Agriculture Reference
In-Depth Information
competition from the undersown or intercrop species has been postulated to
inhibit thrips feeding and development.
Unfortunately, undersowing with clover slows leek growth and reduces
total yield, even though crop quality is improved because of less thrips damage
(Weber
et al
., 1999). However, by sowing the clover only between the rows of
transplanted leeks, competition was reduced and leek yields almost equal to
those attained in bare soil were attained. Ninety nine per cent of the leeks from
this undersowing treatment were marketable, 89% in the best-quality grade,
whereas from bare soil only 44% were marketable with just 1% best quality
(Theunissen and Schelling, 1998). Therefore it may be possible to refine
undersowing or intercropping systems so that the benefits of reduced pest (and
disease) pressure are gained without much loss of yield, or possibly, with
intercropping, with a gain in overall financial return per unit area.
These techniques are not yet used in practice. This may be because more
development work is needed to optimize suitable systems (see Fig. 5.4).
However, they are inevitably more complex than monocropping and would
require more husbandry skill. Also, as described in Chapter 4 and as illustrated
by leek-celery intercropping experiments (see Fig. 5.4), alliums are weak plant
competitors and it will be difficult to devise systems where they are not severely
suppressed by their intercrop, although carrots are fairly slow growing, not
highly competitive and can occupy the ground for a similar period to leeks, so
this could be a commercially viable system for leek production, requiring little
or no insecticide.
Onion fly or maggot
The onion fly or onion maggot,
Delia antiqua
, is one of the 22 species of two-
winged flies (Diptera) listed as pests of alliums by Soni and Ellis (1990). The
onion fly is a specialist feeder on alliums, and attacks all the vegetable alliums.
The most severe damage is caused by the maggots burrowing into the base of
seedlings in the spring, causing wilting and collapse. When seedlings die, the
maggots migrate to neighbouring plants and may destroy successive plants in
a row, resulting in gaps in the crop (see Plate 3b). Similar damage to young
allium crops is caused by
D. platura
, the bean seed fly or maize seed maggot,
which normally attacks spring-sown crops a week or two before the onion fly.
Delia platura
, which looks very similar to the onion fly, has a much wider host
range. In many onion-growing regions there are three generations per year of
the onion fly. Maggots of the second and third generations burrow into the
bases of large plants and developing bulbs and thereby predispose them to
rotting by secondary infections of fungi and bacteria (see Plate 3c).
Adult onion flies are 8-9 mm long, grey and resemble the common
housefly. The females lay eggs in soil near the base of allium plants. The larval
stage is the damaging maggot, which is dirty white and grows to 8 mm in
