Cotton Seed Distributors - Web on Wednesday

Dr. Lewis Wilson, Principal Research Scientist - CSIRO and CRC, and Dr. Moazzem Khan, Research Entomologist Queensland Department of Primary Industries, outline the benefits of beat sheets in crop monitoring and explains Mirid ecology.

Dr Lewis Wilson
What are the current control thresholds for Green Mirids?

The thresholds for Mirids at the moment are based on visual samples and, in cool regions we’re using a threshold of 0.5 or half a Mirid per metre, basically season long and in warm regions, we’re using a threshold of 1 Mirid a metre season long. So I reiterate that’s in visual samples; if you’re using beat sheets to sample Mirids, you take your beat sheet count and divide it by 3 to get a visual equivalent.

In terms of thresholds, with Mirids it’s important to know how many you’ve got there so that’s the trigger for knowing that there’s a problem with that particular pest, but you also need to know how much damage they’re causing because, if the plant is growing very well you might be prepared to take a bit more damage than if the plant is growing poorly.

Early in the season the main thing that Mirids will do is tip plants out. We have published in the IPM guidelines and in the Cotton Pest Management Guide some recommendations for monitoring tip damage and we give some threshold levels for tip damage. These should be used in combination with your actual Mirid counts to decide if they need to be controlled.

Likewise as you move out of that seedling stage and into the squaring phase you want to be doing your Mirid checks and along-side that be looking at first position retention, so top five retention and combining that with some fruiting factor counts as well.

That way you can get a clear picture of how the plants are going and whether or not you have anything to worry about with the particular Mirid population you’ve got.

Finally as you move into the boll fill stage, which is where we’re probably the most uncertain about and in terms of our experience with managing Mirids, you should be sampling Mirids to find out how many there are there but not using that as your sole way of deciding if you’re going to control them.  You should also be sampling bolls and cutting them to look for the signs of Mirid damage so that you can establish that, in fact, if you’ve got Mirids there, that they are in fact causing damage or if you’ve got damage it is in fact due to Mirids. That way, by combining the two you’ll end up with a fairly effective means of deciding whether the controls are warranted or not.

Dr. Moazzem Khan
In a normal cotton season, where do the Green Mirids come from?

Mirids use different hosts for their development, and actually cotton is not their preferred host. They build up in lots of wild hosts like wild turnip and variegated thistles growing around the cotton paddock. They will over- winter as an adult or eggs in those hosts and when the temperature starts to rise from August they start to build-up. First recruitments, in large number, occurs in October and around November when these wild hosts die off, are cultivated or sprayed.  The mirids move into cotton at seedling stage. That’s where they come from.

Is this a long or short distance?

Short distance usually, although they could come from a wider area with a weather front. From wild hosts they are usually bred locally.

During a cotton season, what sort of factors will influence the Mirid populations within a crop?

We have both conventional and Bollgard® II cotton. Usually in conventional cotton insecticide is a measured factor. Insecticides that kill Helicoverpa also limit mirids incidentally, so that is the important limiting factor for Mirids in conventional cotton.

There are other factors that apply for Bollgard® II cotton, one is very hot temperatures. From our studies we found that if temperatures are above 35ºC for at least 3 days Mirids will die, there is a deterrent effect from that sort of temperature.

Another factor is rain or heavy storms. These take out many Mirids in the paddock.

There are generalist predators such as spiders, ants, assassin bugs and, damsel bugs. They can take out Mirids as well.

You’ve mentioned that Mirid numbers decrease during hot weather or after rain. How should that influence your sampling?

Yes, that’s very interesting because before rain you may do sampling and you have some numbers there and after rain when you are sampling you will end up with a fewer numbers because of  nymphs dying or adults flying away.

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