Cotton Seed Distributors article: David Kelly speaks to Lewis Wilson, CSIRO Entomology/ Cotton CRC about research being undertaken on some of the ‘emerging’ pests in Bollgard II® crops; late season thrips, jassids, and aphids.

A video version is available at www.csd.net.au/  

In the last 5 years with the advent of Bollgard II cotton the whole spectrum of managing sucking pests has really changed.  I am talking here today with Lewis Wilson who has conducted a lot of new research in regards to managing sucking pests.  

Aphid management might be slightly different in the Bollgard system with less other insecticides cruising around.  What sort of new developments have you got in regards to aphid management and thresholds?

Well, I guess with aphids one of the questions that we have really had is that in the late 90’s we saw aphid populations in cotton crops much earlier than we have seen them before and in that situation we realized we didn’t have any knowledge as to how much yield loss those aphids might cause.  So we were encouraging growers to allow aphids to build up because they were food for other beneficials as well as a pest but we didn’t really know whether that was risky in terms of yield.  So we have done experiments over the last 4 years now looking at aphid densities and yield loss doing manipulative field experiments and we have put that data all together now and we have shown pretty conclusively that if you get aphids earlier in the season and they increase quickly and you don’t control them, they can cause quite significant yield loss.  So we processed the data into a format, a bit like we did with the mites, where we have a relationship now that predicts yield loss based on when your aphids are in the crop, how fast they increase and we are hoping to put that into a format that can be useful for growers and consultants in the field  making decisions about aphid control.

So when are we likely to see a new threshold or new way of making decisions in regard to aphid management, what sort of timeframe?

What we are aiming for is to try and have that information available for the start of next season.  I am just in the process  of thinking about it critically to make sure that what we do come up with is actually practical and will capture the information the right way.

Another pest is thrip.  Thrips usually have been associated with early season and then late season. It’s not unexpected with low spray systems in Bollgard II that they have increased.  What have you found in regards to thrip?

The point that you make is really good.  What we were seeing in some of these low spray systems is thrips coming in quite often very high numbers last season - two species mainly.  One is Frankliniella schultzei which is the tomato thrip, a dark coloured thrip we have commonly had, and the other species is F.occidentalis which is the western flower thrip.  It’s a recent introduction.  It is a problem in that it is difficult to control if you want to control it, but I will come back to that later.  So what we have been doing is looking at those thrips coming in, damaging the terminal areas of plants, the younger leaves and also getting very high densities in flowers.  So this raises two main questions.  One is the damage to the flowers, is it actually affecting their potential to set as fruit, and is the damage to the leaves actually having a yield potential influence.  Balancing that is the fact that we know those thrips are having a big impact on mite populations. We have been into lots of fields where there is a mite population and the water has been a real problem so we can see quite clearly from the mortality that the thrips have moved.  So we are doing experiments now to look at how much damage you can tolerate from thrips and what the yield outcomes might be and then the next level is if you can identify damage levels that require control, it needs some recommendations for that control.  Also for selectivity of the control and also what will work against western flower thrips.  So we put that all together.

Another pest now regularly seen late season is jassids.  There have been a number of instances of high levels in the last couple of years.  What have you discovered there?

Jassids are a little bit in the same basked as thrips in that  normally in the past they would have been controlled by other sprays.  When there is a low spray situation, the jassid populations is able to build up from the early crop onwards.  Jassids don’t have the really fast lifecycle of mites or aphids.  So it takes longer for the populations to build, but what we have seen in some fields is towards the end of the season, the crop is almost going white with damage starting in the middle of lower canopy, spreading through the middle canopy and up to the upper canopy.  So again you see high levels of damage in some leaves; we know that it affects photosynthesis if it is severe, but we don’t really know how that translates to yield loss.  We have been doing experiments with artificial damage, saying ‘what is the worse case scenario’ if we pull off all these leaves from the bottom of the canopy. If the jassids took the lot, what would that do to yield.  Same with the middle canopy, the upper canopy and all those combinations so that’s a fall back strategy - it gives us some good knowledge. We are also doing experiments with real jassids where we are allowing them to build up in crops and then controlling them at different densities and seeing what the yield outcomes are.  We have got one experiment that has worked quite well there.  We are having some challenges and in fact we are really looking for sites where a grower or consultant identifies that they have got a jassid population building and would be willing to let us come in there and carry out some trial options.  

So next season, if  someone has got big jassid numbers in the field they should speak to you?

Yes, let us know.  It is a priority for us at the moment.

Cotton bunchy top is still obviously an issue on your radar.  What new discoveries have you made in that regard?

We have done a lot of work in the past looking at the basic epidemiology of bunchy top. We showed that aphids spread it and we have done a lot of work working out how many aphids it takes to spread it, how long it takes and those shorts of issues.  We are still trying to refine that work in the glasshouse.  It’s fairly difficult and steady work to get done.  We are also working with the cotton breeders to look for lines that are resistant to bunchy top.  We have got quite strong resistance and now the breeders have being trying to include that resistance in their elite commercial lines so we do screening with them for that and we are getting some quite good results there as well. 

 The other thing  everyone is concerned about is are we going to get another bunchy top year or has it gone for good because we haven’t seen it.  It was a problem for a few years and I guess the way to answer that is two ways.  One is if we got the right season, rain through winter, heavy weed buildup through the winter and spring, we could easily have a situation where we could generate lots of aphids that can then potentially carry the disease into cotton early in the season.  The second thing is ‘well how prevalent is the disease anyway?’ and I don’t really know the answer to that.  It is not an easy question to tackle but one bit of information we have is that when Grant Herron and myself go around the regions collecting aphids for resistance testing, I also am on the lookout for bunchy top plants, and last year in six of the eight sites that we went to, we actually found bunchy top plants, often in the middle of a aphid hot spot which signals to me that the disease probably isn’t rare in aphid populations, it might be reasonably common. It comes in and the fact that it doesn’t build up and spread very quickly actually saves us so it doesn’t get across the field in a hurry.  Situations where it could become a problem is where you have lots of aphids coming in early and spray with a product for instance that didn’t control the aphids well. If you then do not have the predators, then you could get a spread of disease throughout the field very quickly. In the normal course of events where growers are managing aphids fairly sensibly, quite often the aphids  feeds on the plant but it isn’t capable yet of re-infecting other  aphids, so you get some spread of aphids initially that aren’t carrying the disease.

Sounds like farm hygiene in terms of controlling stub cotton is going to be pretty important?

Keeping a track of your aphids early season is important and farm hygiene to prevent carryover as well, because one  thing  that we do know is that stub cotton is a key way to carry the disease from one season to the next.