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

The reasons for this are complex but again in partly due to the
introduction of Bt cotton. The reduction in sprays, and particularly
endosulfan, caused aphids to require more tactical targeted control
with Pirimor or an OP so exacerbating resistance. From the early
1990s, sustainable cotton aphid control quickly became an industry
priority with the recovery of Pirimor considered most important.
An IPM based resistance management strategy was developed
that did eventually recover Pirimor (Figure 3). The recovery of
Figure 3: Per cent strains of cotton aphid
containing Pirimor resistant individuals
between 2000–01 and 2011–12
*13 out of 37 strains complete in 2011–12.
Figure 4: Per cent strains of cotton aphid
containing neonicotinoid (e.g. Cruiser or Shield)
resistant individuals between 2004–05 and
2011–12
*11 out of 37 strains complete in 2011–12.
Pirimor is again complex and multi factorial but in part due to
the introduction of new group of insecticides known as the
neonicotinoids. This group of insecticides containing products
such as Cruiser, Shield and Confidor was initially very effective but
again cotton aphid showed its resilience and developed resistance.
resistance was not restricted to one product but all the
neonicotinoids used against it. resistance was initially only detected
in two strains at low frequencies but within a season resistance had
dramatically increased in both level and abundance with resultant
control failures (Figure 4). resistance then increased up until season
2010–11 when nearly all strains tested were neonicotinoid resistant
but last season resistance was less frequently found (although all
testing is not yet complete).
The reasons for this are again complex because the amount of
neonicotinoid seed dressing used has not diminished and a higher
rate product called Cruiser extreme was made available. I consider
it likely that the foliar use of neonicotinoids against pests other
than aphids has changed and aphids are not receiving the same
concurrent selection they once did (i.e. being in the wrong place
at the wrong time when another pest is sprayed).
The development of neonicotinoid resistance in cotton aphid is a
blow to the industry because it is both cost effective and efficacious.
A possible solution to the evolution of neonicotinoid resistance
forms part of a PhD study being undertaken by Kate Marshall with
funding by the CrDC.
One of the initial tasks undertaken by Kate is to ascertain the
practical implications of neonicotinoid resistance with respect to
seed dressings and to investigate alternatives. Kate achieved this
via two separate greenhouse based trials that firstly investigated
neonicotinoid seed dressings challenged with neonicotinoid
resistance aphids (Figure 5). Secondly a separate trial is planned
with no insecticide seed dressing but rather a Thimet soil granule
that was challenged with Pirimor/dimethoate resistant aphids.
It is clear from Kate’s study that Cruiser will not control
neonicotinoid resistant aphids. The higher rate Cruiser extreme
product worked better than Cruiser but still survivors were
evident at the second 14 day post sowing challenge. Additionally,
Kate reports she did see a survivor against Cruiser extreme
at day seven in an earlier preliminary trial. If Kate’s second
trial is successful and Thimet can used as an alternative to a
neonicotinoid seed dressing, foliar use of neonicotinoids for
aphid control may again become a reliable option.
So aphid control in Australian cotton has now come full circle
in about a decade. IPM friendly Pirimor that was once lost to
resistance is now again a mainstay control. The neonicotinoids
continue to be at risk but their resistance profile is improving.
Bean spider mite all but vanished from Australian cotton but
now seems to be making a resurgence.
Figure 5: Average number of live neonicotinoid resistant aphids per plant on untreated (control) or
neonicotinoid treated (350 or 600 g/L) cotton 7–49 days post sowing (destroyed seven days post each
challenge)
22 — The Australian Cottongrower August–September 2012