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Overwinter host plants of Cotton Aphid (Aphis gossypii) and implications for managing abundance and resistance.

Tanya Smith 1 , Lewis Wilson 1 , Simone Heimoana 1 , Grant Herron 2 and Bernie Franzmann 3
Australian Cotton CRC and 1 CSIRO Plant Industry, 2 NSW Agriculture, 3 Qld DPI&F.

From Proceedings of the 13th Annual Cotton Conference
8-10 August 2006
Organised by ACGRA

Key Points

• ∙Cotton aphid uses a wide range of hosts in winter when cotton is not available. 
• Farm gardens providea habitat for cotton aphid especially when conditions are dry. Aphids sampled from such sites showresistance to traditional chemical controls.
• Aphids maintain chemical resistance within populations by reproduction by cloning.
• Reducing the abundance of aphid hosts on farm through winter, especially weeds, may help reduce abundance on cotton. A list of key hosts is provided.

Winter survival in Cotton Aphid
Cotton aphids (Aphis gossypii) do not have a diapause stage under Australian conditions and must have plant hosts on which to survive. Through winter, populations contract to a smaller subset of available hosts, and population growth slows in the cold conditions. These populations provide the nucleus for populations which in spring move to cotton and contribute to later aphid outbreaks.

Reducing overwinter survival of aphids may reduce the risk of aphid outbreaks on subsequent cotton crops. However, there is little local knowledge of the hosts cotton aphid uses. To address this, a range of sites was selected across the NamoiValley, from 2001 - 2004,at which the abundance of aphids on the succession of weed and native species was studied through regular sampling. Similar studies were also donein the Darling Downs. This study showed that cotton aphid maintained low population levels over winter, on a large number of hosts including weeds, ratoon cotton, and gardenplants (Tables 1 & 2).

Additionally, to clarify if particular winter-growing crops would host cotton aphids we caged groups of cotton aphid oncrop plants in the field and monitoredaphidpopulation growth. These crops included woolly-pod vetch, faba bean, canola, lupins, and lucerne. Aphids did not survive on lucerne, vetch or canola, but did survive, albeit poorly, on both lupins and faba beans.The studies with crop hosts suggest that common rotations such asvetch are unlikely to contribute to aphid survivalthrough winter, unless these crops were infested with host weeds.

Due to the dry winters at the start of this project, data on overwinter survival of aphids was scant due to the scarcity of suitable hosts.The winter of 2003 was slightly wetter and there were more recordings of aphids and in the winter of 2004 where there were regular moderate rains the recordings of aphids on weed hosts were more frequent. This highlights the strong effect of season conditions, particularly winter rainfall and temperatures, on aphid population levels through winter and their build up in spring. This is supported by the consistent findings of cotton aphidinfarm and domestic gardens through winter months where water provision was good and temperature buffers, e.g. plants growing near a wall, were sometimes present. They were often scarce elsewhere during these drytimes, especially on farms where there were few suitable crops or weeds.

Good rainfall over winter encourages growthof host material for cottonaphid. This cancontribute tohigher abundance on young cotton and the risk of faster population growth, particularly if beneficial populations are disrupted with broad-spectrum insecticides. It will also provide hosts, such as marshmallow, which is an alternative host for Cotton BunchyTop Disease. These conditions occurred in the 1998-1999 seasonand drastically affected yields. Removal of the aphid hosts is therefore an important aspect in aphid control. Without hosts the aphids have no food source and will not survive.

Table 1. Hosts of cotton aphid (Aphis gossypii) recorded from regular sampling on cotton farms and weedy areas in the Namoi Valley, between May 2002 and September 2004.

Plant Species	Common name	Warm-Oct-May	Cool -Jun-Sept
Bidens pilosa	Cobblers pegs	xxx	xxx
Bidens subalternans	Beggars ticks	xxx
Boerhavia diffusa	Tarvine	x
Cajunus cajan	Pigeon pea	xx
Chamaesyce drummondii	Caustic weed	xxx	xx
Chenopodium pumilio	Small crumbweed	x
Chrysanthemum sp.	Chrysanthemum	xxx
Citrullus lanatus	Camelmelon/watermelon	xxx
Citrullus vulgaris	watermelon	xx	xxx
Convolvulus erubescens	Australian bindweed		x
Conyza bonariensis	Flax leaf fleabane	x	x
Cucumis myriocarpus	paddymelon	xxx
Cucumis pepo	pumpkin	xx
Cucumis sativus (cucumber)	cucumber	xx
Cucurbita maxima (pumpkin)	Winter squash	xx
Cucurbita pepo(zucchini)	zucchini	xx	x
Echium plantagineum	Paterson’s curse		xx
Datura ferox	Fierce thornapple	xxx
Fallopia convolvulus	Black bindweed	xx
Glycine max	Faba bean	xx
Gossypium hirsutum	Cotton	xxx	xxx
Helianthus annus	Sunflower	xxx	xx
Hibiscus esculentus	Okra	xxx
Hibiscus rosa-sinensis	Hibiscus	xxx
Hibiscus sabdariffa	Rosella	xxx	xx
Hibiscus trionum	Bladder ketmia	xxx	xx
Impatiens hawkeri	Impatiens	x
Lamium amplexicaule	Deadnettle	xx	xx
Malva parviflora	Marshmallow	xx	xx
Malvastrum coromeliandrum	Malvastrum	xx	xx
Medicago polymorpha	Burr medic	x	xxx
Physalis ixiocarpa	Annual Ground Cherry	xxx
Portulaca oleracea	Pigweed	xx
Rapistrum rugosum	Turnipweed	xx	xx
Silybum marianum	Variegated thistle		x
Sida rhombifolia	Paddy’s lucerne	xx	x
Solanum nigrum	Blackberry nightshade	xx
Sonchus oleraceus	Milkthistle	xx	xx
Trianthema portulacastrum	Giant pigweed	xxx
Triticum aestivum	Wheat		x
Tribulus terrestris	Cat-head	xx
Urtica urens	Small nettle	x	x
Verbena bonariensis	Purpletop	x	x

