Agriculture Reference
In-Depth Information
Female thrips lay about 80 eggs, which they oviposit into the leaf tissue at
the top of the pseudostem where new leaves emerge. Here, the newly hatched
nymphs are protected and relatively free from predators. The insects develop
through two larval (nymphal) stages to the pupal stage. All stages are of similar
slender shape, but the larval stages are pale yellow or green, whereas the adults
are darker and have narrow, hair-fringed wings. At the end of the second
nymphal stage the thrips move into the soil near the base of the plants and go
into the pupal phase. The winged adults emerge from the pupae and act as the
dispersal stage, the delicate insects being easily carried by wind. Nymphs are
heavier and more localized feeders than adults and favour particularly young
leaves emerging from the pseudostem, thereby disfiguring the elongating
leaves and reducing photosynthetic capacity. The insects can remain quiescent
in soil or litter in the adult or larval stages, and may also survive the winter in
the necks of infested plants or bulbs.
As with so many processes in crop science (see Chapter 4), the rate of
thrips development is a near linear function of temperature above a base
temperature, T
b
(see Fig. 5.7). From this relationship the thermal time for the
development of a thrips generation from egg to adult can be quantified.
Thrips
tabaci
from England required 261°C-days above a T
b
of 5.9°C. Strains from
warmer regions appear to have a higher base temperature and lower day-
degree requirement; for example, a T
b
of 11.5°C and a thermal time of 179°C
days was required for thrips from onions in Texas, USA (Stacey and Fellowes,
2002). Hence, the duration of the life cycle decreases as temperature increases
and, at 30°C, it takes only 10-11 days from egg to adult. As a consequence
there are more generations per year in warm climates. In the Treasure Valley
onion growing area of Oregon, USA there are five to seven generations per year
(Jensen, 2005), whereas three generations per year are typical of Belgium (de
Clercq and van Bockstaele, 2002).
Fig. 5.7.
The relationship between mean development rate and temperature for
Thrips tabaci
from York
in the UK (from Stacey and
Fellowes, 2002. Courtesy of the
European Journal of Entomology
).
●
, Cambridge
and Surrey
