Of the range of insecticides that are registered
for silverleaf whitefly control there are two key products that have been in use for whitefly
management over the last 15 years. These 2 products are the insect growth regulator pyriproxyfen
and the knockdown product diafenthiuron. To get the best out of these products it’s really
important to understand how they work and also consider their use in the framework of
a resistance management strategy. Pyriproxyfen works at 3 points within the
whitefly lifecycle. It sterilises the adult females and prevents them from laying viable
eggs. Any eggs that are within the crop are also prevented from hatching. And it prevents
the emergence of the pupae into adults. So what you see over time is a reduction in numbers
as the adults die of old age and are not replaced by nymphs coming through. So this product
takes a considerable period of time to work, and you won’t see significant field reductions
quite often until around day 18-21. Pyriproxyfen is trans-laminar, so once it’s applied it
absorbs into the leaf surface and is active within the plant for a 2-3 week period. So
as the whitefly move around the canopy, even though they might be in the lower reaches
of the canopy, they only need to move up and feed on that treated foliage once every 3-4
days to remain infertile. One of the key strengths of pyriproxyfen is
that it�s very specific to silverleaf whitefly and is very soft on the natural enemies that
feed on whitefly. Those natural enemies remain very active in the crop canopy, compounding
the control provided by pyriproxyfen, and remain long after the pyriproxyfen’s effects
have worn off. So in order to get the best out of pyriproxyfen,
we really want to use it as part of an integrated pest management program where we’ve set the
crop up to conserve and protect the natural enemies that are also helping us manage this
pest. Diafenthiuron in comparison is a knockdown
product. After application, diafenthiuron enters a fumigant phase where it’s activated
by sunny conditions. And that fumigant action is very effective at targeting the whitefly
that are on the undersides of the leaves. A key consideration for using diafenthiuron
is that you have 3-4 sunny days after application. Using this product during cloudy weather or
when rain is expected is really going to take away from the efficacy of this product, so
conditions during application are very important. Pyriproxyfen and diafenthiuron have been used
extensively for silverleaf whitefly control throughout the cotton industry for the last
15 years. At the Department of Agriculture and Fisheries we’ve been monitoring resistance
levels to these products and what has been particularly concerning has been rapid increases
in tolerance to our cornerstone product pyriproxyfen. Jamie Hopkinson, our entomologist who leads
the resistance testing program will talk you through some of the changes that he’s observed
over the last couple of seasons and the implications that it will have for pyriproxyfen usage going
forward. In this lab we test for silverleaf whitefly
resistance to a number of insecticides including pyriproxyfen. With pyriproxyfen, over the
last few seasons we’ve seen a shift from susceptible populations to tolerant populations and in
the 2015/16 season we’ve found the first evidence of resistance in the northern NSW area. So
that trend has continued to evolve this season – we now have several regions with pyriproxyfen
resistance, including St George, Macintyre, Gwydir and Namoi Valleys. The levels of pyriproxyfen
resistance seen this season wouldn’t translate to field failures at this stage, but the trend
that we’re seeing clearly demonstrates the product could fail in the near future. So
in light of these changes to resistance levels, the TIMS technical panel is reviewing the
use of pyriproxyfen. The objective of these changes is to show the development of resistance
and also protect the efficacy of pyriproxyfen going forward.
In the coming seasons it will be critical that people use the most up-to-date guidelines
for pyriproxyfen use. These can be found in the most recent Cotton Pest Management Guide.