NZPPS pesticide resistance management strategy

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Lettuce aphid resistance management and prevention strategy

Lettuce aphid Nasonovia ribisnigri was first seen in
New Zealand in 2002. (Photo: Whitney Cranshaw,
Colorado State University, www.forestryimages.org)

M.A.W. Stufkens¹, G.P. Walker² and N.A. Martin^2
1Crop & Food Research, Private Bag 4704, Christchurch, New Zealand
²Crop and Food Research, Private Bag 92169, Auckland, New Zealand

(Revised November 2004)

Reason for strategy and update

Lettuce aphid, Nasonovia ribisnigri, also known as currant-lettuce aphid, is capable of becoming resistant to a variety of insecticides. Pest management strategies aimed at preventing or minimising resistance will help maintain control and conserve the effectiveness of existing products. This is an update of the resistance management strategy (Stufkens et al. 2002).

Background

The lettuce aphid is a major pest of lettuces (Lactuca sativa L.) and Ribes spp. (e.g. currants, gooseberries) in Europe where it originates (Keep & Briggs 1971; Blackman & Eastop 2000). In recent years this insect has spread to other continents, notably North America (Arizona) in 1998–99 (Palumbo 2000; Palumbo & Hannan 2002), causing major problems for growers. The first record of the lettuce aphid in Australasia was in the vegetable growing area of Marshlands, on the outskirts of Christchurch, New Zealand, in March 2002 (Stufkens 2002).

Egg laying sexual females of lettuce aphid move on to the primary hosts, Ribes spp. (e.g. currants and gooseberries), in autumn. The eggs, which are laid around the buds, hatch in the spring and multiply on primary hosts for a few generations, causing leaf curling and decreased stem elongation. The adult female aphids then fly on to secondary hosts, mainly liguliforous Asteraceae (e.g. lettuce, chicory, hawksbeard, hieracium), some Scrophulariaceae (e.g. speedwell) and Solanaceae (e.g. tobacco, petunia) (Blackman & Eastop 2000). Lettuce aphid is found on all growth stages of the lettuce plant. Observations indicate that it causes leaf distortion and reduced vigour in seedlings, while at harvest the presence of aphids can be a cosmetic quality problem. On hawksbeard (Crepis capillaries), a common weed in New Zealand, the lettuce aphid is mainly found on the upper flower stem and flowers. The aphid is hard to control on lettuce crops because it lives deep inside lettuce hearts where few insecticides can penetrate. The aphid is a vector of gooseberry veinbanding disease, cauliflower and cucumber mosaic virus (Blackman & Eastop 2000).

In Europe, lettuce aphid has shown varying levels of resistance to organophosphates, carbamates and synthetic pyrethroids (Barber et al. 1999). Imidacloprid is one of the few insecticide groups that have been effective in controlling the lettuce aphid both in New Zealand and overseas. The development of resistance to imidacloprid by pest insects is of significant concern (e.g. whitefly in Spain, Colorado potato beetle in the USA and low levels of tolerance have been detected in green peach aphid in Japan and UK) (Foster et al. 2003).

Current status of lettuce aphid resistance in New Zealand

Exposure to the recommended concentration for field application of 12 insecticides showed that lettuce aphid in New Zealand was partially resistant to two insecticides, acephate and methomyl, with only 80.6% and 90.7% mortality respectively. These two insecticides failed to give good field control of the aphid (Workman et al. 2004; Stufkens & Wallace 2004).

Products with label claims for aphid control in lettuce, black currants and gooseberries in New Zealand

¹VC = aphid control on various vegetable crops.
²AC = aphid control on various crops.
Note 1 = caterpillar control on vegetable brassicas. No aphid control..
Note 2 = widely used to control aphids on lettuce and has been effective in experimental trials.

Resistance management and prevention strategy

The general strategy is to reduce the need for control of lettuce aphid by reducing virus sources and aphid reservoirs and by non-insecticide controls, such as aphid resistant plants. Selection pressure on aphids in crops can then be reduced by applying insecticides only when necessary to reduce feeding damage and aphid numbers. Only use insecticides that will reach to where the aphids are living in the plant.

Note: Control failure does not always imply resistance

The following guidelines are recommended.

• Maximise virus control by standard management practices. These include:
  • removal of infected plants within a crop
  • elimination of weed sources that may harbour viruses
  • use of screens to prevent aphids entering greenhouses
• Remove alternative host plants for lettuce aphid, e.g. chicory, hawksbeard. Plough under residue lettuce crops.
• Identify the lettuce aphid, since different aphid species may require different strategies (most other aphid species only live on the outer leaves while lettuce aphid prefers the new inner leaves).
• Maximise biological control by the use of selective insecticides to preserve predators and fungal pathogens of aphids.
• Use lettuce aphid resistant cultivars.
• Monitor plants to ensure foliar insecticides are applied only when necessary. Check predator levels since predators and aphid fungal pathogens have given good control of the lettuce aphid in late spring, autumn and winter.
• Maximise the effectiveness of imidacloprid and minimise risk of resistance developing by:
• having an imidacloprid free period every year. Select a time when using aphid resistant cultivars and/or aphid free flight periods and/or when biological control of aphids is effective.
• using imidacloprid only as a seedling drench not as a foliar spray. No more than one application per paddock per year is recommended.
• correct application of imidacloprid to seedling transplants. Good even coverage over the whole tray and good penetration into the potting mix is important.
• Choice of effective insecticides should be based on a knowledge of insecticide resistance in local populations and the position in the plant the aphids are found in.
• Use the correct label rates and application procedures.
• Alternate between insecticide groups.
• If control failures are suspected, crops should be treated with an insecticide from a different chemical group.

Ribes (currants and gooseberries)
Some of the insecticides recommended by Blackcurrants New Zealand LTD for use in the spring to control aphids (1-2 sprays if necessary) are the same as those used on lettuces. These are Pirimor, Thiodan and Vydate. The recommendation to use Pirimor may be replaced with Chess in 2005. This implication is that some insecticides are likely to be used for aphid control in both Ribes spp. and lettuce in the same season in the same area. This may increase the chances of insecticide resistance developing. Insecticide usage on these two crops needs to be coordinated and recommendations checked annually to prevent over use of insecticides from any one chemical group.

Glasshouse lettuces
Lettuces are grown in glasshouses throughout the year and the plants develop more quickly than in the field, providing an environment that suits aphids. Insecticide resistance may be more common in glasshouses, due to the more frequent use of insecticides. Considerable care is needed to ensure that insecticides from different chemical groups are used. Also the incorrect use of insecticides increases the chances of insecticide resistance developing (e.g. using the wrong dilution rates in hydroponic systems).

Research strategy

The distribution of resistance among the major cropping areas should be assessed and the best window for an imidacloprid-free period determined for the different lettuce growing regions of New Zealand.

Implementation

Growers should implement virus control strategies. Growers should monitor plants to ensure foliar insecticides are only applied when necessary. Insecticides registered for use on lettuce should carry the following label statement:

IMPORTANT - RESISTANCE MANAGEMENT
Resistance to this pesticide may develop from excessive use. To minimise this risk use strictly in accordance with label instructions. Avoid using this insecticide exclusively all season and avoid unnecessary spraying or other methods of application. Maintain good cultural practices.

References

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