Next step in lettuce aphid research

“I view the currant-lettuce aphid as an interesting challenge, not only because it’s such a troublesome pest in lettuce crops, but also because it’s important to Rijk Zwaan,” says Anneke. “It was one of the first resistances our company introduced, back in 1996. Unfortunately the resistance was overcome in 2007 because the aphid adapted. That’s why I am keen to play a role in finding a new resistance.” 

Devoted to the aphid 

Anneke, who joined Rijk Zwaan three years ago, is able to pursue that goal in the company’s phytopathology department, where around a hundred colleagues are focused on pests and diseases. For the past three years, Anneke has enjoyed devoting every workday to the currant-lettuce aphid, which is mainly found in temperate climates worldwide. Her aim? To develop tests as the basis for research into Nasonovia ribisnigri resistance. 

Lettuce specialists 

In order to develop a testing method, she has to study the currant-lettuce aphid closely. Today, she is an expert on it. “The aphids are usually green, but they can also have a yellow or reddish-brown colour, with characteristic black markings on their legs. They feed on phloem – the living tissue in plants that transports things like sugars and proteins – using an efficient mouthpart called a stylet,” she says. 

Another distinguishing feature of these aphids is that they can only survive on lettuce or endive. “They are real specialists. They have completely adapted themselves to lettuce, making it easy for them to evolve,” explains Anneke. 

Blackcurrants

However, the aphid actually originated in a different crop: blackcurrants. “That’s how it got the name ribisnigri. The aphids survive the winter as eggs on blackcurrant bushes, where they hatch into nymphs in the spring. After maturing into adults, the aphids then migrate to lettuce crops,” continues the phytopathologist. 

Giving birth to live young

Notably, in lettuce, the adults not only reproduce asexually to lay eggs, but also give birth to live young. “That contributes to their success. After all, eggs are vulnerable. It’s much more beneficial to give birth to live offspring that can feed immediately. Young aphids can produce their own offspring after just one week, so they multiply very rapidly.” 

Besides that, adult aphids can develop wings and fly to another plant if their existing plant becomes overcrowded. That’s how the aphids spread through a lettuce field so quickly. 

Difficult to detect and tackle

Besides rapidly multiplying and spreading, the aphid has found even more ways to ensure its survival. For example, it hides and forms colonies deep inside the lettuce head. This is a clever strategy, according to Anneke: “It not only makes it hard for growers to detect the aphids, but also makes it difficult to tackle them with pesticides or biologicals. And lettuces infested with aphids are unsaleable.” 

Wild lettuce plants

All of this underlines why a resistance to this extremely troublesome pest is so essential for lettuce growers. Anneke explains that a team of Rijk Zwaan specialists has been working on that for many years: “It starts with setting up large-scale testing based on ‘accessions’, in which our team introduce the aphid onto wild lettuce. Needless to say, plants that occur in the wild suffer from pests too, so there’s a chance that they have developed the resistance we’re interested in – which would give us a new source,” states the phytopathologist. 

A step closer

Additionally, she is investigating whether – and if so, for how long – the aphid can survive on a resistant lettuce plant and whether it produces offspring. If the aphid struggles, that’s good news for growers. Every research activity brings the industry a step closer to lettuce varieties with high resistance against Nasonovia ribisnigri. “My work will still continue when we achieve that as Rijk Zwaan. My goal is to find resistances against all aphid types,” concludes Anneke.