Cotton Seed Distributors - Web on Wednesday

Martin Dillon, CSIRO Entomologist, highlights the importance of pupae busting in the resistant management of cotton

Martin it’s a critical time of year with growers finishing picking and the next job that their going to contemplate is some sort of cultivation. Pupae busting is very topical. Can you tell us why pupae busting is so important?

Sure Adam, pupae busting is crucial. Remember that the pupae are the survivors of the previous seasons whole spray regime and if there are any resistant individuals amongst them they’re going to be amongst those pupae that rest through the winter in a state of diapause, so there’s no way that resistance can pass from one season to the next without going through those pupae. If you can kill those pupae you are absolutely minimizing the amount of resistance coming through to the next season.

And so Martin, that’s going to mean some sort of cultivation. Do you want to make some comment on the depth of that sort of tillage?

What happens is when caterpillars get to their full size they drop down to the soil and dig a burrow down in it; usually within about 30cm either side of the plant line and they can go down up to 10cm. Most of them are about 5 but up to 10cm and the thing they do is they dig a little escape tunnel and seal it off with saliva and that is what they are going to use when they finally turn into a month and emerge and the whole idea of pupae busting is to destroy those escape tunnels and trap the pupae and the months under the ground where they can’t dig out. You know a moth can’t really dig. It needs those escape tunnels so you’ve got to disturb the soil down to a depth of 10cm well enough to make sure that you’ve absolutely minimized the chance of any escape tunnels surviving in tact.

And what sort of timeframe have we got to carry out this pupae busting? What would be your recommendation and when we should get in and do that?

You’ve got all winter but the sooner you can get in the better. The reason for that is that once you’ve disturb the soil, any rainfall events you get, any weathering of the clods all helps to reduce and puts a hard crust over the top, reduces the chance of any cracks and things left for the months to emerge out of, so the sooner you can get in the better it is.

And, if you leave it to later potentially the soil is too wet to till?

That’s right, you know if it’s too wet and you’re not getting a proper job done then you’re not going to kill enough pupae to do the job you need to do.

If we consider the Bollgards and our transgenics Martin, have you got any comment on how to handle the pupae there?

Certainly, well as you know it’s compulsory to cultivate to destroy pupae under Bollgard and Ingard crops; that’s part of the licence agreement. The reason for that is to minimize the chance that any of the rare resistance survivors might come through. Now the flip side of the coin there is that your refuges are designed to generate moths, so really you don’t want to cultivate them. Ideally you would let that refuge sit there right through to next spring to let any months that are underneath it that haven’t been exposed to Bollgard or Ingard to emerge and come out and help you increase that population of susceptible months that are going to dilute any resistant individuals out there.

Colin Tann: Scrape the top one or two cm of soil just like this; this is fairly hard but what we’re doing is looking for emergence holes as the larvae drops off the canopy of the plant it just bores into the ground and then it come up and produces an emergence hole and is just short of the surface, so you’re scraping the surface looking for those emergence holes about the size of a pencil width, dig down there and there you’ll find the pupae.

In cotton such as this it’s likely to be fairly sporadic. You might find nothing in quite a few meters and then find quite a few in one meter, so you just get these little hot spots. It’s important to just very, very carefully scrape the top of the soil just looking for those emergence holes.

Further Information:  Robert Eveleigh, John Marshall,  Craig McDonald or David Kelly