Cotton aphid (Aphis gossypii Glover) is a late season pest of cotton, with the potential to

reduce the value of lint by contaminating it with sticky honeydew secretions. Recently, cotton

aphid emerged as a threat during the early and mid season - due to its potential to cause yield

loss and its role as a vector of the disease 'Cotton Bunchy Top'(CBT). This project aimed to

improve understanding of the ecology of this pest, especially season abundance and

overwinter host use, and of its distribution on cotton, which is important in developing

sampling techniques. This project was complemented by two other CRDC funded projects;

'Aphid bio-control' (DAQI 19C), and 'Incorporating aphids, insecticides and early season

plant compensation in PM' (CSP147C). Key outcomes are:

1. Cotton aphid is a mid-late summer pest. A wide range of summer and winter hosts were

identified and included a range of broad leaf weeds, crops and garden plants. Among the

winter crop hosts are faba beans and lupins, although aphid do poorly on them. Aphids

failed to persist on vetch, canola or lucerne. In particular ratoon cotton will carry-over

aphid populations, which is a problem for resistance management and for carry-over of

CBT. This led to recommendations for good overwinter field hygiene.

2. It is likely that winter conditions affect the size of the overwintering populations, with

drier conditions supporting fewer hosts and lower overwinter aphid populations.

3. Cotton aphid uses hosts in farm and urban gardens through winter. Farm gardens in

particular can serve as reservoirs for insecticide resistant cotton aphids and are particularly

important in dry years when other hosts are scarce.

4. The seasonal abundance and host range of cowpea aphids was documented. This species

has two peaks in abundance, in early spring and autumn. It also uses a wide range of hosts,

but is particularly noticeable on medics.

5. Green peach aphid is a cool season specialist. It has a narrower host range than cotton

aphid and cowpea aphid, but uses some brassicaceous hosts that are widespread.

6. A wide range of other aphid species and their hosts was also recorded. This information

has proven valuable, for instance in warning industry of influxes of Rhopalosiphum spp.

which settle on seedling cotton, but which will not establish and breed. This led to the

recommendation not to treat aphid populations unless it is clear they are breeding.

7. Cotton aphids show a preference for the upper canopy leaves of plants. However, at higher

numbers there is a significant population found in the lower canopy. This population is

difficult to reach with insecticides and may be a major source of recolonisation of plants

following insecticide application.

8. Cotton aphid shows a highly clumped distribution, a product of their biology, whereby

females produce live young that do not move far away from the parent. This makes

sampling more difficult as populations may be missed.

9. There is a curvilinear relationship between the proportion of plants infested with aphids

and the mean number of aphids per leaf. This may be useful if it can be linked effectively

with the thresholds being derived in another project.

Outcomes of this research have been widely reported to industry. Future research on aphids

could emphasise selective control, improved understanding of colonization and spread in

cotton, contribution of trap crops or relay crops as hosts and link with spread of CBT.