of biocontrol
techniques in controlling insect pests of crops in sub-Saharan Africa,
and identified patterns across biocontrol interventions, pest taxa, crop
types and experimental design. Using a meta-analytical approach, we
found that biocontrol interventions effectively reduced pest abundance
and crop damage by over 50%, while increasing crop yield by more than
60%. The size of the yield increases highlights the great challenge
posed by insect pests to smallholder crop production, which is in line
with recent evidence estimating high crop losses to pests, especially in
the absence of any control intervention (Oerke 2006; Savary et
al. 2019). The substantial yield increase that biocontrol can provide
could have an enormous impact on sub-Saharan food security if these
practices are scaled up to regional level. Crucially, we showed
comparable performance of biocontrol and synthetic pesticides on pest
abundance, crop damage and crop yield, and a significant reduction in
the loss of natural enemies, particularly following botanical pesticides
application.
Biocontrol effectiveness across biocontrol intervention
techniques
Pest abundance and crop damage were negatively affected by biocontrol
across all interventions. Push-pull and botanical pesticides had the
greatest effect on crop yield, increasing production by 92% and 80%
respectively. This may be due to the highly effective companion crops
utilised in push-pull technologies, which release bioactive chemicals
that repel pests and attract natural enemies, while also suppressingStriga , a parasitic weed which causes up to 100% yield losses
across SSA (Khan et al. 2014). The large yield increase observed
in our synthesis may be due to a combination of the pest repellent and
weed suppression abilities of push-pull implementation. Our findings
indicate the potential of botanical pesticides to be an effective method
of pest control in SSA. However, two thirds of the studies included here
were carried out on research farms, which may be under more controlled
settings compared to more realistic field conditions, potentially
inflating the observed effect size.
Our review captured a small number of studies on classical biocontrol
interventions, including augmentation, despite successful examples such
as the control of the Cassava mealybug (Phenacoccus manihoti ) by
the Encyrtid wasp (Anagyrus lopezi ) (Norgaard 1988). Conceivably
these interventions may be hampered by the high costs involved in their
research and production, such as insect rearing facilities
(Neuenschwander 2004), and the growing concerns on the environmental
risks of releasing exotic species (Van Lenteren et al. 2006).
Therefore, they may only be implemented for highly widespread and
devastating pests such as the Cassava mealybug or the Tomato leaf miner
(Tuta absoluta ).
Biocontrol effectiveness across crop type and pest
taxon
Cereals were the most studied crops in our meta-analysis, conceivably
because they play a central role in the region’s food security,
accounting for about 50% of total crop area and caloric intake
(Robinson et al. 2015). Nonetheless, other crop types such as
fruits, pulses and fibre should be included in future research in this
area. Our study provides strong evidence of the effectiveness of
biocontrol across all taxa, particularly against lepidopteran crop
pests. The potential of biocontrol to reduce cereal crop damage by 60%
is encouraging given the devastating damage caused, particularly on
maize, by caterpillars including fall armyworm (Spodoptera
frugiperda ), Diamondback moth (Plutella xylostella ), Crambid
cereal stemborer (Chilo partellus ) and Maize stemborer
(Busseola fusca ).
Biocontrol effect on natural enemies and non-target
pests
Understanding the effect of biocontrol on natural enemy populations is
crucial as they are both an indication of pest control potential and a
measure of the impact of the pest control method on non-target species.
Our results showed no overall change in NEA following biocontrol
application when compared to untreated fields. Although, we found a
significant decline in natural enemy abundance following botanical
pesticides application. The most likely explanation for this is that the
interventions have reduced prey availability for natural enemies, making
them move to other more profitable foraging locations, but the direct
negative impact of some interventions, such as some broad-spectrum
botanical pesticides, cannot be excluded (Ndakidemi et al. 2016).
The existing evidence for the effect of botanical pesticides on
non-target species is conflicting, with some research showing that plant
extracts such as neem, garlic and eucalyptus may cause mortality and
have sub-lethal effects on beneficial insects (Simmonds et al.2002; Maia & Moore 2011), while other studies found no detrimental
effect of pepper and garlic extract on natural enemies populations
(Fening et al. 2013; Amoabeng et al. 2020). More research
is needed to draw robust inferences on the repercussion of botanical
pesticides on beneficial/non-target species before considering
large-scale adoption.
Evidence is more consistent on the positive response of natural enemy
populations to biocontrol interventions such as push-pull and field
margins (Koji et al. 2007; Midega et al. 2008), which is
in line with evidence from the global north on the benefits of habitat
enhancement on natural enemy density and diversity (Blaauw & Isaacs
2012; Holland et al. 2017). However, we found that only 14% of
the studies measured NE abundance following biocontrol application in
sub-Saharan Africa. Natural enemy abundance should be measured more
consistently in future studies to further elucidate direct and indirect
effects of biocontrol on non-target species.
