Agroecological Intensification of Smallholder Farming - The Oxford Topic of Food, Politics, and Society - page 112

Pesticide sales increased steadily between 1960 and 2004 (Oerke 2005). The concept of
integrated pest management (IPM) arose in response to the widespread problems asso-
ciated with chemical pesticides. IPM components include host plant resistance, cultural
practices, repellent plants, natural enemies (natural and introduced), biopesticides, and
the judicious use of synthetic pesticides. In some contexts, intensive IPM strategies can
be successfully implemented in systems that inherently favor pests, such as monocul-
tures of high-value horticultural crops or potato crops grown without rotation. In post-
Green Revolution contexts in the developing world, successful IPM efforts have focused
on integrating local farmer and research knowledge, knowledge transfer and learning,
and collective action, often through the use of farmer field schools (Bentley 2009).
A focused IPM approach makes sense for highly destructive pests of high-value crops
and main staples. Examples of the latter include Banana Xanthomonas wilt in East
Africa, millet head miner in West Africa (Payne et al. 2011), rice blast in Asia, and late
blight in potato in many regions of the world (Nelson et al. 2001). In many smallholder
contexts, however, it is not feasible for farmers to deploy such intensive methods to deal
with the diverse pest problems that afflict their systems (Orr 2003). Nor does conven-
tional IPM make sense for various secondary pests of secondary crops. For such sys-
tems, it is more useful to implement diverse farming systems that are inherently more
resilient with respect to pest population dynamics (Nelson 2010). For farmers with a
range of crops and an assortment of constraints, it more often makes sense to focus on
system health than on any particular problem.
Polycultures tend to have lower pest pressure than the corresponding pure stands. In
a review of 209 studies involving crop mixtures, over half were found to have lower pest
levels, while 15 percent had higher numbers of pests (Andow 1991). In a meta-analysis
of plant diversification studies involving 552 experiments published in forty-five
papers, diversity was found to reduce pests and damage overall, but also to incur a
mean reduction in yield (Letourneau et al., 2011). Thus, while polycultures can outyield
their corresponding monocultures, they do not inevitably do so, and some intercrops
can actually increase pest pressure (B. Medvecky and J. Ojiem, personal communica-
tion). Polycultures can reduce pest pressures through a number of mechanisms, such as
rotations, which can break pest cycles. For example, when Striga is a problem on cere-
als, rotations with nonhost legumes such as cowpea can cause “suicidal germination”
(Oswald and Ransom 2001) of the parasitic weed, reducing the seed bank. In a mixture,
there is a lower host density for a given pest. Nonhosts serve as barriers for pests look-
ing for their hosts. Chemical ecology can be manipulated to disfavor pests in a variety of
ways, attractant and repellent plants can be used to reduce pest damage, and plants can
produce compounds that attract natural enemies of pests. Intraspecific diversity can be
effective for pest management; varietal mixtures and multilines have been used exten-
sively for management of crop diseases in particular (Wolfe 1985).
The push-pull system illustrates the potential of chemical ecology in pest man-
agement, as well as the potential of systems design to improve overall systems health
and productivity. It also illustrates the vulnerability of a fixed-package approach. The
stresses affecting maize yields in eastern and southern Africa include pests (principally