Agriculture Reference
In-Depth Information
Thrips are usually controlled by insecticidal sprays and, in a bulb onion
crop, two or three sprays of an effective insecticide when pest populations are
likely to increase can give adequate control (Richter
et al
., 1999; MacIntyre
Allen
et al
., 2005). Onion thrips have many alternative hosts, and the timing of
invasions can vary with the neighbouring vegetation - in particular, nearby
crops and crop operations. Consequently, thrips infestations are not as easy to
predict using day-degree models as are invasions of the onion fly or leek moth,
where the host range is restricted to alliums.
To avoid unnecessary spraying, 'scouting' systems have been devised in
several countries to determine when thrips populations in allium crops are
building up to a threshold level that warrants control (Richter
et al
., 1999;
Lorbeer
et al
., 2002; Jensen, 2005). Scouts need to be trained in standard
procedures in order to obtain reliable results. For example, in German trials
fortnightly samples of five plants from each of ten positions in a field were
simply scored for the presence or absence of thrips, and insecticide was applied
if more than 50% of plants contained thrips (Richter
et al
., 1999). The data
showed that a 50% plant infestation score corresponded to an average of about
three thrips larvae per leek plant and just 2% of damaged leaf area, which is
acceptable for good-quality leeks. Adoption of spraying according to this
scouting system in Germany reduced insecticidal application for thrips control
in onions by 60% and in leeks by 30%, without compromising quality or yield.
To try to understand better the reasons for local and seasonal fluctuations
in
T. tabaci
attacks, a model has been developed that combines within-farm,
small-scale geographical information on vegetation patterns, including crop
layouts, along with temperature-driven (e.g. Fig. 5.7) dynamic simulations of
thrips development and migration (Booij, 2003). The aim is to understand
how local differences in agroecosystems lead to the between-farm variations in
thrips damage that occur.
In many allium-producing regions growers have noticed a decline in the
effectiveness of insecticides for thrips control because of the evolution of
resistance in the local populations. Onion thrips resistance to both pyrethroid
and organophosphorus insecticides has been reported in many regions
(MacIntyre Allen
et al
., 2005). To maintain acceptable control, growers
frequently respond by increasing the dosage and frequency of treatments until
the pest becomes totally resistant (Theunissen and Schelling, 1999). When a
new insecticide is introduced it is important that its use is rotated with
insecticides with a different mode of action, even if they are not as effective, to
avoid continual selection pressure for resistance to the novel chemistry and a
repeat of the familiar 'boom and bust' pattern in insecticide effectiveness.
Ideally, such 'resistance management' occurs within a wider context of IPM,
which could include the thrips scouting described above to avoid unnecessary
sprays and cultural controls like maintaining the plants well-watered, which
reduces the impact of thrips damage. In addition, sprinkler irrigation can
directly wash many thrips off onion leaves.
