Green mirids (Creontiades dilutus) have emerged as a significant problem in commercial Bt-cotton crops, especially in Bollgard II crops. This is believed to be due to the reduced spray regimes in these crops, which allow mirids to survive and reproduce, where in the past they were co-incidentally controlled by insecticides applied against the primary pests Helicoverpa spp. Our past experience with mirids has been as early season pests, and although thresholds were available it was not known if these were appropriate for populations occurring later in the fruit setting and maturation period. The level of damage to fruit that can be tolerated without yield loss was poorly understood and this undermines confidence in tentative thresholds and encourages growers and consultants to use low thresholds. This can lead to increased costs, disruption of beneficial population and increases the risk of outbreaks of secondary pests such as silver leaf whitefly, spider mites and aphids. This project investigated the effect of late mirid damage to young bolls through the early, mid and late fruit set and maturation period, using real and simulated damage (boll injecting with weak pectinase solution). The results of a range of glasshouse and field experiment show;

1. Boll injecting was a useful tool to simulate damage by mirids, though the damage caused is more severe than that caused by mirid feeding.

2. Cotton was able to recover fully from damage inflicted in the early fruiting period (3 weeks after flowering began) when damage of 5 or 20 bolls per metre was inflicted.

3. Damage in the mid (5 weeks after flowering began) or late (8 weeks after flowering began) tended to reduce yield at heavier levels (20 or 50 bolls damaged per m) but had no effect at 5 bolls per m)

4. Yield reductions were due to reduced gin out-turn, resulting from selective removal of damaged lint, reduced boll numbers (in some cases), and reduced boll size of damaged bolls.

5. There was strong evidence of compensation for bolls shed due to injecting. This compensation was due to retention of extra undamaged bolls which partially made up for those shed due to damage. In the heavier injection treatments it is possible that too many bolls were lost for immediate replacement by substitution, possibly explaining reduced boll numbers.

There is now a large data set available which can be used to test and calibrate the OZCOT model, which can then be used to generate ‘rules of thumb’ for the degree of damage that can be tolerated. This can also be used as part of a decision support system which integrates mirid information with the wealth of research already captured in OZCOT, allowing interactions between growth, climate and pest damage.