by Georgina Hall

New Zealand lettuce growers now have another weapon in the fight against the devastating lettuce aphid.

The AphidWatch website (www.aphidwatch.com) now has weekly updates of the numbers of lettuce aphids in flight during the growing season.

Counting the numbers of lettuce aphids collected from suction traps gives a good indication when numbers are peaking - alerting growers to check if their crops need spraying. Four traps are located in Canterbury and one in Hawkes Bay, with another planned for Pukekohe.

The programme is funded by New Zealand's Ministry of Agriculture and Forestry's Sustainable Farming Fund and Vegfed working with Crop & Food Research.

Crop & Food Research aphid researcher, Marlon Stufkens, says lettuce aphids have spread from Kaitaia to the Bluff since they were first reported in New Zealand in March 2002.

The aphids can live all year round on lettuce but a percentage fly to other host plants, such as currants and gooseberries in autumn to lay their eggs. In late spring, when these eggs hatch, this creates a small peak in winged aphid numbers.

However, a much larger peak occurs around January. "This is the worst period to control. The aphids are flying in every day and growers may think their spray programme is not working."

A further peak in numbers occurs in autumn. The timing of the peaks can shift somewhat according to the weather, Mr Stufkens says.

In warmer North Island areas growers may also see a rise in aphid numbers as they leave a favoured weed host, Hawkesbeard, as its flowers die off in early winter. Mr Stufkens says the yellow-flowered Hawksbeard, and Hieracium, can harbour over 73 aphids on their flowers or buds. "They can be a huge source of lettuce aphids."

Mr Stukens says the AphidWatch website provides useful lettuce aphid information: a map of where they are found; photos of host plants, such as Hawksbeard; and articles on ways to limit aphid resistance to insecticides and on the damaging viruses they can transmit.

He advises growers to plough in any unwanted lettuce plants as they can be another source of infestation.