Orchardists beware Argentine invaders

KATHY WEBB
Argentine ants could trigger widespread pest infestations and sooty mould in Hawke's Bay fruit trees, says a scientist.
While holidaymakers are being warned to ensure they don't take Argentine ants home in their tents and camping gear, MAF spokesman Simon O'Connor warns that the ants could create a headache for orchardists and other horticulturalists.
The ants establish farms of insect pests in trees and plants to produce food for them. They physically pick up and carry pests such as aphids and scale to the growing tips, where there is plenty of sugar for them to eat and produce honeydew, which the ants eat.
While the pests are busy producing honeydew for the ants to eat, the ants protect them from natural predators such as wasps and ladybirds, which could normally help to control pests by laying eggs inside them and killing them.
"Lots of ants do that, but Argentine ants excel at it," Mr O'Connor said.
High numbers of pests sucking sap from the most productive areas could seriously affect the health of trees and plants, while excess honeydew created an ideal environment for sooty mould.
Pockets of the ants have already been found in the Bay, so it was important for growers to be on alert, particularly around packhouses.
A surveillance programme and immediate eradication of any infestation would be a very good idea, Mr O'Connor said. Pipfruit New Zealand technical manager Mike Butcher, of Hastings, said growers already monitored their orchards closely for aphids and scale as part of their integrated fruit production systems.
If they were found in any great numbers they could be sprayed with refined or vegetable oils to smother them at susceptible stages of their life cycles. Organic orchards could use lime sulphur.
Sooty mould was not a huge problem, Mr Butcher said. It grew only on honeydew, not fruit.
If any fell on to apples it would be washed off by high-pressure water before packaging, and trees could be cleaned up by rainstorms or irrigation.
Argentine ants, first found in Auckland in 1990, have also been found in Northland, Coromandel, Bay of Plenty, Waikato, Wellington, Nelson and Christchurch.
A survey in Hawke's Bay in 2001 found them in Hastings and Napier. Landcare Research scientist Dr Margaret Stanley said they were likely to have spread much further since then.
Mr O'Connor recommended that anyone finding an infestation should call in a pest control firm. Spraying with fly spray would only cause the ants to disperse and create even more nests.