Agriculture Reference
In-Depth Information
Choice of variety can also help. In the western USA, 'Spanish-type' onion
cvs with shiny bright green foliage are generally less susceptible to thrips
damage than are onions with grey/green foliage (Jensen, 2005). Resistant
varieties produce leaves widely separated on the pseudostem and have a wide
angle of divergence between the leaves emerging from the neck, thereby
minimizing the shelter for thrips between leaves. Ploughing to bury over-
wintering thrips in soil or on crop debris will reduce the infection source for the
following year. Overwintering leek or onion crops are likely to act as a 'green
bridge' for overwintering thrips, and situating spring sowings or plantings
remote from these may delay infestation.
IPM principles are embodied in the use of 'soft' insecticides like Spinosad,
which control plant-eating pests but cause little damage to the predatory
arthropods that help to keep pest numbers in check (Williams
et al
., 2003).
Experiments in Oregon in which Spinosad was applied to bulb onion crops that
were also mulched with straw gave better control of thrips and significantly
higher yields and monetary returns than those treated with the usual
insecticide schedule (Jensen, 2005). Populations of thrips predators under the
Spinosad/straw treatment were almost four times those under the standard
insecticidal treatment, and increased predation may have supplemented the
insecticidal effect of Spinosad to produce the high level of pest control. Reduced
thrips damage had previously been noted on onion fields mulched with straw
to improve irrigation infiltration, and similar benefits have been observed in
trials of straw-mulching on leeks (Weber
et al
., 1999).
Undersowing or intercropping allium crops with another species offers a
more radical potential solution to thrips control. Numerous experiments have
shown that leek crops undersown with clover do not suffer much thrips
damage compared with normal monocrops (see Fig. 5.8; Theunissen and
Schelling, 1999). Leeks intercropped with carrots also have much lower thrips
populations (Legutowska
et al
., 2003). The reasons for this effect are not fully
understood, but there is evidence that the females of pest insects do not oviposit
until they have made several successive landings on leaves of a suitable food
crop. If an acceptable species is intermingled with a non-host species, the
female may not receive sufficient stimulus to oviposit and usually departs the
crop without laying eggs (Finch and Collier, 2000).
However, when pots of leek plants that had been grown with clover
undersown but from which the clover leaves had been mown off were placed in
a thrips-infested field, only about one-third the number of adult thrips were
found on these plants as were found on potted leeks grown without clover (den
Belder
et al
., 2000). This experiment indicates that something other than the
visual, olfactory or chemical signals that thrips detect from adjacent clover
leaves makes the undersown leeks less attractive to thrips than leeks grown
in bare soil. Experiments have established that neither increased predator
levels nor decreased nitrogen content in undersown leeks can explain this
suppression of thrips. Some physiological change in leeks brought about by
