Agriculture Reference
In-Depth Information
We will use a theoretical approach to the characterisation of pests and diseases in agricul-
tural systems, based on some of the ideas from invasion ecology. Invasion ecology is a complex
and dynamic intellectual conversation often focused on exotic plants, mammals and birds.
Here, we don't consider only exotic species as invaders but also any pathogen or pest species
not yet present in a crop in a particular growing season. To apply some basic concepts of
invasion ecology to crop protection, we consider three phases of invasion:
1 colonisation
2 establishment, and
3 population outbreak.
We explore whether certain invasion trends may be distinguished for herbivorous arthro-
pods, nematodes, fungi, and bacteria in organic versus conventional farming systems. By bor-
rowing the term
invasibility
, we can examine how organically managed crops may present
barriers against the invasion by pests and pathogens, and compare them, when possible, with
conventionally managed crops. In natural ecosystems, low invasibility has often been related
to high biodiversity depending on the scale of observation (Peterson
et
al
. 1998), and this rela-
tionship may also hold for managed agroecosystems (Knops
et
al
. 1999). Organically managed
agroecosystems are generally more diverse than their conventional counterparts. This has
been shown for above-ground natural plant and crop species, insects and birds, as well as for
below-ground arthropods, nematodes, fungi and bacteria (e.g. Mäder
et
al
. 2002, Aude
et
al
.
2004, Asteraki
et
al
. 2004, Oehl
et
al
. 2004). The reasons for this difference in biodiversity are
manifold, but in particular, the:
1 absence of herbicides reduces detrimental effects on various microbial species;
2 absence of synthetic nematicides and insecticides reduces broad-spectrum effects on bene-
ficial fauna;
3 absence of general fumigants reduces broad-spectrum activity on all soil life;
4 absence of easily available plant nutrients reduces the selective enhancement of fast-growing
microorganisms, and
5 addition of various plant and animal-derived organic materials enhances the soil food web
and, indirectly, the above-ground food web.
In addition, organic farmers frequently purposefully plant strips of controlled natural veg-
etation, which affects not only above-ground biodiversity, but also soil biodiversity. If higher
biodiversity in agroecosystems reduces invasibility, then we can expect a reduced spread of
pests and diseases in organic compared to conventional farms. We suggest that this expecta-
tion is largely met, but that there are exceptions.
We compare the range of pests and diseases that challenge crop productivity in organic and
conventional farming systems around the world. Our emphasis here is on arthropod pests and
diseases, but we include some observations on vertebrate and other invertebrate pests. We then
describe organic and conventional strategies for pest and disease management, including the
three elements:
1 prevention of colonisation or establishment;
2 population regulation through biological processes; and
3 curative interventions.
To illustrate these elements of pest and disease management in practice, we critically
review comparative research programs on conventional and organic pest control in many
parts of the world, and illustrate the constraints and opportunities of organic crop protection
under different farming conditions. We conclude by providing suggestions on future research
