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citrus thrips to reduce leaf damage, studies
of young navels and Valencia oranges
conducted by Drs. Morse and Grafton-Cardwell
in the 1980-90s demonstrated
that continuously treating on
to three year old trees did not improve
the growth or yield of the trees. Citrus
trees can tolerate extreme amounts of
leaf damage and continue to grow and
produce well. However, there are conditions
where protection is helpful. For
example, the combined impact of citrus
thrips and citrus leafminer (arrived in
California in 2000) on young trees needs
to be studied. Secondly, we have had five
years of severe drought and these hot,
dry conditions promote citrus thrips
populations which can then have an
impact on trees that normally withstand
their damage.
Managing Citrus Thrips in Mature
Citrus Orchards
If a grove is more than three years
old, leaf damage caused by citrus thrips
should be ignored and the focus of
management should be on the new fruit
during the period from petal fall till the
fruit is 1.5 inches in diameter. Minimizing
sprays is important for several
reasons: 1) to reduce the impact on
natural enemies needed for citrus thrips
or other pests and 2) so that resistance
to pesticides doesn’t develop in the
thrips. The more generations of citrus
thrips you treat, the faster resistance
will develop. Starting at petal fall, Pest
Control Advisors (PCAs) sample 100
fruit per site and determine the percent
of fruit with immature citrus thrips. At
petal fall this can be a bit tricky because
western flower thrips will be present and
this species does not damage citrus fruit.
The body shape and activity of these two
species of thrips is very different—citrus
thrips are quick moving, short and stout
while western flower thrips move slowly
in a serpentine fashion and have longer
cigar-shaped bodies. It is important to
wait till there are immature citrus thrips
on the fruit, because in some years, petal
fall does not coincide with the appearance
of immature citrus thrips. During
the organophosphate era, the recommendation
for navels was treat when
there were five percent of fruit infested
with immature thrips and if predatory
mites were in sufficient numbers to
assist (> 0.5 predatory mites/leaf) wait
until there were 10 percent of the fruit
infested. In the current era, of more selective
and slower acting insecticides, the
threshold for tolerance may be lower, but
it is still important to wait till the thrips
immatures are present on young citrus
fruit before treating. No research has
been done on the impact of citrus thrips
on mandarin production and some
varieties may be highly susceptible and
need protection, while others may need
no protection. Finally, citrus thrips are
much harder to control in drought years
than cool wet years. Wet weather helps
to reduce the pupae in the soil, while hot
weather accelerates thrips development
and allows the pupae to survive in the
soil.
Pesticide Resistance
Citrus thrips are very prone to developing
resistance. The developed resistance
to DDT in the 1940s, the organophosphates
and carbamates in the 1980s,
and the pyrethroids in the 1990s. Some
of these insecticides seemed to make the
Page 8 Progressive Crop Consultant March/April 2017