Furthermore, the most common outcomes measures reported in the studies
focussed on the abundance of pests and/or natural enemies, while we did
not find studies measuring their species diversity or functional group
diversity. However, it has been shown that biocontrol is strengthened by
increased natural enemy richness (Griffin et al. 2013; Katanoet al. 2015) and this is consistent across temperate and tropical
regions (Letourneau et al. 2009). Ecosystem functioning can be
stabilised by functional redundancy, by enabling functional groups to
compensate for individual species fluctuations and increase the
resilience of ecosystem against species loss (Rosenfeld 2002; Hooperet al. 2005). This is particularly relevant to understand the
long-term impact of biocontrol on natural enemy communities and their
pest suppression ability and should be explored in future research.
Biocontrol effectiveness compared to synthetic
pesticides
When compared to synthetic pesticides, biocontrol interventions had a
similar impact on pest abundance and crop damage, which is a critical
finding for farmers who cannot access or afford chemicals. Crucially,
natural enemy abundance was significantly reduced after synthetic
pesticides application even over the short time scales of the studies
examined. In the long term there could be greater reductions in pest and
crop damage following biocontrol as a result of more abundant and
diverse communities of natural enemies. In terms of a reduction in the
negative environmental impacts associated with chemical pesticides, the
benefits provided by more resilient natural enemy populations could be
one of several indirect positive effects of opting out of conventional
pesticide use. It is worth noting that most comparisons with synthetic
pesticides were measured against botanical pesticides, therefore
inferences for other biocontrol methods should be made with caution.
Future research should aim to determine the effectiveness of biocontrol
approaches, such as push-pull, when compared to synthetic pesticides to
fill this knowledge gap.
A possible limitation of this study is the potential selection bias
towards significant results, causing an overrepresentation in the
published literature, a criticism that could be levelled against all
meta-analyses. The two tests we used to assess publication bias yielded
conflicting results, hence it is hard to know with certainty the scale
of publication bias towards results where an effect was found. However,
we show that crop losses to pests are significantly higher in untreated
fields, supporting the idea that any crop protection intervention has
the potential to improve yields substantially. The size of the yield
gains shown in the current meta-analysis suggest there is a big
opportunity to raise yields with biocontrol interventions.
Landscape composition and
biocontrol
Our study set out to answer the question, “does the surrounding
landscape composition affect the effectiveness of biocontrol
interventions?”. However, we found a paucity of studies investigating
either the effect of landscape composition on biocontrol effectiveness,
or the relationship between landscape composition and natural enemy
abundance. The research we found indicated a significant decrease of
natural enemy density and predation/parasitism activity with isolation
from natural habitat (e.g., Henri et al. 2015; Soti et al.2019). This is in line with recent research showing a similar effect of
landscape complexity on pollinators and natural enemies in sub-Saharan
regions (Ratto et al. 2021; Jordon et al. 2022) and a
larger body of research particularly in the global north (Bianchiet al. 2006; Chaplin-Kramen et al. 2011; Shackelfordet al. 2013).
Furthermore, the sparse evidence we found focusing on the effect of
landscape composition on biocontrol effectiveness showed inconsistent
results. Midega et al. (2014) found that semi-natural habitat
acted as a source of lepidopteran pests to the maize crop fields in
Kenya, while Kebede et al . (2019) demonstrated that landscape
simplification overrode the effect of intercropping practices and was
the main driver of pest infestation levels. A key avenue for future
research would involve large scale studies to identify clear patterns in
the relationship between landscape complexity and natural enemy activity
and the ecosystem service delivered to sub-Saharan agricultural systems.
Furthermore, recent evidence from SSA showed that natural enemy
diversity in crop fields is dependent on the land management of
neighbouring fields (Tripathi et al. 2022). This highlights the
need for further multi-scale studies to identify potential variation in
biocontrol effectiveness across different land management contexts.
In conclusion, our findings provide the first quantitative synthesis of
biocontrol effectiveness in SSA, indicating that biocontrol
interventions have the potential to substantially reduce crop damage,
increase crop yield while maintaining natural enemy populations within
sub-Saharan agricultural systems. Our results further suggest that
biocontrol has comparable performances to synthetic pesticides with
reduced adverse impact on beneficial insects and ecosystems. Our results
suggest that biocontrol represents an effective intervention for farmers
who do not have access to pesticides, while it can maintain crop yields
without associated negative pesticide effects.