December 2, 2002

Growing lettuces in the home garden will never be as easy as it was, now New Zealand has the lettuce aphid, says a Crop & Food Research aphid specialist.

Marlon Stufkens recently sampled six Christchurch home gardens and found an average of 60 lettuce aphids on each of the lettuce seedlings.

Once infested, the lettuce becomes largely unusable. Lettuce aphids produce a clear honey dew which attracts flies, soil mites and other undesirables into the lettuce, and, under high humidity conditions, the aphids are attacked by a particular fungus which kills them and sticks them firmly to the leaf.

The aphid, which was first found in New Zealand in March this year, has already caused substantial financial damage to commercial lettuce producers in Canterbury, Pukekohe, Nelson and most recently, Hawke's Bay. Gisborne, the Manawatu and Kapiti Coast are thought to have escaped the pest so far.

The lettuce aphid is yellow to green with brown markings on the back of the adults. They may be winged or wingless. Unlike most aphid species, it prefers the new leaves in the lettuce heart and once the lettuce has 'hearted up' there's very little a gardener can do - most insecticides will not penetrate that far.

"Once infested, you can find 100 plus aphids in one lettuce - usually right through to its centre."

Mr Stufkens says summer and autumn will be the worst times for infestation and he recommends checking lettuce plants at least weekly and spraying as required.

Garden centers can recommend the correct chemicals to use.

"However, its also important to check the label to find how long the lettuce must remain untreated in the ground before it can be safely eaten.

"If you don't want to spray the only way round is to keep the crop protected."

He suggests a cloche made of a fine fly screen mesh could be used for protection from planting to harvest while allowing air movement. Before planting out under cover, any aphids should be brushed off or the leaves washed in soapy water.

Mr Stufkens says lettuce aphid is "one of the worst" pests of lettuces in Europe. While resistant lettuce varieties have been developed, they're still experimental in New Zealand and seed is quite expensive.

The aphid has also developed full or partial resistance to a number of pesticides in Europe and has some resistance to two of the four chemicals registered for lettuces in New Zealand.

To help limit its spread, Mr Stufkens recommends good crop hygiene such as burying, not composting, infected lettuces. "Any aphid contaminated leaves should also be wrapped in plastic before going in the household rubbish."

It is important that lettuces are not sent to areas where the aphid has not yet been found.

"The more we can slow the spread of this aphid, the longer period of grace we have to work out ways of minimising its impact on lettuce crops."

The aphid is known to winter over on blackcurrants and gooseberries and on a brighter note, it does not seem to have caused any major problems on these crops so far this season.