The lettuce aphid – Nasonovia ribis-nigri – is a major pest of salad crops, particularly lettuce. Originating from Northern Europe, it was confirmed in Tasmania mid-March 2004. A dangerous feature of the aphid breeding cycle is that most species do not need a male to reproduce.
Immature aphids are yellow-green and wingless while adults can be winged or wingless, about 2-2.5 mm in length, greenish to yellow-green with irregular narrow dark patches on their back. Hosts include: Lettuce, Ribes (gooseberry, and red, black and white currants), endive, chicory, Hawksbeard and Hawkesweed.
Lettuce aphid is primarily a contamination pest with head contamination by lettuce aphid making them unsaleable. In general, the direct damage from the aphid is limited on lettuces, though large numbers of aphids may stunt young plants and cause leaves to become pale in colour and slightly deformed. Lettuce aphid can also transfer cucumber and lettuce mosaic virus.
- Resistant varieties – Some seed companies have utilised an aphid resistance gene in some of their lettuce varieties. Lettuce aphid does not feed or reproduce and immatures do not reach adulthood on resistant plants. Other seed companies are now also using this gene and have a range of fancy lettuce ready for commercialisation, but head lettuce varieties are not yet available.
- Source control – Care should be taken that lettuce aphid is not introduced via transplants or movement of other plant material.
- Monitoring – Crops need to be closely inspected. In seedlings and prehearted lettuce, attention needs to be paid to the innermost leaves and in folds or crinkles in the leaf. Once the lettuce has hearted some destructive sampling is needed. Since lettuce aphid may occur non-uniformly across paddocks, a number of widely dispersed sites need to be sampled. Weed hosts should be sampled, including Hawksbeard and wild lettuce if nearby.
- Beneficial insects – Aphids are eaten by a range of beneficial insects which are likely to be more efficient at reaching and killing them than insecticides, particularly in hearted lettuce. However the use of broadspectrum insecticides is likely to kill many beneficials.
- Soft foliar insecticides – Pirimicarb (Pirimor®) is a soft aphicide registered for use on aphids in lettuce in Australia. Although resistance has been reported to pirimicarb in Europe, tests in NZ show that their lettuce aphid is not resistant. There are some newer potentially softer chemistry that is not currently available that may get permits or registration.
- Broadspectrum insecticides – Dimethoate, maldison, methidathion (e.g. Supracide®), and pyrethrins are the other registered aphicides for lettuce in Australia. In Europe resistance to cypermethrin, dimethoate, and endosulfan has been recorded. Broadspectrum insecticides are undesirable in an IPM system as they kill most, if not all, beneficial insects.
- Seedling drenches – An emergency permit has been granted for imidacloprid (Confidor®) as a seedling drench in lettuce.
- Sanitation – Control surrounding weeds. Any infested lettuce, including recalled product, should be buried as soon as possible.
- Post harvest washing – Washing of head lettuce will not disinfest. Loose leaf lettuce washing processes with fine water filtration systems can reduce numbers of aphids present in packed lettuce.