This project evaluated options for selective control of aphids, studied the field epidemiology of cotton bunchy top disease and determined the suitability of relay crops for aphid pest management. Key conclusions were;

1) Acetamiprid plus pulse provided good control of aphids but flared mites, presumably because it reduced beneficial numbers. This will also be a risk if this product or other neonicotinoids are used against mirids.

2) None of the ‘soft’ soap or oil options alone provides adequate control of heavy aphid populations. Canopy oil may be effective if applied to lower density aphid populations (e.g. less than 10-15 aphids per leaf) in a regular program.

3) The biopesticides evaluated are unlikely to provide the high level of control provided by acetamiprid plus pulse. However, efficacy should improve as more field adapted strains are selected. Used early, these products may be effective at preventing aphid populations from increasing without flaring mites and with reduced resistance risk.

4) CBT appears to be relatively common in aphid populations. CBT affected plants were often observed in the centre of aphid hotspots – often only one or two infected plants are found. In 2005-06 we found CBT affected plants in aphid hotspots in 6 of 8 sites.

5) The latent period of CBT in cotton plants can be relatively short (about 10 days) but is likely to be influenced by aphid numbers (lower densities could have a longer latent period). This is complex and needs further experimentation to sort out.

6) Yellow dwarf forms of cotton aphid transmit CBT poorly compared to ‘normal’ sized aphids.

7) The rate of spread of aphid populations while they are in the apterous form (non-winged) is relatively modest e.g. it took about 40-50 days to travel 8m, both across and along rows. Once populations reach densities that produce alates (winged forms) the spread of aphids could be much faster.

8) Spread of CBT was much faster and higher when the infestations in the field were initiated with a CBT affected plant carrying a high aphid population, compared with those initiated by infesting a field plant with CBT infected aphids. This is likely due to movement of CBT infected aphids off the CBT affected plant compared with movement of clean aphids off the aphid infested plant due to the latent period.

9) In terms of risk of CBT outbreaks – generally low densities of aphids colonising plants means transmission efficiency is low and the latent period likely to be longer, which may severely limit the level of infection in field. Exceptions would be in years were season conditions generated high numbers of aphids and of alternative hosts with CBT – which could then colonise cotton crops at higher densities or situations where stubb cotton was abundant in a field and acted as a source of CBT carrying aphids.

10) The effect of CBT on yield is greater the earlier that plants are infected.

11) Lucerne and sorghum are good relay crops for aphid management as they host aphids that don not attack cotton on which beneficials and parasites could build up and move into cotton.

These outcomes significantly advance our understanding of aphid and CBT epidemiology and will be valuable to industry in developing management strategies.