The Royal Society Report - Push-Pull

It is possible to recycle phosphorus (super phosphate
fertiliser, produced by treating animal bones with
sulphuric acid was the first synthetic fertiliser), particularly
from animal sources. However, loss to water and
adsorption in soil mean that the supply of phosphorus in
agricultural systems needs to be continuously replenished;
mined rock phosphate represents the only substantial
supply. The primary rock phosphate reserves in North
America, North and South Africa, Russia and southeast
Asia are likely to be exhausted before the end of the
21st century if trends continue (Smil 2000b; Zapata &
Roy 2004).
2.6.2 Secondary, micro and functional
crop nutrients
In different crops and cropping systems as well as different
regions, yield and quality can be constrained by the
availability in soil of nutrients that are required by crops in
small concentrations. Deficiencies of sulphur (S), calcium
(Ca) and magnesium (Mg) which are classed as secondary
nutrients cause significant yield reductions in some crops
and regions.
There are six micronutrients essential for plant growth:
boron (B); copper (Cu); Iron (Fe); manganese (Mn);
molybdenum (Mo) and Zinc (Zn). Micronutrient deficiency
can usually be rectified when diagnosed and the
significance of elevating the levels of some of these
elements (eg Fe) in crops relates to their importance in
human nutrition as much as crop nutrition.
There are five elements considered to be functional in
plants but not essential: sodium (Na); vanadium (V); cobalt
(Co); silicon (Si) and chlorine (Cl). Of these, Si has
relevance in the context of crop production as a competitor
for arsenic (As) uptake (Ma et al. 2008). Arsenic may
accumulate at dangerous levels in the diets of those who
depend on rice grown in soil and water containing high As
concentrations and low Si.
2.7 Pests, diseases and weed competition
Pests, diseases and weeds have a significant impact on the
sustainability of food crop production. Disease-induced
losses essentially represent wasted inputs of energy, water,
nutrients and labour. Worldwide crop losses due to weeds,
pests and diseases have been estimated for eight major
crops (wheat, barley, rice, maize, soy, cotton, sugar beet
and potato) as 26–40%. In the absence of control
measures such as resistant varieties, crop protection
chemicals and crop rotations, losses would be 50–80%
(Oerke & Dehne 2004).
Locusts, larvae of Lepidoptera, and other herbivorous
chewing insects can cause very substantial crop losses as
can root-attacking nematodes and sucking insects such as
aphids and leaf-hoppers; the latter are also important
vectors of diseases caused by viruses and phytoplasma.
Corn borer and corn rootworm cause much damage;
rootworm also affects nitrogen and WUE by damaging the
root system. Damage to cobs by corn borers facilitates the
entry of fungi such as Fusarium and Aspergillus species
that contaminate the seed with poisonous mycotoxins.
Many crops, especially fruit and vegetables, are prone to
rot after harvest and before or during transport to
consumers. Seeds from cereal and legumes are prone to
losses from bruchid beetles, grain and meal moths.
Temperature and humidity control can reduce, though not
eliminate, these losses.
Arthropods and nematodes can also act as disease vectors.
Aphids and leaf hoppers, for example, can act as vectors of
viruses and phytoplasmas. Many different genera of
nematodes cause plant disease, usually by infecting and
colonising roots. Feeding occurs through a hollow stylet
that can penetrate plant cell walls. Most are endoparasites,
invading root tissues and carrying out most of their feeding
from inside the root. Two genera of endoparasitic
nematodes are the source of much crop damage in wheat,
potato, soya beans and many other crops. These are the
cyst nematodes (Heterodera sp. and Globodera sp.) and
root knot nematodes (Meloidogyne sp.). Nematodes are
particularly difficult to control with pesticides. Soil
fumigation with methyl bromide has been widely used until
recently, but the use of this toxic chemical is now severely
restricted although there are few alternatives.
Vertebrate pests are also a significant problem. Rodents
and other large herbivores can inflict significant losses on
crops during their growth and development as well as post
harvest. In industrialised countries, these losses are usually
adequately controlled by regulating the populations of rats,
rabbits or deer using poisons, gassing or shooting. In
developing countries, recourse to such methods of control
is more limited and losses can be considerable in field as
well as plantation crops (Sridhara 2006).
2.7.2 Diseases
Diseases have an impact on loss of crops, pre and post
harvest. There is a cost associated with their control
through crop-protective chemistry and resistant varieties.
Significant losses are caused to crop yields from a variety
of fungi and oomycetes (microscopic fungus-like
organisms), bacteria and viruses across a range of crops.
Some examples are summarised in Table 2.2.
2.7.1 Pests
Pests can cause significant losses of food production,
and there are chemical and non-chemical approaches to
minimising these losses (Yudelman et al. 1998). Table 2.1
lists the major pests of maize, rice and wheat.
2.7.3 Weed competition
Among biotic constraints on crop protection, weeds have
the highest loss potential (32%), followed by pests and
pathogens (18 and 15% respectively) (Oerke & Dehne
2004). Losses due to weed competition represent a
16 I October 2009 I Reaping the Benefits The Royal Society