IPM for outdoor lettuce

July 2008
Lettuce aphid was discovered in New Zealand in 2002 and spread rapidly to all growing areas. A big science programme with funding from government and the growers, through Horticulture NZ, has resulted in an integrated pest management (IPM) system for control of the key insect pests, being aphids and caterpillars, and also controls for plant diseases on outdoor lettuces, which has greatly reduced the use of foliar sprays.

Integrated pest management (IPM) systems have been developed and proven for process tomatoes and vegetable brassicas. Essentially they are a regular monitoring programme by growers to reduce chemical inputs and encourage predators and other biological control agents. IPM is becoming increasingly popular worldwide as certain pests and diseases become resistant to chemicals that have been used on crops for many years. When the lettuce aphid was detected in NZ it spread rapidly and built up huge numbers on outdoor lettuces. Up to 6000 aphids on one plant were counted and the market for the vegetable was decimated. Fortunately the IPM development process was underway and funding was obtained quickly from the government and industry to involve more technicians and scientists. In total $2 million has been spent on the IPM process.

Features of an IPM programme

Effective pest and disease control

Production of crops that meet market standards

Use of techniques that emphasise crop monitoring in some form

Reduction of pesticide risks

Use of selective pesticides in preference to broad spectrum materials

Minimal impacts on the environment

Lettuce aphid is the worst pest of outdoor lettuces around the world, but until 2002 NZ and Australia were free of this pest. When it arrived here it spread quickly to all growing regions and no outdoor grower was spared. The aphid flies onto a lettuce and quickly moves down into the growing region where it is out of reach of foliar spray applications and protected from most predators. In fact, foliar spraying of common chemicals like broad-spectrum insecticides kill predators and beneficials and the aphids, protected inside the plant, continue to feed and multiply.

The IPM trials identified that the brown lacewing, native to New Zealand and Australia, is a very effective predator of the lettuce aphid because the small larval stage of brown lacewing can move down deep inside the plant and eat out the protected aphids. Lacewings and other beneficial insects can be encouraged to breed up to numbers which control all the insect pests. Our trials have determined an action threshold based on the number of aphids and predators in a crop, so that growers can be confident that the predators will control the aphid infestation.

Aphids can also be controlled with early treatment of lettuce seedlings by nurseries with a systemic insecticide which basically disappears by the time the lettuce is harvested. There are also European varieties of lettuce resistant to the lettuce aphid but these are generally not suitable for NZs growing and market requirements.

The other important insect pests, caterpillars and thrips may be controlled by a range of other beneficials, mainly hover fly larvae and spiders, so that in some seasons of the year, no insecticide treatments are required at all.

Management strategies have also been developed for the major plant diseases, Sclerotinia and downy mildew.

IPM kit

All 170 outdoor lettuce growers in NZ have received the IPM system on CDROM, plus a glovebox-sized guide to the pests and diseases of lettuce and the predators and other beneficials.

Graham Walker, entomologist at Crop & Food, says most of the insects in a field crop are not pests of that plant. However there are 14 pest species of aphid and two caterpillars which have threshold levels established, plus a minor pest species called thrips.

The glove-box guide will assist growers to get to know the beneficial organisms that live in their crop including predator insects such as the brown lacewing, tiny parasitic wasps, the eleven-spotted ladybird and many others.

Mr Walker says a key message of the programme is the need to monitor.

There is only one secret to getting an IPM regime working well and that is the need to put your boots on and scout the crop. In this way you will get to know if you have enough beneficial insects or too many problem insects. Once you know this, you can make a decision to spray or not to spray and most importantly, what to spray with. Iceberg head lettuce grown outdoors is covered under the IPM, not the hydroponic varieties grown in greenhouses.

Perfect Produce

Allan Fongs Perfect Produce company in South Auckland grows over 200ha of outdoor lettuce each year, with production counted in the millions of head. This is all head lettuce, called Iceberg, with a very small amount of Cos lettuce. Lettuce growing was decimated by the aphid in 2002 said Perfect Partners general manager Kirit Makan and a great deal of time and money was put into finding solutions. It was very impressive how quickly the aphid spread and the level of infestation. Fortunately the IPM initiative was underway and there was a quick response to the problem. We spent money on research also. The lacewing predator response came along about two years later and is effective in spring if the aphid numbers build. But we rely on early systemic insecticide applied by the nursery which supplied our plants. The IPM material will be very useful for the staff to know what to look for and when to control it.