X, more than one aphid found on a plant on one sampling occasion
XX, more than one aphid found on one plant on more than one sampling occasion, but never more than 100/plant
XXX, more than one aphid found on one plant on more than one sampling occasion, and more than 100 aphids found on a plant at least
once.

Table 2. Hosts of cotton aphid (Aphis gossypii) recorded from town and farm gardens in the Namoi Valley, between May 2002 and September 2004.

Plant species	Common name	Warm Oct-May	Cool Jun-Sept
Abutilon sp.	Abutilon		xx
Argyranthemum	chamomile		x
Begonia sp.	Begonia	x
Calibrachoa	Calibrachoa	x
Chili	Chili	xx
Chrysanthemum sp.	Chrysanthemum		xxx
Citrus (lemon)	Lemon		xx
Citrus (Lime)	Lime		xx
Citrus (orange)	Orange		x
Cuphea llavea	Tiny mice	xx	x
Gazania sp.	Gazania		xx
Hypoestes phyllostachya	Polka dot plant		x
Lagenaria siceraria	Bottle gourd	x
Lantana spp.	Lantana	x	xx
Melissa officinalis	Fennel	x
Mentha sp	Mint		x
Nerium	Nerium		x
Origanum marjorana	Marjoram	xx
Ozothamnus  sp.			x
Rosa sp.	Rose	x	x
Salvia sp.	Salvia	xx	x
Solanum melongera	Eggplant/aubergine	x	x
Zinnia peruviana	Zinnia		x
Zinzibar Ginger			x
Citrus hystrix	Kaffir lime		x
Hibiscus rosa-sinensis	Hibiscus		xxx
Malus	Apple	x	xx
Vinca	Vinca		x

X, more than one aphid found on a plant on one sampling occasion
XX, more than one aphid found on one plant on more than one sampling occasion, but never more than 100/plant
XXX, more than one aphid foundon one plant on more than one sampling occasion, and more than 100 aphids found on a plant
at least once.

Resistance in relation to winter aphid populations
Cotton aphid uses hosts in farm and urban gardens along with other sites, through winter. Farm gardens in particular canpossiblyserve as reservoirs for insecticide resistant cotton aphids and may be particularly important sources of aphids in dry years when other hosts are scarce. We investigated this possibility by sampling aphids from farm and domestic gardens and testing their resistance to the key insecticides used for aphid control in cotton crops.

Samples collected from farm gardens and from some town backyards during the study period often showed high levels of insecticide resistance to insecticides used to control them in cotton (Table 3). Also aphids off several of the weeds, such as deadnettle, fierce thornapple, beggars ticks, bladder ketmia and caltrops showed resistance, reinforcing that weed hosts may be important in maintenance of resistant aphid populations. This reinforces that insecticide resistant clones do overwinter on farms and therefore farm hygiene is very important.

Control of overwinter hosts is particularly important if the aphids are resistant. Under Australian conditions cotton aphids reproduce asexually, with adult female aphids giving birth to live young. As such there is no mating, so there is no opportunity for dilution of resistance by matings between susceptible and resistant individuals (as there is for instance with Helicoverpa armigera). Some clones of cotton aphid have developed resistance to organophosphates and pirimicarb, though there are some clones that are still susceptible. If resistant aphids are allowed to overwinter on farms the chance that aphid populations encountered in the next season will also be resistant increases. Eliminating the hosts of these aphids will reduce survival, at least giving the chance that aphids that enter the farm subsequently may be susceptible.

Conclusions
Farm hygiene is an important part of farm management for good cotton aphid control. Cotton Aphid overwinters on non-cotton, and cotton hosts. Removal of these hosts is a tactical way to control the aphid prior to the cotton growing season. Pesticide resistant clones of cotton aphid have been found to overwinter on various hosts, as indicated by collections of resistant aphids fromfarm gardens. Future work will compare the resistance status of aphids collected from weeds on farms and at some distance from farms to see if there is a difference, as this may be important for management. However, it is important to remember that farm gardens were only sampled because other hosts were scarce and in these years, aphid populations on cotton occurred very late in the season– do to the low starting point in spring. In years where rainfall promotes growth of weeds on farm, and hence the overwinter survival of aphids is likely to be higher, it will be important to control the weeds to reduce the overwinter survival of resistant clones.

Acknowledgements
CRDC provided funding for much of this research (CSP103C, CSP145C, CSP147C, DAN163C, DAQ119C). Thanks to Dee Hamilton, Deon Cameron, Lex Cameron, Nemrod Mor, Mark Strahle, Lawrence Smith, K

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