Leek and shallot 377
tunnel mines in the leaf tissue, sometimes causing distortion, and are reported to
occasionally attack the bulb and stems. Damage to the leaves of leek can make them
unmarketable. Symptoms include mining and perforations.
On leek, larvae prefer to feed on the youngest leaves, but can consume leaves
more than two months old. They bore through the folded leaves towards the centre of
the plant, causing a series of pinholes on the inner leaves. Larval mines in the central
leaves become longitudinal grooves in the mature plant. It is reported that pyrethroids
and Bt products are effective tools for the management of infestations. Insecticides
are rarely required in the United Kingdom. Cultural controls including crop rotation,
delayed planting, removal of old and infested leaves, destroying any obvious pupae
or larvae, early harvesting (to avoid damage by last generation larvae and population
build-up), positioning susceptible crops away from infested areas and destruction of
plant debris following harvest may be effective in reducing populations below damaging
levels. German literature suggests covering leeks with netting prior to female activity
and cutting off all outer leaves before the winter leaves appear in late season may
reduce damage to leek. In Europe, a number of predators, parasites and pathogens are
known to attack the larvae and pupae of the leek moth.
Thrips
The major pest of leeks is the thrips (Thrips tabaci Lindeman). These 2 mm long
insects hide between the inner leaf blades, where they feed on cell fluids. The green
leaves lose their colour as the empty surface cells form thousands of highly visible
grey spots. The economic damage due to quality loss is serious. Thrips damage is
most severe when plants are water-stressed in hot, dry weather. In these conditions
leaf expansion is slow and the increase in thrips number is fast. At 30 ∞C, it takes only
11 days for the insect to develop from egg to adult (Edelson and Magaro, 1988).
Given the low damage threshold for thrips, much research has been done to control
this pest efficiently. The better chemical products are carbamates, including methiocarb
and furathiocarb. Some phosphorous compounds, such as acephate and malathion,
and also pyrethrins, may have protective effects. Novel pesticides for use against
thrips are being tested.
In the Netherlands, seed-coatings with fipronil were effective in protecting seedlings,
with no apparent phytotoxicity (Ester et al., 1997). Another approach suitable for
‘organic’ production is intercropping with legumes or other plants to discourage
thrips from feeding in large numbers on leeks. Belder and Elderson (1998) studied
the feasibility of intercropping in pot and field experiments in the Netherlands.
Intercropping with clover led to reduced thrips populations on leeks, even when the
legume was trimmed. Theunissen and Schelling (1998) reported that intercropping
leeks with clover (Trifolium fragiferum), either throughout the field or in between
rows, suppressed both larval and adult thrips populations.
22.3.17 Harvesting and yield potential
When harvested by hand the plants are usually lifted mechanically, then the roots are
cut, the outer damaged and senescent leaves are removed, the remaining leaves
shortened, and the plants are packed into boxes. When mechanical harvesting is used,
hand cleaning and machine washing are necessary. The crop is ready to harvest once
the blanched basal portion of the leaves is at least 1.25 cm in diameter. However,
because the plant does not form a bulb, there is no rush to harvest, and growers often