growers@sgcotton.com.au Roger Tomkins - Greenmount Press

The changing face of aphids and
mites in Australian cotton
■■By■Grant■Herron,■Senior■Research■Scientist,■NSW■DPI,■Camden There has now been a long history of resistance monitoring
for both aphids and mites from Australian cotton with
much of this being done by our team at NSW DPI.
Mites
Up until 1980, both bean spider mite and two-spotted mite
were collected and tested for resistance, but for 30 years bean
spider mite disappeared from Australian cotton. This is unusual
because normally bean spider mite will displace two-spotted
mite. The reason probably relates to two-spotted mite’s ability to
develop resistance. For instance, bean spider mite resistance to
the OP monocrotophos never exceeded about 10 fold, yet twospotted
mite resistance to the same chemical was often hundreds
of fold (Figure 1).
This gave two-spotted mite a huge advantage over bean spider
mite in heavily sprayed cotton and bean spider mite disappeared.
Jump forward a few decades and the cotton ecosystem is
dominated by Bt cotton and the sprays are all but gone. Now
bean spider mite again has the advantage over two-spotted mite
and in theory it should start to displace it. Anecdotally this is
exactly what seems to be happening with mite samples collected
by myself in Queensland invariably containing bean spider mite
while two-spotted mite is more abundant in NSW.
The reason for this NSW preference is not clear, as two-spotted
mite is still quite resistant to some chemicals, but fortuitously not
to its mainstay control Comite (Figure 2). The bean spider mite
resurgence is complicating the laboratory based resistance testing
because bean spider mite will displace two-spotted mite in the
lab prior to testing being done.
Additionally, if it happens that again bean spider mite needs
to be tested for resistance, the chemicals used in cotton are now
different to what they used to be. reference baseline data for
newer chemicals is not available and this baseline information
would have to be generated. Finally, the damage done to cotton
by bean spider mite looks different, less severe. Bean spider mite
is likely a whole new management ball game.
PhD student Kate Marshall with PhD supervisor grant Herron
testing neonicotinoid resistant cotton aphid.
Aphids
Aphid problems in cotton up until the mid 1990s were
little more than deciding what to spray. At the end of 1998
a resistance issue developed in cotton aphid with resistance
detected to all registered chemicals used for its control.
The biggest loss to resistance was the IPM friendly chemical
Pirimor. In laboratory tests with high level resistant aphids, the
product could surround and cover aphids like snow but do
nothing.
Figure 1: Monocrotophos resistance detected
in two-spotted mite (Tetranychus urticae) and
bean spider mite (Tetranychus ludeni) between
1976 and 1994
Figure 2: Percent strains of cotton aphid
containing propargite (e.g. Comite) resistant
individuals between 2004–05 and 2011–12.
NB: No strains tested in 2009–10.
20 — The Australian Cottongrower August–September 2012