Agriculture Reference
In-Depth Information
of natural insect control through insecticide reduction is well documented (Heinrichs
et al.
, 1982, 1986; Gallagher
et al.
, 1994; Heinrichs, 1994).
The required reduction in insect populations for effective reduction of insect
transmitted diseases may be beyond that which can be achieved by diversity alone
(Power, 1988). However, simultaneous reduction of insect vectors and disease
inoculum can be effective as has been shown with the beet western yellows disease
and its aphid vector
Myzus persicae
(Andow, 1991). Recent results have also shown
that there is a potential to reduce insect transmitted viruses such as PVY using straw
mulch (Saucke and Döring, 2004). This result is relevant because the mechanisms
involved can be made use of in diversified systems. It is commonly thought that
reduced visual apparency through the use of coloured or reflective materials is the
reason for the reduced aphid and white fly attacks in mulched stands. However,
experiments with straw mulch in potatoes have shown that it is likely that the
surface structure of the straw induces a long distance take-off flight in aphids rather
than the change in visual cues (Döring
et al.,
2005). Thus, structural cues may also
be of importance here and it might be possible to achieve better virus control by
selecting plants that induce long-distance flight.
Examples at different scales
Intrafield diversification for the management of insect pests has to a large extent
focused on the use of polycultures or intercropping (e.g., Bach, 1980; Risch, 1983;
for reviews see Altieri and Letourneau, 1982; Risch
et al.
, 1983; Andow, 1991). One
successful example is the restriction of carrot root fly by growing the host crop in a
legume background (Finch and Kienegger, 1997).
The wholly green crop
comprising 60% of non-host confused the pest which was unable to find and/or
settle on its normal host.
A more complex, but highly successful example based on semiochemical effects
is the 'push-pull' strategy developed in Kenya for control of stem borer in maize
(Khan
et al
., 1997). Two intercrop plants repel the adult stem borers while two edge
or trap crop plants attract the stem borers. It also emerged that one of the 'push'
plants, the legume silverleaf (
Desmodium uncinatum
), used as an intercrop, also
gave excellent control of the parasitic witchweed,
Striga hermonthica
(Pickett,
1999).
Natural enemies increased greatly and soil erosion was reduced by the deliberate
planting or maintenance of flowering weeds and grass in established vineyards in
Switzerland and southern Germany (Boller
et al.
, 1997). This practice is
increasingly popular in Californian wine-growing areas and in apple production
(Finckh, personal observation; Crowder, 1996). A low-input system for growing
wheat with a permanent understorey of white clover (
Trifolium repens)
can reduce
the major pest aphid species and slugs (Jones and Clements, 1993); in addition,
reductions in splash-dispersed diseases, such as those caused by
Septoria
spp. were
shown by Bannon and Cooke (1998), because of the clover covering the soil surface.
There may be beneficial effects of greater intra-varietal diversity in the oat-frit
fly (
Oscinella frit
) system (van Emden, 1966). The flies can attack the host plants
