Agriculture Reference
In-Depth Information
pests, and this has given impetus to the more judicious use of insecticides within
a context of IPM and 'resistance management'. Pest invasion can also be
prevented by covering crops with insect-excluding netting or non-woven
translucent fabrics, but this is expensive and labour intensive and only likely to
be economic on 'organic' crops sold at a price premium.
Onion thrips
The most damaging pests worldwide are the insignificant-looking thrips or
thunderflies. These are slender insects, only about 2 mm long as adults. They
are found wherever alliums are grown, but are most severe in the warmer
production regions. Thrips belong to the order Thysanoptera family Thripidae
.
Soni and Ellis (1990) list seven species of Thripidae as allium pests, the most
important of which is
Thrips tabaci
, the onion thrips, which attacks all the edible
alliums. The western flower thrips,
Frankliniella occidentalis
, is also an important
onion pest in the south-west of the USA. These small insects concentrate
between the young leaf blades at the top of the neck. In this sheltered part of the
plant they rasp and pierce leaf cells and feed on the sap released. The resultant
air spaces in the leaf cells give the foliage a silvery appearance (see Plate 3a).
Large numbers of thrips can cause extensive damage to leaves, and also to
flowers in seed production. Damage is most severe when plants are water-
stressed in hot, dry weather. In these conditions leaf expansion is slow and
increase in thrips numbers is rapid. Heavy rainfall or sprinkler irrigation can
wash many thrips off leaves and increase their mortality. For example, in a dry
year in New Zealand thrips populations peaked at 500 per plant on untreated
onions whereas in the following year, which was wet, the peak population was
70 per plant (Workman and Martin, 2002). In Oregon, USA, leaf damage from
thrips can reduce total yields by up to 27%, and the percentage of the most
valuable large bulb grades to a much greater extent (Jensen
et al.
, 2003).
In the 1990s the thrips-vectored iris yellow spot virus (IYSV) emerged as a
potentially devastating disease of onion bulb and seed crops (see Viral Diseases,
below), particularly in the south-western USA. The severity of this disease
correlates with thrips populations, and control of thrips is the best way to
reduce IYSV infection (Jensen, 2005). In leek crops, thrips larvae populations
must be kept below about five per plant to ensure that the majority of the crop
is in the top two quality grades at harvest. Populations in excess of 15-20 per
plant can cause so much visible damage that the crop will be downgraded to
unmarketable under current European quality standards (Theunissen and
Schelling, 1997). Thus, the disfiguring effect of thrips symptoms on leaves
assumes particular importance for the quality of allium vegetables harvested
green like leeks, salad onions and bunching onions, and very low pest levels
must be maintained to prevent total economic loss even though losses of
weight yield may be slight.
