As the cotton season gets underway aphids are starting to be detected in crops, on ratoons, volunteers and weeds. Last season, there were some regions where Cotton Bunchy Top (CBT) was detected in crops towards the end of the season. To assess and manage the risk of yield loss caused by CBT, and minimise the difficulties of controlling insecticide resistant cotton aphids it is important to i) be able to correctly identify aphids in the crops and ii) consider how early season infestations (in crop and on nearby weeds) will be managed.
There is a lot already written about aphids and aphid management in cotton. Follow the link to the Cotton CRC website to read more about aphid biology and ecology, alternative hosts, and strategies to manage (including managing insecticide resistance).
http://www.cottoncrc.org.au/industry/Publications/Pests_and_Beneficials/Aphids__Bunchytop
Identifying aphids in cotton
There are a number of species of aphid that will establish in cotton, Cotton Aphid, Cowpea Aphid and Green peach aphid are the most common. Cowpea and green peach aphid will colonise cotton, but tend only to persist in cooler conditions. Cowpea aphids were present in cotton crops in Emerald in early November this year, but numbers have since declined. We have received samples of cowpea aphid and cotton aphid from cotton on the Downs this season.
Key features used in the identification of aphids are illustrated below. Use a x10 hand lens to examine the aphid to see the tubercles, cornicles and cauda features.
Illustration from aphid identification key: http://ipm.ucdavis.edu/TOOLS/KEYAPHID/typical.html
Aphid impact on cotton
Aphids have the potential to reduce yield by direct feeding and exude honeydew, resulting in sticky cotton. Early season infestations may result in CBT transmission, and subsequent yield loss caused by CBT.
Research to date (Lewis Wilson et al.), has only detected CBT transmission by Cotton Aphid, although it is possible that other species may transmit at very low levels.
Deciding if aphids need control
Here I will cover some basic points to consider when monitoring for aphids, and in making an assessment of the need for, or timing of aphid control.
- Identify the aphids
- Make an assessment of the level of infestation – % of plants with aphids. Look for aphids towards the top of the plant, at 3-4 nodes down from the terminal, under leaves.
- Mark, or record, infestations and revisit to determine if the population is spreading, increasing in size, and whether there is beneficial insect activity.
- If the infestation is spreading – this is the point at which to make a decision about control.
- Don’t wait until populations are out of control (>30% infestation) to act.
- Suppression of populations for example with a spray oil, may be sufficient to minimise the risk of spread and in situations where there is a high risk of CBT transmission.
- Large influxes or rapidly spreading infestations may warrant the application of pirimicarb.
- Early season spray decisions should take into consideration the potential disruption to beneficials that contribute to the subsequent control of aphids, SLW and mealybug.
CBT and aphid infestations
- Crops at high risk of CBT are those close to reservoirs of CBT and aphids (weeds, particularly malvaceae species; ratoons).
- Early infection will result in more severe symptoms.
- However, do not start spraying aphids at first appearance in the crop. Large influxes of cotton aphid from CBT reservoirs represent a larger risk, in terms of CBT transmission, than a slow influx over a longer period of time.
- CBT symptoms will not appear in the crop for up to 8 weeks after transmission – don’t act on symptoms.
Article by Melina Miles
Watch out for the Blues
Grass blue butterfly larvae feeding results in windowing of the leaves (this is mostly cosmetic) and destruction of vegetative terminals.
The latter damage is of greater concern. Terminal death (tipping) can result in crops setting pods low to the ground where they are more difficult to harvest. Damage in most crops has been uneconomic to date.
Thresholds are determined at 25% terminal loss.
However after the current rain event dies down, growers should re-inspect crops for any terminal death symptoms. Other early signs of infestation of this pest include the small blue-winged adults (butterflies) flitting over the crop, and the presence of their relatively large (0.6mm diameter) pale blue eggs.
Grass blue larvae are often confused with hoverfly larvae. They are of similar appearance and size but hoverfly larvae are key aphid and whitefly predators.. Hoverfly larvae are ‘true fly’ larvae or maggots and they differ from grass blue larva in that they do not have a head capsule, true segmented legs and fleshy prolegs.
To confirm that you have pest (the grass blue larvae) and not the beneficial, flip them over to check for the (normally hidden) head capsule, 3 pairs of proper legs (at the head end) and 4 pairs of ventral prolegs. These legs are positioned towards the larva’s mid-line and are not visible from above. Another way to confirm identity is to place the larvae in a jar with some leaves – the hoverfly larvae will not eat the leaves but instead rear up to look for prey while the plant pest will eat the leaves.
Hoverfly larvae Grass blue butterfly larvae
Management
There are no registered insecticides for the control of grass blue butterfly in pulse crops. However, insecticides used for the control of helicoverpa will incidentally control this pest as well.