Sandra Deutscher, Experimental Scientist - CSIRO discusses Mirid sampling and Dr Brian Duggan, Research Scientist - CSIRO outlines Mirid damage.

Sandra, you did some detailed studies on mirid sampling last season. Could you describe the work that you did and some of the key outcomes?

Last season we particularly concentrated on mirid sampling. We compared three different sampling methods; the visual, the beat sheet and the sweep net sampling method. In the United States they use the sweep net method to sample for Lygus bugs, so we thought we would trial it in the Australian conditions to see how it matched up with the more popular ‘beat sheet’ technique.

We looked at the timing of each method. We found that the beat sheet and sweep method were both very quick compared to the visual. The beat sheet was quicker than the sweep net, coming out at about 4 minutes as opposed to 6 minutes

We looked at whether there was any variation between scouts using these methods when you are sampling for mirids. We found that there were no significant differences between the three methods, so that was really good.

Can you tell us how you managed the accuracy of the different methods and also how they compared as far as the conversion factor?

We came up with our own method of collecting an absolute count in the field and we called that our ‘pounce net’ method.

Compared to the ‘pounce net’ densities per metre the ‘beat sheet’ and the sweep net were much on par but the visual was quite poor in ‘finding what was actually in the field. When we looked at the correlations between visual and sweep net to get a conversion factor (because all the thresholds within the industry are based on per metre counts visually so we thought we would have to convert the sweep nets back to visuals), it came back as a three to one. So, for every three mirids that you find in a sweep net you are looking at about 1 per metre visually.

Could you give us a summary then of what the key results were?

In summary, the beat sheet and the sweep net methods to find mirids are on par. They are both quick, they are both accurate. They are both finding about the same amount, so about 3 to 1 conversion. When the fields are wet you would probably use a sweep net method and when the fields are dry you might use a beat sheet. Another thing is that when you are using a sweep net, you are likely to find a few more adults, but when you are using the beat sheet you will find a few more nymphs. So both methods are quite complimentary really.

Dr Brian Duggan, CSIRO Division of Plant Industry

Brian can you describe some of the work you did on mirid damage last season and what are the implications to growers?

We had trials right throughout the cotton belt, from Emerald in the North through to Hillston in the South. What we were doing was injecting bolls to simulate mirid damage and we inflicted the damage at various times throughout the growing season.

These included, shortly after first flower, mid season and then post cut-out. We did that because we are really interested in developing more accurate thresholds for mirid numbers. So in some of the work we have done, we have injected bolls throughout the growing season and then look at the ability of the plant to compensate following that simulated mirid damage.

Have you done some work correlating the simulated damage to real world damage?

Yes. We have had two years of the simulated damage where we have used the pectinase injections. But, like you said, something that has always troubled me is relating that to actual mirid damage. So what we have done is cage sections of crop and then introduced mirids into those cages and compare that to the boll injection. That should give us a better idea of how many bolls we inject and how that relates to how many mirids there are actually in the crop and how many bolls the mirids will actually feed on.

And how is that looking at this stage?

That’s looking pretty good. We have always thought that our thresholds are probably a bit low. We can probably have a few more mirids than what the current thresholds are. We have only had that at one site. We had that at Narrabri this year and we hope to have that at Emerald, Narrabri and Hillston so we can get a full range of the cotton belt this year.

To summarise at this stage, what do you think the key aspects of your findings are?

A couple of aspects. Firstly, the damage is going to be more severe in cooler, shorter season sites (from Narrabri and south). What we are going to end up with is a more dynamic threshold. Instead of having a set number of ½ or 1 mirid per metre, depending on where you are in the cotton belt, it is going to be more looking at the retention, the time of season, which you have got the mirids and where you are in the cotton belt.

The example I always give is if you are in Emerald, (a nice long growing season area), damage shortly after flowering when you have got a very high retention is probably not something you are going to worry about, whereas if we had a shorter season site, it is later in the season and your retention is quite low, you are probably going to be more concerned about your mirid numbers.

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